Frequently Asked Question-27
- By Jayesh Patel
- Published 01/31/2010
Tech Quest
1. Two motors are having same H.P. but in different RPM, Which will take more amps?
2. what is the use of lantern ring in the stuffing box?
1. There is no direct relationship between motor HP, motor RPM and current.
Wattage = 1.732*V*I*cos(phi) is a basic formula.
And 746 W = 1 BHP. Metric HP= 736W.
Values of V and I for putting in the formula will be values related to input to motor. But motors are rated by their output HP. So, if V and I at input to motor will be proportional to (Motor HP/Motor Efficiency). If two motors are of same HP, one 4 pole and another 2 pole, have same efficiency and are working with same voltage and power factor they should draw equal current (amps).
For same HP, different RPM will give different torque from the motor. But that is on the output side of the motor. Current is measured at the input to the motor. That is why there would be no direct relationship between motor HP, motor RPM and current.
2. Lantern ring acts as a container for the flushing, cooling, lubricating feed. Since lantern is fed with liquid under pressure, it also acts as the barrier against ingress of atmospheric pressure, thus safeguarding the vacuum being developed at the eye of the impeller. Since liquid at the lantern ring should also serve flushing of the particles loosened from the rope packing, a drop by drop leakage is recommended for stuffing boxes packed with rope packing.
Injectable sealants which are coming into vogue do not need lantern ring, because no offensive particles get loosened out, they have lubricant impregnated and work with much less co-efficient of friction and hence do not need much cooling. In effect they work with almost zero leakage.
Q. Why does a star delta starter not function if the delivery valve is slightly open while starting the pump. When shifting from star to delta the breaker trips. Assuming That the delivery valve is open to 1/10 th of its capacity that will not exert a great amount of load on the pump but the starter ceases to function unless the valve is closed tightly.
The starter would trip, if it is not able to exert enough torque to drive the load. Although delivery valve being open only to 10% of its capacity does not appear to be excessive load, prima facie, the 'sense and respond' logic of the starter itself is causing it to trip. Maybe, the excessive load is caused due to bent shaft, misalignment at the coupling, worn bearings, rubbing at wearing rings. All other causes for overload apart from amount of opening of delivery valve need to be explored.
Pump Efficiency
Q. How to improve Pump Efficiency? In all pumps. Please explain briefly. I want to know how can I identify the H.P of Motor in the field.
There would be two points of view regarding improving efficiency of a pump, one, in the field and the other, at manufacturing.
Improving efficiency in the field is the same as using a pump efficiently, which in turn is same as energy conservation in pumping. A comprehensive note on this can be found at www.pumps.org There, seven ways for saving energy in pumping are detailed. Briefly they are as follows.
1. Design systems with lower capacity and total head requirements. Do not assume these requirements are fixed.
2. Avoid allowing for excessive margin of error in capacity and/or total head.
3. Despite the tendency to emphasize initial cost, you will save in the long run by selecting the most efficient pump type and size at the onset.
4. Use two or more smaller pumps instead of one larger pump so that excess pump capacity can be turned off.
5. Use variable-speed drives to avoid losses from throttle valves and bypass lines, except when the system is designed with high static heads.
6. Use pumps operating as turbines to recover pressure energy that would otherwise be wasted.
7. Maintain pumps and all system components in virtually new condition to avoid efficiency loss.
Efficiency in manufacturing is through better design, better surface quality, especially of hydraulic surfaces of casing and impeller, close running clearances, least leakage, less wear at bearings, proper dynamic balancing and alignment.
