Reason for
Bearings failure !

What do we mean by good bearing life? Most of us change the bearings every time we disassemble the equipment to replace the mechanical seal or the packing sleeve. Is this really a sensible thing to do? If you think about it for a minute there is nothing in a bearing to wear out, there are no sacrificial parts.

Bearing life is determined by the number of hours it will take for the metal to *fatigue" and that is a function of the load on the bearing, the number of rotations, and the amount of lubrication that the bearing receives. Pump companies predict bearing life measured in years.

To understand the term "fatigue" we will conduct an experiment:

Straighten out a standard paper clip.

Flex it a little and then let it go. you will notice that it returns to the straightened position. You could repeat this cycle many times (many years actually) without breaking (fatiguing) the metal because you are cycling the metal in its "elastic range" (it has a memory similar to piece of rubber).

Now we will bend (stress) the paper clip a lot further and you will note that it did not return to the straightened position. This time you stressed the metal in its' "plastic range" where it did not have a memory.

If you bend the metal back and forth in this plastic range it will crack and break in less than twenty cycles. The metal fatigued more quickly because it "work hardened" and became brittle. The more you stress the metal by flexing it the quicker it will work harden and break.

You have just demonstrated that fatigue is a function of stress and cycles.

When the bearing is pressed on a rotating shaft the load passes from the inner race (inside ring) through the balls to the bearing outer race (the outside ring).

Each ball carries a portion of the stress as the balls roll under the load. It is this stress that will eventually fatigue the metal parts.

When a pump is operating at its best efficiency point (B.E.P.) the only load the bearing has to carry is:

The weight of the rotating assembly.

The stress caused by the interference fit on the shaft.

Any bearing preload specified by the manufacturer.

The fact is that most bearings become overloaded because of:

The wrong interference fit between the bearing and the shaft (the shaft was out of tolerance).

Misalignment between the pump and its' driver.

Bent shaft

An unbalanced rotating element.

Pushing the bearing too far up a tapered sleeve.

Operating the pump off of it best efficiency point (B.E.P.).

Shaft radial thermal expansion.

A futile attempt to cool the bearings by cooling the bearing housing with a water hose or some other similar system. Cooling the outside diameter of a bearing causes it to shrink, increasing the interference and causing additional stress.

Cavitation.

Water hammer.

Axial thrust.

The bearing housing is sometimes out of round.

Pulley driven designs.

Vibration of almost any form.

The impeller is located too far away from the bearing. This is a common problem in many mixer/agitator applications.

A bad bearing was supplied. This is becoming more of a problem with the increase in counterfeit parts we are finding in industry.

This overloading will cause heat to be generated, and heat is another common cause of premature bearing failure.

Heat will cause the lubricant to :

Decrease in viscosity, causing more heat as it loses its ability to support the load.

From a "varnish" residue and then "coke" at the elevated temperature. This "coking" will destroy the ability of the grease or oil to lubricate the bearing. It will also introduce solid particles into the lubricant.

In addition to the heat generated by overloading we get additional heat from:

The oil level is too high or too low. Too often pumps are aligned but not leveled.

The bearing was over greased.

The shaft material is conducting heat from the pumpage back to the bearing housing. This is a common problem in heat transfer oil pumps, or any time a metal bellows seal is used in an application and the stuffing box cooling jacket is shut off or inoperative.

A loss of barrier fluid between double seals causing a temperature rise that conducts heat back to the bearings.

A failed cooling jacket in the bearing housing around the stuffing box or built into the seal gland.

Grease or lip seal contact on the shaft, right next to the bearings.

A failed cooling "quench" in an A.P.I. type seal gland.

A leading bearing manufacturer states that the life of bearing oil is directly related to heat. Non contaminated oil cannot wear out and has a useful life of about thirty years at thirty degrees centigrade (86 F.). They further state that the life of the bearing oil is cut in half for each ten degree centigrade rise (18 F.) in temperature of the oil.

This means that oil temperature regulation is critical in any attempt to increase the useful life of anti friction bearings.

Probably the major cause of premature bearing failure is the contamination of the bearing lubrication by moisture and solids. As little as 0.002% water in the lubricant can reduce bearing life by 48%. Six percent water can reduce bearing life by 83% percent.

There are several methods used by pump companies to keep this water and moisture out of the bearing housing :

A finger ring to deflect packing or seal leakage away from the bearings. A silly arrangement at best.

Keeping the bearing oil hot to prevent the forming of condensation inside the bearing case. A ridiculous system when you consider that bearing life is directly related to heat.

The use of "so called" sealed bearings. You can call them any thing you want, but the seals will not seal anything, especially moisture or water.

Grease or lip seals that have a useful life of about two thousand hours (84 days at 24 hours per day) and will cut the expensive shaft directly under the seal lip. Double lip seals will cut the shaft in two places.

Labyrinth seals that are superior to  lip seals. but not totally effective because you are still trying to seal with non contacting surfaces that are useless Statically.

The moisture comes from multiple sources :

Packing leakage flows back to the bearing area.

Because of packing leakage a water hose is used to wash down the area. This washing splashes on to the pump bearing case also.

Aspiration, moist air enters through the lip or labyrinth seals when the bearing case cools down.

A seal quench gland that often has steam, condensate or cooling water leaking out and directed at the radial bearing.

The moisture causes several problems :

Pitting and corrosion of the bearing races and rolling elements that will increase the fatigue of the metal components.

Free atomic hydrogen, in the water, appears to cause hydrogen embrittlement of the bearing metal accelerating the fatigue.

A water and oil emulsion does not provide a good lubricating film.

We get solids into the lubricant from several sources :

Metal seal cage wear. This is the part the separates the balls that are held between the bearing races. It is often manufactured from brass or a non metallic material.

Abrasive particles leach out of the bearing housing casting.

Often solid particles were already contaminating the grease or oil we are using for the lubricant.

Solids were introduced into the system during the assembly process because of a lack of cleanliness.

Airborne particles penetrate the bearing seals.

Particles worn off of the grease or lip seals penetrate into the bearings.

How to keep solids and moisture out of the bearing housing.

Seal the inside of the bearing housing with epoxy or some other suitable material to stop rusting and to prevent solids from leaching out of the metal case. If you do this be careful about using some of the new high detergent lubricants. They might be powerful enough to remove this protective coating.

Replace the grease or labyrinth seals with positive face seals. In the future, you are going to need these seals to prevent hydrocarbon fugitive emissions.

Install an expansion chamber outside of the bearing casing to accept the air (approximately 16 oz. or 475 ml, in a typical process pump) that expands as the bearing casing increases in temperature. Without this expansion chamber approximately one atmosphere of pressure will build up in the bearing housing. This is not a problem for a mechanical seal, but during long periods of shut down the pressure could be lost.

Clean the oil in the bearing casing by installing a simple oil circulating and filtering system or change the oil frequently.

When do you go from anti-friction ball and roller bearings to hydrodynamic (sleeve) bearings in a centrifugal pump?

Any time the DN number exceeds 300,000 (Bearing bore times rpm)

If the standard bearings fail to meet an L10 life of 25,000 hours in continuous operation or 16,000 hours at maximum axial and radial load and rated speed.

If the product of the pump horsepower and speed in rpm is 2.7 million or greater.

The past several years have seen a decrease in the quality of the bearings available for rotating equipment. We find prepacked bearing being shipped with too much, or no grease at all. Stabilization temperatures have changed and overall quality has diminished. If you adopt the above suggestions you should not have to be changing your bearings as frequently as you are now.

Why Copper Rotors Only in Large
Submersible Sewage Pump motors
Under Indian Operating Conditions?

Material for Rotor is the most critical but least appreciated aspect of the large Submersible Sewage Motor Pump Specifications.

The situation is further confounded by the prevalent misunderstandings amongst the users about relative merits/demerits of the two candidate materials viz, Copper and Aluminum.

The ensuing article, after dwelling upon all the relevant aspects, strongly advocates exclusive incorporation of Copper Rotors for ensuring operational reliability and efficiency.

1) Comparative Material Properties Analysis : Enclosed Comparative analysis of properties (Refer to Table 1.) conclusively establish that due to superior electro mechanical properties; Copper is decisively better both on efficiency and several operational reliability considerations and therefore Copper would always remain the first choice material for rotor.

Critical advantages of copper rotors over Aluminium rotors can be elaborated as follows :

a) Motor Efficiency : Electrical conductivity of copper is 60% higher than Aluminum, therefore efficiency of electric motor with copper bar rotors is higher than with Aluminum Die cast rotors by at least- one percentage point.(Refer to Table 2.)

This is due to around 15 to 20% reduction in overall energy losses (rotor resistance and windage/friction losses) achievable with copper bar rotors. Hence, for the same required bkw energy cost with copper bar rotors would be significantly less.

Still, less efficient Aluminum die cast rotors were considered fair enough due to their substantially lower cost until the explosion of global energy crisis in 1970 which eventually led to the promulgation of the path breaking Energy Policy Act , 1992 in U.S.A. which incidentally is the largest global consumer of electricity (25% of the annual global consumption of 15 TKWh)

Estimated electrical motor energy consumption in U.S.A. is 63% of the total industrial energy consumption. According to the findings of Copper Development Association, U.S.A: one percentage point improvement in motor efficiency saves approximately 20 billion KWh equalling around 1.4 billion U.S. dollars.

After the enactment of the above Energy Policy Act in 1992, the subsequent trend in U.S.A is to reconvert the more economical aluminum die cast rotor motors to more efficient Copper rotor motors in recognition of the fact that in Life Cycle Cost of the electric motor purchase cost forms just 2% and energy cost constitutes a whopping 97%.

The ongoing Aluminum die cast replacement by copper bar activities can be tracked on

This lead taken by U.S.A. is being closely followed by Canada, China, Australia, Brazil and Mexico. These countries have so far adopted mandatory high (eff. 1) Minimum efficiency Performance Standards (MEPS). European Union is also expected to follow suit. (Refer to Table 4.)

