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Thanks again. Maybe I got this wrong, but just by measuring preesure on the suction header (which is 36&quot of 1.5 psi, right before it branches off to the pumps' suction side, that means that the available NPSH at that point (neglect vapor pressure) would be 1.5 + 14.3 - VP, little bit over 15 psi or 35 feet. I will loose some of it from 36" reducing to pumps' suction but that is maximum 4-5 feet. That still leaves me with at minimum 30 feet available NPSH. or 1.3 NPSHr. I think from that, theoretical side I should be OK, unless something weird is going on.

Thanks again

Thanks again,
I thought that I answered but I must have pressed wrong button, in any case:
The pressure gauge on suction manifold shows 1.5 psi. Unless I am missing something that should read as at least 14.3 + 1.5 - VP= more than 15 psi or around 35 feet of NPSHa. Now I have say about 5 feet of headloss through eaach pump suction, but it should still be around 30 feet or at least 130% of the NPSHr.

It seems to me that the whole problem is in flow pattern to a double suction pump. Unfortunately I don't have a chance to check that out? I am thinking of installing Vane Flex or something similar, but how would I know if that will help?
Thanks again

,

Are you sure you are having a reading of 1.5 psi from a pressure gauge?

Okey lets not go on to details on NPSHa. But I guess your missing something on a NPSHr. NPSHr is given by pump manufacturer based on thier impeller and casing design. Please confirm the NPSHr from the manufacturer.

Please give me TDH and rated capacity, or if you want give me the pump model, brand and size.

And I will check at my side as well.

Thanks for your effort. NPSHr is listed as 23 feet at the design point which is 2,500 gpm @ 275 feet. I have the NPSHr curve infront of me. The pumps are Ing.Dresser (now Flowserve)Type 6 LR-18 double suction impeller type, running at 1770 rpm. pump has a 10" suction with a 6" discharge.

assuming all four pumps are rebuilt to oem specs w/identical trim impellers, & running at the same speed; i would guess (since very little information is given) that you have two issues - the suction piping is not to HI recommendations - causing flow & NPSHa issues in this side, and you have little knowledge of what the system head curve is - which is imperative to determine where the pumps should be running on their curves. i suggest you find someone with some experience to assist you.

Hi,
Thanks for your input. I do have plenty of experience in designing pumping systems (over 20 years of sucesfuly designed and implemented projects. I was hoping that this forum would provide a stimulative discussion on various topics especially pump systems troubleshooting, design and similar challenging applications. An advice: go find someone with some experience does not fit those expectations.

Anyhow, this is an interesting application, I thought I explained it in a lot of details, I have a pretty good idea what caused pumps to failed, but was trying to get some feedback from the peers that may have some different ideas.

Thanks again to all who made that effort.