Q. Whether API-682 seals can be fitted into ISO-2858 pumps? If so, which edition 1 or 2?
ISO-2858 is adopted in IS-13518. Standard on "Dimensions of Stuffing Box Cavities" is IS-11382. It suits pumps as per IS-13518, i.e. ISO-2858. ISO-2858 was also derived substantially from DIN-24256 and the cavity dimensions derived from DIN-24960. DIN-24960 can be considered as the 'father' standard for API-682. But Americans are still attached to FPS system of units, whereas in European and in ISO standards the approach is emphatically metric and wherever possible, even SI system of units. American also have started giving metric converted values in their standards. But basic or reference dimensions are in FPS units and metric values are only conversions. So, dimensions in IS-11382 derived from an ISO-standard (I do not have the reference ready at hand), itself derived from DIN-24960 is metric in its spirit. So there would not be exact matching. Typical example is of Nominal size 3", which becomes 80 mm nominal in metric, but the difference is large, 3" being 76.4 mm. Even 25 mm being metric nominal of 1" also gives a difference of 400 microns, which is a large difference for shaft diameter, since there has to be much fi Further to my reply I would like to add the following. For requirements of pumps not necessarily as per API-610, ISO-2858 seems appropriate. ISO-2858 is already adopted in IS-13518. Actually, conditions such as vibration limits, minimum nozzle loads and moments as specified in API-610 are specified for ISO-2858 pumps in ISO-5199, which is adopted in IS-13537 . But ISO-2858 (IS-13518) can be read with or without ISO-5199 (IS-13537). It is for the purchaser to tune up the purchase specification to be appropriate and economical for the application. In America also they have ANSI-B-73.1, which is more akin to ISO-2858 and serves general purpose requirements which do not need specifications as stringent as API-610.
Q. We are a company dealing in various torque measuring instruments. We would like to know the importance of torque measurement in Various Pumps.
A torque measuring instrument is important only where the measurement of torque is needed. Pump-users do not need measurement of torque. They would need measurement of parameters of pump's output, i.e. primarily flow and differential pressure. So a torque measuring instrument is irrelevant to pump-users. For pump manufacturers measurement of torque is somewhat relevant only during pump testing, if one wants to segregate input to pump from input to motor. Input to motor is anyway electrical and is measured by electrical measuring instruments. If pump is connected to the motor directly, without any transmission unit or gear box in between, input to pump is output of motor. And this can be derived from known performance characteristics of motors. This implies indirect measurement of torque. A direct measurement of torque using a torque measuring instrument has the advantage of it being direct. But the instrument out not to intervene or interfere in the transmission of the drive. If it interferes, it will falsify the measurement. But torque or torsion is a phenomenon, which does not seem amenable to measurement without intervention. There has been some talk of non-contact type torsion dynamometers, which again, if all feasible, would be a relative measure based on a calibration technique. To such extent it becomes again an indirect measurement only!
In assembly shops, for critical mechanical assemblies, people do use torque wrenches. But torque wrenches are 'tools' and not measuring instruments. There, torque is to be set to a 'control' and not exactly to be 'measured'. Setting the wrench to a level of control would imply as much measurement. But that calibration is provided on the wrench. If it is set to a torque of, say, 40Nm, it would not allow the exerted torque to exceed 40Nm. But it would not tell whether the torque actually applied was only 20Nm or 10Nm or 32Nm. That would be measurement but that again is not required in the use of torque wrenches.
Design Problem
Q. Please advise the criticality of machining process for between bearing split case pump.
Since there are two halves of casing and both halves are castings which have cored portions for wearing rings, impeller, volute cross sections, stuffing boxes and bearings, the cored portions have to match over each other and all machining is to be done by putting the two halves over each other with the gasket also in place. All this machining is thus blind and yet needs to be done within tolerance, especially at the wear ring and bearing portions, which have H7 or H8 tolerance. This machining is best done on a horizontal boring machine with a support column to support the special boring bar with number of boring tools for the different bore dimensions from one bearing to the other. If the pump is to be provided with mechanical seals, the stuffing box portion also becomes a close tolerance machining.
Some designers separated the stuffing boxes in "through bore" design. But that increases the number of parts and number of surfaces to be machined and access to the rotating element for repair and maintenance does not remain as easy. Different approaches, different ideas, different pros and cons.
Q. I want some clarification regarding a problem in Tubewell Submersible pump.
We have a water cooled single phase Tubewell submersible pump with motor core length above 400 mm. To support both ends of the shaft, 27mm journal bearings are used. Distance between the bearing supports is 540 to 550mm. The inner diameter of stator is 50 mm and outer diameter of rotor is 49 mm We ore using 24 slot stator stamping and 16 slot rotor stamping. The total weight of (rotor stamping + rotor conductors + rotor end rings) is around 4.25 kg. The diameter of motor shaft is 27mm.
