3 replies to this topic

[ecaliani]

Dear fellows,

I am assessing a project where it was proposed the association of one centrifugal pump (operation point:160m3/h @ 5.22 barg ) in series with a positive displacement pump (NPSHr = 18 mcw, flowrate: from 25m3/h to 150m3/h with VSD) for pumping diesel. Please see the attached file for a more claer description.
Have anybody ever seen such kind of association?
Does anybody have any advice to be included?
Do I need to take into account the effect of acceleration head for in the calculation of NPSHa of the reciprocating pump?
I am not confident this system will work properly and I am kindly asking any suggestion from your previous experience.
Regards

Ecaliani

Attached Files

•  Pump association.JPG   17.85KB   30 downloads

[djack77494]

Please see the attached file for a more claer description.

The thumbnail does not provide a clear description. I see only one pump. Does the right hand side rectangular box represent the PD pump? It's definitely NOT clear.

Have anybody ever seen such kind of association?
Does anybody have any advice to be included?
Do I need to take into account the effect of acceleration head for in the calculation of NPSHa of the reciprocating pump?
I am not confident this system will work properly and I am kindly asking any suggestion from your previous experience.

I have seen a booster pump located upstream of a second pump to provide needed NPSH for the second (normally) very high head pump, so I am not concerned by that. If the PD pump is downstream, then you won't have any NPSH problems. (But if you want to calculate NPSHa you do have to include the acceleration head [always].)

[chemsac2]

Section 3.5 "Displacement pump flow control" in Pump Handbook by Karassik, 3rd edition explains in series operation of displacement pump and centrifugal pump.

Extract from the book is below:

FLOW CONTROL IN COMBINED DISPLACEMENT AND CENTRIFUGAL PUMPS

Systems that involve both centrifugal and reciprocating positive displacement pumps deserve some special consideration. Centrifugal pumps are often used as suction boosters to overcome acceleration head requirements peculiar to reciprocating pumps but are rarely used to supplement flow. Some unique characteristics of each type of pump that affect the other type must be considered in the design, operation, and control or the interrelated system.

Series Operation, Suction Boost

The flow rate is always determined by the positive displacement pump capacity and is independent of the centrifugal booster pump. The capacity is therefore controlled as with any positive displacement pump.

The purpose of the centrifugal pump is to supply sufficient pressure to satisfy the suction requirements of the positive displacement pump. The centrifugal pump must be sized to provide the maximum rate of flow of the positive displacement pump at a pressure high enough to meet the NPSH requirement of the latter. With a flow-controlled positive displacement pump, speed control of the booster might be considered, as illustrated in Figure 4. The system pressure requirement can be satisfied by the positive displacement pump at all flow rates; therefore, total energy requirements may be reduced by using the less efficient centrifugal pump to develop no more than the required suction head. If the centrifugal booster pump is run at constant speed, its share of the total head requirement will increase as system capacity is reduced.

Regards,

Sachin

[ecaliani]

Thanks fellows.
I´ll take a look at the reference quoted.
Just to add: the right hand side rectangular box represent the PD pump!