Back to Basics
Positive Displacement
Pumps

The last decade has seen changes on technology from that was not witnessed over 50 years in the past. While we are trying to keep pace with technology, we have little time to ponder over the basics of equipments used.

In the present scenario, an engineer is faced with a choice of multitude of pumps. It is this choice that makes the selection difficulty Selected properly, a pump will continue to operate as designed without ant difficulty.

However, the selection of pump for a given duty is the first task that must be free from error.

Apart from the usual parameters such as flow rate, suction and discharge pressure, liquid characteristic, pumping temperature etc, other considerations such as requirement of constant flow, non-emulsifying pumping, ability to handle variety of liquids using the pump must also be made clear for correct selection.

Since a vast majority of the pumps used in the Industry are of Centrifugal type, much of the information available is based on these types. There are a variety of pumps apart from such pumps whose functions and applications have not been understood well by users as well as suppliers.

In this paper an attempt is made to bring awareness among users about the merits of such pumps. Classification of Pumps is shown in Fig 1.The list of pumps is not complete as several special design pumps also exist but their manufacturers and users are limited.

Among the PD pumps, we have Rotary and Reciprocating designs.

PD Pumps should be considered only when:

1) a constant flow regardless of the system pressure is desirable

2) viscosity is above 200 cP

3) very small flow rates at higher pressures and viscosities above 10 cP

4) High pressure requirements regardless of the speed

5) special needs to handle slurry -PC/Peristaltic pumps

6) Space restrictions/reliability - such as vane pumps for water in hydroelectric plants

Reciprocating pump design is the oldest. Before the invention of Centrifugal pumps these were the only pumps known to industry. A number of applications involving low heads and high volumes have been successfully replaced by Centrifugal Pumps. Reliability of Recips has been high but so do the requirement of maintenance.

In the present day context, Reciprocating pumps still occupy an important place:

Metering pumps from a few litres/hour to gigantic transfer pumps to 1500 cubic, metres/hr capacity and pressure range up to 700 bar are routinely used In terms of power 0.5 hp to 3000 HP. Small sized pumps are made in several materials including plastics and exotic alloys while large pumps are usually with steel fluid ends.

Reciprocating motion is achieved either thru a cam mechanism from a rotary motion or directly thru a reciprocating hydraulic or pneumatic cylinders. In AODD pumps, a special air diverter valve makes the compressed air to impart reciprocating motion to the diaphragms.

Pumping elements include a cylinder, piston or plunger and a set of valves - referred as fluid end parts. In a diaphragm pump piston/plunger may still exist but the actual pumping is done by a diaphragm in a chamber (instead of cylinder or liner). Compatibility of pumped fluid with pumping elements is essential.

In case of Air operated pumps with double diaphragm, the discharge pressure of fluid is equal to the air pressure available for driving the pump. Special design allows for twice the air pressure as the fluid end pressure.

Due to pulsating discharge rate, the discharge pressure also reflects pulsations. If this is objectionable a Pulsation dampener is installed. Two types are known : in-line type and across the line.

Hydro testing pumps using low pressure air are used for high pressure testing - over 1000 times the inlet air pressure due to favouable ratios of fluid plunger to air Piston area.

Clean liquid to abrasive slurries are handled by reciprocating pumps routinely.

In addition to the normal motorized drive or pneumatic drives, solenoid operated plunger pumps are also used for small flow rates while handling clean fluids.

Flow rate adjustment is by varying the SPM (strokes per minute) or by changing the stroke length or by a combination of both. The other method would be - use a by-pass line and throttle the by-pass to achieve desired flow rate in the main line. This, of course, would waste energy but its simplicity sometimes will justify its use.

A very interesting idea was to use an adjustable spring loaded mechanism opposite to the pumping cylinder that will "store" a part of the delivery during forward stroke and "return" the same during back stroke.

In the Rotary design of Positive Displacement pump, the metallic rotating parts have close working clearances. Presence of a lubrication film is essential for satisfactory life of such pumps - gear, screw, lobe and vane are some common pumps in this category. For viscosities above 10 cSt most rotary PD pumps are suitable. For low viscosity handling specify pumps with non-contact rotating parts. Timing gear driven Twin Screw or lobe pumps can handle low and high viscosity fluids.

For viscosities above 200 cSt, PD pumps should be preferred to Centrifugal pumps unless there are valid reasons like very large flow rates and presence of some solids.

Positive Displacement pumps are self priming. However, it is always recommend that these operate with enough suction pressure to prevent cavitation.

Application areas : Hydrotesting of vessels, pipelines, hydroblasting, drilling mud pumping in Oilwell application, as homogeni4ers in dairy industry, spray drying applications, general transfer of fuels, cement slurry pumping, slurry pumping thru pipelines ,metering of chemicals as dosing pumps, filter press application and others.

All positive Displacement pumps need a pressure relief valve to protect them and the system from overpressure damage. A closed valve in the discharge line can lead to dangerously high pressures leading to self destruction hence the discharge should never be throttled for flow control. At manufacturer's work you will see a throttling valve - contrary to what is just said.

There is no mistake here. The valve is used for simulating the discharge pressure conditions and used with utmost care. In addition, there is always a relief valve (RV) between the pump and throttling valve to safeguard the system. The same throttle valve is also use for testing the RV functionality.

Wherever frequent on off is involved, it is recommended to use an "unloader" valve. This permits pump to continue to run at "no load" condition when the main flow is stopped. The valve permits maintaining a desirable pressure in the main line. As soon as the pressure reduces for any reason in the system, the unloader will stop by-passing the fluid. This is a self governing and needs no external pilot or energy source to operate. Such valves are common on all high pressure washers.

There are no "built-in" relief valves on reciprocating pumps; A line mounted RV or a rupture disc, is used with the discharge always open to atmosphere or to a nearby tank open to atmosphere or with no backpressure. The discharge line from relief valve must ensure emptying the content by gravity - by providing a downward slope. A mistake here may lead to non functioning of RV and lead to explosion

For absolute safety a rupture disc is preferred. These can't be reused and can't be fabricated locally with certification.

For Rotary or Reciprocating Pumps following applies:

Selection parameters:

1. Product pumped

2. Flow rate - LPM, cubic metres/hr, GPM etc

3. Suction and discharge pressures bar,kg/cm^2,psi..

4. Viscosity - cP, SSU , Engler, mPa.s...

5. Pumping temperature

6. Special needs - direction of rotation

7. Shaft Sealing option

Without the product knowledge it is not proper to select the pump and its metallurgy for fluid end parts. Chemical compatibility, if not checked can cause serious problems. This is especially relevant for metering pumps handling chemical additives.

Flow rate: Minimum and maximum anticipated flow rate should be specified while making the enquiry itself. Flow rate being directly proportional to pump speed, manufacturer will check the suitability of his pump for the speed range required. It is safe to operate BELOW the speed specified without any harm to the pump but always consult manufacturer if the pump speed is to be above the max speed specified. Power requirement, NPSHr values change with speed - both increase!!

Controlling change of flow rate by throttling suction should never be tried.

You can predict the output at different speeds if the output at any given speed is known:

Q1/Q2=n1/n2

Viscosity : For 200 cP and above the first choice should be a PD Pump.

Viscosity affects pump performance. The percentage of slip is reduced with increase in viscosity while Power requirements increase Maximum slip occurs at the lowest viscosity.

The capacity is chosen for the lowest viscosity while the power is selected for highest viscosity. Variations in temperature can bring about a large change in viscosity, usually; the viscosity drops with temperature for a given product. Sometimes one pump is intended to handle several products having different viscosities. The wear on rotating parts is affected by viscosity changes: lower the viscosity-higher is the wear for same pressure duty.

Pump speed is to be lowered while handling higher viscosity. See table (2) as a sample of speed reduction vs Viscosity for a shuttle block pump.

Pumping Temperature: If the pump was not designed to operate for a given temperature, please consult manufacturer, Usually operating at lower than specified temperature should not cause any alarm except in cases of cryogenic or temperature below the freezing point of liquids. Most liquids show increase in viscosity as the temperature decreases. PD pumps will require lower speeds and higher power when dealing with higher viscosities.

Special needs: Noise, vibration levels, direction of rotation if critical must be specified at the time of enquiry as the pump selection itself will depend on these inputs. Mechanical seals, Lantern Ring arrangements, location of inlet/outlet ports, drain plug location etc also form a part of this review.

Pressure capability: PD pumps do not "develop" pressure as erroneously stated by many manufacturers. .PD Pumps are "capable" of overcoming a system pressure. Clearances between working parts, the rigidity of components determine the pressure rating of Pumps.

Pump shaft and rotating elements are designed for a particular torque/HP. Exceeding these values -by trying to operate the pump at higher pressure-will lead to damage to components.

Reciprocating pumps - plunger, piston and diaphragm are on the top ofthe list in this regard. 1500 bars is quite easily achievable. Hydraulic gear pumps also handle high pressures but strictly are meant for very clean fluids of specified grade of oil only.

