parallel pump problems 1

[hitchhiker]

Dear fellow members,

I'd like to ask your expert opinion on my current situation. I'm currently handling left-over projects that have 5 booster pumps in paralell delivering liquids from storage tank (produced water) to a pig launcher. 3 of the booster pumps are old existing pumps and another 2 are fairly recently bought with matching H/Q characteristic with the existing (not really identical). These pumps fail frequently and they took turns (imagine that!). so operation guys came to my department (project) to look for solutions and hopefully create something to solve their predicament.

the first thing when I look up when I step in is the calculation by FEED contractor. The old pump itself basically reused from another project, rated at 600 gpm @ 735 ft, water temp 180 F. the newer 2 pumps also have the same rating, 600 gpm @ 735 ft to suit the old one. So before they actually bought the 2 new pumps, FEED contractor have studied the system and made recommendation for 5 pumps in parallel. So the combined pump curve only intersect the system curve at 4 & 5 pumps operating. At single, double and triple pump operation the system is well below the pump curve (way to the right). So I wonder does this possible?

by right, operating 1,2 or 3 pumps will have cavitation problem, since it will push operation way to the right curve (it doesnt even intersect the system curve!)and while the pumps do have problems and regulary in-out service for maintenance, there were no report of cavitation damage on impellers. there are erosion damage on the casing, however (it turns out there were sand sediment in the fluid that was not considered). there are even report on casing leak and damage. I still wonder if this is because of the sand or other things.

So back to the my question, can parallel pumps designed this way? that the system curve only intersect the combined curve of 4 & 5 pumps?

 

[clay87]

Separate recircs. This allows testing, startup, and pump swaps while minimizing system disturbance.

 

[BigInch]

Agreed. With a large number of pumps, what you can do with recirc off the header could be done by turning on/off individual pumps to a large extent. I also think that, in general, recirculation lines are better on individual diesel driven pumps, where the pumps must run for awhile at low power as the diesel warms up. On electric units, they start fast so power is available nearly immediately and long recirculation times where products heat up are avoided. You would only need recirculation for flow control, which you can do with a large number of pumps just by the on/off button, with, or in some cases without, the 2 CVs to control downstream pressure to the pipelines. A recirc off the header could still be installed, if it turned out to be necessary at all. That depends on how closely you can operate these pumps to the duty point you want without the CVs, or if you will have enough flow variations that operating at within efficient range of a basically one-head duty point, or close to it, isn't possible.

Independent events are seldomly independent.