Boiler Feed Pumps- cavitation or dry running damage

[chadvic53]

Hi all,

we recently overhauled 2 boiler feed pumps, and they started tripping after 2-3days of a run-
1. pump A would be running lead and would trip without any alarm in SB, leaving a fault on HMI. (on investigation found that flow has been stable before and after trip, no overcurrent trips in MSB).
2. when pump A and B both are running, if pump A stops pressure dips and comes back up??? both pumps have ARC to feedwater tank.

colleague thinks it might have sucked air/steam-water mixture from DA, when a level in DA was 60% full (highly unlikely as there are two DA's in Parallel.(sister DA's and almost same setting of heating steam pressure) -

if in case BFP sucks some steam-water mixture, wouldn't the steam bubble would collapse as the pressure goes up from one stage to the last stage???
what is science behind pump gassing up and losing suction, how much steam/vapor is required to get 0 flow or gall the pump parts?

if the pump was cavitating wouldn't motor Amps drop and when would pump trip, would it trip on galling being happening???

we have KSb multistage pump, where the balance line feeds the suction bearing acting as orifices I guess? what conditions would it require for pump to cavitate - pls do include if pump was running overcapacity?? going out of curve??

 

[LittleInch]

To have any idea about this we really need some more detailed information such as a P&ID / schematic with pressures, temperatures, flows, level etc and the pump curve.

We can only see what you tell us and a description really needs to be accompanied by a drawing so we can join the dots.

The issue with getting two phase flow is often that you get surges and spikes in pressure / load which can trip things very quickly.

You mention galling? Is this what you found when stripping the pump down?

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

 

[MJCronin]

Chad...

Pump transient operation, NPSH issues etc, should be discussed in the Pump Forum, IMHO

Suggest that you repost there....

If you are a newbie to BFP suction piping systems, realized that the design requirements of these systems are unlike those of all other power plant pumped systems.

! .... (See the pdf file referenced below)
.
First, a few questions ...

1) What did the KSB Pump rep say when you contacted him about this problem ?

2) Can you run the DA at a slightly higher level and replicate the problem ?

3) Have you repeatedly reconfigured the complex boiler feed pump suction piping since the plant has been started up and was this piping "redesigned" on the fly by the plant or maintenance staff ? (i.e. no engineering involvement)

4) Sometimes, in boiler plants where there is a confused, complex system of many BFPs and new/old deaerators, you can get into the situation where the common pump suction line is "overwhelmed" on pump switchover. By overwhelmed, I mean that the combined draw of the BFPs cannot accelerate the mass of water in the suction line in enough time. This appears very similar to BFP cavitation or no NPSH.

5) Have you calculated the NPSHa for each of the pumps and compared it to the NPSHr (plus a 10 ft margin) ?

6) What does "galling being happening" mean ?

See the comments on the last page of this document regarding BFP suction piping velocities. I. Kasarick also has discussed this unique problem in his famous seminars.

https://www.pumpfundamentals.com/pumpworld/suction piping 2.pdf

Also, Frank and Bruno have written this seminal TAMU paper on this subject

https://pdfs.semanticscholar.org/0548/90f8b255e577740254c342e529230747412b.pdf

MJCronin
Sr. Process Engineer