General purpose transfer pumps are meant for 8 bars.

All Positive Displacement Pumps are prone to damage if the discharge line valve is closed. Some may develop dangerously high pressure and may cause a permanent damage to system or itself while the others may suffer from high temperature damage with high pressure. To avoid this accidental damage, a relief valve either line mounted or mounted integrally on the pump itself is necessary. Built-in relief valve will protect only the pump and not the system. It also offers no protection against high vapour pressures that can result from closed discharge valve. API does not permit built-in relief valves and insists on line mounted relief valves only.

Flow rate control: Capacity regulation in PD Pumps is to be done by using one or more combinations of the following methods only:

a) Changing of speed - rpm / spm

b) by-passing apart of the flow

c) adjusting the stroke length (for piton / plunger)

Flow control should NEVER BE DONE BY THROTTLING THE DISCHARGE ORSUCTIONVALVE!

Precautions during start-up: For any PD Pump the method is same :

1. Keep suction and discharge valves fully OPEN!

2. Make sure a filter with adequate mesh size is in the suction line to protect the pump against any solids that might get flushed into the Pump suction. Do not install fine mesh filters in the slurry lines!

3. Ensure liquid is present in the pump casing at the time of starting - even if the pump is self priming. Dry run will damage most PD Pumps permanently.

4. Ensure before starting the pump that the motor/ driver will have the correct direction of rotation.(if not sure, de-couple the pump and separately run the motor to check the correctness of Direction of Rotation

5. Use the pump for the same fluid for which it is intended. If not, please consult the manufacturer before using it in any other liquid. Elastomers / metal parts may or may not withstand different fluid.

6. Install suction gauge - a compound type and a pressure gauge at the discharge to monitor pump performance.

7. If the discharge pipeline is long - provide a by pass between suction and discharge. A bleed line near the discharge is very helpful.

8. Ensure that a Relief Valve, either built-in or pipeline mounted, is in the system - do not start without this safety devise.

9. Starting a steam jacketed pump or heat traced pump ensure the required temp is achieved before staring the pump operation.

Useful Tips :

For rotary pump that carry the liquid along the periphery the rotor peripheral speed should not exceed 5 m/s. Peripheral speed is calculated as (3.14 *D*n/60) where n is rpm and D - diameter of rotor is in metres. At higher speeds the centrifugal force acting on the liquid actually "opposes" the entry of liquid into the Pump.(true for external gear and lobe pumps)

Internal Gear Pumps have lower NPSHr requirements compared to the External Gear Types at same speed.

Screw Pumps carry liquid "axially" within the pump .Therefore, the limitation of 5 m/s does not apply to these pumps

Specify jacketing when pumped liquids tend to solidify or change its properties with temperature.

It may steam, hot water, hot oil or even cold water depending upon the need.

When pumping clean liquids, a vertical inlet and vertical outlet configuration is most beneficial. This ensures that some liquid will always be retained in the pump and there will be no "dry starts".

As opposed to above, a CIP pump is preferred with top vertical inlet and bottom outlet to ensure that no liquid will be retained in the pump after flushing.(these are usually metallic lobe pumps driven by a pair of timing gears)

When handling heterogeneous slurries with PC pumps try to mount these vertically- with suction from the TOP and discharge at BOTTOM. This arrangement ensures that flushing is assisted by gravity.

Gravity - the only force in Nature that has not changed its value or direction and has never failed. Make full use of this. Users in Pharmaceutical and Food industry should particularly note this aspect since their requirement to drain pump is of critical importance.

When and where possible, make use of expansion bellows at the suction and discharge of the pump. Benefits: pump vibrations and noise will not be transmitted to pipelines. Similarly, the Pipeline stress will never be passed on to pump casing. Rotary pumps with fine working clearances are especially prone to malfunction due to pipeline stresses.

When handling oils, resins and other sticky matters it is advisable to specify a lantern ring in the stuffing box and relieve the former back to suction. Suction pressures are usually low and this allows the leakage to be reduced to almost nil without the detrimental heating of shaft and need to tighten the gland to minimize the leaks.

To avoid alignment problems and bulky baseplates, specify a monoblock version of pump. If this is not available use a flange mounted pump with an adapter bracket to accept a flange mounted Motor. (see fig #3). Misalignment is a major contributor to bearing failures.

Vertical pedestal mounted pumps will save on installation space and cost. It will ensure alignment at all times and save the motor from getting inundated in the event of floods.

Installations where, reverse rotation is not permissible, even for a fraction of second, protect the pump with a one- way clutch type coupling. Specify a phase discriminator with the starter to prevent reverse direction start. For example- motorized valve actuators on pipelines at remote locations - under no circumstances the initial direction of rotation of motor should get reversed. Without a phase discriminator, the actuator will try to open an already open valve or close an already closed valve - in both cases damage to motor/valve is imminent.

Extending useful life: Like all other pumps, PD pumps are also prone to wear .Over a period of time, the performance starts to deteriorate. Where permissible, one may use the option of increasing the speed till maximum permissible speed is reached.

Some sort of variable speed mechanism is necessary for this.

While selecting the pump for prolonged use, check the design if it permits to restore the original performance without changing the pump casing. Replaceable liners, vanes, screw cartridges are among the spares that can restore the original performance of the pump.

In Twin Screw and Three Screw Pumps it is possible to change the pitch of the screws in the same liner/body to get larger or smaller capacity at the same speed - this may come in handy in case of plant expansion.

Dry running and operating a pump for liquids with lower viscosities than for which it was specified may be detrimental to its useful life.

With exception of PC (single screw) and peristaltic pumps all other rotary positive displacement pumps must be reviewed for their ability to handle water. Twin screw pumps with bearings located outside the pumped liquid are capable of handling water.

All other pumps, where a metal to metal contact forms a part of pump design, must never be run in water.

During hydrotesting of plant piping, such pumps must be isolated failing which it may cost the life of the pump!

Common errors:

1. Often PD Pumps are specified for thin clear liquids. This should be avoided. There are better alternatives: both cost wise and durability wise. Centrifugal, Centripetal, Turbine (peripheral) pumps choice must be exhausted first. Special needs like constant flow regardless of pressure may be one exception for specifying PD pumps for thin liquids.

2. Pump capacity, system pressure and viscosity of fluid are inter-related. When pressure requirement is not met at rated flow and given viscosity, it shows error in calculation of system pressure and not malfunction of Pump.(since pump does not develop pressure)

3. The term "Flooded" stated against suction condition/pressure does not convey sufficient information, except that pump suction line is below the tank level. NPSHa is a better term.

4. GD^2 value ( often asked in data sheets) carries no relevance for Progressive cavity pumps and other pumps where the rotor is not free running ! Where needed, this value should include gearbox and coupling along with the pump to check the suitability of motor.

5. The term "self- priming" shows ability to handle air/vapour with some liquid present in the pump. It does not mean pump will prime when dry. Dry self priming is a specific term to signify that pump is capable of running dry till it is primed. Most pumps are not Dry Self priming type!!

6. Don't over specify: like asking API 676 standards for non-critical and non refinery service.

Otherwise it would only add to the costs and reduce choice of vendors.

7. Pumps that need to work under vacuum condition require special attention. Discuss this at length with supplier. Special sealing arrangements are needed for such pumps. Do not take for granted that self priming pumps automatically meet this requirement.

Recent developments in science and technology has provided alloys and polymers with unique properties for pump industry.

Manufacturing techniques now permit production of one off units.

Magnetic couplings, canned motor designs are capable of meeting the zero leak requirements.

Where options permit, usage of a tank mounted unit solves the leakage problem using standard pumps with additional benefits like reduction of noise and vibrations.

Well proven record forms the basis of specifications for pumping needs. However, without innovations and lateral thinking progress would be unthinkable.

Here are a few examples of non- traditional approach that led to successful applications :

(it is suggested to log on to the respective websites for details)

Centrifugal pumps for viscous and abrasive slurries to replace PC/Diaphragm Pumps (Discflo)

Single stage, high speed centrifugal pump against a multistage "normal" speed pumps (Sundyne)

Compact three screw pumps with special metallurgy for handling water at low capacity at high pressure in place of high speed multistage centrifugal pumps. (Allweiler/Knoll)

Modified peristaltic/PC pump combination to handle four different liquids at the same time independently with zero leakage to replace four separate pumps (Seepex)

Impulse technology that shattered the concept of barometric pressure as the limiting factor for suction lift (10 m or 32 - feet of water) (Clavis Impulse Technology, Norway)

Conclusions : When the limitations and operating principle of any, including PD pump, are understood well, it helps to specify a correct pump.

SELECTION OF PUMP
USE OF ALGORITHM MAKES EASY

A) Should Centrifugal Pumps Be Default Selection?

One would often select a centrifugal pump by default, because most thinking of pumps focuses on centrifugal pumps. This is so, because centrifugal pumps are made in such majority over other types of pump. However an algorithm should rather ensure that there is no default selection. This algorithm hence provides for checking whether the application would warrant a pump other than a centrifugal pump. In short, it takes into account the limitations of centrifugal pumps. From the algorithm, one would realise that centrifugal pumps do have many limitations.