The problem which we are facing in the motor with above mentioned configuration is described below:
We observe a vibration and noise when the motor is running at no load itself. No load current consumption is around 7A and it increases up to 18 to 20A if we are providing a pump load. It is observed that there is a considerable speed reduction (from 2850 RPM to 2100 RPM) during pump testing. When we dismantle the motor after running of few minutes, we can seen that about 40 to 50mm in the middle of the rotor stamping is severely rubbing inside the stator. Only around 60 degree out of total 360 degree on the rotor stamping periphery is rubbing.
We are now trying to rectify the problem thinking that this may be due to a mechanical unbalance. We got the magnitude of total unbalance of the rotor shaft as 21g. Kindly give your suggestions and clarification regarding my doubts related to the above issue.
Q. Why the rotor is rubbing inside the stator? What may be the real reason behind it?
Q. Whether there is any electrical force or magnetic pull affecting the dynamic balance of rotor?
Q. Whether I am now proceeding in the right way to solve the problem?
Q. In which way I can proceed if the problem persists after using a dynamically balanced rotor?
Q. Is there any requirement of motor shaft hardening; if am using a dynamically balanced rotor shaft assembly?
Q. Kindly give some suggestions regarding different ways to solve the problem.
For a visual, draw two circles one of dia 50 and another of dia 49,the center of circle of dia 49 should be 0.5 mm away from the centre of dia 50. The diagram will tell you why the rubbing marks are only on a small portion of the stator ID. The problem is either due to eccentricity during machining of the rotor, which also be a cause for imbalance in the rotor or due to too much clearance at the bushes or some foreign particles or burrs on the stator ID. The rubbing marks on the stator and rotor would be prominent enough to distract one's mind from looking also at the bushes. Since the rubbing marks are at the middle of the core length, that too on a vertical motor, bushes could be okay. But it would be good practice not to miss that. Any foreign particles would have got rubbed off, when you open the motor after running.
Rubbing is rubbing, no-load or full load. In fact, high no-load current should be considered as the right and first symptom of the problem and should be investigated right away, without carrying on with testing on load. Carrying on further is only worsening the damage. Why do that?
Q. We would like to provide a water filling nut on our 0.50 hp pump model. Please advise whether it is possible. Our pattern maker views are that it can't be provided. Please suggest us on this subject.
This is a problem of individual design and would need to be considered separately and in the forum.
Possibility of providing the nut needs to be checked by examining the design/drawing. A good designer should know what is possible in a pattern and what is not. It cannot be the other way round, that a pattern maker tells what is possible or not.
Q. What should be the sizing of the priming pot for Centrifugal pumps with Negative suction. Is there any L/D ratio. Can a pump intended for positive suction be fitted for negative suction with a priming pot
Volume of priming pot should be about six to seven times the volume of air-filled suction pipe. By this proportion water in the priming pot will be able to entrain some 15% air by volume. Smaller volume of priming pot will need longer priming time and more seriously may cause dry running of pump. Pumps intended for positive suction mean pumps having high NPSHr. Priming pot provides a virtual positive head upstream of the pump's suction. And the priming pot will have to be tall enough to provide the needed positive head. This would often be impracticable. It would be more logical then to install the pump with pump submerged, as a sump pump or a vertical turbine pump
For any centrifugal pump what is the minimum required NPSH for flow rate 5.0 m3/hr and 3.0 Kg/cm2 pressure head. In my case I have 0.3 meter available NPSH for above flow rate of fluid at its boiling point, please suggest me. Is it possible to have double suction centrifugal pump.
Available NPSH of 0.3 m is too small and does not provide for even the minimum recommended margin of 0.5 to 1 m between NPSHr and NPSHa. The required flow-rate of 5m3/h is very small. In a double suction impeller each side would get only half of the flow-rate, only 2.5 m3/h. There would be no double suction pump designs for such small flow rates. I would recommend simply putting the pump down into a pit and thereby increase the NPSHa. you further mention the liquid being near to its boiling point. That is dicy. It would be better to cool down the liquid to ensure that it is not in the vapour state. otherwise the pump would get vapour-locked and would not do any pumping.
New Excise Policy
Q. We are the subscribers of your magazine almost since beginning. We would categorically like to state here that your magazine is very good and extremely useful for our business. Especially the topics you cover are extremely good and useful.
