last night we started testing new two pumps at the field. these pumps are a vertical pumps with same size and speed, 26HP and 3545RPM.

we started them one by one. the first one went so good with steady pressure indication and no . However, the other one was completely different. once we were opening the discharge valve some noise started to appear and pressure was going down. the sound was likely a cavitations sound.

we went to throttle the discharge valve very slowly and observed the noise is started going down and pressure back up.

wondering if that kind of cavitations symptoms even the two pumps are operated to pump the same sump ?

Hello Jawal,

is this a new installation or direct replacement?

the design of the sump has a great influence on the pump operation characteristic.

Where the two pumps run in parallel? In which case is the sump symetrical and both pumps 'see' the same conditions? For two pumps to run in parallel sump design is even more critical....


You need to check that there remains enough NPSH to avoid cavitation. Dimensional checks are required to confirm this and then the manufacturer's data. For example if the sump is not deep enough vortices may be created and air could even be sucked in from the surface.


try to work the NPSH required against the available to see if you have enough.

Hi Rennie
this is new pumps for new sump system. we are in stage of commissioning new plant. and these two pumps are working in parallel. That is why I think there is no problem with the sump level, because it would affect on the other pump as well.

so you think i should do the check NPSH ?

I agree to check NPSH for both pumps. If both have different NPSH, one pump will give more discharge than the other and there is no option but to throttle discharge valve of the pump with lower NPSH.
Regards
Irshad

AKHTAR
In case of finding lower NPSH in one of the pumps, do you think throttling discharge wont cause bad impact on the pumps performance?! even it would restrict some of flow

Axial thrust will be higher and tell upon thrust bearing.
Regards
Irshad

so.. you telling me its not a good practice to throttle discharge line if its going to cause an impact on thrust bearing.

Hello Jawal,

I would stop the OK pump and let the 'problematic' pump run by itself.....if the problem disappears then this demonstrates that the problem is flow induced and a matter of NPSH. If switching on the second pump the problem is transfered to this pump then I don't think you have much to do but to address the issue of sump design.

Rennie
the setting of operating the pumps to run one and keep the other as a stand by pump. we already run the main pump for a while and it was ok, then we shut it off and turn on the other one but it was running with this noicy sound. we left it online for about 10 min but nothing changed. turned off then try tune onn nagain with discharge valve semi close we found the noice is disappeared but with low discharge pressure. once we started slightly opening the discharge valve, the sound came up.

is what we have done is the same what you said here "I would stop the OK pump and let the 'problematic' pump run by itself.....if the problem disappears then this demonstrates that the problem is flow induced and a matter of NPSH"

hello jawal,

if i understood you correctly, with the main pump off, the problem persists.

the way you have it described..

discharge valve closed discharge pressure is high
discharge valve partially open, discharge pressure falls and noise comes in

if noise is from cavitation then i would say problem is flow related. Are you sure that both pumps see the same inlet conditions. I would still take a go at NPSH calculations.

could you please state the name, and model of the pumps..

Rennie
I am not sure if you understood what i meant about the pumps normal operation. we only run one at the time and leave the other as stand-by. i just wanted to clear the operation philosophy of these two pumps it might help you in troubleshooting problem.


yes the two pumps see the same inlet conditions. they installed next to each other to pump oily water from the same sump tank.

pump data:
Manufacture: KSB pump
Model/TYPE/SIZE:KRT K40-250/172WC2

thanks

check there is no restrictiom of the inlet to the "problematic" pump

No noise with discharge V/V closed means the discharge flow capabilities match that of the inlet
Noise comes in with discharge open, means discharge flow capability is greater than inlet and now you have cavitation.

New installation? maybe some rag or other inside left by fabricator???

With the discharge valve wide open the pump operates off the curve and it would cavitate. It is fine to throttle the discharge valve and bring the pump back on the curve.

Get the pump curve and record pump flow rate, suction and discharge pressure gauge readings and rotational speed and you will know what is going on.

If you operate the pump with wide open discharge valve you will see high frequency activity indicating cavitation.

If it has cavitation, check vibration. High frequency energy mounds will be there in the spectrum. Second thing, check that the pump has no obstruction in flow path. Third thing, the pump is not very close to the wall of the sump as compared to the first pump. Fourth thing, if there is a bell mouth or cruciform, check whether these are installed correctly and not damaged. Check the height of the bellmouth from floor. If it is less than as mentioned in OEM MI, there may be cavitation. If the pumpm is provided with suction strainer, check it for cleanliness otherwise it will reduce available NPSH and cause cavitation. Check for proper lift adjustment of the rotor otherwise it will affect discharge pressure if it is not same for both pumps.Check for any change in layout of suction pipe of the second pump as compared to the first which may increase pressure loss.
Regards
Irshad

One more important thing:

Is concentric pipe reducer used on the pump suction close to pump suction nozzle / flange. If eccentric reducer pipe is used the sloping side should be at the bottom and the straight one should be at the top.

When the sloping side is at the top, there are formation of air bubbles leading to cavitation.

Never use a concentric reducer on pump suction.