13 replies to this topic

[dineshperu]

Does the increase in suction pressure effect the pump ? 

 

 

There is a centrifugal pump which handles water which trips on full load current ..Suction is throttled and it runs fine ( there is lot of margin between NPSHa and NPSHr  ) . When we tried to investigate the problem we haven't found any discrepancy expect suction pressure ( designed for 2kg/cm2 .. available is 5.6 kg/cm2) . Pump is developing the differential head (2kg/cm2) as per its design . What could be the possible reasons for pump trip ?

 

Thanks 

Edited by dineshperu, 25 November 2015 - 04:13 AM.

[fallah]

dineshperu,

 

Please specify if there is a control valve at discharge; if so what's the type of control valve...a simple sketch can help a bit more to furnish  a proper response...

[breizh]

Hi ,

probably good also to see the pump curves and the operating point .

 

my guess , you are on the right side of the curves ...and you may consider to revise your start up procedures .

 

My 2 cents

 

Breizh

Edited by breizh, 28 November 2015 - 02:03 AM.

[fallah]

dineshperu,

 

No response from you; but in general:

 

1) If there is a FV at discharge; the discharge pressure will be increased equal to the increment of suction pressure...

 

2) If there is a PV at discharge; the discharge pressure will remain unchanged but the flow will be increased...

 

3) If there is no CV at discharge line; all depends on the discharge destination conditions, then if it kept unchanged the flow will be increased due to an increase in TDH...

[shan]

You should first identify where is the trip signal come from.  Over pressure? Excessive temperature? Undesired power load? Or severe vibration?

[dineshperu]

Sorry for the late reply,

 

@ fallah : there is no control valve in discharge.It is a cooling water pump which delivers water to various coolers in a compressor.

 

@ shah : Pump trips at high current

 

@ art motemayor : Sorry this was my first post after posting only realized i posted in wrong forum. I didn't know how to delete the post . I will make sure it will not be repeated.

 

@Breizh : I tried but @ rated flow and rated head operating point fall in between the pump curves of different impeller diameter

[fallah]

 

@ fallah : there is no control valve in discharge.It is a cooling water pump which delivers water to various coolers in a compressor.

 

 

dineshperu,

 

Then, if you're not satisfied by the 3rd item of my post no.4, please upload a simple sketch of the system as I mentioned before...
 

Edited by fallah, 26 November 2015 - 03:12 AM.

[dineshperu]

@ Fallah : ok I will do it by evening. And one more part of my question is still unanswered. Does the increase in suction pressure pose any problem ? 

[manojkaila]

Dear Dinesh,

 

Increase in suction pressure has no much harm on pump with water as a fluid.( Just check your seal design). But as per my opinion throttling suction is not good practice. I think your pump is giving more flow at given condition which creates high amp. This pump you are starting with discharge valve full opening? You can try with discharge valve close during start up & slowly open by looking amp meter , keep below full load current & still you can fulfill your process condition ( Discharge flow) then your pump may over rated for this condition.

 

Regards,

 

Manoj 

[shan]

Please check your pump suction curve.  Higher suction corresponds to larger flow rate.  Larger flow rate results faster pump rotation (Q2/Q1=n2/n1).  Faster rotation needs more power (bhp2/bhp1=(n2/n1)^3).  The excessive power demand (electrical current Amp over specification) tripped your pump. 

[dineshperu]

Mr. Shan ,

           

       Initially I came to to the same conclusion as yours.

 

" Higher suction corresponds to larger flow rate. " As you quoted. When the size of suction pipeline is same for two similar pumps and suction pressure differs , pumps tends to operate at different flow rates ?

[dineshperu]

Dear Dinesh,

 

Increase in suction pressure has no much harm on pump with water as a fluid.( Just check your seal design). But as per my opinion throttling suction is not good practice. I think your pump is giving more flow at given condition which creates high amp. This pump you are starting with discharge valve full opening? You can try with discharge valve close during start up & slowly open by looking amp meter , keep below full load current & still you can fulfill your process condition ( Discharge flow) then your pump may over rated for this condition.

 

Regards,

 

Manoj 

 

Mr.manoj,

 

         I agree suction throttle is not good practice.There is no discharge valve ( it supplies CW to multiple inter coolers in compressor . So there are valves at downstream of the inter cooler CWR .We already throttled these valves to say like 30% ).

 

I have made arrangements for the flow measurements . Will let you know what are the results.

 

Thank you

[Padmakar Katre]

Hi,

 

Does the increase in suction pressure effect the pump ? 

 

 

There is a centrifugal pump which handles water which trips on full load current ..Suction is throttled and it runs fine ( there is lot of margin between NPSHa and NPSHr  ) . When we tried to investigate the problem we haven't found any discrepancy expect suction pressure ( designed for 2kg/cm2 .. available is 5.6 kg/cm2) . Pump is developing the differential head (2kg/cm2) as per its design . What could be the possible reasons for pump trip ?

 

Thanks 

 

Hi,

Why you at all operate pump in this case when the increase in suction pressure is more than the required differential pressure? Just open the suction and discharge valves and you should get the desired flow.

[S.AHMAD]

Throttling centrifugal pump suction is not a good practice that may lead to pump cavitation. We may have the same effect by throttling the pump discharge. We must first of all that understand the job of a centrifugal pump is to deliver the volume rate required by the system.technically speaking, the pump curve meets the system curve. When we throttle the pump suction pressure we are actually controlling the flow-rate delivered by the pump in which we can also achieve the same effect by throttling at the pump discharge. That is why a good practice is to install flow control at the discharge of the pump.

 

 

Centrifugal pumps trip is normally triggered by current due to higher power requirement. Increasing pump suction pressure but the flow rate is maintained the same by controlling at the pump discharge will not increased the pump power.We must also understand that pump power is proportional to pump head and flow-rate but inversely proportional to pump efficiency.When pump suction increases the net effect is to increase the discharge pressure accordingly resulting in increasing the system flow rate as simply explained by Bernoulli equation.The pump is then responded by delivering the system flow requirement this increases the pump power accordingly such that it triggered the pump tripping current.

 

Hope the above comments triggered your though. We may be able to solve the problem if you can provide more detailed data such as pump suction and discharge pressure and the corresponding flow-rate and most important t is the pump performance curve.

 

 

Best regards from S. Ahmad.