Sir, for your information, we are the contractors dealing in Pumping machinery in Maharashtra and nearby States. We execute water supply schemes on turn key basis. As we require some information urgently, we request you to kindly let us have your professional guidance immediately In this regard, we refer Circular from Govt. Of India wherein it has been stated that Excise Duty is exempted on all those material which has been used for Water Supply Schemes. We also refer recent Vote on Account and declaration placed by Hon. Finance Minister wherein he has given benefit of exemption of Excise Duty on all the Water supply Schemes. For availing this benefit, we have to just collect letter from Collector or Asst. collector or Executive Magistrate confirming that the material has been used for Water Supply scheme. On submission of the letter to manufacturer, the material billed for the schemes will be excise free.
For getting confirmation of the same, we approached central Excise Office, Nagpur. However, we were surprised to here from the authority that such exemption is not at all available for Pumping Machinery and such exemption is available only for machineries used for WTP including pipe-line. However, there is nothing written or specified as far as Raw and Pure Water machinery and allied material is concerned. We also approached some leading civil contractors from Nagpur wherein they had informed us that they are regularly availing this facility from other few states like Gujrat etc.
Q. Can we avail this benefit or not? We are in a genuine difficulty of whether to avail this facility or not. We therefore request you to kindly guide us in the matter along with necessary details regarding Notifications/Circulars from Govt. Of India and also from Central Excise Deptt. so that we can go ahead accordingly. Kindly quote necessary Notification and Circular Nos. for our records for further necessary action.
It may be of interest to refer to Central Excise notifications, Nos. 46/2002 and 47/2002. Vide these notifications exemption was given in Central Excise for indigenous manufacturers, since import duty was also made nil vide customs notifications Nos. 91,92 and 93 all of September 2002.
The matter was discussed during council meeting of IPMA in Nov 2002. If I remember it right, it was not to be a blanket exemption. But, as I understood and if I remember it right, the onus was with project authorities to get the project notified and the exemption was to manufacturers and/or importers only for notified projects. The notification covered "All items of machinery". This would obviously include pumps.
It may have to be also checked whether there is any change in status during vote on account. However, vote on account is usually for administrative convenience of carrying on Governmental expenses. So no major policy changes are supposed to be effected during vote on account.
1. Two motors are having same H.P. but in different RPM, Which will take more amps?
2. what is the use of lantern ring in the stuffing box?
1. There is no direct relationship between motor HP, motor RPM and current.
Wattage = 1.732*V*I*cos(phi) is a basic formula.
And 746 W = 1 BHP. Metric HP= 736W.
Values of V and I for putting in the formula will be values related to input to motor. But motors are rated by their output HP. So, if V and I at input to motor will be proportional to (Motor HP/Motor Efficiency). If two motors are of same HP, one 4 pole and another 2 pole, have same efficiency and are working with same voltage and power factor they should draw equal current (amps).
For same HP, different RPM will give different torque from the motor. But that is on the output side of the motor. Current is measured at the input to the motor. That is why there would be no direct relationship between motor HP, motor RPM and current.
2. Lantern ring acts as a container for the flushing, cooling, lubricating feed. Since lantern is fed with liquid under pressure, it also acts as the barrier against ingress of atmospheric pressure, thus safeguarding the vacuum being developed at the eye of the impeller. Since liquid at the lantern ring should also serve flushing of the particles loosened from the rope packing, a drop by drop leakage is recommended for stuffing boxes packed with rope packing.
Injectable sealants which are coming into vogue do not need lantern ring, because no offensive particles get loosened out, they have lubricant impregnated and work with much less co-efficient of friction and hence do not need much cooling. In effect they work with almost zero leakage.
Q. Why does a star delta starter not function if the delivery valve is slightly open while starting the pump. When shifting from star to delta the breaker trips. Assuming That the delivery valve is open to 1/10 th of its capacity that will not exert a great amount of load on the pump but the starter ceases to function unless the valve is closed tightly.
The starter would trip, if it is not able to exert enough torque to drive the load. Although delivery valve being open only to 10% of its capacity does not appear to be excessive load, prima facie, the 'sense and respond' logic of the starter itself is causing it to trip. Maybe, the excessive load is caused due to bent shaft, misalignment at the coupling, worn bearings, rubbing at wearing rings. All other causes for overload apart from amount of opening of delivery valve need to be explored.
