Acceleration head

[Alanator]

I have been reading recently about a phenomenon called acceleration head. It seems to apply only to reciprocating pumps. My question (s):
If you had a glycerin-filled pressure gauge at the reciprocating pump inlet and another gauge at the booster pump discharge could you see the pressure difference? I would assume the difference would be larger than the difference due to friction losses. Would the glycerin act as a filter and mask the true magnitude? I'm wondering if this might account for cavitation in the reciprocating pumps even though we think we should have good quality fluid at the inlets.

I have heard cryogenic pump manufacturers talk about the need to mount pumps close to the supply vessels. Now I think I see why.

Thanks for any thoughts or insights.

 

[Ashereng]

One reason to mount pumps to the supply is NPSH(a). The closer you are, the less pressure lost in the inlet line. This is usually a good thing as you can never have too much NPSH(a).

A glycerine filled pressure gauge is specified usually to keep the needle from bouncing back and forth, making it a bit easier to read.

It is a bit of a filter, I guess, in that regard. But, if the pressure is X, the gauge will read X. It just slows down the needle movement a bit.

I am a bit confused with regards to friction loss, pressure difference and acceleration head as it pertains to the pressure gauges though.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Montemayor]

I’ve found Ashereng’s comments right inline with my field experience. The basic question asked is, in my opinion, “loaded”. With pressure gauges on the suction and discharge side of a reciprocating pump (or any other type of pump) you will always see the difference – whether or not the gauges being used have a glycerin filler fluid or not. However, the “difference” will be fluctuating in the case of a reciprocating pump – primarily due to the cyclic nature of the working piston(s). This effect can be markedly reduced (never eliminated) by using pressure “snubbers”, or simply hydraulic pressure shock absorbers which are nothing more than “wide spots” in the line. The nature of reciprocating devices is cyclic and this can’t be eliminated. The observed effect can be “veiled” or “evened” out, but it is still there mechanically: the pressures fluctuate and are not constant. There is nothing abnormal or wrong with this observed effect. Depending on the installation, it could have a negative effect on the pump’s NPSHa.

However, reciprocating pumps are not only very rugged, but they are also very dependable. Theorists would be vastly impressed at what these “clunkers” can do and what they can put up with out in the field. You have to take one apart, work, and maintain it for a couple of years in order to appreciate it. Yes, it is smart to locate a cryogenic reciprocating pump as close to its suction source as is physically possible. Nevertheless, for many years I installed and operated Oxygen and Nitrogen cryogenic reciprocating pumps with spherical ball check valves acting as suction and discharge valves and never had problems pumping cryogenic, saturated liquids from a source that was 10 feet away (most of it overhead). My point is that the cryogenic liquid(s) was always saturated – at best – and still I was able to take it from 5 psig to 3,000 psig in one plunger stroke! This has to be a remarkable accomplishment on the part of a pump when one considers the precarious condition of the cryogenic fluid(s) in question. (Don’t forget that in calculating NPSHa for a saturated liquid you can’t take credit for the vapor space pressure above the suction liquid surface!)

If you concentrate on having a proper, engineered pump installation and keep your fluids as clean as possible you shouldn’t have to worry about such things as cavitation. If I’m able to take a saturated cryogenic fluid to 3,000 psig in one stroke, you should have a picnic with a supercooled liquid. I would be more concerned with maintaining a good mechanical running condition on the pump.

 

[stanier]

Reference The Reciprocating Pump by Miller

 

[DiaphragmPumps]

1)There wouldn't be a lot of point having a pump, if the pressures were the same or the inlet was higher than the discharge.
2)Glycerin filled gauges are ideal for filtering out the pulses.
3) Acceleration Head is not a term I have heard, but symptoms come under many titles. Are you referring to the pressure spike caused by the shock wave of liquid coming to a sudden halt and then starting to move again?

If not then sorry stop reading

If you are then consider using a suction stabiliser to remove the pulses from your pump inlet, reduce cavitation and improve the working life of your pump.

http://www.blacoh.com

 

[Alanator]

I get the feeling most of the replies stemmed from an assumption that the booster pump and reciprocating pump were one and the same. I should have stated that the booster pump is a centrifugal pump, which feeds a reciprocating pump. It's the pressure loss between the two pumps I'm curious about, as compared to the loss between two centrifugal pumps in series (assuming identical flow rates in both cases).

Sorry about the confusion.

 

[checman]

Alanator; The pressure drop you might see would depend on how flat of a curve the centrifugal pump and the capacity of both pumps. If the feed pump curve is flat and the capacity of the feed pump exceeds the reciprocating pumps capacity the pressure may not fluctuate much. The glycerin filled gauges will dampen some of the pulsation so the sudden change in flows may not be fully shown on the gauge. When you say inlets is the centrifugal pump feeding more than one pump on a header?

 

[djack77494]

Alanator,
The concept of acceleration head is an important, though often overlooked, concept. For a simple, single stage piston pump, to pick an example, you start with a static volume of liquid in the suction pipe. You must (nearly instantaneously) accelerate that fluid to its maximum velocity. That doesn't occcur for free. The cost is head loss due to acceleration and is expressed in the GPSA book as length x average velocity x speed (rpm) x C (factor=0.2 for single acting duplex) / K (factor depending on the fluid = 1.4 for water) / g (32.2 in Imperial/FPS units). Besides the hydraulic losses (which should be calculated at the maximum and not average flowrate), the above equation shows the importance of having minimum length suction lines for reciprocating pumps. (Calculation of the acceleration loss is also important on the discharge side of the pump.)
HTH,
Doug