Back in India, Government of India has also already passed Energy Conservation Act in 1991. As a follow up measure many State Governments have also issued specific directives for using energy efficient equipments in all their schemes which dictates the selection of more efficient (and also reliable) copper bar rotors only.

With projected rapid industrial growth In India, Energy Generation at any point of time would always remain way behind rapid increase in Energy Demand; in such scenario, energy conservation through more efficient copper bar rotors would be the need of the hour.

b) Longer insulation life : Insulation is the most vulnerable component in the electric motor; Therefore longer insulation life would automatically ensure longer motor life and greater reliability.

Rotor temperature regulation plays a direct and critical role in increasing insulation Life and in turn motor life and reliability.

In contrast to the normal water pumping, sewage pumping is inherently more arduous as these pumps are constantly subjected to frequent on/offs caused by

1) Power failures.

2) Irregular and erratic pattern of sewage in-flow to the sump.

3) Frequent intermittent impeller choking during pumping operation caused by prevailing unsatisfactory solid waste management and unsatisfactory quality/maintenance of mechanical screens.

Further, normally, sewage pump capacities are determined on the projected sewage inflows up to 15 years. Hence during the initial commissioning, most of these pump capacities happen to be oversized resulting in far more frequent on-off operations of the motor.

Consequently ,compared to maximum 314 start/stops per hour operation of water pumps for which incidentally standard motors are designed , sewage pumps are subjected to many more(10 to 15) start/stops operations per hour.

The maximum damage to the motor (rotor) occurs during restarting of each such frequent on/off cycle of the pump. During each such starting; current is around 6.5 times the rated current of the motor resulting in corresponding intense heating of the rotor and the insulation. 4 times the reduced starting current attainable with auto transformers/soft starters is still damaging enough from this consideration.

Due to its Physical properties (specific heat and density) Aluminum rotors get much hotter than copper bar rotors. Generally for the same motor rating and for the same design temperature, temperature rise inside the motor due to above on -off sewage pumping cycles with Copper Bar rotors is about 5 degrees lower than with Aluminum die cast rotors.(refer table 3)

In motor industry it is accepted that one degree lower temperature rise within motor increases the insulation life and in turn motor life roughly by 10% This 5 degree lower temperature rise thus would result in the increase in insulation and motor life with these Copper Rotors by about 50%.

Hence for rotor application copper bar material would always be better and preferable both in conventional and submersible sewage pumps.

c) Special relevance for large Submersible Sewage pumps: Surface motors and submersible motors though apparently similar are fundamentally different in their cooling mechanism.

As the name suggests T.E.F.C surface motors are totally enclosed fan cooled whereas submersible sewage pump motors are dry type without these in built cooling fans and depend entirely on submergence in surrounding sewage itself for their cooling by convection. Naturally extent of submergence would determine extent of cooling with full submergence providing most effective cooling.

For Small and Medium rating Submersible sewage Pumps up to around one meter pump submergence is required and can normally be provided at site.

Due to large height (around two meters) and difficulties in providing the required full submergence, these large pumps, often operate under partial/full dry running conditions resulting in inadequate heat dissipation. Further, Service factor in large rating motors is usually one which means that there is no in built buffer in these large Submersible sewage motors to take care of such anticipated operational overload conditions caused by the combination of lower supply voltage/frequency, frequent on/offs cycles, impeller choking, together with ineffective cooling due to partial/full dry running conditions.

Integral cooling jackets are sometimes recommended as a solution for cooling where full submergence is not feasible due to site problems, Contrary to the claims, however, these jackets can never be as effective as full submergence and the situation can get still worse in the event where this jacket gets choked with plastic material in the sewage circulating through these jackets.

In this scenario Copper Bar Rotor Material due to its temperature restricting property emerges as the exclusive natural choice. Aluminium Die Cast rotors with inferior electromechanical properties under such imperfect motor cooling conditions would be risky and should be avoided.

d) Longer Bearing Life and Reduced Down Time: Reduced operating motor temperatures also ensure increased bearings life and reduced maintenance cost and down time

e) Longer rotor life: For a given temperature change in motor during on-off cycles, thermal expansion for Aluminum is 35% higher than in Copper and at the same time aluminum has lower tensile and fatigue strength which makes Aluminum die cast rotors more prone to Premature mechanical failure than copper bar rotors

f) Better thermal withstanding capacity: Due to higher melting point copper (1083 degree centigrade for copper and 660 degree centigrade for Aluminum) can better with stand thermal cycling over the life of the motor and thereby ensuring longer life.

g) Greater Resistance to Fatigue failure: Due to greater fatigue endurance limits Copper bar rotors are less sensitive to fatigue and thereby ensure longer motor life

h) Design Flexibility: Copper bar rotors come in many different alloys with relative resistivity ranging from 1.0 to 10.0. Aluminum rotors come in a few alloys ranging from 1.8 to 3.0 range. This provides greater design flexibility with copper bar rotors than aluminum Die cast rotors.

i) Better Corrosion Resistance: Sewage is corrosive and copper has relative immunity from oxidation and corrosion in this environment. in which , in the event of leakage into the motor there is a likely hood of formation of Aluminum Oxide powder in Aluminum rotors which adversely affects the electrical properties of the rotor.

i)Superior Torque speed Characteristics : Both during starting and normal running, torque speed characteristics of double cage copper rotors motors are superior to Aluminum Die cast rotor motors.

k) Rotor Material quality: Copper bar rotor is a mechanically worked product. Die cast Aluminum Rotor is a cast product. As a general rule inherent quality of mechanically worked product is always better than cast product.

2) Easy Reparability:- In the event of rotor failure, it is Practically impossible to repair Aluminum die cast rotors which needs entire replacement only whereas repairs are comparatively much easier for fabricated copper bar rotors

3) Cost Benefit Analysis : Although copper is about four times costlier than Aluminium, its impact on the composite total submersible pump price goes on progressively diminishing with increase in pump size and rating. This difference is only around 7.5% for large submersible Sewage pump sets which would be highly justifiable in view of the extra operational reliability that can be ensured through this marginally extra cost.

4) Capital Budget Vs Maintenance Budget: This actually is a part of cost benefit analysis only but dealt with independently due to its distinct perspective.

Incorporation of Copper rotors in essence implies upgrading the tender specifications and in turn marginally increasing the Capital Budget. Increasing capital budget is relatively easier (since it is normally provided by financially stronger Apex bodies such as ADB, World Bank, N.R.C.D etc), than providing the subsequent maintenance Budget which has to be invariably borne by financially (perennially) bankrupt user local bodies.

It would therefore make a great practical sense to upgrade the technical specifications and increase the capital budget which would, through lesser operational problems, significantly reduce the subsequent load on the Maintenance Budget besides ensuring longer trouble free pump/motor performance.

This aspect has now acquired far greater practical relevance due to the emerging emphasis on longer operation and maintenance periods (up to 6 years instead of earlier 1/2 years) in the Tender Specifications for Sewage pumping/Treatment schemes.

5) Practical Feasibility : Manufacturing copper bar rotors is far more traditionally established and easier. Manufacturing copper bar is a simple procurement (required silicon stampings, copper bar and copper end ring material is readily available in the market) and manufacturing (the process involves simple insertion of copper bars in the rotor slots and brazing them at both ends with copper end rings) activity,

Therefore incorporating copper bar rotor, when required/specified, is absolutely feasible.

6 Internet Search: Interestingly, while there are thousand of articles available on the internet highlighting the electromechnical advantages of Copper over Aluminium, there's not a single site where Aluminium is claimed to be electromechanically superior to Copper. The farthest these articles go is to propagate aluminium on cheaper cost and 'just fair enough properties' considerations

7) Emergence of Die Cast Aluminium Rotor: It needs to be emphasized that Aluminium does not have a single electro mechanical or physical property better than Copper to technically justify, its emergence as an alternative Rotor Material.

Aluminium emergence as an alternative (second choice) rotor material is solely attributable to its most abundant availability on earth and substantially lower cost (Presently Aluminium is around Rs l25/kg compared to copper which is around Rs 410/Kg) and 'fair enough 'electro mechanical properties for many relatively simpler motor applications.

Mass manufacturing Aluminium Die casting process further enhanced the material cost benefits inducing irresistible temptation to extend indiscriminate Aluminium substitution for Copper for all rotor applications.

In submersible sewage pumps Aluminium Die Cast rotors are sought to be promoted mainly on following two apparently convincing grounds.

a) Misleading Comparison with Standard Motors: 'Even reputed manufactures like Siemens and Compton are using aluminium die cast rotors.'

While Siemens and Crompton may now be supplying motors in India with Aluminium die cast rotor it would be interesting to note that they do not claim superiority of Aluminium rotors over Copper.

Further it also needs to be noted that these motors are with efficiency 2 standards which is unacceptable in U.S.A where the same motor manufactures are required to supply the motors with mandatory efficiency 1 standard with more efficient Copper rotors only.

And last but not the least, these are all surface fan cooled motors and as explained earlier, it would be risky and wrong to over extend the logic of surface fan cooled motors to submersible non fan sewage pump motors particularly of higher ratings which are inherently susceptible to imperfect cooling.

b) Misleading Comparison with European Makes: 'Even some of the European makes are using Aluminium Die Cast rotors.'

In this context it is necessary to understand the huge difference between ideal European Operating conditions and arduous Indian Operating conditions.

Due to better solid waste management and excellent quality of screening; the quality of European Sewage (in terms of presence of clogging material) is almost similar to clear water. Additionally site voltage and frequency variations are also non existent. Such near ideal pumping conditions place lesser operational demands on the electric motors thereby rendering any special attention to the motor rotor material specification relatively unnecessary.

In sharp contrast, Indian operating conditions are far more arduous. Due to absence of solid waste management and poor quality of screening, Indian sewage contains practically all the clogging matter under the sun leading to frequent undesirable on -offs operations. Besides site electric voltage and frequency variations are even to the extent of -15% and -5% respectively. Such combination of arduous operating conditions put far more demands on the submersible sewage pump motors (particularly of higher ratings) requiring special more reliable rotor Copper material stipulation which in effect acts as an indirect additional design safety factor for ensuring operation reliability.

