variable speed centrifugal and reciprocating pump

[Alanator]

Does a variable speed centrifugal pump make sense as a booster for a reciprocating pump? I believe some of my colleagues are proposing such a system. Perhaps they think that a control valve and return line are unnecessary, or they believe the centrifugal pump can be made to operate close to its BEP.

I'm thinking that once the flow rate of a reciprocating pump is set (by its speed), that's it. One cannot send any more fluid than the pump will take and if one sends less, the reciprocating pump will cavitate. Looks like some kind of return line is needed.

Also, since the flow rate to reciprocating pump should remain constant, is it fair to say that the discharge head has a largely static component? I understand that variable speed drives are not worth using if the load consists of static head.

Thoughts/insights are welcome.

 

[hydromech]

The only thing I can relate to this are some Towler hydraulic axial piston pumps that were boosted by a single centrifugal pump.

There were 4 pumps each delivering 500+ litres per minute at 350 BAR.

The slippers and the piston were not tied together so the pump could not pull oil in.

The centrifugal pump ran all the time at a fixed speed. The piston pumps were also fixed speed.

Thinking about it...why would you have a variable speed centrifugal pump with a fixed speed reciprocating pump?

The demand of the reciprocating pump is known. If you overspeed the centrifugal pump the flow will drop, then the reciprocating pump will cavitate.

As I understand it, it is better to have a fixed displacement pump and run it against a closed head. When the reciprocating pump starts up, the pressure in the centrifugal pump will drop off and pump will run along its efficiency curve.

I would imagine the max flow rate of the centrifugal pump should be about 10-15% above that of the reciprocating pump just to allow for any drops in performance.

Hydromech

 

[BigInch]

If you have a variable flow requirement, it can make sense to me.

The idea is to boost inlet pressure to the recip. If you need to do that at all recip speeds covering a wide range of flows, a centrifugal upstream could certainly supply a boosting pressure over its operating flow range.

The centrifugal could do that using either a fixed speed or variable speed drive.

If using a fixed centrifugal speed drive you will deliver flow to the recip according to the one curve the centrifugal has at that speed. If the pressure is too high for the recip suction for some reason, (adding that to the recip's suction pressure would exceed the inlet pressure rating or casing pressure or discharge pressure of the recip) you will have to control it. If you control it, you will be forced to waste some energy.

If you use a variable speed drive for the centrifugal, you will be able to adjust the centrifugal's curve hopefully to make a closer match to the recip's suction requirements. If designed correctly, you will probably be able to keep pressures and flows within the recip's requirements and NOT need a control valve and thus you will not waste the energy that the control valve would consume. That is the cost benefit that counters the cost of the variable speed drive. Compare those for the life of the equipment and see if it makes economic sense too.

BTW ..never run a recip against a closed valve without a full flow safety relief between them.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[hydromech]

For clarification, I meant to run to the fixed displacement centrifugal pump against a closed head.

Things go bang if you run a fixed, positive displacement pump against a closed head without a relief valve installed.

Hydromech

 

[JRLAKE]

I have done this before but not without problems. I recommend using an accumulator, like a diaphragm tank, between the pumps to "smooth" out the pressure and flow spikes.

A reciprocating pump will cause pressure and flow fluctuations in both suction and discharge lines. If you are controlling the centrifugal VSD with pressure or flow, the VSD will have problems chasing the signal.

 

[Alanator]

Thanks for the insights. I had not thought about the pulsations. What makes this proposal more interesting is the fact that the fluid will be liquid nitrogen and the flow rate will be somewhere in the neighborhood of 25 GPM. The downstream vaporizer will not handle much higher flow.

 

[dcasto]

the cost of the vfd, controls and such on such small gains on a 25gpm systems says, don't install a VFD.

 

[BigInch]

Right... Probably cheaper to replace the existing pump with a constant speed centrifugal, unless you need a high pressure ratio. You've got a constant flowrate.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com