Which Is Better-Recycle or Variable Speed?
Which Is Better-Recycle or Variable Speed?
(OP)
Consider a centrifugal pump acting as a booster for a reciprocating pump operating at a fixed flow rate. The ideal is that the centrifugal pump operates at its BEP and provides the desired NPSP (a).
Now suppose the reciprocating pump/lines are being chilled down before starting the reciprocating pump. Flow rate is much less. What is the best way to keep the centrifugal pump at its BEP in order to prevent vibrations, deflection, leakage, etc.? Should one run excess liquid through a recycle/diversion valve or operate the centrifugal pump at a reduced speed? When the reciprocating pump starts I can imagine a quick transition between reduced and normal flow rates. Which method responds quickest?
Now suppose the reciprocating pump/lines are being chilled down before starting the reciprocating pump. Flow rate is much less. What is the best way to keep the centrifugal pump at its BEP in order to prevent vibrations, deflection, leakage, etc.? Should one run excess liquid through a recycle/diversion valve or operate the centrifugal pump at a reduced speed? When the reciprocating pump starts I can imagine a quick transition between reduced and normal flow rates. Which method responds quickest?





RE: Which Is Better-Recycle or Variable Speed?
RE: Which Is Better-Recycle or Variable Speed?
Yes, agreed. The problem has nothing to do with vibrations, deflection, leakage, etc....
However....
Some cryogenic pump manufacturers, most notably ACD, have a 'system' in which a centrifugal pump is installed immediately upstream of a triplex recip. The centrifugal is there because the NPSH of those triplex pumps are so high. This is partly a suction valve issue and partly a line size/pressure drop issue.
Anyway, recirculating liquid nitrogen (LIN) around a centrifugal pump will fairly quickly result in boiling of the LIN and cavitation of the centrifugal. I don't believe there's sufficient refrigeration to cool down the pumps while recirculating. As far as I'm aware, the LIN is forced through the triplex and dumped. To reduce flow, I believe the most common method is to backpressure the centrifugal using a valve.
RE: Which Is Better-Recycle or Variable Speed?
I can understand not wanting to run liquid from the centrifugal discharge back to the suction, that would add heat. Dumping the liquid somewhere else makes sense.
I am confused about some of the statements, though. I thought that running a pump off its BEP would cause imbalances that lead to shaft deflection. Restricting flow would move the operating point to the left along the pump curve away from the BEP. Am I missing something?
RE: Which Is Better-Recycle or Variable Speed?
On a huge, say 2000 HP centrifugal pump, there maybe a criticle speed or criticle point on the pump curve. But you only see those criticle points on older units.