An algorithm is better readable as a flowchart. An algorithm has been attempted and is at Annex l. Notes below explain the logic of the algorithm.

B) Most Common Parameters of Pumping Duty

Pump selection should start with data on hand about some most common parameters of pumping duty, viz. Head and/or pressure, flow-rate required, preferred speed, number of pumps.

C) Factors Governing Flow-rate Per Pump

There are statutory codes to be followed, especially when selecting pumps for fire-fighting. Fire fighting pumps (IS-12469) have nominal discharge ratings specified in the code itself. Depending upon the degree of hazard, one has to select a discharge rating from the nominal ratings given in the code.

For pumping water to the overhead reservoir in a multi-storey residential building, number of hours of inflow of municipal supply, capacity of Ground Level Reservoir (GLR) and total daily requirement of the residents will together decide the flow-rate. For example, if there are 50 families in a building, i.e. a population of 250 people at an average of 5 persons per family, then at 200 litres per capita per day, daily requirement becomes 50 m3. If number of hours of inflow of municipal supply is 3 hours and the capacity of GLR is only 20 m3, balance of the total requirement i.e. 30 m3 will have to be pumped to OHR in the 3 hours of inflow. So required flow rate of pumping becomes 10 m3/h.

In a sewage pumping station, flow rate for pumps has to vary at different times of the day, depending upon the rate of inflow of sewage, size of sump, and time within which sewage should be pumped so that it does not become septic, also taking care that the pump should not suffer too many starts and stops at too short intervals. Traditionally this is managed by running different number of pumps in parallel. One may as well have pumps of different flow rates to run in parallel. Variable Speed Drives (VSD) make a good, energy-saving alternative for such regulation of flow-rate.

D) Factors Influencing The Total Head

Total head primarily comprises the level difference and/or pressure difference and the hydraulic friction. Hydraulic friction depends upon the selection of types and sizes of pipes and fittings.

All this data on the most common parameters of pumping duty may often prompt the default selection of a centrifugal pump. However there are many other parameters to be considered to decide the type of pump.

E) Factors Influencing Operating Speed

Wear life, bearing life, preference for a compact design and variety reduction influence selection of operating speed of the pump. Abrasive and corrosive wear will be more pronounced at high operating speed. Bearing life will also be less at high operating speed. But pumps designed to run at high operating speed are compact and space-saving and are better portable. If pumps have to operate even optional drive from internal combustion engines, or if stand-bye pumps for emergency operation have to be with internal combustion engines, variety of pumps and carrying of inventory of spares can be less if both electrically driven and I. C. engine-driven pumps are of common design.

F) Initial Procedure of the Algorithm

1. Is viscosity > 350 cSt?

1.1 If yes, select a positive displacement pump

    Check if viscosity is shear sensitive?

    1.1.1 If yes, select positive displacement pump, where liquid will not suffer shear at the     pumping element. A helical rotor, progressive cavity pump or a diaphragm pump may be     considered.

1.2 If not, i.e. if viscosity < 350 cSt, proceed to 2

2. Is distortion or damage to entrained solids acceptable?

    2.1 If not, select a helical rotor, progressive cavity pump

    2.2 If yes, i.e. if distortion or damage to entrained solids is acceptable, proceed to 3

3. Does duty need metering or dosing?

    3.1 If yes, select a positive displacement pump

    3.2 If not, proceed to 4

4. Is multi-phase medium to be handled, e.g. oil, gas and mud in oil exploration? If yes, consider piston rod or plunger pumps. Pumps called as sucker rod pumps in the oil industry are basically plunger pumps.

5. Are solid-contents > 10%?

5.1 If yes, select a positive displacement pump, but get back to Step 2.

    5.1.1 Is distortion or damage to entrained solids acceptable?

    5.1.2 If yes, consider a positive displacement pump.

    5.1.3 Is flow-rate of micro scale? if yes, consider a peristalic pump.

5.2 If solid-contents < 10%, consider a non-clog centrifugal pump.

6. Is specific speed <6?

    Calculate the specific speed from the pumping duty, viz. Q, H and rpm. If pressure is     dominant factor of the duty, convert pressure to equivalent head to find the specific speed.

    6.1 Check if running at higher speed acceptable, so that higher speed will make higher     specific speed and will make a centrifugal pump feasible. Then go to 5.6.

    6.2 Check if liquid is clear, i.e. if turbidity is <500 ppm, If yes, check whether multi-stage     construction is acceptable, so that specific speed per stage will again be higher

    6.3 If checks (5.1) and (5.2) are negative, and liquid is clear, check if pulsating flow is     acceptable.

    6.3.1 If pulsating flow is acceptable, a positive displacement pump can be more energy-    efficient than any impeller pump. But with thin liquids, there will be slip flow, which will     cause the efficiency of pumps to be less than normal.

    6.3.2 If pulsating flow is not acceptable, one may consider a regenerative turbine type     pump. But these pumps are not energy-efficient. But the advantage is that they can run at     high running speed and are in turn compact, have non-pulsating flow, and also have a fair     degree of self-priming capability.

    Check whether this capability has cost-benefit advantage in the application.

    6.4 Further option for selecting centrifugal pumps is to select the pump for higher discharge     and bye-pass the excess flow back to suction. This again will not be energy-efficient.

    6.5 Selecting from feasible options should be by Life Cycle Cost (LCC) analysis.

    G) Selecting Among Positive Displacement Pumps

    6.6 Specific applications where helical rotor, progressive cavity pumps, metering or dosing     pumps, peristaltic pumps, diaphragm pumps would become selection options have been     mentioned above. Where the algorithm lends selecting positive displacement pump as a     general option, various types of positive displacement pumps can be considered, viz. piston     or plunger pumps, internal gear pumps, external gear pumps, lobe pumps, vane pumps,     flexible vane pumps, multiple screw pumps, etc. Again, selecting from feasible options     should be by Life Cycle Cost (LCC) analysis.

H) Selecting the type of mounting

Most common arrangement of a pump is with its working axis horizontal. Centrifugal pumps with horizontal working axis often operate with a suction lift, unless the pump is to draw the liquid from a suction vessel above the pump.

Vertical mounting becomes an option for following reasons.

a) NPSH available may not be adequate. If so, mounting the pump to be submerged in the liquid will improve NPSH available. Vertical turbine pumps become an option to large mixed flow or axial flow end suction pumps or axially split casing horizontal pumps. But dismantling vertical turbine pumps for overhauling becomes quite an exercise. Also it becomes important to ensure structural stability, non- resonant frequency and limited and controlled vibration in case of vertical turbine pumps.

b) Mounting more than one pumps in a pit would warrant proper design of sump.

c) Volatite or high temperature liquids with high vapour pressure would warrant pump to be mounted in a vertical can with enough depth, to take care of NPSH required by the pump in the depth of the can itself. d) Vertical mounting often makes a compact, space saving installation. Vertical in-line pumps are an eminent example.

e) Submersible pumps in tube wells are vertical pumps. Most pumps for drainage and dewatering are also vertical, since they then are compact and hence portable.

f) Submerging pumps in the liquid, often called as "wet pit" mounting has become feasible with the development of submersible motors. Various types of submersible motors are in vogue, mechanically sealed dry motors, water-filled motors, oil-filled motors and encapsulated motors.

I) Selecting Other Features

7. Shaft-sealing : See Table l.

Table I

6. I Traditionally seal-manufacturers supply seals to be assembled by the user. Now seals are also available as ready, &s sembled seals, assembled in a cartridge. Seals are also available as split seals for in situ installation or replacement without the need to dismantle the pump. API-682 details various plans for using seals. When to use which plan, is detailed in Table 2.

1. Heating Jacket

If the liquid has high melting point and in turn, a tendency to solidify at ambient, the pump will not restart after shut down. A heating jacket will help to liquefy the solidified mass blocking the suction.

2. Cooling Jacket

    2.1 When pumping temperatures are high, to prevent the shaft seal from malfunctioning and     bearings having short life, cooling will be needed around stuffing box and bearing housing.

    2.2 If pumping duty is less than safe minimum flow, pump will experience rise in     temperature, vapour pressure will be higher and pump may suffer cavitation. To control the     rise in temperature, cooling jacket will be necessary for the pump casing also.

J) Selecting Materials Of Construction (MOC)

The main considerations behind selecting material of construction for different components are to have

Wear-life, especially against wear due to corrosion and/or abrasion

Creep-strength to withstand the temperature of liquid to be pumped.

Wear-life against cavitation erosion

Corrosion, abrasion, cavitation-erosion and liquid temperature will primarily affect the 'wetted' components of the pump. In case of centrifugal pumps 'wetted' components include the casing impeller, impeller nut, casing cover and seal housing, shaft sleeve, gaskets, shaft seal materials for whichever shaft sealing arrangement is selected.