It may be further noted that despite simpler operating conditions even in Europe for larger pumps for the same reasons more reliable Copper Rotors only are preferred.

It would thus be incorrect to get over influenced by the above comparisons which are not fully relevant in the context of rotor material for large submersible Sewage Pumps.

8) Plausible arguments against Copper Rotors: Since for rotor application, copper is superior to Aluminium for all the relevant electro mechanical, corrosion and metal forming properties, attempted arguments to still technically justify Aluminium die cast rotors as better material can at best be only plausible and are separately replied in the enclosed annexure point by point.

Conclusion: "Horses for the Courses" needs to be the sole guiding principle for deciding the rotor material for multifarious motor applications.

For Large Submersible Sewage Pump application, as long as Indian Operating Conditions are what they are; temptation for cheaper Aluminum Die Cast Rotors should be resisted and First Choice Copper Bar Rotor Material only should be insisted upon and used for greater reliability in sewage pumping schemes involving huge amount of Public Money.

Copper rotors will also simultaneously ensure much desired energy conservation.

The logic behind this exclusive superior rotor material selection is identical to the logic of S.S material stipulation for Hydraulic Components for Critical pumping application.

Finally, Copper Bar Rotor Selection is purely a technical matter and should never be misconstrued as something in favor of or against a particular Pump manufacturer.

*********************

COMMENTS ON PLAUSIBLE ARGUMENTS AGAINST COPER ROTORS

Point No. 1:-The copper bar technology is a very old technology and in today's world almost 99% motor manufacturers have switched over to the Al die cast rotor technology due to their advantages and reliability.

Comment: This statement is incorrect and misleading.

Aluminium die cast rotor is neither more reliable nor more advantageous than copper Rotor and also does not represent latest technology.

The latest trend in the world, led by U.S.A is the exclusive adoption of efficient Copper rotors only manufactured by the latest revolutionary Copper Die casting technology. This latest die casting technology (introduced only after year 2000) however, is yet to arrive in India and till such time reliable and efficient traditional Copper Bar Rotors should continue for all the critical applications.

Point No 2:- The basic draw back with Cu Bar rotor during the manufacturing process itself is that it has to be assembled leaving immense scope for manual errors making it highly unreliable.

Comment: - This statement is also not correct. Cu Bar rotors are not assembled manually. But by Jig & fixture only and therefore no manual errors are left out.

Point No 3: Under Indian operating conditions Aluminium die cast rotors has better starting torque characteristics which are more suitable for submersible pump handling clogging material.

Comment: The statement is also not correct thanks to double cage copper rotor technology In fact - selection of proper resistivity copper alloys (form wide available 1 to 10 range) for the outer and inner cage ensures both better starting torque characteristics through higher resistance Cu alloy in the outer cage and better running torque characteristics through lower resistance Cu Alloy in inner cage.

Point No 4: Aluminium die cast rotor being single piece assembly, has better strength and reliability. Copper bar rotor is unreliable in the longer run.

Comment:- The statement is also not correct.

Due to combination of higher coefficient of thermal expansion, lesser thermal conductivity, lower melting point, lower tensile strength and lower Fatigue strength Al-rotor is more susceptible to damage and premature failure.

When all the relevant electromechanical and physical properties of copper are superior to Aluminium it should only logically follow that Copper should be more reliable.

Point No 5: The rotor stampings need to be held together under high pressure leaving no air gaps between two stampings to maintain the exact length of the rotor stock. This is well achieved during the process of Aluminium die casting. In case of copper bar rotors, it is difficult to maintain the exact dimension of rotor stack leaving certain air gaps in the stamping affecting performance of the motor.

Comment: This statement is also not correct.

For Al. Die Casting process. the rotor stamping is heated up to 800°C before pouring Al. metal into the rotor stamping slots; end rings and varnish on rotor stamping on both sides burn out and gap occupied by varnish remains unfilled and in turn leaves air gaps & rotor performance is affected. Whereas for Copper bar Rotor stamping does not required to be heated up. The rotor stamping is pressed by special Jigs & fixture under pressure, as in case of stator stamping is pressed. So that no Air Gap is left out.

Point No 6: The argument that copper is more expensive, is also not valid as the so called higher cost of copper rotor is more than off set by the initial high Aluminium die cost.

Comment: Die casting is essentially a mass manufacturing process where the one time cost of Die is distributed over these mass manufactured rotors where as cost of costlier copper is directly proportional to the quantity. If aluminium rotors were indeed costlier as claimed, they could never have replaced superior copper rotors.

Point No 7: Due to draw backs of copper bar rotor over aluminium die cast rotors have been adopted by the motor manufacturers all over world including Siemens and Crompton Greaves.

Comment: This statement is also misleading.

Switching over to Aluminium Die cast rotors from Copper rotors can be attributed only to the substantial lower cost of Aluminium and absence of any mandatory MEPS standards in India so far but with acute increase in energy conservation awareness this trend is bound to change in line with the global change for copper rotors.

Table 1.

Comparison of Properties

 
Characteristics    Pure
Aluminum    Pure
Copper      
Conductivity, %IACS@20°F          
Specific Heat, BTU/Lb. °F.@68°F      
Density, Lb./In.3@68°F              
Melting Point, °F              
Coef. Of thermal expansion,/°C      
Coef. Of thermal expansion,/°F      
Thermal expansion mils/in/100°C      
Thermal Counductivity,BTU/Ft.2/ft/hr° F
Yield Strength, psi x 1000****          
Annealed                  
Tempered                  
Ultimate Strength, psi x 1000          
Annealed                  
Tempered                  
,% in 2"                  
Annealed                  
Tempered                  
Fatigue endurance limit, psi x 1000****    62   
0.233   
0.098   
1195 - 1215
0.0000238
0.0000132
2.38   
135
   

4   
24       
12   
27       
23   
1.5   
7            101   
0.092 
0.323 
1981  
0.0000176
0.0000098
1.76  
226   
      

10    
53    
      
32    
57    
      
45 - 55
4 - 40
11 - 17     

Table 2

Motor Efficiency Comparison Efficiency

 
HP    kW    Poles    Aluminum    Copper    Difference    Lost Reduction      
4
7.5
10
15
25
40
120
270
Average    3
5.5
7.5
11
19
30
90
200
    4
4
4
4
4
4
2
4    83.2
74
85
89.5
90.9
88.8
91.4
92
    86.4
79
86.5
90.7
92.5
90.1
92.8
93
    3.2
5
1.5
1.2
1.6
1.3
1.4
1
    19.00%
19.20%
10.00%
11.40%
17.60%
11.60%
16.30%
12.50%
14.70%     

 

Valve Terminology

POSITION SWITCH: A switch that is linked to the valve stem to detect a single, preset valve stem position. Example: Full open or full closed. The switch may be pneumatic, hydraulic, or electric.

POSITION TRANSMITTER: A device that is mechanically connected to the valve stem and will generate and transmit either a pneumatic or electric signal that represents the valve stem position.

POSITIONER: A device used to position a valve with regard to a signal. The positioner compares the input signal with a mechanical feed ba.ck link from the actuator. It then produces the force necessary to move the actuator output until the mechanical output position feedback corresponds with the pneumatic signal value. Positioners can also be used to modify the action of the valve (reverse acting positioner), alter the stroke or controller input signal (split range positioner), increase the pressure to the valve actuator (amplifying positioner), or alter the control valve FLOW CHARACTERISTIC (characterized positioner).

POST GUIDE: A guiding system where the valve stem is larger in the area that comes into contact with the guide busings than in the adjacent stem area.

PUSH-DOWN-TO-CLOSE: A term used to describe a LINEAR or GLOBE STYLE valve that uses a DIRECT ACTING plug and stem arrangement. The plug is located above the seat ring. When the plug is pushed down, the plug contacts the seat, and the valve closes. Note! Most control valves are of this type.

PUSH-DOWN-TO-OPEN: A term used to describe a LINEAR or GLOBE STYLE valve that uses a REVERSE ACTION plug and stem arrangement. The plug is located below the seat ring. When the plug is pushed down, the plug moves away from the seat, and the valve opens.

PRESSURE RECOVERY FACTOR: See (F1).

QUICK OPENING: A FLOW CHARACTERISTIC that provides maximum change in flow rate at low travels. The curve is basically linear through the first 40% of travel. It then flattens out indicating little increase in flow rate as travel approaches the wide open position. This decrease occurs when the valve plug travel equals the flow area of the port. This normally happens when the valve characteristics is used for on/off control.

RANGEABILITY: The range over which a control valve can control. It is the ratio of the maximum to minimum controllable FLOW COEFFICIENTS. This is also called TURNDOWN although technically it is not the same thing. There are two types of rangeability - inherent and installed. Inherent rangeability is a property of the valve alone and may be defined as the range of flow coefficients between which the gain of the valve does not deviate from a specified gain by some stated tolerance limit. Installed rangeability is the range within which the deviation from a desired INSTALLED FLOW CHARACTERISTIC does not exceed some stated tolerance limit.

REDUCED TRIM: Is an undersized orifice. Reduced or restricted capacity trim is used for several reasons, (1) It adapts a valve large enough to handle increased future flow requirement with trim capacity properly sized for present needs. (2) A valve with adequate structural strength can be selected and still retain reasonable travel vs. capacity relationships. (3) A valve with a large body using restricted trim can be used to reduce inlet and outlet fluid velocities. (4) It can eliminate, the need for pipe reducers. (5) Errors in over sizing can be corrected by use of restricted capacity trim.

REVBRSE ACTING: This term has several deferent meanings depending upon the device it is describing. A REVERSE-ACTING ACTUATOR is one in which the actuator stem retracts with an increase in diaphragm pressure. A REVERSE-ACTING VALVE is one with a PUSH-DOWN-TO-OPEN plug and seat orientation. A REVERSE-ACTING POSITIONER or a REVERSE-ACTING CONTROLLER outputs a decrease in signal in response to an increase in set point.

