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Variable Frequency Drive For Centrifugal Pumps Operating In Parallal

vfd multistage cetrifugal pump

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#1 shin29

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Posted 12 June 2013 - 03:41 AM

Hello all of you

 

At our facility we have five centrifugal pumps Operating  in parallel . Some times the overall load is such that  the pumps operate on lower than rated  flow . I therefore studying feasibility for installation of VFD in one of the pump so that variation in load  could  be adjusted and power could be saved .

Main feature for our system are as follow :

 

System pressure is governed by static head 

It is an explosive liquid 

All pumps are identical and having  rated flow and head respectively 400 m3/hr and 2090m (fluid density is 460Kg/m3)

 

My doubts in this regard and for your kind attention are as follow :

 

1. I  have a understanding that while we lower the RPM of one of the pumps running in parallel , that pump start to follow all new Characteristic  curve and deliver a flow  at  common header pressure according to new curve. I seek somebody to verify and add to this understanding .

 

2. How could  I  relate efficiency of the pump at various flow with reduced RPM, does it remain constant with varying flow .

 

3. Can somebody suggest a road map for finding out power saving through it .

 

Regards ,

Sahil

 

 

 



#2 breizh

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Posted 12 June 2013 - 03:57 AM

Sahil,

It should be useful for you to use the search engine in this forum , key words are : Variable speed drive.

 

Hope this helps

Breizh



#3 ankur2061

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Posted 12 June 2013 - 06:43 AM

Sahil,

 

The one pump which you will hookup as a VFD pump will float with the required flow rate in the common discharge header. This will require that you have a flowmeter in the common discharge header and that flowmeter provides a signal to the speed controller of the pump with the VFD to maintain the required flow rate. This is generally known as a cascade control where the FIC (Flow Indicator Controller) acts a Master controller which in turn provides a signal to the speed indicator controller (SIC) which is the slave controller.

 

Refer these two blog entries to understand how pumps work in parallel (flow split / common head) and effect of variable frequency drives on rpm, flow, differential head and BHP of the pump.

 

http://www.cheresour...s-and-parallel/

 

http://www.cheresour...equency-drives/

 

 

Regards,

Ankur



#4 shin29

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Posted 17 June 2013 - 03:18 AM

Thank you sir for your prompt response.

Sirs,  could your assist me in examining in viability of this VFD installation Viz how can i find out the power saving through RPM reduction.

Does efficiency of the pump varies or follow the same curve. 

Does the Power which is stated  in affinity  law ,is driven power of bkw

 

Regards,

sahil 



#5 ankur2061

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Posted 17 June 2013 - 04:14 AM

sahil,

 

1. Refer the attached file for performing energy savings and payback period time for pumps and fans when using a Variable Frequency Drive. The results you will obtain are approximate and it is the pump vendor who will provide you final details on energy savings. Also remember that In general, Variable Frequency Drives become more attractive economically for large motor ratings such as for motors with ratings exceeding 250 kW.

 

2. For a pump defined for a BEP (Best Efficiency Point), any point on the right or left of the BEP on the pump curve decreases the efficiency.

 

3. If you refer my blog entry it uses the term BHP which is Brake Horse Power which is nothing but BkW in Metric units.

 

Hope this helps.

 

Regards,

Ankur.

 

 

Attached Files



#6 Padmakar Katre

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Posted 18 June 2013 - 04:26 AM

Hello all of you

 

At our facility we have five centrifugal pumps Operating  in parallel . Some times the overall load is such that  the pumps operate on lower than rated  flow . I therefore studying feasibility for installation of VFD in one of the pump so that variation in load  could  be adjusted and power could be saved .

Main feature for our system are as follow :

 

System pressure is governed by static head 

It is an explosive liquid 

All pumps are identical and having  rated flow and head respectively 400 m3/hr and 2090m (fluid density is 460Kg/m3)

 

My doubts in this regard and for your kind attention are as follow :

 

1. I  have a understanding that while we lower the RPM of one of the pumps running in parallel , that pump start to follow all new Characteristic  curve and deliver a flow  at  common header pressure according to new curve. I seek somebody to verify and add to this understanding .

 

2. How could  I  relate efficiency of the pump at various flow with reduced RPM, does it remain constant with varying flow .

