Which Is Better-Recycle or Variable Speed?

[Thealanator]

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?

 

[dcasto]

since when does BEP have anything to do with vibrations, deflection, leakage, etc.... BEP means its highest effiency. So what if you run a pump at 53% effiency vs 71% for a few hours. You just need to make sure you keep the pump above its minimum flow. In that case (and I believe that is what you really meant) do the economics. Since you only operate off point during short time periods, there will not be a payout of the VFD, but you need to run the economics.

 

[iainuts]

What is the best way to keep the centrifugal pump at its BEP in order to prevent vibrations, deflection, leakage, etc.?

dcasto responded:

since when does BEP have anything to do with vibrations, deflection, leakage, etc.... BEP means its highest effiency.

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.

 

[Thealanator]

Thanks for the comments.
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?

 

[dcasto]

iainuts, You are on point with the LIN vaporizing. I was only thinking about a point where they were pumping downstream only and that the centifugal pump was chilled. I wouldn't even turn the pump on until it was chilled and there was no vapor from a vent valve downstream of the pump. Then I'd turn the pump on and use it to push through the PD pump and out a vent downstream of the PD pump. This is when a recirculating valve on the centrifugal would be used.

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.