Corrosive effect of a liquid is mainly due to alkalinity or acidity. Liquids with pH value between 6.5 to 7 are neutral and hence are most comfortable to handle with commonplace material like Cast lron. Until the development of stainless steels, for alkaline corrosion, especially for alkalinity as of sea water copper-based alloys like Naval Bronze were much in vogue. Many stainless steel materials however have good corrosion resistance both against alkaline and acidic corrosion.

Many commonly used materials are better known by acronyms or codes. For the sake of accuracy, one should be conscious of finer distinctions in the codes. For example, 18-8 Cr-Ni stainless steel is commonly known as SS-304. However since components like casing and impeller are made by casting process, the code name is CF8, derived from grade CFB in specification A-351 of American Society of Testing and Materials (ASTM). American Iron and Steel Institute (AISI) designated same material in wrought form as Grade304.

Corrosion-resistance and abrasion-resistance do not go together. Abrasion- resistance would require high degree of hardness. Alternatively if the abrasive particles would impinge on the boundary surfaces at some angle of impedance, resilient materials would sort of cause the abrasive particles to bounce off and not cause wear. Abrasion with low or zero angle of impedance becomes erosion. So, for abrasion-resistance one should choose hard materials for erosive wear and resilient materials for bouncing particles, e.g. particles of sand. For components like pump casing resilient materials like rubber are better provided as linings in a cast casing.

Often components running close to each other, even when made of appropriate corrosion-resistant materials will suffer galling and electrolytic corrosion because of their chemistry being identical. A differential hardness or difference in chemistry of the close running surfaces can reduce or eliminate this galling and corrosion. Simple example to note this is by realising that a piece of glass moves harder over another surface of glass, than how hard a piece of steel would move on a glass surface.

K) Selecting Pumps for Parallel Operation

When flow to be pumped needs regulation, one can get increased flow by running more pumps to run together in parallel. Switching off some of the pumps can reduce the flow. The flow is however not directly proportional to the number of pumps running. Study of system characteristics along with pumps' characteristics is necessary to devise a trouble-free system for pumps running in parallel. It is also necessary that H-Q characteristics of individual pumps to run in parallel should be continuously rising towards shut-off. Also shut-off head of pumps should be identical as far as possible.

L) Life Cycle Cost Analysis

Most situations of pump selection can provide opportunity to explore options.

Most economic selection would emerge by Life Cycle Cost analysis.

LCC=PSC+ICC+OC+MIDC

Where

PSC = Pumping System Cost as the sum total of costs of Pump, Driver, Controller, Coupling, Piping, Valves, etc. Options in sizes of pipes and fittings should be considered as independent options of the Pumping System.

ICC = Installation and Commissioning Cost inclusive of costs of excavations and building of sump and pump room and material handling tackle, on-site assembly and testing

OC = Operating Cost over the life. Period for all options should be life of that option, which has longest life.

MIDC = Maintenance, Inventory and Downtime Cost over the same Period as for Operating Cost.

M) Conclusion

This article has attempted an algorithmic approach to Pump selection, so that any exercise in pump selection would result in selection of not only the most appropriate pump, but of the most appropriate pumping system.

Q.22 Conduct Life Cycle Cost (LCC) Analysis for all feasible options.

LCC=PSC+ICC+OC+MIDC

Where

PSC = Pumping System Cost as the sum total of costs of Pump, Driver, Controller, Coupling, Piping, Valves, etc. Options in sizes of pipes and fittings should be considered as independent options of the Pumping System.

ICC = Installation and Commissioning Cost inclusive of costs of excavations and building of sump and pump room and material handling tackle, on-site assembly and testing

OC = Operating Cost over the life. Period for all options should be life of that option, which has longest life.

MIDC = Maintenance, Inventory and Downtime Cost over the same Period as for Operating Cost.

Twenty-two questions as above help analyse a pumping application to identify options in types of pumps and also important constructional features.

Questions 9, 10, 11 also suggest that for ease of selection the liquid duty should be translated into equivalent water duty.

Questions 3,4,5,17,20 and 21 suggest that there would be multiple options. If answer to a. 6 is 'yes', then the suggestion to select a Positive displacement pump is also a generic answer and would need analysing multiple options of types of positive displacement pumps.

In fact, most situations of pump selection can provide opportunity to explore options. Most economic selection would emerge by Life Cycle Cost analysis.

(Author Er. S.L. Abhyanakar is Technical Advisor of Indian Pump Manufacturers Association (IPMA) & Hon. Editor of Pumps India. Email : sl-abh@yahoo.com

 
Various options in shaft-sealing with degree of sealing      
Type of Sealing    Degree of Sealing      
Gland with soft packing
and lantern ring



Labyrinth sealing





Mechanical Seal



Injectable Sealant



Hydrodynamic seal




Canned Motor Pumps





Magnetically coupled
Pumps
    Zero leak sealing not desirable, since leakage
would flush the wear-out away and also cool
and lubricate the packing, carrying the heat
away

If over-designed, may draw in air into the
pumpOil Seals or U buckets Susceptible to
work hardening and wear of sleeve. Common
in reciprocating pumps for degree of sealing
better than rope packing

Commonplace seal is seemingly zero leak,
though there are invisible fugitive emissions.
Various designs and Plans are available. See 6.1

Competes with mechanical seals, substantially
reducing the cost of repairs and replacements
as compared with mechanical seals

Designed integrally with each pump. Cannot
be interchanged across pumps, unless
pumping duty, viz. head discharge, speed are
all identical.

Are truly zero leak. Detailed consideration
needed, if liquid does not have lubricity or
pumping temperature is high. Part of discharge
is circulated through the motor. This reduces
efficiency.

Are truly zero leak. Detailed consideration
needed, if liquid does not have
lubricity or pumping temperature is high.
Magnetic induction itself generates heat,
causing transmission losses and reducing
overall efficiency.
 

A Few Reminders

Pump Selection :

Do not oversize pumps. This leads to uneconomical operation and generally narrows the safe operating range of capacities.

Do not try to select pumps with excessively low required NPSH (Net Positive Suction Head).

Do not falsify real available NPSH, trying to keep a margin up your sleeve. This leads to selection of pumps with excessively high Suction Specific Speeds and high minimum flows.

Do evaluate economical advantages of variable speed operation. It is more efficient and results in longer pump life.

Don't overestimate value of pump efficiency if it's obtained at cost of reliability.

Do not use a mechanical seal when packing is more than adequate for the intended service.

Installation :

Do provide sufficient submergence over intake piping to prevent vortex formation.

Do not use suction elbows in a plane parallel to the shaft; place them in the plane perpendicular to the shaft.

Do not use the pump casing as an anchor for the piping. If you use expansion joints, support and anchor them independently of the pump.

Do provide adequate flow, pressure and temperature instrumentation for each pump.

Pump and driver alignment must be rechecked under normal operating conditions.

Operation :

Do not operate pumps below the recommended minimum flow.

Do not operate pumps with suction valve closed.

Do not run two pumps in parallel when a single pump can carry the reduced system load.

Do Not stop a pump while it is cavitating. Reestablish normal operation first and then stop the pump if you have to.

A pump handle liquids. Keep air out.

Do not run a pump if excessive noise or vibration occurs.

Do run spare pumps occasionally to check their availability.

Maintenance :

Run a performance test at reasonable intervals of time, to follow effect of increased internal clearances.

Do not open pumps for inspection unless factual or circumstantial evidence warrants it.

Do not over lubricate grease lubricated bearings.

Do not overcool outer races of ball bearings. Inner races continue to expand and balls are squeezed out of shape.

Packing stuffing boxes is an art. Do not assign this task to inexperienced personnel.

Do not tighten stuffing box glands excessively. Let enough leakage flow to cool and lubricate packing.

Do monitor the pressure drop across suction strainers. An excessive pressure drop indicates clogging and may reduce available NPSH to a dangerous degree.

Do keep an adequate stock of spare parts.

Except in an emergency, use original equipment manufacturer's replacement spares.

Consider upgrading material for parts that wear or corrode too rapidly. This lengthens interval between overhauls.

Do examine and recondition, if necessary, all metal-to-metal fits.

Do examine parts for corrosion, erosion or other damage.

Do check concentricity of all parts of the rotor before reassembly.

Do use new gaskets for complete overhaul.

Mine dewatering with
submersible pumps

A crucial aspect of most mines is the dewatering issue. In fact, there are very few mines around the world that don't need to bother about dewatering. Often there are enormous quantities of water that needs to be pumped away-water, which would make mining impossible if not dealt with properly.

There are several pump types available for mine dewatering. The biggest advantage with electrical submersible pump is their simplicity to install and use, just plug it in and pump. This becomes handy when the number of pumps needed changes. Generally an electric submersible pump is easier to service than other pump types. As the weight of a submersible pump varies from 25-550 kg, the smaller once can be handled by hand. The bigger pumps can be handled by the normal lifting equipment in the mine quite easy.

