Noisy Pump

[ChrisConley]

Hello all,

We are currently looking at an application where there has been a complaint that the pump is too noisy. The pump has been replaced once and the system is only two years old.

The system is a boiler loop feeding a plate heat exchanger, the pump is an inline circulator and temperature is controled with a three-way valve. My concerns with the system is that: there is no air seperator/air release, there is no check valve after the pump, there is no expansion tank, and finally the system was never balanced properly.

The problems I envision are: air in system causing cavitiation-like damage, flow from other pumps spinning impeller in reverse while not operating and causing start-up problems, pump running through the pump curve as head is lower than expected and velocity through pump and heat exchanger is too high.

I am going out to check the installation. Is there anything else I should be looking for, and do my assumptions about the possible (probable) causes of the problem look reasonable?

Thanks all,

 

[quark]

What are the temperatures after PHE and in the bypass line of 3 way valve? If the difference is high then you will have some hammering.

I think you covered everything on hudraulics. Also check bearings and floating of shaft(impeller may be touching the casing).

Regards,

 

[PUMPDESIGNER]

Consider hiring a pump expert for this.
But, if you have the situation whre no one minds you learning on their dime, here are some things to start with.

You did not state why the pump was replaced, where the damage was, what kind of damage. Cavitation damage is very specific and easy to diagnose, well maybe easy for experienced knowledgeable persons anyway.

Air cannot cause cavitation, ever. Air does not usually damage pumps unless the amount of air is sufficient to cause vibration, expose the seal faces, etc.

Your application often has pumps with very high specific speeds and high suction specific speeds. If qualified experts selected the pumps they are probably Ok, unless someone changed the specs (you know, value engineering).

If the pump is as I said high Ns and Nss, then those buggers are very tricky suckers to work with indeed and you really should find help.

PUMPDESIGNER

 

[ccfowler]

The first things that I would want to check out for myself are:

1- How much STRAIGHT pipe is connected to the pump suction port? If it isn't at least 10 diameters, the orderliness of the flow into the pump is likely to be compromised, and the "real" NPSHr can be greatly increased.

2- How does NPSHa compare with NPSHr under all potentially troublesome operating conditions? Bear in mind that the the stated NPSHr represents the pump already having lost 3% of head to cavitation. For quiet operation, your pump may need NPSHa to be anywhere from 2 to 15 or even 20 times the "3%" NPSHr.

3- Make sure of the accuracy of all of the pump "curves." It matters very much whether they are from actual pump tests or just generic curves for the pump model.