High Pockets on Pump Suction Line
High Pockets on Pump Suction Line
(OP)
Dear Collegues,
Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required
Your feedbacks will be greatly appreciated.
Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required
Your feedbacks will be greatly appreciated.





RE: High Pockets on Pump Suction Line
To make a long story short (too late), a high point vapor trap is a common villain. Many engineers believe it is always a bad idea and will always be a problem. But that is not necessary the case. I believe that if you have adequate NPSH to keep from vaporizing product, and if you vent the high point well when the pump is first flooded, you may have no negative affects at all.
RE: High Pockets on Pump Suction Line
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
I am not sure if I understand your comments. Perhaps I did not make my point clearly. Eliminating the high point vapor trap did nothing to solve our problem. Reducing the Nss was the correct solution. Since that time, we actually eliminated the suction vessel and went back to the original piping configuration and have seen no negative consequences.
If a high point can be easily avoided, it is a good practice to do so. But the presence of a high point will not necessarily cause any problems. Without futher details, I cannot be more specific than that.
I will try to be more clear in the future.
JJ Pellin
RE: High Pockets on Pump Suction Line
You did not indicate whether this is an existing installation or one that is being planned.
My opinion:
1. If existing, then it sounds like you are stuck with what you have got.
2. If it is still in the planning/design stage, I give this advice. "WHEN IN DOUBT, DON'T". Don't allow any high pockets. Either make the pump suction line step down from the source to the pump or make the pump suction line step up from the source to the pump. This goes for any pump and any commodity.
RE: High Pockets on Pump Suction Line
I understand there are +/-s and its good to discuss them both. I was trying to agree with the bulk of your comment, that there seems to be a heck of a lota' problems that crop up whenever you try to do this, and even though they might be solved, there's no guarantee the system will operate successfully. So, even though it can be done, it sure doesn't sound like a good idea.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
It is interesting that the problem of “cavitation” you experienced was at low flow and not at the designed operating point and that you are now back to the original pipework design which seems to indicate that there wasn't an NPSHa/NPSHr problem at the design flow or poor pipework design.
My reading of this is that at part flow condition you were experiencing poor entry flow into the impeller which can sound like cavitation and could well be as damaging. The change of impeller to one of different NPSHr would be in fact a different design, - maybe with a different number or different vane angle or both which was more tolerant to operating at the part flow condition.
However, it overcame your problem which was a good result irrespective of the hydraulics involved.
Phitsanulok
Thailand
RE: High Pockets on Pump Suction Line
The other reason I like this example is that it slows the problems of using a high dollar consultant to solve a problem. In this and other instances, the consultant came up with a complex and expensive solution that not only did not solve our problem, but it created several other very serious problems.
RE: High Pockets on Pump Suction Line
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
With that said, I agree with Pennpiper. Avoid complicated suction lines. You run the risk of dry running and cavitation. There are ways to make it work but it is always best to keep it simple.
RE: High Pockets on Pump Suction Line
NPSH - having a large NPSHa/r to me does not represent a problem excepting as you have pointed out it might very well put you into a much larger pump than necessary with associated problems.
Phitsanulok
Thailand
RE: High Pockets on Pump Suction Line
I2I
RE: High Pockets on Pump Suction Line
Having been on both sides of the fence as well as in the middle, I have found the only way to really get down to what is needed is to meet with all parties who had input into the system design and define exactly what is needed and what margin of "just in case" needs to be applied. Only then can the manufacturer make an informed decision and demonstrate why they are making that decision - this also overcomes the second guessing and helps the supplier and purchaser ensure a correct outcome for the project.
Phitsanulok
Thailand
RE: High Pockets on Pump Suction Line
Well stated! The cumulative effects of multiple "just in case" margins often seems to be a major factor in operating and maintenance problems in pump and piping systems.
Reliance on specifications can provide a very false sense of security. If the specifications are not very simply and clearly written, they can cause more problems than they avoid due to misundertandings or variations in interpretations.
A "star" for you!
tkdhwjd,
Regarding the high pockets in a suction line, unless there is some compelling physical constraint that forces their existence, I can't imagine why all possible efforts would not be made to avoid them. Why invite trouble?
Regarding NPSHa being greater than NPSHr, what problem? Within bounds of practicality, the greater NPSHa vs. NPSHr the better. Where NPSHa = NPSHr, the pump is already suffering performance deficiencies due to cavitation!
RE: High Pockets on Pump Suction Line
I2I
RE: High Pockets on Pump Suction Line
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
I am suprised at the responses you have gotten to a very simple question.
"Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required"
The way I see it you are asking first about the pump suction piping. You did give us important data that relates to that specific question. That was good.
My suprise is at the responses that go so far afield even to suggesting that writing a better pump spec will prevent high-point problems in a pump suction line.
I stand with my original recommendation:
Don't allow high points in pump suction lines.
RE: High Pockets on Pump Suction Line
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: High Pockets on Pump Suction Line
A pump on the ground floor was supposed to pump paper stock up and into the bottom of an open tank on the third floor.
The plant's engineer kind of overdid it, and bought a pump that was too big, by a big factor.
On startup, the paper stock squirted out of the tank entry with enough velocity to reach the third floor's high ceiling, and splatter, everywhere but into the tank.
We had to throttle the pump's outlet rather severely, which wastes energy, and doesn't do paper stock any good either.
Mike Halloran
Pembroke Pines, FL, USA
RE: High Pockets on Pump Suction Line
tkdhwjd,
I am suprised at the responses you have gotten to a very simple question.
"Will there be any issues for allowing high pockets on pump suction line provided that:
1) fluid is heavy hydrocarbon - no flashing at high points
2) NPSH_available is greater than NPSH_required" ---------"
Actually the question was adequately addressed and answered with the 2 early relies from JJPellin - everything else has evolved from the various inputs of others.
Surely this is the essence of the forum - wide ranging input and discussion which unfortunately often deviates away from the original question but allows for answers to other questions and comments raised by members.
Another interesting points is, the discussion is now between members with exception of the original poster who hasn't had any further input. Is he / she still there, or have we answered the question?
However, it seems we all agree that high points can be a problem and better avoided when ever posible - but this isn't a perfect world and if you care to look round any process plant you will see a multitude of pipelines crossing road ways etc via pipe bridges (high points). That would seem to indicate a properly engineered pipe line with high points isn't really a problem.
Phitsanulok
Thailand
RE: High Pockets on Pump Suction Line
Thank you all for great inputs.
I am aware and fully agree that it is a "good" engineering practice to avoid (or minimize) high pockets on a pump suction... I wanted to see if I was missing anything. Two thinking hats are alaways better than one, in my opinion. Once again, thanks!