Centrifugal pump discharge pressure problem

[Saad Ahmed]

Hey,

I work in power sector and just want to discuss a problem with the centrifugal pump that arose and affecting the performance of the condenser. We have a vacuum pump alongwith it a small recirculation centrifugal pump that is responsible for providing the sealing water in the vacuum pump. Between the centrifugal pump discharge and vacuum pump inlet, there is a shell and tube heat exchanger that decreases the temperature of demineralized water coming from recirculation pump and entering the vacuum pump inlet. Temperature is decreased from 45 degree C to 30 degree C.

Now the issue is that the recirculation pump design pressure is 1.7 bar and now it is not creating the desired pressure and just giving 0.5 bar pressure when the main vacuum pump is not put into operation. We have a pressure guage after the heat exchanger and not immediately after the pump discharge. When the vacuum pump is put into operation it pressure reduces from 0.5 bar to -0.5 bar and no decrement in motor amperes are observed. We have test the pressure guages and also cleaned the heat exchanger which has no issues but could not get rid off this problem. The vacuum in the condenser is affected and have become around -88 KPa as it should be maintained at -97 KPa

The pump should atleast create around 2 bar as the same assembly is also there for backup developing the pressure around 2 bar when the main vacuum pump is not put into operation and around 1 bar when the main vacuum pump is put into operation.

Kindly give yout valuable comment on this issue.

Thanks
Regards,
Sandy

 

[LittleInch]

Can you sketch your system and also provide some pump details such as flow, head, the pump curve etc

I assume you mean "..now it is NOT creating..."

Do you have flow control on this pump?

If the identical unit is creating a higher pressure then you need to look for differences.

Is the pump rotating the correct way? Three phase motors can turn in either direction.

Any filters/ valves, difference in piping?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Saad Ahmed]

http://files.engineering.com/getfile.aspx?folder=778dd335-4ab8-403e-afb0-2514a90ef455&file=asds.jpg

 

[Saad Ahmed]

The sketch of our system has been attached.

Unfortunately we dont have the pump performance curve, head and flow rate etc
What differences are to be measured with the identical unit?

Yeah the pump is rotating in the same direction.

Furthermore, there is no only one strainer and discharge valve in the line

 

[LittleInch]

That's not really a lot to go on here without that info.

What I meant was if one unit works and the other doesn't work so well then are there any difference between the two to try and explain it?

Can you install pressure guages in more locations? However little can be done without knowing the pump details and what flow you're getting.

Your sketch is quite schematic. What keeps the pressure in the loop? Is this a closed system? Does the vacuum pump somehow affect the pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Saad Ahmed]

The difference is that the recirculation pump of the one unit is developing 2 bar which is fine and the one creating only 0.5 bar which is much lesser ultimately affecting the vacuum in the condenser. The design head of recirculation pump is 1.8 bar but we are getting 0.5 bar.

Yes when only recirculation pump is put into operation the pressure is different and when the main vacuum pump starts the discharge pressure gets lesser (For example the value goes from 2 bar to 1 bar). Yes this is a closed system but make up lines are also provided there.

 

[LittleInch]

Ok, So does your system look like this?

If so is there any physical difference between your two pumps? height of pump, location of pressure guage etc?

Is one pump duty and the other mainly standby?
Have you stripped the pump down?
Have you been cavitating the pump and hence destroyed the impellor?
Do the pumps use the same power when working?
Can you install more pressure gauges to see what is happening at different points ( pump inlet, discharge).

Where is the filter?
Is it blocked?

You need to find the pump details and pump curve. If you don't have it, find the pump serial number and contact the vendor or look it up on their website.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Saad Ahmed]

The height of pump and location of pressure guages are same. Both pumps are put into operation alternately. Both pumps are consuming the same power. Recently the pump having issues, its impeller got damaged, repaired and installed. One thing more, we have installed other pumps also in its place but having the same lesser pressure at discharge.

I guess we need to install more pressure guages to monitor the parameters at required location and also need to check out the condition of strainer.

Thanks mate !

 

[LittleInch]

When you "repaired" the pump, was it tested?

how was it repaired?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

I'll suppose that either the impeller is the wrong one, or the internal clearances are wrong.
If they didn't flow test it then I wouldn't trust it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Have you checked the makeup water level transmitter / controller ? Is there some way of verifying its level readout? If the actual level is too low, and gas also flows down the recirc. pump suction line, the pump wont be able to produce the required dp.