Centrifugal and Reciprocating Pumps
Centrifugal and Reciprocating Pumps
(OP)
I have been trying to understand the flow vs. head business in the case of a centrifugal pump feeding a reciprocal pump.
Since a reciprocating pump is a constant-flow device, the flow rate in the centrifugal pump is fixed. Therefore, the centrifugal discharge head is the sum of the suction head and the differential head at the particular flow rate. If NPSP (A) is greater than NPSP (R), the reciprocating pump is happy. If the flow rate is at or near BEP flow, the centrifugal pump is happy.
Is my logic correct?
Since a reciprocating pump is a constant-flow device, the flow rate in the centrifugal pump is fixed. Therefore, the centrifugal discharge head is the sum of the suction head and the differential head at the particular flow rate. If NPSP (A) is greater than NPSP (R), the reciprocating pump is happy. If the flow rate is at or near BEP flow, the centrifugal pump is happy.
Is my logic correct?





RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
You should be right on the BEP. The centrifugal pump will not have constant flow, but it also shouldn't care what part of its TDH is split between static head and velocity head.
RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
Johnny Pellin
RE: Centrifugal and Reciprocating Pumps
Draw the load on the Combined Diagram (CD).
Draw the NPSHA curve.
Add each centrifugal differential head to the NPSHA as you transfer its curve to the CD to show the total discharge head (TDH).
Draw the family of Positive Displacement Pump's "Curves" from each Centrifugal's Total Discharge point.
Now from any flowrate, go up to the Centrifugal's TDH, then follow the Recip Family Curve at that point to its intersection with the Load Curve to see what the Recip Pump will have to do to get you to the combined system operating point.
Take flowrate 1, go up to Load Curve. Recip Pump is not rquired to run and could be bypassed. Actually a CV would need to be installed to run at flowrate 1 or less and knock the centrifugal's TDH down, or the operating point for the centrif alone would move to the intersection of load and Centrif curve (near flowrate 2).
Take flowrate 2, go up to Load curve. Recip must provide a little bit of head to run at flowrate 2.
Take flowrate 3, go up to Load Curve. Recip must provide significant head to run at flowrate 3.
To see when the system could be run with the centrifugal bypassed, draw a second diagram placing first the recip head, then add the centrifugal head on top of that.
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RE: Centrifugal and Reciprocating Pumps
Cetrifugal pumps feeding PD's is not that complicated. The centrifugal pump is selected based on NPSH available and its differential is the NPSH required by the PD at the PD's rate (heck add 100% more, it won't hurt anything. Pipe the discharge of the Cent to the suction of the PD, put a recycle valve from the the discharge of the PD to the suction of the centrifugal keep the flow constant if the feed source fluctuates.
RE: Centrifugal and Reciprocating Pumps
http://virtualpipeline.spaces.msn.com
RE: Centrifugal and Reciprocating Pumps
I am a bit confused by the diagram, but maybe I can add some assumptions to my "case":
RPM's, bore and stroke on the reciprocating pump are fixed, so flow rate cannot change (hence my confusion over a family of curves).
The reciprocating pump discharges to vaporizers and into receiver tanks whose pressure runs between 2500 and 5000 psig.
RE: Centrifugal and Reciprocating Pumps
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RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
If you recycled from recip discharge to recip suction, you would be able to hold centrifugal discharge pressure, but the high pressure liquid arriving from the recip's discharge would tend to increase interstage pressures anyway, so possible of little benefit. Hence I would look for the driving cause of the process going over 5000 psig and deal with that. If that didn't work, then yes, shutdown the whole system.
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RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
I just always go back to basics and line up whatever type of flow element curves I have with differential head references to the system curves referenced to absolute head. Works EVERY time.
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RE: Centrifugal and Reciprocating Pumps
RE: Centrifugal and Reciprocating Pumps
Then again, if the pump is 20 GPM, not much to save in energy.