The content of mine water

The water, which must be dealt with in mines, is seldom-clean H2O. It often contains solids and dissolved chemicals that will cause wear on pumps, valves and other equipment used for dewatering. The solids in the mine water are normally the most important factor when choosing pump type.

Drainage pump

A drainage pump is the most commonly used pump type in mines. It is used for pumping water with less abrasive solids, like clay. Sand and solids in suspension can also be pumped, up to the size of the strainer holes (normally 7-12mm). As sand is quite abrasive to the pump, it must not be too concentrated.

Sludge pumps

Suitable for pumping water with bigger solids, as well as for pumping sludge. The solids can be up to the size of the pump inlet (normally 32-80 mm).

Slurry pumps

Designed to handle abrasive solids in suspension, like sand gravel and concrete, in high concentration. To cope with the abrasives, the hydraulic parts of a slurry pump are often made of Ni Hard 4, one of the hardest materials available today. For improved performance, slurry pumps are generally equipped with an agitator.

As most drainage and sludge pumps are made of aluminium, water with corrosive contents will decrease the lifetime of the pump. If the water is less corrosive, the pump can be equipped with sacrificial zinc anodes. Another way of handling the corrosion problem is to coat the aluminium parts of the pump with a material that withstands the corrosion, like epoxy or urethane type. Pumping untreated chemical mine water out in the nature is an environmental issue and is often regulated by local laws.

Pump arrangements

The general method of dewatering a mine is to use small pumps (2-6 kW) for dewatering the mine galleries, pumping the water to larger collecting pump pits. As the smaller submersible pumps weights 25-50 kg, they can be easily moved to different spots when necessary.

The collecting pit is equipped with big pump (8-35kw) that pump the water up to the pump station in the next level. These pumps weigh from 60-285kg. If automatic control of the pump(s) is necessary it can be arranged through a level control system.

In the pump station all water is collected from the levels below. The bigger pumps(s) at the station is weighing 285-550 kg and have a delivery head of upto 100m. The pumping of the water up to the ground level is done in steps, either by staging tanks or by connecting several pumps in series.

When pumping from one level to another, the uses of check valves are important. The check valves stops the water form running back in the pump when the pump stops As the water pillar gets quite high in mines, the pump may suffer quite hard from wear if the water runs back from high heads uncontrolled.

Sedimentation

If the water contains sand, gravel or smaller stones. it might be necessary to use sedimentation tanks. In the sedimentation tanks the solids will settle at the bottom, leaving the water easier to handle. The emptying of the  solids can be simplified by using transportable containers for the tank system.

Open pit mines

The pumping capacity required in open pit operations depends very much on the size of the mine, groundwater conditions and the amount of precipitation. Submersible wear-resistant pumps are always preferable since they can operate under the extremely difficult conditions, which usually prevail in open pit mines. In wide-open pit mines it is recommended to arrange with several small submersible pumps and have them pump to a dedicated pumping station.

As the water level may vary widely from a time to another, it is advisable to place the pumps on a raft, which follows the surface of the water. Otherwise, the pump may dig itself down in sand and clay. To manage the delivery head, several pumps can be connected in series. Intermediate tanks can also solve the problems.

Pumps sizes

The portable pumps are normally 1-6 kW with a flow of 10-60 1/sec and weights 15-50 kg. The pump in the collecting pit is usually from 10 kW and up. If the water shall be pumped to a higher level, a more detailed calculation might be necessary. The bigger pumps are sized from 30 kW and up. One big pump can, of course, be replaced by several smaller pumps. By switching one or two smaller pumps on/off, the flow can be adjusted to match the inflow. This can be done by float switches that automatically switch the pump on and off as required by the water level in the pit. This saves both electricity and extends the service intervals at the pump.

Tips

Avoid kinks and sharp bends on the hoses as they affect the efficiency of the pumping.

Make sure that the pump doesn't burrow itself into sand or clay. This can be done by letting the pump stand on a plank or a bed of coarse gravel. The pump can also be placed into a cut-down and perforated oil drum.

Summary

Choose pump type after the different kind of solids in the water. Drainage pumps are most common.

Arrange with several small pumps evenly spread in the area that needs dewatering.

Dedicate a collecting pump pit; with one big (or several smaller) pumps installed. These pumps may be automatically controlled by float switches.

Arrange with check valves when pumping to another level in the mine. The water might have to pass sedimentation tanks before leaving the site.

Never pump untreated chemical mine water out in the nature without checking the local laws for this environmental matter. Dewatering a mine is not very high tech, but need to be done properly to avoid flooding.

(Courtesy: World Pumps, UK)

Air- vs. Oil-Filled
Submersible Motor

Long, trouble free life may be the most important consideration in specifying and installing a submersible pump system. No component is more important than the motor in attaining this goal. Many design features determine the quality and ruggedness of a submersible motor. One of the most important is the motor type, air filled or oil filled. An air filled motor is superior to an oil filled motor in many ways.

1) An Air Filled Motor Has A Lower Power Cost Than An Oil Filled Motor.

The higher power cost of the oil filled motor is the result of a lower efficiency compared to the air filled motor. The lower efficiency of an oil filled motor is the result of two major issues:

The oil filled motor has a significant hydraulic drag resulting from the rotor spinning in an oil bath. The extra power required to spin in oil is like walking in water compared to air.

In addition, the hydraulic drag requires the gap between the rotor and stator to be larger in an oil filled motor. Because the motor operating principle is based on the magnetic field generated between the rotor and stator, a larger gap automatically results in lower efficiency. If the gap was small as in an air filled motor, the hydraulic drag between these two components would be exorbitant.

2) An Air Filled Motor Runs Cooler Than An Oil Filled Motor.

Because of the lower efficiency discussed in No.1 above, an oil filled motor design generates more heat than an air filled motor design. Most oil filled designs have a layer of oil between the stator and the outer motor wall that is in contact with the cooler surrounding water. This oil acts as a layer of insulation, retarding the dissipation of heat. So, the combination of the additional heat generated due to lower efficiencies and the oil insulation layer between the stator and the outer shell results in a higher winding temperature in an oil filled motor. The higher winding temperature means a shorter life.

You may have noticed that most oil filled motors have windings with only an insulation Class B. This is because the flash point (boiling point) of the dielectric oil is lower , than the Class F insulation temperature of 155°C. So, the oil filled motor runs hotter than the air filled motor, but yet has a lower maximum allowed operating temperature than the air filled motor.

3) Air Filled Motors Have A Longer Bearing Life Than Oil Filled Motors.

There are several major differences between air filled and oil filled motors that determine the life of the bearing:

The metal surfaces in the bearing (the balls and the raceways) must not contact each other during operation. The role of the lubricant is to maintain a molecular grease film between the two surfaces preventing contact. The higher viscosity grease used in the sealed bearings in the air filled motor will maintain the lubricant layer between the surfaces better than the thin low viscosity dielectric transformer oil used in the oil filled motors. Generally, the transformer oil does an acceptable job of lubrication at lower temperatures but becomes marginal as the temperature approaches 100°C. Many motors operate at or above 100°C (Class A insulation allows 105°C maximum temperature and Class B insulation allows 130°C). If they did not, why would a manufacturer spend money on anything better than a Class A insulation rating?

Lubrication and cleanliness is of critical importance in assuring long bearing life. The air filled motors maintains a clean lubricant because the bearings are sealed at the bearing manufacturer. Conversely, the bearings used in an oil filled motor are open allowing the contaminated circulating oil to pass through the bearings and between the bearing surfaces. Drain any oil filled motor and you will see the contamination and debris in the oil.

Sealed bearings used in the air filled motors have another advantage in that a variety of lubricants can be selected based on the application. You are not limited to transformer oils that must serve other purposes in the motor "system". The ability to select translates into a longer bearing life and a more dependable pump.

4) Maintenance Is Easier And Less Costly On An Air Filled Motor.

Maintenance on an air filled motor is as simple as disassembling, rebuilding as necessary and reassembling. Oil filled motors require additional maintenance. The old oil must be drained and properly disposed. It may not be discharged into any sewer. Upon reassembly, the motor must be thoroughly cleaned, including all interior components to eliminate the contamination discussed above that will lead to premature bearing failure. After final assembly, the oil filled motor must be refilled with new oil.

5) An Air Filled Motor Does Not Expose The Engineer, Contractor Or Owner To An Undefined Future Liability Resulting From Environmental Contamination By Oil Leaks From The Motor.

Any operating piece of equipment filled with oil has a potential to develop a leak. An oil leak from a submersible pump will directly contaminate the water in which it operates. Where a discharge of dielectric oil into any sanitary sewer is illegal, it is an especially critical in non- sanitary applications because of the visible nature of an oil slick.

6) An Air Filled Motor Is Less Likely To FaiI Prematurely.

Any oil leak in an oil filled motor will result in premature motor failure. The upper bearing will fail immediately after the oil drops below the bearing. The oil filled motor relies on the oil to dissipate the heat. As the oil level drops, the motor will begin to overheat.