Pump Efficiency
Q. How to improve Pump Efficiency? In all pumps. Please explain briefly. I want to know how can I identify the H.P of Motor in the field.
There would be two points of view regarding improving efficiency of a pump, one, in the field and the other, at manufacturing.
Improving efficiency in the field is the same as using a pump efficiently, which in turn is same as energy conservation in pumping. A comprehensive note on this can be found at www.pumps.org There, seven ways for saving energy in pumping are detailed. Briefly they are as follows.
1. Design systems with lower capacity and total head requirements. Do not assume these requirements are fixed.
2. Avoid allowing for excessive margin of error in capacity and/or total head.
3. Despite the tendency to emphasize initial cost, you will save in the long run by selecting the most efficient pump type and size at the onset.
4. Use two or more smaller pumps instead of one larger pump so that excess pump capacity can be turned off.
5. Use variable-speed drives to avoid losses from throttle valves and bypass lines, except when the system is designed with high static heads.
6. Use pumps operating as turbines to recover pressure energy that would otherwise be wasted.
7. Maintain pumps and all system components in virtually new condition to avoid efficiency loss.
Efficiency in manufacturing is through better design, better surface quality, especially of hydraulic surfaces of casing and impeller, close running clearances, least leakage, less wear at bearings, proper dynamic balancing and alignment.
Q. Whether API-682 seals can be fitted into ISO-2858 pumps? If so, which edition 1 or 2?
ISO-2858 is adopted in IS-13518. Standard on "Dimensions of Stuffing Box Cavities" is IS-11382. It suits pumps as per IS-13518, i.e. ISO-2858. ISO-2858 was also derived substantially from DIN-24256 and the cavity dimensions derived from DIN-24960. DIN-24960 can be considered as the 'father' standard for API-682. But Americans are still attached to FPS system of units, whereas in European and in ISO standards the approach is emphatically metric and wherever possible, even SI system of units. American also have started giving metric converted values in their standards. But basic or reference dimensions are in FPS units and metric values are only conversions. So, dimensions in IS-11382 derived from an ISO-standard (I do not have the reference ready at hand), itself derived from DIN-24960 is metric in its spirit. So there would not be exact matching. Typical example is of Nominal size 3", which becomes 80 mm nominal in metric, but the difference is large, 3" being 76.4 mm. Even 25 mm being metric nominal of 1" also gives a difference of 400 microns, which is a large difference for shaft diameter, since there has to be much fi Further to my reply I would like to add the following. For requirements of pumps not necessarily as per API-610, ISO-2858 seems appropriate. ISO-2858 is already adopted in IS-13518. Actually, conditions such as vibration limits, minimum nozzle loads and moments as specified in API-610 are specified for ISO-2858 pumps in ISO-5199, which is adopted in IS-13537 . But ISO-2858 (IS-13518) can be read with or without ISO-5199 (IS-13537). It is for the purchaser to tune up the purchase specification to be appropriate and economical for the application. In America also they have ANSI-B-73.1, which is more akin to ISO-2858 and serves general purpose requirements which do not need specifications as stringent as API-610.
Q. We are a company dealing in various torque measuring instruments. We would like to know the importance of torque measurement in Various Pumps.
A torque measuring instrument is important only where the measurement of torque is needed. Pump-users do not need measurement of torque. They would need measurement of parameters of pump's output, i.e. primarily flow and differential pressure. So a torque measuring instrument is irrelevant to pump-users. For pump manufacturers measurement of torque is somewhat relevant only during pump testing, if one wants to segregate input to pump from input to motor. Input to motor is anyway electrical and is measured by electrical measuring instruments. If pump is connected to the motor directly, without any transmission unit or gear box in between, input to pump is output of motor. And this can be derived from known performance characteristics of motors. This implies indirect measurement of torque. A direct measurement of torque using a torque measuring instrument has the advantage of it being direct. But the instrument out not to intervene or interfere in the transmission of the drive. If it interferes, it will falsify the measurement. But torque or torsion is a phenomenon, which does not seem amenable to measurement without intervention. There has been some talk of non-contact type torsion dynamometers, which again, if all feasible, would be a relative measure based on a calibration technique. To such extent it becomes again an indirect measurement only!