REVERSE FLOW: Flow of fluid in the opposite direction from that normally considered the standard direction. Some ROTARY VALVES are considered to be bi-directional although working pressure drop capabilities may be lower and leakage rates may be higher in reverse flow.

ROTARY VALVE: A valve style in which the FLOW CLOSURE MEMBER is rotated in the flow stream to modify the amount of fluid passing through the valve.

SEAT LOAD: The contact force between the seat and the valve plug. When an actuator is selected for a given control valve, it must be able to generate enough force to overcome static, stem, and dynamic unbalance with an allowance made for seat load.

SEAT RING: A part of the flow passageway that is used in conjuction With the CLOSURE MEMBER to modify the rate of flow through the valve.

SELF-CONTAINED REGULATOR: A valve with a positioning actuator using a self-generated power signal for moving the closure member relative to the valve port or ports in response and in proportion to the changes in energy of the controlled variable. The force necessary to position the CLOSURE MEMBER is derived from the fluid flowing through the valve.

SEPARABLE FLANGE: Also known as a SLIP-ON FLANGE. A flange that fits over a valve body flow connection. It is generally held in place by means of a retaining ring. This style of flange connection Conforms to ANSI/ISA 275.20 and allows for the use of different body and flange materials. Example: A valve with a stainless steel construction could use carbon steel flanges. This type of valve is very popular in the chemical and petro-chemical plants because it allows the use of exotic body materials and low cost flanges.

SOFT SEATED: A term used to describe valve trim with an elastomeric or plastic material used either in the VALVE PLUG or SEAT RING to provide tight shut off with a minimal amount of actuator force. A soft seated valve will usually provide CLASS VI seat leakage capability.

SPLIT BODY: A valve whose body is split. This design allows for easy plug and seat removal. Split-bodied valves are made in both the straight-through and angle versions. The Masoneilan 2600 or ANNIN is an example of a split body valve.

SPRING RATE: A term usually applied to SELF-CONTAINED REGULATORS describing the range of set point adjustment available for a particular range spring.

STATIC UNBALANCE: The net force produced on the valve stem by the fluid pressure acting on the CLOSURE MEMBER and STEM within the pressure retaining boundary. The closure member is at a stated opening with a stated flow condition. This is one of the forces an actuator must overcome.

STELLITE: Also called #6 Stellite or Alloy 6. A material used in valve trim known for its hardness, wear and corrosion resistance. Stellite is available as a casting, barstock material and may be applied to a softer material such as 316 stainless steel by means of spray coating or welding.

STEM: The VALVE PLUG STEM is a rod extending through the bonnet assembly to permit positioning of the plug or CLOSURE MEMBER. The ACTUATOR STEM is a rod or shaft which connects to the valve stem and transmits motion or force from the actuator to the valve.

STEM GUIDE: A guide bushing closely fitted to the valve stem and aligned with the seat. Good stem guiding is essential to minimizng packing leakage.

SUPPLY PRESSURE: The pressure at the supply port of a device such as a controller, positioner, or transducer. Common values of control valve supply pressures are 20 psig. for a 3-15 psig. output and 35 psig. for a 6-30 psig. output.

STROKE: See TRAVEL.

THROTTLING: Modulating control as opposed to ON/OFF control.

TRANSDUCER: An element or device which receives information in the form of one quantity and coverts it to information in the form of the same or another quantity. (See I/P)

TRAVEL: The distance the plug or stem moves in order to go from a full-closed to a full-open position. Also called STROKE.

TRIM: Includes all the parts that are in flowing contact with the process fluid except the body, BONNET and body flanges and gaskets. The plug, seats, stem, guides, bushings, and cage are some of the parts included in the term trim.

TRUNNION MOUNTING: A style of mounting the disc or ball on the valve shaft or stub shaft with two bushings diametrically opposed.

TURNDOWN: A term used to describe the ratio between the minimum and maximum flow conditions seen in a particular system. Example: If the minimum flow were 10 G.P.M. and the maximum flow were 100 G.P.M. the turndown would be 10:1. This term is sometimes incorrectly applied to valves. See RANGEABILITY.

VALVE: A device which dispenses, dissipates, or distributes energy in a system.

VALVE BODY: See BODY.

VALVE FLOW COEFFICIENT: See Cv.

VALVE PLUG: See CLOSURE MEMBER.

VENA CONTRACTA: The location where cross-sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level, and where fluid pressure is at its lowest level. The vena contract a normally occurs just downstream of the actual physical restriction in a control valve.

MAKE YOUR, CENTRIFUGAL
PUMP 'MATURED'

Every industrial process involves transfer of fluids from one level to other. Thus the pumps have become essential part of the industry. Indirectly the pumps are integral part of modern economy and social development.

The pump industry plays an important role in improvement of economy, hence efforts must be made for increasing the life of the equipment with lesser expenditure and educate the pump user in the practices which in turn pump consumes less energy and gives trouble free services and reduces the down time.

Audit the Existing Pumps

Every pump industry should make the thorough audit of the existing pump line in respect to the Hydraulic Performance and Mechanical design, in comparison with the competitor's product. Check out the program for the improvements to be made. There may be some pump of the old design, which is to be made obsolete because they do not become competitive in the market.

In the hydraulic performance improvement, the points to be considered are; Improvement in efficiency; the margin to be provided for NPSH and effect of internal recalculation in the pump

Pump Efficiency

Based on the statistical data and the experiments conducted by the hydraulic research association, check the real efficiency against the specific speed. The difference will give the improvement in your pump efficiency. This improvement in pump efficiency can be achieved by improving the surface finish, the internal clearances, vane shapes and smoothness of casting. But the same is to be achieved for the mass production and economically it should be viable. Reliability is also to be considered based on the present working conditions. Hence there should be consistency of the performance in the batch of ten pumps and the mean value to be considered.

Improvement in the NPSHR by the Pump

The institutes work out the NPSHR for individual specific speed pump. The higher NPSHR for the pump will further increases the value of NPSHA i.e. change in the installation lay out.

Every pump manufacturer should study the existing pump NPSHR and compare the same with Hydraulic Institute standards and take up the goal to reduce the NPSHR. At present the end customer and the consultants prefer low NPSHR Pumps.


Internal Recirculation

Abnormal flow would occur in and around the impeller under certain conditions. The pressure water coming out of the impeller gets circulated to suction from the clearance between impeller and wearing ring and secondly through the clearance between back wearing ring and connected to suction chamber through the balancing holes. The pressure developed per single stage pump get increased and because of the variation in the pressure, wear takes place between the wearing ring portions and the vane tips. The suction specific speed values to be considered in setting the maximum flow restrictions.

Working Duty Point

The mean time between the failures of the standard single stage pump is far from satisfactory. This short fall in the life expectancy are directly related to improper operating and maintenance practices. There are normal standards for centrifugal pump and the highest standard is API 610. In order to increase the reliability and the life of the pump, an intermediate standard can be worked out.

To improve the work conditions, use -

a) Over sized stuffing boxes

b) Magnetic bearing housing to eliminate lubricant contamination

c) Use of angular contract bearing back to back

d) Heavy duty shaft to reduce shaft deflection

e) One piece bearing frame

Thoughts on Material

Change of material to suit to the liquid handled should be considered; as the down time of the equipment of the material failure, will be too costly. We can think of additional material of coating on the wetted portion material to reduce erosion and improve efficiency.

Pump Users Training

There is lot of improvement to be done in this section. By conducting the users Seminars, publishing the technical articles, Pump users symposiums lot of knowledge is parted out to user to understand the theory and improve maintenance methods.

Every pump manufacturers submits the instruction manual with each pump because all the pump operators do not have the access to the technical articles nor all of them attend the technical seminars. The other suggestion is to organise the seminars by the pump manufacturers with the help of institutes, universities so that the engineers get the feel of the pumps, its principal operation and maintenance.

Energy Conservation

Due to increase in the power cost and the scarcity of the power, any rotating equipment should be studied carefully about the energy consumption. Normally power saving in the pump installation can be achieved by

a) Avoidance of over sizing

b) Use of variable speed prime mover

c) Proper selection of pump to avoid two pumps running at a time.

d) Restoring the internal clearance to avoid recirculation

e) Use of variable frequency drive, provide soft start i.e. requires low starting current.

Suction Piping

No doubt all the pump user takes the precaution for the calculation of duty point like Total Head, Range of Discharge and Speed. But lot of problems have arised because of the improper suction piping. Some of the most glorying errors are observed in wrong positioning of the Elbows, improper slopping of the pipe, incorrect use of reducers, inadequate submergence over the piping in let which lead to formation of varieties and entrant of air in the pump. It is preferable for the pump user to send the suction piping layout to the pump manufacturer for study and guidance.

Suction specific speed can be useful to evaluate and predict suction performance of the pump impeller design and its ability to operate away from BEP conditions.

A lack of proper monitoring can lead to unscheduled shut downs with the costly reduction in production, Monitoring prevents a reasonable assessment of the need to replace the renewable parts so that the original efficiency can be restored.

The bearing life in the process pumps very between 30 to 40,000 hours provided proper lubrication and recommended lubrication is used. If the records are examined for the bearing failure the main causes are -

a) Not using the recommended lubricant

b) Actual load on the bearing changes

c) The load which the bearings can carry falls short of the basic load rating because of the environmental conditions The variation of the load can be caused by varieties of circumstances such as misalignments excessive forces of the piping, pump construction, operating above the minimum flow conditions, poor suction piping, poor lubrication for bearings, not using the recommended lubricant, inadequate cooling or excessive cooing water, water entering the lubrication area. Hence educate the pump user on lubrication practices is the only way to reduce the failure rate.

CONCLUSION

Constant improvement in the product and educate the end customer for proper installation and maintenance of the equipment can improve the life cycle of the product. Better guide lines for the installation operation and maintenance practices should be provided to all the personals involved in the selection and application process.

