problems with NSPH for piston pump 3

[mustafa2014]

Dear All Good day,

our case is :we have a piston pump in platform used in TEG package , this pump start to have problem because of NSPHR .

My question could we use a booster pump up stream the piston pump? If yes where should I pay attention to?

 

[LittleInch]

Try looking at the post about 5 down from yours for starters...

http://www.eng-tips.com/viewthread.cfm?qid=363467

some more data would be useful - type of pump, number of cylinders, flow, pressure, temperature, why you're getting NPSH issues (fluid lift?)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

Yes you can use a booster for TEG. Theoretically, just about any small centrifugal would do the service; I'm guessing it is a fairly low capacity.

First look to see if it is possible to modify system to allow for more inlet pressure (changing elevations, less drop across filters, etc.). Failing that, you must look at the end-user spec requirements however, you may be forced into using an API pump as the booster....very expensive.

In that case, it may be cheaper to get a bigger piston pump so that it can run slower and require less NPIP (that is the term (Net positive Inlet Pressure) the authorities are telling us to use for positive displacement pumps in lieu of NPSH).

 

[mustafa2014]

Dears LittleInch & DubMac ,

Thank you for your reply . Please check the attachment s for more information you asked for please advice sooner

 

[DubMac]

You might check with your Peroni supplier to see if larger pistons/liners can be used in this pump in order to run it slower and require less NPSH (NPIP).

Or check to see if you can use other than an API pump as charging pump.

 

[mustafa2014]

Dear DubMac,

Could you explain to me the second option more please

 

[DubMac]

If end-user has specifications (either company wide or location or zone specific) that require compliance with API 610 specification for ALL pumps in chemical (TEG) service, then your solution using a centrifugal pump just got very expensive. If you are not required to use API, then I would use an ANSI pump or maybe just a good non-metallic (GRP, etc) centrifugal to charge the Peroni. You would barely need much head at all out of the booster pump (maybe 10m, I can't remember the Peroni's inlet requirements), you just need to size for the capacity.
The real consideration is the seals in the charge pump and making sure you have compatible with TEG, and good drip pan system to collect any inadvertent leakage.
Figure out the capacity and head needed to "charge" the Peroni and ask recommendations of a couple of good pump distributors in your area.

 

[micalbrch]

As previously mentioned in another thread: As the Peroni pump is a triplex pump your booster pump must have around 30 % more flow than the Peroni pump.

 

[DubMac]

I think the poster probably has enough info to chew on, so really just to further the theoretical discussion.....

The 30% over comment I know is just a rule of thumb, but opens up one of the least understood, and researched areas in all of pumping: feeding the suction side of a valved recip pump.

I'm guessing this rule of thumb came about at some point as a safety margin after a "properly sized" centrifugal was in fact undersized after all and ended up running out on the curve and ITSELF being starved. Just a guess.

I do know that, at some point of the recip being "overfed", there is risk of overcoming the spring tension that closes the suction valve and it stays open a bit too long in the cycle. Instead of closing at the end of the suction stroke, it will stay open a tad bit long into the discharge stroke and you will get leakage back through it. This can lower volume out of the recip and in abrasive services (especially drilling mud) can lead to washing out the suction valve prematurely.

Very few studies done on what the proper NPIP is for a recip pump.....I think because the term acceleration head scares so many people. Sure does me.

 

[bingopin]

A little off topic here...As I look at the data sheet, I would be concerned with a 150# suction flange. He may have RV protection on the suction side but I'm used to seeing the discharge flange rating carried back to and including the suction block valve.

 

[micalbrch]

DubMac, my English is not good enough to explain the correct physic behind the rule of thumb but the value 30 % is valid for a triplex pump only. The reason is the offest of the crank (120°) and thus the plungers (pistons). In a triplex pump there is an overlapping of plunger movement during the suction stroke (of course also during the discharge stroke but that does not matter here). For a certain time (degree of crank revolution) two plungers perform a suction stroke at the same time. And as the flow of a reciprocating pump is not constant over one crank revolution but varies, the booster pump must be oversized. I can't explain it better.

Correct what you write about the spring tension and the backflow but the loss in flow is rather negligible as long as the valves are not worn out. The valves close as soon as the plunger moves forward as it (the plunger) always creates a pressure higher than the suction pressure as long as the fluid is incompressible.

 

[DubMac]

No problem with your English, better than many native born.....

I have heard the 30% rule before (actually just have heard "oversized"); and it is surely better for the charge pump to be oversized than undersized! And yes, it is correct that the charge pump will be feeding two cylinders rather than one, for a time.

So yes, I think it is agreed that oversized is good, I'm just thinking about the limits: where does oversizing start to hurt performance and why.

I recall as a young hand in the oilfield having suction valves pushed open so forcefully that the discharge stroke could not close them anywhere near in time and a very appreciable amount of flow was lost (as well as very rough treatment of all parts concerned). In this case, we would put either a stronger spring, or double spring on top of the suction valve to make sure it closed quickly. But we were young and dumb, and that made the problem go away so we never considered further. The point is, there is a line you cross when oversizing the charge pump that can make bad things happen.

Mainly, what I don't get is why there have not been more published studies on exactly how to properly charge a recip. There are hundreds of incredibly intense technical papers written around every imaginable topic on the suction side of a centrifugal. Not so with the poor recip pump.

Again, I say people are scared of acceleration head.

 

[mustafa2014]

Dears DubMac & micalbrch,

I am really thank you very much for your kind support.
please could tell how the 30% calculated for triplex pump?
and where could we consider that the flow is over sized so much and will effect our pump ?

Many thanks for your help

 

[JJPellin]

Acceleration head can definitely create challenges in dealing with cavitation problems in reciprocating pumps. The acceleration head component of NPSHa is proportional to the length of the suction piping. The resulting reduction in NPSHa can be corrected by reducing the effective length of the suction line. The use of a properly design accumulator can de-couple the pump from the long run of suction piping. Before I would spend the money to add a booster pump, I would fully evaluate the suction piping configuration. Adding an accumulator would be a lot cheaper than adding a booster pump. And the energy to drive the booster pump makes that option even less desirable.

Johnny Pellin

 

[DubMac]

Johnny Pellin, I'm familiar with a bladder type pulsation damper that is sometimes built into the suction manifold of a triplex (especially in mud pumps), but not with any other device like the accumulator you describe.

Would you mind elaborating on what this is, sounds very interesting. Yes, anything to forego a booster pump would be a huge plus.

 

[JJPellin]

The term accumulator has more to do with the function than the design. I am referring to a rubber bladder damper. But, the damper and the connection it attaches to would need to be sized to take the flow swings to limit the acceleration head in the longer run of suction piping. I have used them on triplex piston pumps with good results.

It might be worth asking mustafa2014 what symptoms lead them to believe that NPSH is the problem. Is it reduced capacity? Is it vibration and noise at the pump? Is it vibration and noise in the piping? There have been papers written that studied the effects of cavitation is piston pumps. Cavitation, in and of itself is not destructive in a piston pump like it can be in a centrifugal. But, if there is high vibration in the pump or piping, it can cause other problems. The OP stated that NPSH is the problem. And we took him at his word that this was the problem. Perhaps we should ask more questions about how he came to that conclusion.


Johnny Pellin