The amount of time the windings are at the operating temperature is one of the determinants of winding life. Longer time means shorter life. In an oil filled motor, the hot oil does not cool quickly when the motor shuts off. The continuing contact of the hot oil with the windings keep the windings hot and has the reverse effect of its purpose of cooling, actually speeding up the deterioration of the coating on the wire of the winding. The result is a shorter life than expected.

7) An Air Filled Motor Cannot Catastrophically FaiI Due To A Defective Internal Connection.

Electrical arcing at a bad connection in an oil filled motor can vaporize the surrounding oil, resulting in increasing internal pressure. Either an oil leak or an explosion is possible. This cannot happen in an air filled motor.

Summary

Why specify an air filled motor? Compared to an oil filled motor, an air filled motor has many advantages:

Lower power costs because of higher efficiency

Operates at a lower temperature

Has a longer bearing life

Maintenance is easier and less costly

No used oil to be disposed of in accordance with EPA regulations

No premature failure due to oil leaks

No undefined future liabilities due to environmental contamination and explosions.

Pump Business
Prospects in US

Within the overall $25 billion pump industry, several market sectors are currently experiencing very active periods. According to Industrial Info Resources (Sugar Land, TX) there are currently 90 new plants under construction in the food and beverage industry 84 in the pharmaceutical and biotech sector, 82 in alternative fuels and 79 in the power industry.

This activity is reflected by the investments occurring within specific market sectors, such as:

Hydrogen

The growing demand for additional hydrogen capacity continues, on top of the new capacity already added over the past three years, according to Industrial lnfo Resources. 33 capital projects in the U.S. and Canada, worth a combined total investment value (TIV) of nearly $952 million, are planned for construction during 2OO7 and 2008.

A large percentage of this new capacity will be used by petroleum refiners to produce low sulfur content fuels and also in manufacturing processes by the Chemical Processing Industry (CPI). $634 million of these new investments are concentrated in the Great Lakes, Southwest, and West Coast market regions where demand from refining markets continues to increase.

Biodiesel

The biodiesel industry projects over $2.4 billion in capital spending during the next two years. The U.S. currently has 26 large commercial-scale biodiesel plants that produce 150 million gallons annually, but 20 new biodiesel plants are currently under construction that could increase production to nearly 500 million gallons by September 2007. Another 50 plants are in various stages of pre-construction and should add another 2.9 billion gallons annually by 2008.

Oil & Gas

93 proposed oil and gas pipeline projects, worth approximately $ 11.3
billion, are planned for construction in the U.S. in 2007. These projects involve the transportation of natural gas, crude oil, refined products, NGLs, and other products. 41 of these projects (approximately $1 billion) involve new and expansions of existing compressor stations. This is the highest level of pipeline spending activity seen to date in the U.S.

$6.3 billion of this spending is in the Rocky Mountain region, where the increasing natural gas production in the region will be exported to markets in the Midwest and the Northeast. $900 million of the total spending is in the late stages of engineering, while $4.7 billion is in various permit stages with state and federal government.

Chemical Processing

North American chemical processing construction is currently projected at approximately $4.1 billion, with the largest project being a, uranium enrichment plant in New Mexico. Another 14 projects, worth $130 million, are also planned in the Southwest. Nearly $380 million in capital and maintenance spending is planned in petrochemicals, plastics, resins, and specialty chemicals, including a new $125 million carbon black plant planned in California.

Pulp, Paper and Wood

Across the country, 162 proposed capital and maintenance projects, valued at $2.8 billion, are scheduled to begin construction this year. Three of the top three projects are a new $200 million oriented strand board plant, a new $140 million engineered wood products mill, and a new $75 million laminated flooring plant. All three plants are located in the Southeast region.

Summary of Findings

According to the latest overall data for 2005 from the Census Bureau of the U.S. Department of Commerce, the value of shipments for pumps (except vacuum pumps), including parts, rose from $5,720.2 million in 2004 to $6,367.3 million in 2005, an increase of 11 percent. Vertical turbine pumps (except drivers) increased in value of shipments in 2005 by 7 percent, from $244.3 million in 2004 to $260.6 million.

Domestic water systems, including drivers, decreased 18 percent in value of shipments for 2005, from $463.3 million in 2004 to $379.6 million. Domestic sump pumps, including drivers, increased in value of shipments by 11 percent in 2005, from $200.9 million in 2004 to $222.6 million.

Oil-well and oil-field pumps greatly increased in value of shipments for 2005, from $366.2 million in 2004 to $448.0 million, &r increase of 22 percent. The value of shipments of parts for pumps made by manufacturers of complete pumps increased 10 percent in 2005, from $1,131.2 million in 2004 to $1,242.4 million.

Pumps for
viscous liquids

There are number of applications involved in any industry where pump has to handle viscous liquids. Broadly there are 3 types of pumps for such applications, Internal Gear Pumps, External Gear Pumps & Vane pumps. We will covet these pumps in series, starting with Internal Gear Pumps.

Internal Gear Pump

Overview

Internal gear pumps are well-suited for a wide range of viscosity applications because of their relatively low speeds. This is especially true where suction conditions call for a pump with minimal inlet pressure requirements.

For each revolution of an internal gear pump, the gears have a fairly long time to come out of mesh allowing the spaces between gear teeth to completely fill and not cavitate. Internal gear pumps successfully pump viscosities above 1,320,000 cSt/6,000,000 SSU and very low-viscosity liquids, such as liquid propane and ammonia. In addition, lower speeds and low inlet pressures provide for constant and even discharge despite varying pressure conditions.

In addition to superior high-viscosity handling capabilities, internal gear pumps offer a smooth, nonpulsating flow. Internal gear pumps are self-priming and can run dry. Because internal gear pumps have only two moving parts, they are reliable, simple to operate, and easy to maintain. They can operate in either direction which allows for maximum utility with a variety of application requirements.

How Internal Gear Pumps Work

1. Liquid enters the suction port between the rotor (large exterior gear) and idler (small interior gear) teeth. The arrows indicate the direction of the pump and liquid.

2. Liquid travels through the pump between the teeth of the "gear-within-a-gear" principle. The crescent shape divides the liquid and acts as a seal between the suction and discharge ports.

3. The pump head is now nearly flooded, just prior to forcing the liquid out of the discharge port. Intermeshing gears of the idler and rotor form locked pockets for the liquid which assures volume control.

4. Rotor and idler teeth mesh completely to form a seal equidistant from the discharge and suction ports. This seal forces the liquid out of the discharge port.

Advantages

Only two moving parts.

Only one stuffing box.

Positive suction, nonpulsating discharge.

Ideal for high-viscosity liquids.

Constant and even discharge regardless of pressure conditions.

Operates well in either direction.

Can be made to operate with one direction of flow with either rotation.

Low NPSH required.

Single adjustable end clearance.

Easy to maintain.

Flexible design offers application customization.

Disadvantages

Usually requires moderate speeds.

Medium pressure limitations.

One bearing runs in the product pumped.

Overhung load on shaft bearing.

Applications

Barge, tanker, and terminal loading and unloading.

Filtering.

Circulating.

Transferring.

Lubricating.

Booster.

General industrial.

Marine applications.

Petrochemical

Light, medium, or heavy-duty service.

Materials Of Construction / Configuration Options

Externals (head, casing, bracket) - Cast iron, ductile iron, steel, stainless steel, Alloy 20, and higher alloys.

Internals (rotor, idler) - Cast iron, ductile iron, steel, stainless steel, Alloy 20, and higher alloys.

Bushing - Carbon graphite, bronze, silicon carbide, tungsten carbide, ceramic, colomony, and other specials materials as needed.

Shaft Seal - Lip seals, component mechanical seals, industry-standard cartridge mechanical seals, gas barrier seals, magnetically-driven pumps.

Packing - Impregnated packing, if seal not required.

What is the best
seal technology?

The "Best Technology" phrase comes up in every day plant conversations. So what is the best Mechanical Seal Technology available today? Here is opinion:

Materials

Identifiable face materials compatible with the fluid to be sealed and any cleaners or solvents put through the lines.

Materials able to handle the full temperature range of the product you are sealing.

Viton® compatible with water.

Hard faces that are not sensitive to temperate change or caustic cleaners.

Unfilled carbon graphite seal faces

No elastomers with shelf life.

No stainless steel springs or bellows.

Design

The seal should shut with spring and systemhydraulic pressure.

Hydraulicalty balanced designs for low heat generation.

Two way balance in dual seal designs.

Built in pumping ring for cartridge dual seals.

Tandem configuration in dual seal designs. No rotating "back to back" designs.

Stationary configuration for non-cartridge applications.

Self aligning design for stationary cartridge versions.

Springs designed out of the fluid.

The elastomer should move to a clean surface as the faces wear.

No spring loaded elastomers.

Non fretting designs.

Independent of shaft tolerance and finish

Static elastomer located away from the seal face

Cartridge sleeve sealed at wet end.

Vibration damping of the seal face.

Seal should be located close to bearing support.

No elastomer in the seal face.

Faces in compression.