Single Phase
Pump Starters

Most of the pump-motor burnout is due to improper selection of pump, wrong installation and poor power supply condition. This article deals with operation of single-phase motor, effect of poor power supply conditions on it, and selection of proper protective devices.

Operation of Single Phase Motor:

Any motor requires a rotating magnetic field to produce torque continuously in one direction. In three phase motor 120° phase difference in each phase is used to generate rotating magnetic field. In single phase motors capacitors are required to generate phase shift between two winding supply. A RUN Winding is directly connected to supply and is responsible for generating torque. Resistance of RUN winding is always less than START winding START winding is energised through capacitor in series with supply. Current through START winding is responsible for starting torque.

At the time of starting more torque is required and so additional capacitor is connected in series with supply. Value of starting capacitor is high and they are electrolytic type. Electrolytic-capacitors are not rated for a. c. supply so if a. c. current passes through it for more than 2 to 3 seconds, life of capacitor decreases and it may damage.

So for operation of single phase a. c. motor, phase is directly connected to RUN winding. START winding is connected to supply through starting & Running capacitor and after 2 to 3 seconds, starting capacitor must be disconnected. After 3 second only Running capacitor should remain in series with supply.

Effect of Power Supply Conditions:

1. Under voltage :

Speed, Torque and Power Output reduces considerably at Low Voltage. This effectively reduces pump discharge. In some case it will also increase current to get required power. Low voltage is also harmful during starting. Contactor will not make proper contact and will chatter at Low Coil Voltage. This is harmful to contact due to high contact resistance, It is harmful to motor also as it will not get steady voltage.

2. Over voltage:

At higher voltage motor flux will increase. This will result in higher current due to saturation of flux. Dielectric strength of insulating material reduces with temperature rise and it will reduce life of motor.

3. Overloading:

Even in perfect power voltage water condition motor may draw higher current due to mechanical problems associated with pump and motor. If these problems are detected in earlier stage, it will save winding, and further mechanical damage can be eliminated. At higher current temperature of winding will increase which reduces life of dielectric.

Selection of Protective Devices (STARTERS)

Selection of starters depends on power supply condition, extra protections required and types of motor. Above factors influence selection due to following reasons.

(A) Type of motor

Varieties of motors are available. HP rating decides selection of switching device. (i.e., contactor, power relay etc.). Some manufacturers claim that their motor opereteas satisfactorily even at 120v. But at 120v, switching device should operate satisfactorily and should withstand high voltage up to 270v.

Depending on torque required and design of motor, manufacturers suggest value of Starting and Running Capacitors. Some manufacturers are suggesting only Running capacitor. Pumps with only Run capacitor will not provide high starting torque. For high starting torque starting capacitor is required. After 2 to 3 seconds, starting capacitors must be disconnected, as they are not for continuous rating.

So from motor manufacturer following information is required for starter.

(1) Full Load Current

(2) RUN Capacitor

(3) START Capacitor

(4) Operating Voltage Range.

(B) Power supply Condition:

Even though pump is designed to operate on wide voltage range, starter also should operate properly in that range. Starting capacitors are rated at 275v, it may damage capacitor and so starter should not operate in that condition. If it is required to operate pump at very low/very high voltage, additions boost/buck transformer is suggested.

(C) Other Protections/Facilities Required.

1. Overload:

Due to wear & tear of mechanical parts, more of sand particles in water etc. pump may draw more current. Manufacturer suggests full load current for motor. If the current is 10% higher than its rated, starter should trip pump. If the current is very high, tripping time should be low.

2.Dry Run:

Cooling is provided to submersible pumps through water passing through it. If pump operates without water, it may damage due to over heating even at rated current. Two types of Dry Run Protections are possible.

(a) Probe Type Dry Run:

In this type of system, probe is immersed in water, at slightly higher level than pump. If water goes down than probe will be open. This condition is sensed and it will switch off starter.

(b) Probe less Type Dry Run:

When pump operates without water, its operation is on NO LOAD condition. IN NO LOAD conditions, pump draws less current from supply. No load current varies from 60 to 90% of full load current depending on design of motor.

In case of problems in Dry Run, selection of dry run percentage setting and overload setting is important, otherwise it will malfunction. Information for setting should be provided by starter manufacturer.

(3) Starting Procedure:

When a START push button is pressed, a second NORMALLY OPEN contact is used to connect starting capacitor in series of START winding. Push button is kept pressed for 2 to 3 second and then released; starting capacitor is disconnected with release of start push button. If push button is kept pressed for more than 3 seconds there it may damage starting capacitor.

Push buttons available in market are rated at 10 Amp. If starting capacitor current is more than 10 Amp, it may damage push button contact also.

(b) Starting Relay/Contactor.

In this type of starting method, additional Contactor/Relay is provided to connect starting capacitor. When start push button is pressed, starting Contactor/Relay is energised and automatically after 2 to 3 seconds relay will be off. It will disconnect starting capacitor automatically.

(D) Starters Available in Market:

(1) Switch Type:

Only DP switch and Run capacitor is provided in starter. It is the cheapest starter but will not provide any protection.

(2) MCB TYPE:

Instead of DP switch, MCB and starting capacitor is provided in the starter. It will not provide proper overload protection but provides short circuit protection.

(3) Contactor Type:

It is available with contactor, overload relay, Start/Run capacitors. Thermal overload is used for overload protection. START push button is kept pressed to keep starting capacitor in series of START winding.

STARTING time is manual. Contactor coils are not designed to operate on wide voltage range so at low voltage contactor will chatter and at high voltage coil will damage due to over heating.

(4) Relay Type:

The main advantage or Relay type panel is wide operating voltage range, it uses power relay with DC operating coil. If DC supply is properly designed, it can make proper contact on wide operating voltage.

Number of varieties available is.

(a) Relay type without protection:

It does not provide any protection. It can be operated with push button on wide voltage range, Starting/Running capacitors are available as per requirements.

(b) Relay type with overload Protection:

It provides overload and low voltage protection. An Electronic circuit is provided for above protections. Starting/Running capacitors are available as per requirement. Starting time is provided manually by start push buttons.

(c) Relay type with Overload and Starting Capacitor Protection:

It provides additional protection for starting capacitor. If START pushbutton is pressed for more than 3 seconds, it automatically switches off starter.

(d) Tow Relay type with all Protections:

It provides all protections to motor. Low voltage, high voltage and overload protection is provided. Separate relay is used for starting purpose and relay will automatically cut capacitor after 2 to 3 seconds.

Moreover, in Relay type panels are also available with probe type or probe less Dry-Run Protection. A Water Level Guard, Liquid Level Controller can be connected easily to relay type or contactor type panels.

From above guidelines proper protection device (STARTER) can be selected after getting proper data for the pump, power condition in your area, safety required, convenience required and costing.

(Courtesy : Gujarat Electronics & Controls, Ahmedabad)

Difference Between Iron & Steel

Q. What are the various types of diamonds? Which is the most precious?

There are four known types of natural diamond (Ia, Ib, Iia, IIb), classified according to the presence of nitrogen in the crystal, and certain other properties. The different types are: i) Pink Diamonds: these are the most rare and valuable. The Argyle mine produces 95 per cent of the world's supply. However, less than one tenth of 1 percent is classified as Pink; ii) White Diamonds: these are produced by mines worldwide in a wide variety of shapes and sizes; iii) Champagne Diamonds: these are naturally coloured diamonds produced in a wide range of colours from light straw to rich cognac; and iv) Pink Champagne Diamonds: attractive champagne diamonds with secondary pink colour are also available and command a higher price per carat than champagne diamonds. These stones display slight to bold flashes of pink in their fire; v) Yellow Diamonds: Fancy yellow diamonds come in a broad range of shades. Vi) Blue Diamonds: These are available in a range of shades from the blue of the sky to a more "steely" colour than sapphire; vii) Green Diamonds: Usually, penetration of the colour is not very deep and is often removed during the fashioning of the stone.

Q. Gorillas can stand on their legs, but why do they walk in a 'four-footed' way?

Animals who walk on two legs are known as bipedal animals. Those who walk upright have a centre of gravity close to their hips. Their upper leg bones fit onto the end of the lower leg bone, the hinge like joint created there is called the knee. This results in an energy efficient posture that allows the bones, and not just the muscles, to help support the weight of the animal. Gorillas do not satisfy these conditions and, therefore, walk on all four limbs.

Q. Which is the most poisonous animal?

The poison arrow frog and certain salamanders are among the most poisonous animals. Just two micrograms of toxin from the poison arrow frog will kill a human. Stonefish store their tox in gruesome looking spines designed to hurt predators and it's the most venomous fish. The most venomous snake is the inland taipan of Australia. However, the most venomous of all is the box jellyfish, found in the waters around Asia and Australia. They have long tentacles with stings at the very ends.

Q. What is the procedure to differentiate between blood groups?

The blood group is found out by adding two chemicals Anti Sera A and Anti Sera B to the sample. If the sample gets agglutinated by Anti Sera A, and not by Anti Sera B, the sample is A. If it gets agglutinated by B, but not by both, the sample is AB, and if by neither, the sample is O. The Rh factor is found out by adding another chemical, Anti Sera D to the sample. If the sample gets agglutinated, it has a positive Rh factor else it has a negative Rh factor.

Q. What is the difference between iron and steel?

The difference is percentage of carbon, the main alloy element. Those irons containing less than 2% carbon are known as steels while those containing more than 2% carbon are known as pig iron. Pig iron is obtained from iron ore by processing it with coke in a blast furnace. This pig iron is then further processed to reduce the carbon content in different furnaces to obtain steels. These steels can be then further processed to obtain alloy steels, stainless steels by adding elements such as silicon, manganese, chromium, nickel, etc.

Q. Where was the first metro rail constructed?

The first metro rail became operational in London on January 10, 1863. The Metropolitan Railway, as it was known then, was run between Paddington and Farringdon. Over the years, this underground and over ground railway system has carried millions of passengers. It's fondly called 'the Underground' or 'the Tube'.


