Pumps in series, does maximum suction velocity apply?

[philruh]

With a multistage centifigal pump, hydrocarbon, NPSHr 18, SG .5, being fed from another pump (two pumps in series). Does the maximum suction velocity guidlines apply to the second pump. If so why. Start up? Bubbles? Erosion?

 

[TD2K]

For the second pump, I'd be more likely concerned with maximum liquid velocities (10 feet/sec or so) rather than the more typicaly pump suction limits (less than 3 ft/sec for a bubblepoint liquid).

NPSHA won't be an issue with any reasonable head from the first pump.

 

[tstead]

If you have two pumps in series with no recirc piping, then the flow will be the same through both pumps....you know, mass conservation and all...

If the suction piping is the same in both pumps, then the inlet velocity will be identical....

Tim

 

[Slagathor]

I do not agree with TD2K as relates to the "general case". He may in fact be correct for this pump, but not correct for all pumps.

Velocity distribution is the reason you should follow accepted design practice for pump inlet piping. If you have pipe turns or elbows close to the suction of the second pump, the high velocities can cause problems even with very high pressures because you can end up with higher velocities and lower velocities at different points on the impeller. Poor velocity profiles result in uneven velocity vectors at different points. Depending on how sensitive the pump is, you may have no problems, or big problems! Consult with the manufacturer(s) and follow their recommendations. If you have an existing plant and are desinged into a corner, roll the dice.

Assuming that you can disregard accepted pump piping design practice because you have high suction pressures is not an assumption I would reccomend making on a regualar basis.

 

[pardal]

I want to keep track of this topic.

Pardal

 

[stressriser]

I think the answer to this question is "Yes". You state the NPSHR at 18. I must assume that this is for the second pump. Therefore, the NPSHA must be comming from the first pump.

With these assumptions, you have to feed the second pump with adequate flow to keep it from cavitating.

I would imagine the reason for the second pump is inadequate pressure in the system. So the first pump feeds it at a specific pressure and flow. The second pump then increases the pressure of the output. So the second pump needs to be fed with adequate flow so it is not starved.