 

3. Can somebody suggest a road map for finding out power saving through it .

 

Regards ,

Sahil

 

 

 

Dear Sahil,

Before I conclude the use of VFD is better or not. I need few inputs,

1. System curves (as you mentioned in your post that the system pressure is governed by Static head, interested to see what range of variation speed your system can have)

2. Variation in Load with frequency (shutting one or more pumps will also work in this case if frequency/demand is well known to you)

3. Schematic of the system indicating suction and discharge side arrangements (headers, control valves etc.)

 

Replying to your doubts;

1. In centrifugal pump curves, it will show you variation in head, power and efficiency with flow. Your second assumption of delivering flow at constant header pressure is somewhat not correct unless you have almost flatter curve or impeller is sized such that shift is nearly horizontal.

2. Efficiency will change with variation in flow

3. Before you switch to power saving scheme, I will suggest you to refer the available literature and understand, you will decide/calculate the potential for making energy efficient system.

 

Hope this helps a little.


Edited by Padmakar, 18 June 2013 - 04:30 AM.


#7 shin29

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Posted 19 June 2013 - 12:10 AM

 

Hello all of you

 

At our facility we have five centrifugal pumps Operating  in parallel . Some times the overall load is such that  the pumps operate on lower than rated  flow . I therefore studying feasibility for installation of VFD in one of the pump so that variation in load  could  be adjusted and power could be saved .

Main feature for our system are as follow :

 

System pressure is governed by static head 

It is an explosive liquid 

All pumps are identical and having  rated flow and head respectively 400 m3/hr and 2090m (fluid density is 460Kg/m3)

 

My doubts in this regard and for your kind attention are as follow :

 

1. I  have a understanding that while we lower the RPM of one of the pumps running in parallel , that pump start to follow all new Characteristic  curve and deliver a flow  at  common header pressure according to new curve. I seek somebody to verify and add to this understanding .

 

2. How could  I  relate efficiency of the pump at various flow with reduced RPM, does it remain constant with varying flow .

 

3. Can somebody suggest a road map for finding out power saving through it .

 

Regards ,

Sahil

 

 

 

Dear Sahil,

Before I conclude the use of VFD is better or not. I need few inputs,

1. System curves (as you mentioned in your post that the system pressure is governed by Static head, interested to see what range of variation speed your system can have)

2. Variation in Load with frequency (shutting one or more pumps will also work in this case if frequency/demand is well known to you)

3. Schematic of the system indicating suction and discharge side arrangements (headers, control valves etc.)

 

Replying to your doubts;

1. In centrifugal pump curves, it will show you variation in head, power and efficiency with flow. Your second assumption of delivering flow at constant header pressure is somewhat not correct unless you have almost flatter curve or impeller is sized such that shift is nearly horizontal.

2. Efficiency will change with variation in flow

3. Before you switch to power saving scheme, I will suggest you to refer the available literature and understand, you will decide/calculate the potential for making energy efficient system.

 

Hope this helps a little.

 

Sir, 

We have a long supply pipe line , pressure of which is constantly maintained at 90 barg , our pumps are pressurizing liquid in such a way  that their discharge header pressure equals to this static head ( 90 barg ) and frictional losses that very with the flow .

As customer increases the received quantity , the pressure of the line is maintained by the pumps discharging more flow .

Thus with same header pressure all  the pump distribute the discharged liquid eually . ( all pump being identical ).

 

Sir , regarding efficiency , does frequency also very by reducing RPM 

for instance  . at 2985 RPM frequency of the pump is 75% @ 400 m3/hr flow and 52% with 200 m3/ hr

 Is it okay that at 2915 RPM frequency of the pump is 75 %@  391 m3/hr flow and 52% with 195 m3/hr 

 

Regards,



#8 TS1979

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Posted 19 June 2013 - 10:52 PM

My first impression is that it is not a good idea to install a VFD on one pump. You probably cannot save power by installing a VFD on one pump. If you want to save energy simply shut down one pump.

 

If you install VFD on one pump, that pump may run slower compared to other pumps. At same time, the pump will create a lower head. Back pressure from the other pumps will keep the check valve closed (if there is a check valve), you will create a dead head for the pump. For the explosive fluid, heat created from the pump dead head may explode the whole plant.



#9 thorium90

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Posted 20 June 2013 - 01:42 AM

Hmm, I always thought it was a good idea to have one that had a VFD.

Say for example, there are 3 pumps each with a capacity of 1000 (arbitrary units) and one of these had a VFD. If the required flow is 2500, 2 pumps will produce 2000 and the VFD pump will produce 500. If the required flow is just 1300, 1 pump will produce 1000 and the VFD pump will produce 300. If only 700 is required, the VFD pump will produce the 700. The scheme would therefore save energy.


Edited by thorium90, 20 June 2013 - 01:44 AM.





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