Q. Who invented the T9 dictionary used in mobile phones?

The inventors of the predictive T9 dictionary used in cell phones are Svensson Henrik Brun (Demark) and Williams Stephen (Finland). This predictive text input method helps in efficient typing of SMS messages. It improves on the common multi-tap method since fewer total button taps are needed. It's achieved by using a small, quick-access dictionary to automatically display the word most often desired for a sequence of keystrokes.

Q. Who found the wreck of the Titanic?

The remains of the Titanic were found in 1985 by Robert Ballard, and oceanographer and marine biologist with the Woods Hole Oceanographic Institution. When he located the Titanic, he saw that, as some survivors reported, the ship had broken apart. He believed the weight of the water-filled bow raised the stern out of the water and snapped the ship in two just before it sank. Debris falling out of the ship was strewn over a large distance across the sea floor. The bow and the stern were found nearly 2,000ft apart.

Q. How is a bullet train able to move at high speed?

Bullet trains run largely on conventional steel rails mounted on concrete sleepers, but the fastest service use dedicated tracks. Every part is aerodynamically shaped to reduce drag. The motors are very powerful. The key to reach high speeds is the power of the traction device. Each carriage has four motor sets to power the axles. If there are 16 carriages, then there are 64 motors. Each motor is rated at 185kW totalling to 11,840kW. Extensive sound proofing reduces the noise level. Bullet trains also use magnetic levitation in which the rails and the train don't touch each other.

Q. Is a ray of light visible in vacuum?

No, rays of light cannot be seen in vacuum. When a ray of light enters an enclosed dark room through an opening, light is scattered by dust particles suspended in the air and thus we see the path of the ray. Actually, we see the dust particles falling in the path of the ray. If there is no air i.e., vacuum, there won't be any suspended substance which can scatter the light. This explains the darkness in space though there are many light sources. We can see only the light sources and the objects which fall in the path of rays.

Q. How do we calculate purchasing power parity?

Purchasing Power Parity (PPP) is a theory which states that the exchange rates between two currencies are in equilibrium when their purchasing power is the same in both countries. This means that the exchange rate between two countries should equal the ratio of the two countries' price level of a fixed quantity of identical type of goods and services. When a country's domestic price level is increasing, that country's exchange rate must be depreciated in order to return to Purchasing Power Parity. The PPP exchange rates are used to compare standards of living.

Q. What is white coal?

White coal is a form of fuel produced by drying chopped wood over a fire. It differs from charcoal which is carbonised wood. White coal was used in England to smelt lead ore from the mid-l6th to the late 17th centuries. It produces more heat than green wood but less than charcoal and thus prevents lead evporating. White coal was produced in distinctive circular pits with a channel, known as Q-pits. They are frequently found in the woods of South Yorkshire.

Q. What is hydroponics?

Hydroponics is often defined as the cultivation of plants in water. Since many aggregates or media support plant growth, the definition has been broadened to read the cultivation of plants without soil, Growers use hydroponics techniques due to lack of water supply or fertile farmland.

Home gardeners have used it to grow fresh vegetables year round and to grow plants in smaller spaces. Greenhouses and nurseries grow their plants in a soil-less, peat or bark-based growing mix.

Flexible Impeller Technology
Pumping viscous Liquids
without damage

Flexible impeller pumps have a number of advantages over centrifugal pumps in the handling of viscous liquids. In this article, Martin Ruse of Jabsco, a pioneer of flexible impeller techonology, discusses the basics of operation and the benefits of flexible impeller pumps during installation, maintenance and use.

Positive displacement pumps such as flexible impeller pumps are often used as an alternative to centrifugal pumps in a wide range of applications throughout many industries. The flexible impeller pump (FIP) was invented and introduced by Jabsco (now part of ITT Industries, Motion and Flow Control) some 60 years ago and offers significant benefits compared to centrifugal pumps for handling viscous liquids.

Figure I shows the basic pumping principle of the FIP. As the impeller rotates past the eccentric cam, the sample cell volume is decreased. This then increases as the cell passes the inlet pipe, creating a partial vacuum that allows atmospheric pressure to push additional liquid into the cell. As the cell passes the discharge line there is a decrease of cell volume, which forces additional liquid volume out and into the discharge pipe.

Key to the ability of flexible impeller pumps to pump viscous fluids is the fact that flow rate is directly proportional to the speed of the pump. There are two important factors to consider. Firstly there are friction losses within the pump itself and secondly there is the friction between the liquid and the walls of the pipe supplying the pump. Friction losses within the pump cause the performance of centrifugal pumps to deteriorate rapidly with increased liquid viscosity. Although viscous friction can be reduced by reducing the pump speed, the centrifugal pump principle relies on liquid velocity through the impeller, so reducing the pump speed results in a significant drop in performance. In practice, they are limited to viscosities up to 200-300 cP. In FIPs, however, internal friction losses due to viscosity can be reduced by lowering pump speeds because this results only in a proportionally lower displacement. The impeller blades are self adjusting to rpm, viscosity and differential pressure.

The main problem, however, is actually getting the liquid into the pump. The pressure drop caused by friction loss inside the inlet pipe is an important factor. To reduce these losses, the size of the pipes must be increased as the viscosity increases. In addition, the inlet piping should be kept as short as possible and as free from bends as is practicable. The more viscous the liquid, the less able the pump is to lift the liquid and, in extremes of viscosity, a flooded suction, hopper-fed arrangement is used with the supply tank above the level of the pump to pRovide a positive head (Figure 2).

Installation benefits

The main benefit of FIPs is that the pump can be positioned where it is most convenient for the particular process; unlike centrifugal pumps, they are not restricted to flooded suction positions below the liquid level. This means they can be mounted off the floor where there are safety issues to be considered, such as the possibility of tripping over the pump or pipe work or risk of damage by passing delivery vehicles. Another consideration is the configuration of the liquid reservoir itself. If this reservoir (typically a drum or similar bulk delivery container) is owned by an external supplier, it might not have the appropriate bottom outlet flange or valve. Some vessels such as rubber lined tanks do not lend themselves to a bottom outlet; here the FIP can be connected by a simple flexible hose through the top opening. Alternatively, the tank may itself be mounted at or even below floor level, precluding a flooded suction.

Ease of first use is also an important consideration, particularly when inexperienced operators are involved in the start-up process. The very first and every subsequent operation of a flexible impeller pump simply requires it to be switched on, with no complex priming procedures necessary.

Maintenance Considerations

Thoughtful positioning of a pump greatly simplifies maintenance. An FIP mounted above the liquid level can be purged so the maintenance technician will not be presence with a pipe and pump full of fluid that could spill when the end cover is removed. By contrast, submersible pumps are usually left permanently immersed in the liquid and tend to be forgotten until they actually fail When retrieved for repair they are, of course, covered in the liquid, which could be sticky, unpleasant or corrosive, Repairing a hermetically sealed pump may be impossible or, at best, much more complex than the FIP.

Factors that affect the self priming capability of an FIP are blockages , air leaks and internal leakages. A closed valve or a blockage on the discharge side will prevent the partial vacuum developing. An air leak on the inlet side will also prevent the partial vacuum developing (ensure all inlet pipe connections are airtight) as will an internal leak back past the impeller to the inlet side, e.g. severe scoring of the pump body after long term pumping of abrasive particles.

However, as the impeller is made from rubber, there is no significant wear of .the rubber material itself and, being flexible, it adapts to the worn shape of the pump body. So an FIP retains its self priming ability and volumetric efficiency considerably longer than, for example, gear pumps relying on fixed clearances between rigid components.

When maintenance does eventually become necessary, most jabsco FIPs have easily removable end covers and end wear plates. Replacing these and the impeller will return all but the most badly worn pumps to full efficiency. These pumps can also be easily stripped down for cleaning, and hygienic versions can readily withstand CIP procedures. Their design ensures cleaning fluids reach all areas of the pump during this process.

Practical examples

Foodstuffs such as cream cake fillings and jams represent the extremes of the practical viscosity scale and, to handle them successfully, speeds of the order of 70-100 rpm must be used. The cream cake filling is thick and light. These types of substances frequently require an additional pressure plate in the hopper to force them into the pump. Jam of the same viscosity may not need this additional help as it has a high specific gravity and the weight of the product will be sufficient to force it through the inlet pipe to the pump. A typical application is the transfer of jam from bulk containers into hoppers feeding the filling machines that deposit the jam onto a moving conveyor carrying long strips of cake or pastries, or directly to the heads of machines where they are extruded in multiple ribbons onto the lengths of pastry or cake passing underneath. A hopper is positioned above the pump into which the bulk containers of jam are emptied. A flexible hose leads to the hopper on top of the filling machine or to the machine itself.

For jams so viscous that they will not flow under their own weight, very low pump speeds (say 20 rpm) should be selected initially and these can be increased until the optimum speed is determined, avoiding emulsification of the jam. Choice of pump speed depends very much on the application and the product being pumped. For example, although low speeds are appropriate for jams, 500 rpm would probably be suitable for treacle and molasses, whilst speed could be increased to 100 rpm for double dairy cream.

Naturally it is important that the pumps are both hygienic and can readily be cleaned between batches and shifts. A hygienic flexible impeller pump such as the 28 Series from Jabsco (Figure 3) is manufactured in 316 grade austenitic stainless steel and has impellers of specially formulated hygienic rubber, including EPDM, to meet the latest 02-09 standard of the 3A Sanitary Standards Administrative Council in the USA.

Flexible impeller pumps also have a number of other properties that make them extremely useful in the process industry. These include the ability to pump shear-sensitive liquids without damage, and also to pump liquids containing hard or soft solids without damage to the pump or to the liquid.

(Courtesy : World Pumps)

Why Seals Fail?