In assembly shops, for critical mechanical assemblies, people do use torque wrenches. But torque wrenches are 'tools' and not measuring instruments. There, torque is to be set to a 'control' and not exactly to be 'measured'. Setting the wrench to a level of control would imply as much measurement. But that calibration is provided on the wrench. If it is set to a torque of, say, 40Nm, it would not allow the exerted torque to exceed 40Nm. But it would not tell whether the torque actually applied was only 20Nm or 10Nm or 32Nm. That would be measurement but that again is not required in the use of torque wrenches.
Design Problem
Q. Please advise the criticality of machining process for between bearing split case pump.
Since there are two halves of casing and both halves are castings which have cored portions for wearing rings, impeller, volute cross sections, stuffing boxes and bearings, the cored portions have to match over each other and all machining is to be done by putting the two halves over each other with the gasket also in place. All this machining is thus blind and yet needs to be done within tolerance, especially at the wear ring and bearing portions, which have H7 or H8 tolerance. This machining is best done on a horizontal boring machine with a support column to support the special boring bar with number of boring tools for the different bore dimensions from one bearing to the other. If the pump is to be provided with mechanical seals, the stuffing box portion also becomes a close tolerance machining.
Q. I want some clarification regarding a problem in Tubewell Submersible pump.
We have a water cooled single phase Tubewell submersible pump with motor core length above 400 mm. To support both ends of the shaft, 27mm journal bearings are used. Distance between the bearing supports is 540 to 550mm. The inner diameter of stator is 50 mm and outer diameter of rotor is 49 mm We ore using 24 slot stator stamping and 16 slot rotor stamping. The total weight of (rotor stamping + rotor conductors + rotor end rings) is around 4.25 kg. The diameter of motor shaft is 27mm.
The problem which we are facing in the motor with above mentioned configuration is described below:
We observe a vibration and noise when the motor is running at no load itself. No load current consumption is around 7A and it increases up to 18 to 20A if we are providing a pump load. It is observed that there is a considerable speed reduction (from 2850 RPM to 2100 RPM) during pump testing. When we dismantle the motor after running of few minutes, we can seen that about 40 to 50mm in the middle of the rotor stamping is severely rubbing inside the stator. Only around 60 degree out of total 360 degree on the rotor stamping periphery is rubbing.
We are now trying to rectify the problem thinking that this may be due to a mechanical unbalance. We got the magnitude of total unbalance of the rotor shaft as 21g. Kindly give your suggestions and clarification regarding my doubts related to the above issue.
Q. Why the rotor is rubbing inside the stator? What may be the real reason behind it?
Q. Whether there is any electrical force or magnetic pull affecting the dynamic balance of rotor?
Q. Whether I am now proceeding in the right way to solve the problem?
Q. In which way I can proceed if the problem persists after using a dynamically balanced rotor?
Q. Is there any requirement of motor shaft hardening; if am using a dynamically balanced rotor shaft assembly?
Q. Kindly give some suggestions regarding different ways to solve the problem.
For a visual, draw two circles one of dia 50 and another of dia 49,the center of circle of dia 49 should be 0.5 mm away from the centre of dia 50. The diagram will tell you why the rubbing marks are only on a small portion of the stator ID. The problem is either due to eccentricity during machining of the rotor, which also be a cause for imbalance in the rotor or due to too much clearance at the bushes or some foreign particles or burrs on the stator ID. The rubbing marks on the stator and rotor would be prominent enough to distract one's mind from looking also at the bushes. Since the rubbing marks are at the middle of the core length, that too on a vertical motor, bushes could be okay. But it would be good practice not to miss that. Any foreign particles would have got rubbed off, when you open the motor after running.
Rubbing is rubbing, no-load or full load. In fact, high no-load current should be considered as the right and first symptom of the problem and should be investigated right away, without carrying on with testing on load. Carrying on further is only worsening the damage. Why do that?
Q. We would like to provide a water filling nut on our 0.50 hp pump model. Please advise whether it is possible. Our pattern maker views are that it can't be provided. Please suggest us on this subject.
This is a problem of individual design and would need to be considered separately and in the forum.
