Operating Pumps in paralle even thoough they are not originally designed for that purpose 2

[Bultum]

HI,

I am currently working on rectifying anissues at a pump station. In short I have three pumps connected to one suction line and one discharge line. The pumps are designed to operate individually, one duty and 2 stand by. The pumps work perfectly fine as long as there is a positive pressure going in. However once the water in the storage drops below the pump intake line, there is negative pressure in the suction. The issue is during this time, we have excessive air leaking through the off-duty pumps (packing gland). I have considered mechanical seals, but they are very expensive - especially for an old pump and I'm not sure how effective they are.

Since the pumps have VSD installed on them, I was thinking running the three pumps in parallel. So reduce the speed of the pumps. I know I will be operating the pumps inefficiently but if I can prevent air getting into the system, I can prevent the pumps from failing.

what are your thoughts?

By the way, i'm a grad engineer. Been in this job for only three months so i'm new to this stuff.

Thank you

 

[LittleInch]

Ok, we only know what your system is from what you tell us.

A distribution system with multiple offtakes and users is virtually impossible to create a system curve for unlike a point to point pipeline.

Even if you pick a number of permutations the shape of your system curve is constantly changing, hence the pressure required for a certain flow will vary depending on usage. Therefore I can understand how at times you need one pump and others two or maybe three.

The packing is rather vague but looks like a long string? There may be alternatives which can act in both directions to prevent air entering - time to talk to a packing / seal manufacturer or pump vendor if they are still around. Or maybe you just need to replace the packing and tighten it up a bit?

Are there some old and knarled pump maintenance guys there? ask them nicely and they might have some good ideas.

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

 

[Artisi]

round and round went the bloody great wheel -- now a name change for more confusion.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[chicopee]

Negative pressure in the suction suggests to me a cavitation problem. Was the problem of cavitation examined?