Inspect the seal drive

Seal designs all use some way to transmit torque from the shaft to the rotary face. Quite often, it is done with pins, set screws and lugs. In a few cases it is done with the single spring. To check for this clue you must first determine for your particular se al where the drive junction is located. Seals are usually loose in torsion, that is, outside the pump you can twist them slightly before they engage. You are looking for signs of wear at the pin, drive lug, dent or spring. In bellows seals the signs are not present because they are usually a solid drive.

Hysterisis

When a stationary ring is not square with the shaft, the sliding elastomer in the face of the seal must move back and forth on each revolution in an axial direction. The amount of motion depends directly on how much misalignment from a perfect 90 degree angle. Misalignment can also be caused by pipe strain, bad bearings, a bent shaft or shaft deflection caused by improper system operation. The seal is alternately pushed away from the stationary ring by the immovable face and back towards it by the spring pressure and by the fluid hydraulic pressure. The spring force must be high enough to overcome the resistance to motion caused by the drag of the elastomer.

Hysteresis is sometimes used to describe the amount of drag caused by the elastomer as measured in lbs., oz., etc. Hysteresis is also used to describe a delay or lag between two events. The rate of motion of the seal face axially must be the same in both directions or the seal faces will separate in not returning as fast as it was thrust away from the stationary ring. This minute separation caused by motion, drag and hysteres is depends then on not only the amount of drag, but the size of the seal and the speed. Hysteresis is the underlying reason for face separation, leakage, premature life, abrasive face damage and a variety of other ills in pumps that are not in perfect alignment.

Face separation

The faces of a seal are normally flat to less than 20 millionths of an inch and are lubricated with a thin film of the sealed fluid. Because this leaves something less than a micron between them, they would normally act as a natural excluder of abrasive particles. When the faces are moved axially on each rotation, there is a tendency for them to separate much greater distances than millionths of an inch. .005" to .030" misalignment is not uncommon. The number of times that the seal has to move axially is over 10 million times a day on a 3600 rpm pump.

The separation of the stationary and rotating face by a few thousands of an inch causes two problems: It allows large abrasive particles to get between the faces and it allows the fluid being sealed to leak out. The leakage out can carry away wear particles causing rapid face wear and it will gum up or hang up the sliding elastomer from the outside where no self-cleaning takes place.

Proper size wear track

This is an important sign because it tells you that the pump is in good alignment and face leakage is probably not the cause of any seal problem you might have. In a clogged metal bellows seal, for example, this is the clue that tells you the seal leaked by the static secondary seal.

Narrow wear track

When the wear track is narrower than the thinnest face, this means that the seal has been over pressurized and has bowed away from the pressure. This bowing causes the seal to seal only on a portion of the face width. This is from improper design and the seal must be changed to a higher pressure, more rugged design if this occurs.

No wear track

If there is no apparent wear on the faces of the seal after they have been in operation for some time and the seal is a rubber bellows type you should examine the springs and stuffing box. This means the faces may have been pressed together with the shaft rotating under the rubber. The springs will be worn and shiny if this has happened. This is because the spring remains stationary and rubs against some rotary part of the pump. This is caused by using the wrong lubricant on the rubber during installation and could be also due to an underside shaft and too good a shaft finish.

In several conventional seals we have seen this symptom where the seal had run against the gland rather than the pressed-in stationary face. This had been caused by the gland slipping in one case and in another case by the gland bore being smaller than the OD of the seal.

No wear track - shiny spots on the face

This is caused by a warping of the face with the spots. Warping is caused by too much pressure, improper bolting or clamping or a bad face on the pump where the face is clamped. This can happen easily on two bolt glands that are not thick enough; it also can happen when the face is severely out of flat before it has been installed.

Cures for the problem include checking to see if the hard face is flat prior to installation, facing off the pump so that it is a clean smooth surface, using four bolt glands or glands that are strong enough to spread the bolt force evenly, and taking pains to draw up the bolts evenly.

This is an important symptom because it indicates the seal probably was leaking from startup. The constant leakage usually causes the elastomer to hang up and the seal is no longer able to clean-itself. This can then lead to clogged springs which might have appeared to be the cause of the failure, but was really a result of the leakage.

Collect the entire seal

Do not try to troubleshoot a seal by using only the parts that look important. You must have both the rotating part and the stationary part. If possible, you should also be able to inspect the gaskets, O-rings or other secondary seals, the shaft sleeve and the inside of the stuffing box.

Guides for the installation of successful mechanical seals

Preparation of the pump

(a) Be sure seal chamber is clean and free of all foreign matter.

(b) Shaft of shaft sleeve on which the seal is to operate must be to size, must be smooth, straight, and free of all burrs, sharp corners, nicks, or excessively deep scratches.

(c) Plug all holes in the stuffing box which are not to be used in the operation of the seal.

(d) Face of stuffing boxes must be smooth, clean, and square with the axis of the shaft.

(e) Halves of boxes on horizontal split case pumps must match perfectly, with the gasket between the halves extending flush with the surface on which the mechanical seal gland-gasket is to seal. Remove all sharp corners and burrs from stuffing box face.

(f) Check the shaft for alignment with a dial Indicator. The maximum allowable runout for optimum seal performance Is .005 " TIR. Excessive misalignment may mean faulty bearings or bent shafts.

(g) Keep shaft end-play at a minimum Recommended macimum end-play is .005 inches.

(h) Check pump wear rings and impeller for proper clearances. Shaft must turn freely. Vibrations caused by rubbing and improper clearances can cause seal failure.

(i) Wherever shaft sleeves are used, make certain the sleeve is properly gasketed to the shaft to prevent leakage under the sleeve.

Installation of the Seal

(a) Always handle mechanical seals with extreme care. Cleanliness is imperative. Never place faces face down on bench or floor. Keep seal in shipping containers until ready to install.

(b) Follow seal Installations Instructions carefully, (See Installation Instructions).

(c) Be sure all seal set screws are tight.

(d) Where set screws are used as a drive between seal and shaft, shaft should be counter-sunk to receive cup point.

(e) Care should be exercised In tightening gland bolts. Tighten evenly and do not spring gland.

(f) Use four equal-spaced gland bolts wherever possible; API-ASME Code for Unfired Pressure Vessels should be followed wherever possible when selecting gland bolt size and spacing.

(g) When tightening gland bolts, check clearances between shaft and gland with feeler gauges. This is particularly important when the gland is not piloted on the stuffing box as glands must be accurately centered.

(h) Test seals statically under pressure before starting pump. Make slight adjustments in gland nuts as necessary to stop any leakage which may occur through gland-gasket.

(i) Never operate mechanical seals dry. Carefully follow instructions for flushing and cooling connections where specified. Be sure suction and discharge of pump is open and a positive head of fluid is present before starting pump. This applies even to that period when checking for proper direction of rotation and adjustment of motor electrical connections.

Troubleshooting

1. Seal spits and sputters in Operation

Product is flashing across the seal faces due to vaporization. Keep in mind a definite liquid condition between the faces is required and take steps to maintain this. Check to determine if pressure, perhaps, requires balanced design rather than unbalanced and, if the seal is already balanced, it may be that pressures are more severe than indicated on the specification sheet. Determine the correct actual stuffing box pressure and temperature and also the specific gravity and vapor pressure at these conditions for the product being handled as this data may provide the clue to the trouble.

2. Seal leaks and icing appears around the gland

Product is flashing across the seal faces due to vaporization. If icing has occurred, undoubtedly some damage has been inflicted on the stationary seat and the carbon seal ring. These faces should be inspected and repaired, if possible, or replaced, if necessary, after the vaporizing condition has been corrected.

3. Seal drips steadily

Check to see that the gland gasket is under proper compression against the face of the stuffing box. With horizontally split case pumps, be sure to check at interface of joint and gland. Faces may be deflected or not truly flat. Improper gland-bolting or overstressing of the bolts may have caused a deflection of the stationary seat under compression. This will occur, primarily, with clamped type seats. Shaft packing on the rotary unit or on the stationary seat may have been damaged in installation. The waring faces may have been scored by abrasives or other fine particles. If a unitary assembly or mounted on a pump sleeve, it is possible that leakage is coming under the sleeve itself.

4. Squealing seal

This indicates dry operation which may be due to lack of liquid at the sealing faces. It is possible that a circulating flush line from discharge or an extemal source of fluid may be necessary. Furthermore, if one is already installed, it is possible that the orifice in it is too small and it may be necessary to enlarge the orifice.

5. Carbon rotating face dusting and this wear showing up outside the seal on gland and along the shaft

Insufficient liquid at the sealing faces. Liquid is flashing due to vapor pressure built up between the seal faces leaving a fine crystallized particle residue or is creating dry contact thus grinding the carbon away. The stuffing box pressure is too high for the seal dgsign and, undoubtedly, some correction has to be made. A balanced seal may be the answer.

6. Seal leaks and there is nothing apparently wrong

The faces may not be flat. This can best be determined by removing the faces and examining the wear pattern as discussed previously. The stationary seat may have been distorted due to excessive gland bolting stressing the clamped stationary seat and distorting same. This can also be determined from the wear pattern on examination. Improper piping to the suction and discharge gland of a pump can actually stress that pump, distorting the seal faces in the alignment with the shaft. If this problem is encountered, it is most common on vertical end suction overhung type impeller centrifugal pumps. Many of these pumps are not of sufficient strength in design, etc. to tolerate the excessive weight which results in misalignment due to same and this will affect the seal. Pipe hangers are the only solution to this problem. Possible shaft vibration can be caused by misalignment, impeller unbalance, cavitation, and bad bearings.

