Low pocket in pump suction

[DGrayPPD]

An internal discussion has been going around about the effects of a low point pocket as shown below on a pump suction line. I have tried searching for answers but all my results are only applicable to "high" points in a pump suction, not low points.

I have already stated my disagreement with this proposed routing, but I am looking for a few stronger arguments to build my case around my disagreement. Some engineers do not believe the pocket will have any adverse effects on the pump performance. This question is essentially the opposite of a high pocket in a pump suction line which can result in cavitation. I am more than familiar with the high pocket effects, but nobody has ever asked me to drop a suction line underground and resurface before tying into the pump. So my argument is a little lacking, and I would rather go back with more than just "you're not supposed to." Do the same effects for a high point pocket apply to a low point pocket? What other effects from this arrangement may arise?

And just to clarify, the pump in this scenario is a vertical can pump taking suction from a bullet tank. The whole purpose of the pocket is because two bullet tanks with about 25'-0" between them are being tied together and they do not want to block off access between the two tanks.

I apologize for such a basic question, as I feel I should definitely know this by now.

 

[MJCronin]

Carefully evaluate NPSHa and NPSHr .....

What type of NPSH margin is required by your pump vendor ?

What type of NPSH margin is typical for the liquid being pumped ?

What is your assumption of the peak summer temperature for this liquid ?

Compare the "straight through" configuration with the on you proposed.

Remember that pump vendors evaluate NPSHr based on cold water service .... which you do not have

You are probably well aware that you will be SIGNIFICANTLY INCREASING the suction piping pressure drop with the four additional elbows.

https://www.cheresources.com/invision/topic/10528-npsha-calculation-using-lpg/

http://www.eng-tips.com/viewthread.cfm?qid=317961

Remember, pumps operating at very low NPSH margins turn into operating/maintenance nightmares

..... and you will be responsible

MJCronin
Sr. Process Engineer

 

[MFJewell]

I wouldn't have any concern with it as long as you have evaluated NPSHa as MJ said. You may have to include a low level switch/alarms to ensure tank level stays high enough to support it. I have been around equipment that has been operating decades with similar configurations and have not had any cavitation issues.

 

[quark]

If V-XXX is tank, then reduction in NPSHa is subjected to extra two elbows and increased length of pipe. I don’t see a bigger issue here. It is just “you are not supposed to”

 

[quark]

Ok, 4 elbows

 

[Compositepro]

High points create gas pockets which can cause priming problems. Low points are only a problem if they create a high point elsewhere.

 

[lilliput1]

Must have accessible strainers below the drop to prevent dirt build up where there is no easy way to clean it out. Include 2 psi strainer pressure drop in calculating available NPSH.