Wide operating range

Low hysteresis.

Equal & opposite clamping of stationary face.

Sealing fluid located at the outside diameter of the seal faces

Leak detection capability

lndependent of shaft finish and tolerance

Compensate for thermal expansion and adjustments.

Meet fugitive emission standards.

Simple installation.

Eliminate all elastomers if possible

Short length leaving room for a shaft support bushing.

Finite element analysis of all components.

A method of supporting the shaft in the event of a bearing failure.

Trapped gaskets.

OTHER

Packaging to survive a one meter drop.

Back up sealing.

Built in seal face vent for vertical applications.

No glued elastomers in split seal configurations.

(Courtesy : DuPont Dow Elastomer)

Pump Operation
Under low flow condition

Operating a pump under low-flow conditions can give to a range of problems, all of which result in premature failure. In this article, Canadian pump reliability consultant Ross Mackay considers first the definition and causes of low flow in pumping systems before examining the design and operation of the different types of pumps that can provide satisfactory performance in low-flow conditions.

The concept of operating at 'Low Flow' first demands a definition. The simplest most practical response would undoutbedly be "a flow rate less than that for which the pump was designed and at which the pump starts to react poorly". In other words, the concept of low flow is obviously relative to the design of the pump that is operating in the system. While 1.89 Litres may be considered low flow for a pump designed to operate at abest efficient point (BEP) of 1500 LPM, it is quite normal for a pump designed to operate at 130 LPM.

Every centrifugal pump has a problem operating below what is usually referred to as 'minimum flow'. The problem is that, on every pump curve, a number of 'mininmum' flow points can be identified, depending on the operating requirements and equipment reliability standards of the individual end user.

Minimum flow points

The earliest minimum flow point used was that point on the curve at which the flow was so low that it would result in a significant temperature increase of the pumpage. As we have since identified a number of other concerns that relate to low flow conditions (as indicated in Figure 1), the problem of the temperature increase has faded into insignificance. One other such condition is suction recirulation, which has similar symptoms to cavitation in that there is the familiar rattling\rumbling noise from the pump csasing, excessive vibration and impeller damage. However, the impeller damage occurs in a different location . with suction recirulation, the damage tends to manifest itself halfway along the vanes of the impeller, while the damage tends to manifest itself halfway along the vanes of the impeller, while the damage caused by pure cavitation takes place on the start of the vanes in the eye area of the impeller.

Suction recirculation is a condition created by operating the pump at low flows, and it frequently dictates the low-flow limit of stable operation of the pump in relation to the percentage of BEP.in the pertoleum industry, it is usually referred to as the 'minimum flow for stable operating condition', and is frequently required to be identified by the pump suppIier for consideration in the pump evaluation process.

Discharge recirculation is another condition precipitated by low-flow operation that takes effect at a lower flow than suction recirculation , and also displays similar symptoms.

Causes of low flow

So, how did we get there in the first place? When discussing any kind of on-going pump reliability problem, I am a great believer in going back to basics and finding out how we got there in the first place . I do not believe there is anything to gain by simply trying to solve the symptom.

So why are we running at low flows in the first place? The answer normally goes back to the reasons the pump was originally selected, and there are usually three possibilities:

1. The pump has to handle multiple services

When considering using one pump for a variety of services, one of the biggest mistakes made is to ignore the length of time at which the pump will run on each service. There is also the problem of selecting the pump for the 'worst' application and allowing the other services to fall where they may.

The problem with such a concept is that, as soon as you decide to size the pump to handle the 'worst' service that set of conditions immediately become the best service because that's the point at which the pump and system have now been matched.

This can be compounded by problems that come into play if the selected condition is required for only a small percentage of the time the pump is actually running. this puts the pump in the situation where for most of its operation, it's running under adverse conditions, which will inevitably result in poor reliability and premature failure.

Consequently, when a pump must be selected for multiple operations, the best selection will be one where the pump is operating closest to its BEP for the longest period of time. This will ensure that the pump will not only require the lowest power draw, but will also operate more quietly and smoothly and with an increased level of reliability throughout its operational life.

2.The pump was purchased to optimize parts interchangeability.

This practice used to be extremely popular because it frequently contributed to a lower capital cost expenditure for the pumps in the original project as the manufacturer could realize economies of scale in the manufacturing process, and thus offer lower prices.

The problem with focusing on parts interchangeability is that it reveals an implicit aceptance that these pumps are going to break down on a frequent basis and will need a regular supply of spares. Such an aceptance is a thing of the past in companies that are working with a focus on reliability rather than on the old fashioned focus of repair.

In spite of a (still) widely held belief to the contrary, pumps do not have to fail on a regular basis . if they are selectively sized , correctly installed, properly operated and well maintained, they will last for many years without faiure and the attendant need for spare parts.

3, An error was made in designing the system or selecting the pump

It happens. Get over it. Now fix it.

Regardless of the reason that the wrong pump is in the system, it is worthwhile to understand that the most costly reaction to such a condition is to continue to live with it without correcting the problem. The initial capital cost of a pump fades into insignificance when compared to the total cost of ownership over a number of years. The combined cost of additional maintenance, extra power draw and unscheduled downtime over just a few years will far execced the initial cost of a new pump. If there is any doubt about a particular case, run the numbers. A simple tracking of the costs and a comparison with new pump prices will easily identify the most economical option.

An extension of this last problem occurs when the pump was selected because the engineer was mentally locked in to the end -suction, single-stage ANSI or API design to such a degree that he didn't even consider any other option .this happens fairly frequently when the pump is not only expected to run at a low flow, but also to generate a fairly high head.

It is very easy to fall into the trap to assuming that, just because the capacity and head combination required are shown on the pump curve, the pump can run there without any problems...even although the operating point is positioned well to the left of the BEP. Most of the time, these pumps will be subjected to the problems identified above, when any centrifugal pump runs at a low percentage of the BEP flow rate.

Special pumps for low flow-high head conditions

Let's consider a number of pump styles that can operate well at low-flow conditions, All of these options have been designed to operate at these combinations of head and capacity that are a problem for most single-stage centrifugal pumps.

There is , however, one style of single-stage pump that may be applicable in some cases. Some manufacturers have a low -flow impeller design that has a series of straight radial vanes, such as that shown in Figure 2. A typical model of this pump might have a BEP of about 260 LPM compared to a conventional impeller in the same sized pump, which has a BEP of approximately 750 LPM. This pump is still classified as an ANSI design.

Multi-stage centrifugal pumps

These pumps are not always considered for low-flow applications as they are often related to major processes requiring high horsepower drivers. However, the two-stage centrifugal pump might be worthy of consideration, and this is available in two different designs.

The between the bearings design is widely used in chemical and petrochemical processes for high pressure requirements with a slightly reduced flow rate.

The two-sage end-suction design is a typical end -suction pump with two impellers mounted on an overhung shaft. This has proved to most effective in many applications such as small package boiler systems. It has proved to be very reliable as long as it is operated close to its BEP. Any operation towards the shut-off condition may result in exterme radial loads being created on the end of an extra long shaft, which can result in significant shat deflection and premature bearing and seal failure.

The multi-stage 'stacked' impeller design utilizes the diffuer design concept rather than having the impellers rotation in a volute casing. A popular version of this style is in a vertical configuration with a close-coupled electric motor driver.

The regenerative turbine pump

This pump style is widely used in commercial applications and is designed with a series of buckets machined into the periphery of an impeller wheel. The pumped liquid circulates in and out of the impeller buckets many times on its way from the pump inlet to the outlet. Both centrifugal and shearing action combine to impart additional energy to the liquid each time it passes through the buckets.

Very high head conditions are achieved in a single-stage unit. The impeller runs at very close axial clearances with the pump channel rings that provide a circular channel around the blade area of the impeller, from the inlet to the outlet liquid entering the channel from the inlet is picked up immediately by the buckets 'from both sides of the impeller and pumped through the channel by a shearing action.

The head-capacity curve in a regenerative turbine design is very steep and is more efficient that a centrifugal pump operating at low flows. These pumps are also available in a two-stage design.

Positive displacement pumps

The positive displacement pump is almost ideally designed for low-flow operation as one of its major advantages is that it can deliver consistent capacities regardless of the system conditions. This is because the out put is solely dependent on the basic design of the pump and the speed of the driver. Consequently, if the liquid is not moving through the system at the required flow rate, it can /always be corrected by changing one of both of these factors.

It should be noted that the maximum pressure developed by this type of pump is limited only by the mechanical strength of the pump and 'system and the driving power available. In view of this, the effect of that pressure should be controlled by a pressure relief or safety valve. in addition, all discharge valves installed with any positive displacement pump must be open before that pump is started. This will prevet any fast buid-up of pressure that could damage the pump or the system.

A variety of positive displacement pumps have been have been available for many years and some of them, such as the piston pump, the gear pump and the screw pump, are better suited to some applications than to others. The two styles that are more widely used are the diaphragm pump and the progressive cavity pump.