7. Short Seal Life

The greatest major cause of short seal life is excessive abrasives getting between the faces and causing rapid wear. The source of these abrasives may either come from slurry condition or they may come from the supercooling of a supersaturated solution or it may occur due to flashing across the seal faces, causing the dissolved solids to crystallize out between said faces and, again, causing wear. Cooling or heating, as the situation might occur, and/or most assuredly circulation of pumpage from discharge to the stuffing box or external clear flushing will alleviate these conditions. Misalignment of equipment. Pipe strain distortions as mentioned above. Seal shows signs of running too hot when a by-pass flush or recirculation may be necessary. Check for the possible rubbing of seal components along the shaft. Throttle bushings and poorly piloted glands can often cause this condition. Attempt more effective cooling of the seal area by connecting all cooling lines, checking to ascertain that all cross drilling of flush lines, etc. , are clear and unobstructed (remove all scale, etc., that may accumulate, in these lines), and by increasing the capacity of cooling lines or open the orifice clearances on circulation lines. Possible improper choice of type of seal may have been selected.

(Courtesy : Mcnally Institute)

India's on the
brink of a water crisis

The World Bank has warned that India might have to grapple with severe water scarcity in the next two decades if the country fails to correct the way it manages its ground water resources. In a draft report entitled India's Water Economy : "Bracing for a Turbulent Future," the World Bank says : "Unless water management practices are changed - and changed soon - India will face a severe water crisis within the next two decades and will have neither the cash to build new infrastructure nor the water needed by its growing economy and rising population."

The report examines the challenges before the country's water sector and suggests critical measures to address them. The report, which was discussed by experts in New Delhi on October 5, is based on 12 papers commissioned by the Bank from Indian practitioners and policy analysts. "Though the country's past investments in large water infrastructure has yielded spectacular results with enormous gains in food security and in the reduction of poverty. The same is now crumbling," the report further notes.

The Bank takes a dig at the country's "build-neglect-rebuild" philosophy and states that much of what currently masquerades as 'investment' in irrigation or municipal water supply is in fact a belated attempt to rehabilitate crumbling infrastructure.

Faced with poor water supply services, farmers and urban dwellers alike have resorted to helping themselves by pumping out groundwater through tubewells.

"Today, 70% of India's irrigation needs and 80% of its domestic water supplies come from groundwater. This has led to rapidly declining water tables and critically depleted aquifers, and is no longer sustainable," the Bank has observed. It outlines a number of areas that are already in crisis situations and further warns that by '20, India's demand for water will exceed all sources of supply.

"Not withstanding the catastrophic consequences of indiscriminate pumping of ground water, government action -including the provision of free power- have exacerbated rather than addressed the problem," the report notes. Referring to various inter-state wrangling over water, the report highlights that around 90% of India's territory is drained by inter-state rivers and the lack of clear allocation rules and uncertainty about it impose high economic and environmental costs.

This has affected the country's important rivers and has made them fetid sewers. "Sewage and waste water from rapidly growing cities and effluents from industries directly flows into rivers. Massive investments are needed in sewers and waste water treatment plants to protect people's health and improve the environment."

With the climate change projections, India's water problems are only likely to worsen, the Bank predicts. "With more rain expected to fall in fewer days, and the rapid melting of glaciers-especially in the western Himalayas-India will need to gear up to tackle the increasing incidence of both droughts and floods," it warns.

The report candidly notes that India can't have a secure water future unless drastic changes are made in the way the state functions.

"The state needs to surrender those tasks which it does not need to perform, and to develop the capacity to do the many things which only the state can do," the study also suggests.

The concept of
Pressure - Boosting

Hydropneumatic tanks offer multiple advantages over conventional systems for domestic usage, while solving common water pressure problems.

Pressure Boosting systems have been an industry standard for over a couple of decades. They involve the setting up of an external system to enhance the pressure in the pipelines/application circuit, as per the requirements of the application. Pre-pressurised tanks have been an integral part of these systems for over twenty years.

Diaphragm tanks are preferred over conventional non-pressurised tanks because diaphragm tanks separate air from water inside the tank, are Pre-pressurised and require little maintenance. Diaphragm tanks provide three times the draw down of water that conventional tanks provide.

Pre-pressurised tanks are used in a variety of applications. They are being extensively used worldwide for numerous applications such as:

Pressure boosting in homes and buildings

Industrial pressure boosters

Reverse osmosis storage

Irrigation

Sprinkler systems

Water hammer arresting

Fire-fighting

Heat expansion

Thermal expansion

Pre-pressurised tanks or hydropneumatic tanks are used to provide pressure to very small public water systems such as resorts, mobile home parks and very small communities. These tanks are available in a variety of models and sizes.

These tanks operate on the same principle as a home water system, in that the pressure-rated tank contains approximately two-thirds water and one-thirds air at full capacity. An air compressor is required to maintain proper volume of air within the tank at the necessary pressure. At a low operating level, the tank will contain about one-third water and two-thirds air. The air is pressurised to provide a system head and operates at about a 20-pound-per-square-inch pressure difference between high and low water levels. A system using a hydropneumatic tank requiring an average operating pressure of 40-psi would then have 50 psi pressure at high levels and a 30 psi pressure at low levels. In fact, these tanks with pre-pressurised water help to reduce the pump run time drastically in applications that need small supplies of water at regular pressure over short intervals. The water stored in the tanks can be supplied at the preset pressure without switching on the pump too frequently.

Hydropneumatic tanks are generally constructed of steel and must meet the standards of the American Society of Mechanical Engineers (ASME) for pressure-rated tanks. The tanks are usually long and cylindrical, and positioned horizontally on concrete support piers. They look similar to propane storage tanks.

A hydropneumatic tank consists of a diaphragm made up of a food-grade EPDM rubber membrane that separates the air from the water inside the tanks. The diaphragm can also be a complete balloon where the air is inside the balloon bag and the water is trapped around the balloon and the tank shell.

Water distribution

Water pressure bears a very important role in various applications whether it is at the domestic level or in industry. The process of water distribution is similar for all applications.

The easiest method of water distribution in a building or a complex comprises of a source of water connected to a storage tank. All the water outlets, as per requirements, are connected to the storage tank. The distribution pattern may vary between a multi-storeyed building complex and a bungalow.

a. Multi-storeyed buildings: In case of a multi-storeyed building, the water is initially stored in an underground tank of very large capacity. This water is then pumped into multiple storage units placed on top of the buildings. It is then distributed to individual floors under gravity.

b. Bungalows: In case of a bungalow, the water may be stored in an underground tank of very large capacity. This water is then pumped into single or multiple storage units placed on top of the bungalow. It is then distributed to individual areas under gravity. Alternatively, the water is pumped into the above-ground storage tank, directly from the source.

Water distribution under gravity: This is the most commonly employed technique for water distribution. It works on the principle of kinetics of flow, wherein the water moves through a closed area under a specific velocity, which it acquires by virtue of the height from which it is flowing. As per the laws of physics, the pressure generated in the water is directly proportional to the height from which it flows. Thus, water flowing from a very high location would have higher pressure as compared to water flowing from a lower height, irrespective of the pipe size and dimensions. Thus, while designing a water distribution system for a building, it is the height from which the water flows which will always determine the pressure of water at the outlet points.

Shortfalls of the conventional systems: The conventional systems though effective have been proven to have several shortfalls, in current times. These are listed below.

1. In a multi-storeyed building, although the height is great, locating the storage tanks on the topmost floor requires extensive design considerations to take care of the excess loadings. Moreover, in case of earthquakes, the buildings are more likely to collapse due to the presence of these tanks as moving loads.

2. Also, in case of multi-storeyed buildings, the pressure generated in the pipeline at the lower levels is as high as that on the higher floors. Thus, you have regular complaints of low pressure at various levels.

3. In case of bungalows, as the height is not much, the pressure is very low at the outlets, and in case multiple outlets are opened simultaneously, the water pressure/flow further drops.

4. In this system, multi-level pumping is required in most cases, as the water pressure in the municipal mains is not sufficient to drive the water up to the storage tanks provided on rooftops.

5. The pressure requirement for the new gadgets- like shower panels, Jacuzzis and dishwashers-employed these days is very high, which conventional systems cannot meet.

The sizing of the tanks is based on Boyle's Law. The effective draw down from the tank can be determined using the cut-in & cut-out pressure required by the given system.

The data required for sizing a suitable boosting system can be listed as:

Capacity- flow rate in lph or m3/hr.

Total volume- flow rate in lpm x pump runtime

Select pump runtime

Pump cut-in pressure- P1

Pump cut-out pressure- P2

Calculate tank efficiency using Boyle's Law.

Divide actual flow rate by tank efficiency.

For example, we can have a simple calculation of the system.

Pump flow rate = 10 litres per minute

Pump runtime = 1 minute

Total volume: Pump flow rate x pump runtime : 10 lpm x 1 minute = 10 litres

Cut-in pressure = P1 = 30psi + 14.7
psi=44.7 psi

Cut-out pressure = P2 = 50psi + 14.7
psi=64.7 psi

According to Boyle's Law, tank efficiency=(P2-P1)/P2
                  (64.7-44.7)/64.7=0.309

Tank size= Total volume= 10.00 = 32.36 litres

            Tank efficiency 0.309

Thus, from the above calculations, we can select a tank having a total volume of 32.36 litres, so that at any given point of time it can deliver at least 10 litres of water, without switching on the pump. The only other precaution to be taken is that an adjustment factor for the atmospheric pressure must be considered while performing the calculations. The atmospheric pressure is considered to be 1 bar or 14.7 psi. Similarly, the pump should also be selected keeping in mind the atmospheric pressure.

Benefits of pressure systems

An HPN tank extends the life of the pump and enhances performance of the pump by reducing the number of cycles.

HPN tanks provide a closed drinking water system. A closed system inhibits bacterial and vermin intrusion. Conventional tanks used in a gravity system are vented and allow impurities to enter the system.

HPN tanks help deliver equal pressure throughout multi-level dwellings. Conventional tanks create pressure disparities in homes.

HPN tanks draw water from wells or cisterns as needed and allow source replenishment.

Unsightly overhead tanks are not needed in a pressure system. A roof can be designed without the worry of making room for an overhead tank.

Water-saving devices can be used effectively with pre-pressurised tank systems. Gravity-fed systems require open flow and waste water.

Overhead tanks typically require multi-line plumbing to various points of use in gravity feed systems. Pressure systems allow single pipe plumbing.