Reciprocating pump NPSHa 1

[AndreChE]

Dear all

I'm having some difficulties to understand what in reality is acceleration head.

I'm designing a system to pump hydrocarbon (gasoline) to another header with a reciprocating pump.

Without acceleration head calculation I have a NPSH of 84m. Considering it I have -5m because the line lenght is 160m.

I was reading API 674 and it is stated a formula for "short suction lines". What is a short suction line?

Should I consider all the line lenght as suction line? Or only the inline suction of the pump?

This phenomena is quite new for me and anyone believes the pump will cavitate...

Suction pressure: 5.6 barg
Vapour pressure: 0.67 barg
Density: 719 kg/m3
Friction losses: 100 mbar
Line diameter: 2 inches

Lenght of "ALL SUCTION LINE": 160m
velocity: 0.27 m/s (flow of 2 m3/h)
pump speed: 200rpm
c = 0.200
K = 2

ha (acceleration head) gives me 89m, reason why NPSHa is -5m, and according to this pump will have strange behaviour.

Pump suction valves just opens, not sucks liquid into to plunger. Isn't the liquid pressure enough to beat this pressure? Really don't understand this.

Regards,
AndreChE

 

[BigInch]

Acceleration head is a necessary evil in using recips. Due to the nature of the piston moving back and forth, the fluid inside the pump is stopped when the pump begins its forestroke and accelerates from zero velocity to maximum linear piston velocity as the piston pushes it along. That acceleration at first creates a pressure reduction as fluid velocity is increased, which requires some additional compensation in the form of a slightly higher NPSH requirement when compared to centrifugal pumps. The same effect is seen at the discharge of recips in the form of pressure pulsations, when the piston velocity and fluid velocity is reduced and converted into a slightly increased pressure bump wave leaving the piston.

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[AndreChE]

I agree with you when you say "requires some additional compensation in the form of a slightly higher NPSH requirement when compared to centrifugal pumps".

Analysing API 674 and Pump Handbook, due to the lenght of the line the NPSH get negative which I don't understand (don't believe!!!) because acceleration head calculation gives a loss of 89m!

 

[BigInch]

Assuming your K factor includes the density effect, yes it should be around 88m.

No its not a negative result, the formula result should be limited to a minimum value of 0 psiA/barA plus the vapor pressure of the liquid. The value of zero is saying that the 2" line can't provide the flow needed at maximum piston velocity. A break in the supply liquid will occur, creating vapor space, and you will have only the vapor pressure pushing any remaining liquid into the pump, which won't be much.

Its a very long supply line. You need to put in a reservoir (tank, or "accumulator") before the pump and reduce the length of that supply line.

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[BigInch]

The root of the problem comes from accelerating each intake of fluid to maximum cylinder velocity in 1/100 min... on a simplex pump, only 1/2 the stroke is effective. If I'm not wrong here, I get a cylinder size of about 2" diam with a 3" stroke, so it makes for an average cylinder velocity of 1.8 ft/s (simplex pump) and its a sine wave, so dividing by 0.707 gives a max velocity of 2.5 fps at 1/2 cycle. So on the backstroke you must be pulling fluid into the cylinder at 2.5 fps. Basically You have to take a 3" to 4" bite of fluid out of the suction line every OTHER 0.15 seconds (starting from velocity of 0 ft/sec), or about 30" (0.76 meters) in a full second. That takes an average kick (or impulse) on that 3 or 4 inches of fluid of about 4 or 5 lbs which effectively has to be supplied by the suction line alone on demand. Its the IMPULSE force that the suction line must supply, so its not a steady state head loss over the length of the suction line that you're really looking at there. When the equation goes to zero or less, you're out of impulse force.

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[25362]

Since reciprocating pumps are surge-producing machines they need sufficient energy to overcome NPSHR, pipe frictional loss and acceleration head.

Acceleration head in the suction line can be reduced by installing a pulsation dampener (Pump Handbook by Karassik et al. McGraw-Hill).

Here's an excerpt from thread407-150618:

"djack77494 (Chemical) 31 Mar 06 16:00

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"

 

[BigInch]

Hmmm ... "a surge producing machine". What a great way to think about recips. That is exactly what I was trying to say, but so much less eloquently than djack has.

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[AndreChE]

Ok, I understood everything except the most important.

With a NPSH of 84m (as centrifugal pumps) and a calculated acceleration head of 89m, my pump as pulsation dampener. For a 4m required NPSH will the pump work without vapour space or not?

Does in fact this acceleration head reduces my +84m NPSH?

 

[BigInch]

It reduces your effective NPSHa.
Looks problematic.
What kind of pulsation dampner are you using and where is it?

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[AndreChE]

We have pulsation dampener in the suction and discharge of the pump.

 

[AndreChE]

In fact I was again outside in the plant studying possibilities and then I realised the obvious...

In these 160m, I have an heat exchanger that cools down my gasoline to be pumped. This HE works in fact as a buffer and the suction line lenght I should consider is not 160m but around 30m. With 30m, NPSHa considering acceleration head is +84-16m = 68m when the required is +4m.

The HE and both pulsation dampeners prevent this high acceleration head loss.

Am I right?

 

[BigInch]

It might hurt more than it helps, depending on its dynamics. I would try a good pressurized volume reservoir located as close to the suction flange as possible with as large a diameter pipe from that reservoir to the pump as you can possibly use. A reservoir breaks the suction piping feeding the pump into a smaller manageable length, allowing you to reduce the 160 m length to say 4 to 6 meters.

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[MJCronin]

To all..

Pulseguard and others size thier products based on the pump operating and design parameters.

Basically, you enter thier website with your pump defined and they suggest inlet and outlet dampeners.

http://www.pulseguard.com/pump-pumps/plunger-pump/packed-plunger-pump-dampener-prices.htm

There are a bazillion types offered for allkinds of service.

http://www.pulsation-dampeners.com/

-MJC

 

[BigInch]

Andre, my last post went in exactly at the same time you wrote yours, so I didn't see the bit about the heat exchanger.

Yes that probably helps and you should use 30m length with your dampners. Good luck.

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[dcasto]

Most larger PD pumps have flanges on each end of the valve boxes (or header). I've added the pulsation dampener on one flange and the piping on the other.

 

[djack77494]

For a pulsation damper to be effective, it must be a "break point" in the pulsating flow. Let's look at a simple simplex piston pump. The flow into the cylinder of the pump looks like a series of half sinesoidal curves. Half the time there is zero flow into the pump. The rest of the time, the flow starts at zero, rises to the peak flow, then falls back to zero.

Let's now look at the pump's suction line. That very same flow curve continues upstream of the pump until it is disrupted. I think of that as occuring at the point where the system stops being liquid-filled. An accumulator or flexible bladder device is effective for this purpose, since it substitutes a compressible fluid (the gas) for the liquid incompressible fluid. The mass, momentum, and kinetic energy changes of the gas are extremely small relative to those of the liquid, so the pulsations are essentially "broken" at that point.

Andre, I do not think your heat exchanger functions as a pulsation dampener, since it does not appear to contain a gas or two phase mixture. It may "look" like nothing more than a "wide spot in the piping" as far as the hydraulics are concerned. There may be several velocity changes as you move into and through the exchanger, but it still looks like a piece of pipe.

BTW, I'm just now paying enough attention to this thread to realize you're talking about a gasoline pump. Gasoline behaves much better than water (for example) in terms of having a small compressibility and a range of vapor pressures and will "forgive" some ventures into the fringes of good design practices. Despite this, I think it best to consider it as incompressible.

Doug