Diaphragm pump

The most common arrangement of this design is the air-operated double diaphragm pump, which uses pressurized air to actuate the pumping action ot the diaphragms. This is basically two pumps in one, where one is on the suction cycle while the other is on the discharge cycle. The air valves alternately pressurize the inside of one diaphragm chamber and exhaust air from the other one.

This pump does not require additional sealing devices. It can also be operated safely against a closed discharge valve as the air pressure automatically balances out on each side of the pump diaphragms and stalls the pump.

Progressive cavity pump

This pump has been referred to as a single-end , single-rotor type of screw pump where the pumping elements comprise a single rotor and a stator. The stator usually has a double helical internal thread with a pitch twice that ofthe single helical rotor. This results in two leads on the stator, and one on the rotor.

As the rotor turns inside the stator, two cavities from at the suction end of the stator, with one cavity closing as the other opens. The cavities progress in a spiral-from one end of the stator to the other. The result is a flow with relatively little pulsation, and the shear rates will also be low in comparison to radial pump styles.

The compressive fit between the rotor and stator creates seal lines where the two components contact the seal lines keep the cavities separated as they progress through the pump with each rotation of the rotor. The elastomeric stator and stainless steel rotor allow the pump to handle some solid particles in suspension and a certain percentage of abrasives.

The manner in which the rotor turns within the stator complicates the mechanical design of progressive cavity pumps. As the rotor turns in the stator, the centreline of the rotor orbits about the centreline of the stator. This eccentric motion means the pump must be fitted with universal joints to transmit power from the concentric rotation of the drive shaft to the eccentically rotating rotor. These joints must transmit torsional and thrust loads. Designs of this drive mechanism range from simple ball-and -pin mechanisms to heavy-duty sealed gear couplings.

Conclusion

It is not necessary to become locked into a low-flow problem with a single-stage pump. If it is causing repetitive failure, you don't need to live with it. In fact you don't need to live with any ongoing and repetitive pump failure for any reason. Just stop it! Get rid of the problem pump, and replace it with one designed to do the job. Low flow does not have to mean low reliability.

(Courtesy : World Pumps)

Exports to UAE
Opportunities for Indian
Pump manufacturers

United Arab Emirates (UAE) came to existence on 2nd Dec 1971 . UAE is a federation of seven emirates namely Abu Dhabi, Dubai, Sharjah, Ajman, Umm al Quwain, Ras al Khaiman & Fujairah, Abu Dhabi is its capital.

The terrain is primarily flat with barren coastal plains merging into the rolling sand dunes of the vast desert, however high mountains are found in East. UAE has some 700 kms of coastline, including 100 KMs of Gulf of Oman. Most of the area is under Abu Dhabi emirate; Dubai is second largest city. Other important cities include Al Ain, Ajman, Dhaid, Fujairah, & Ras Al Khaiman.

Climate: The UAE has sub tropical basically arid climate. Rainfall is infrequent & irregular. The average rainfall is about 13 cms, a year. Temperature ranges from a low of about 10° C. To high of 48° C. The mean daily temperature in Jan is around 22°C & around 41°C in the month of July.

Language: Arabic is the native language in UAE, though English is widely spoken. Most of the business transactions are in English.

Resources: The UAE is one of the world's wealthiest countries, with large proven reserves of Oil & Gas. Most of the oil reserves are under Abu Dhabi. The economy of Dubai mainly depends on trade and tourism; where as other emirates are mainly supported by Abu Dhabi & the central Govt.

Economy & Industry: The economy & industry have been growing with prosperous trends over the years as local & foreign business investments are increasing in the country due to:

01. Encouraging Govt. Policies & incentives to support business.

02. Easy & transparent Govt. Procedures.

03. Easy transfer of profits / money any time anywhere, without taxes etc.

04. Low crime rates, stable political & financial systems.

05. Very good infrastructure.

The Exports of the country include petroleum, base metal, minerals, chemical products, textile, ceramic and precious stones. Re exports generate very large revenue.

Key Characteristics of ,UAE market:

1. Large: Despite very small area & population, lot of new products, ideas, trends find market, as Govt. Is keen on development by using latest technology, new ideas. The second most important aspect is that UAE has a larger % of floating population. Dubai is being developed as tourist spot. Apart from this, Re Exports to various countries also is a major contributing factor.

2. Growing: The non-oil exports expanded over 200% Most of the economies of the region served by UAE are still at relatively early stage of development, so there is a scope for diversification & expansion in future.

3. Diversified: Potential for various types of products & services. In the prosperous but sparely populated Gulf, there is demand for foodstuff, equipment & luxury products.

4. Free: There are no trade exchange control, quotas or trade barriers. Import duties are extremely low & consistent over the years and some products are exempted.

5. Accessible: The most important business cities have very good infrastructure facilities. Over 100 shipping lines serve the ports & international airport serves over 70 airlines. Lot of foreign banks, advisor is available. English is main business language.

6. Competitive: Most of the well-known brands are present in market for any commodity, making the market very competitive.

7. Accepting new ideas / technology.: Market is willing to change over, to new ideas / products based on latest technologies / fashions / treads.

Ihe main business activities include Oil field related activities, Service sector tourism, Manufacturing, Trading - for own consumption & for re exports, Construction activities, Infrastructure development projects, Banking, Information Technology etc.

There is a great & ever growing demand for various types of pumps as under:

Market Scenario:

At present almost all the leading international brands of pumps are available. Especially European brands are very popular due to:

1. Reputed pump companies from Europe concentrated their efforts on this region long back & developed good distributors with terms and condition suitable for the market.

2. These manufacturers . have understood the nerves ofthe market I needs of the customers very well and offered / developed most suitable pumps for different segments/ applications. Then with their marketing efforts most of their pump features became standard / basic requirements / tender specifications generally.

3. Due to quality and reliability of the products and services offered by their distributors these brands have become popular and standard.

4. In most of the cases the brand approving authorities (Consultants) are from Europe, in fact most of the consulting firms are from UK / Europe. In most of the cases, the consultants know the products & capabilities of various pump manufacturers very well.

At present the contribution from Indian pump manufacturing is very limited. Some manufacturers especially in the field of submersible well pumps are successful in getting break through, however compared to the total business of various types of pumps, the present business is far below the expected levels. For Indian pump manufacturers there is a good potential for supply of variety of pumps to cater to the huge and ever growing demand of the pumps.

Points to be kept in mind:

1. In depth market study is required to know exact requirements & type of competition, to judge the potential for company products in the region.

E.g. a. Even for utility water Stainless Steel (sheet metal) pumps are being used.

b. In most of the pump systems two pumps as duty and stand by are used instead of operations depending on one pump.

2. The product quality should be at par with international brands. Reliability of operation is the key factor. The surface finish & packing quality etc should be like an international branded product.

3. A quick on line response system & order execution system is required. Overall dealings should be with very positive attitude and at par with international levels. The delivery commitments have to be strictly adhered to.

4. Brand approval takes time, the manufacturers / exporters should be willing to invest in Marketing activities initially with own manpower working in region to promote the brand. This period can be longer & would depend on type of pumps and the market segments.

5. An appointment of energetic / enthusiastic distributor is the next step. A distributor, supported on marketing front, can then fetch very good results / business & can offer effective service locally.

6. There are at least two or three bigger exhibitions being held in UAE / Dubai every year like BIG 5 Show, which could provide good platform for exporters to display the range of products to attract dealers / distributors & consultants / customers towards them.
 
Function    Type of Pumps generally used    Typical characteristics      
Irrigation Projects
Submersible wells
Pumps    Stainless Steel submersible (well) water
pumps    1. The requirement is by tenders.
2. Brand approval is required.
3. Technical bid and price bids are Involved.      
Building services              
1.    Transfer / Booster
Pump systems
2.     Sewage / waste water
Pump sets
3.    Fire fighting pumps set
4.    Sump pumps
5.    Pumps for other
Application    1. St. St. Vertical Multistage
pumps or St. St. Horizontal end suction pumps.
2. Submersible sewage/dewatering Pumps
3. Horizontal CI Pumps
4. Smaller submersible sump
pumps & non-metallic pumps.    1. Brand approval is required.
2. Consultant approval for any supply
Each time is a required.
3. The orders are placed by the Civil /
Electro-mechanical contractors.
4. Fire fighting contractors are
Involved in Fire fighting pump sets.      
Govt. Projects    Variety of pumps depending on type of services & applications.      
Municipality Projects    Water supply schemes, Waste
Water treatment projects, City
Beautification Projects etc.    Variety of pumps depending on type
of services / applications      
Counter sales
Off the shelf selling.    1. Submersible well pumps
2. St. St. Horizontal end suction & vertical multistage pumps.
3. Smaller sump/dewatering pumps.      
Re Exports    Variety of pumps depending on type of services / applications.      
Replacement of pumps
Used in the original
Equipment.    St. St. Horizontal end suction and vertical multistage pumps.      
Oil Field services    These are special pumps and the replacement market depends on pumps used
for Original equipment.