Possibility of providing the nut needs to be checked by examining the design/drawing. A good designer should know what is possible in a pattern and what is not. It cannot be the other way round, that a pattern maker tells what is possible or not.
Q. What should be the sizing of the priming pot for Centrifugal pumps with Negative suction. Is there any L/D ratio. Can a pump intended for positive suction be fitted for negative suction with a priming pot
Volume of priming pot should be about six to seven times the volume of air-filled suction pipe. By this proportion water in the priming pot will be able to entrain some 15% air by volume. Smaller volume of priming pot will need longer priming time and more seriously may cause dry running of pump. Pumps intended for positive suction mean pumps having high NPSHr. Priming pot provides a virtual positive head upstream of the pump's suction. And the priming pot will have to be tall enough to provide the needed positive head. This would often be impracticable. It would be more logical then to install the pump with pump submerged, as a sump pump or a vertical turbine pump
For any centrifugal pump what is the minimum required NPSH for flow rate 5.0 m3/hr and 3.0 Kg/cm2 pressure head. In my case I have 0.3 meter available NPSH for above flow rate of fluid at its boiling point, please suggest me. Is it possible to have double suction centrifugal pump.
Available NPSH of 0.3 m is too small and does not provide for even the minimum recommended margin of 0.5 to 1 m between NPSHr and NPSHa. The required flow-rate of 5m3/h is very small. In a double suction impeller each side would get only half of the flow-rate, only 2.5 m3/h. There would be no double suction pump designs for such small flow rates. I would recommend simply putting the pump down into a pit and thereby increase the NPSHa. you further mention the liquid being near to its boiling point. That is dicy. It would be better to cool down the liquid to ensure that it is not in the vapour state. otherwise the pump would get vapour-locked and would not do any pumping.
New Excise Policy
Q. We are the subscribers of your magazine almost since beginning. We would categorically like to state here that your magazine is very good and extremely useful for our business. Especially the topics you cover are extremely good and useful.
Sir, for your information, we are the contractors dealing in Pumping machinery in Maharashtra and nearby States. We execute water supply schemes on turn key basis. As we require some information urgently, we request you to kindly let us have your professional guidance immediately In this regard, we refer Circular from Govt. Of India wherein it has been stated that Excise Duty is exempted on all those material which has been used for Water Supply Schemes. We also refer recent Vote on Account and declaration placed by Hon. Finance Minister wherein he has given benefit of exemption of Excise Duty on all the Water supply Schemes. For availing this benefit, we have to just collect letter from Collector or Asst. collector or Executive Magistrate confirming that the material has been used for Water Supply scheme. On submission of the letter to manufacturer, the material billed for the schemes will be excise free.
For getting confirmation of the same, we approached central Excise Office, Nagpur. However, we were surprised to here from the authority that such exemption is not at all available for Pumping Machinery and such exemption is available only for machineries used for WTP including pipe-line. However, there is nothing written or specified as far as Raw and Pure Water machinery and allied material is concerned. We also approached some leading civil contractors from Nagpur wherein they had informed us that they are regularly availing this facility from other few states like Gujrat etc.
Q. Can we avail this benefit or not? We are in a genuine difficulty of whether to avail this facility or not. We therefore request you to kindly guide us in the matter along with necessary details regarding Notifications/Circulars from Govt. Of India and also from Central Excise Deptt. so that we can go ahead accordingly. Kindly quote necessary Notification and Circular Nos. for our records for further necessary action.
It may be of interest to refer to Central Excise notifications, Nos. 46/2002 and 47/2002. Vide these notifications exemption was given in Central Excise for indigenous manufacturers, since import duty was also made nil vide customs notifications Nos. 91,92 and 93 all of September 2002.
The matter was discussed during council meeting of IPMA in Nov 2002. If I remember it right, it was not to be a blanket exemption. But, as I understood and if I remember it right, the onus was with project authorities to get the project notified and the exemption was to manufacturers and/or importers only for notified projects. The notification covered "All items of machinery". This would obviously include pumps.
It may have to be also checked whether there is any change in status during vote on account. However, vote on account is usually for administrative convenience of carrying on Governmental expenses. So no major policy changes are supposed to be effected during vote on account.
