Control Valve versus Variable Speed Pump 1

[Khansahib]

Gentlemen I have to maintain Normal level in a production separator. I am using a variable speed pump to draw the liquid and pump to a high pressure (from 30 barg to 100 barg). Normally, the liquid flow will remain constant with the exception when pigging operation is in progress. To control the level an LCV with a constant speed motor is also an option. What are the merits in using either of the options.

 

[unclesyd]

There was a recent thorough and lengthy discussion on the subject.

Checkout the following thread by clicking on the link.

thread407-260203

 

[JLSeagull]

Begin by defining a normal level.

It is common for people to know a target value for temperature, pressure, perhaps flow and sometimes level. think about the difference between a steam drum and a suction scrubber. Instead of a normal level I like to think in terms of the normal range for level. Often starting and stopping a sequence of pumps can accommodate a level applications. Is there a specific target level, do you need to maintain sufficient level for the npsh of the outlet pump etc.? With either system perhaps you can establish a level-flow cascade to maintain the level.

The discussion regarding a valve or VFD seems moot if you already have a variable frequency drive. Are you considering a control valve to replace the VFD or for a similar application?

 

[davefitz]

Others can provide a more complete analysis procedure for each individual case. But a pump coupled to a VS drive will have flow proportional to rpm only if the required developed head varies by the square of the rpm; if the required developed head is a constant value, then there is no value in the VS drive, as the available developed head will be less than required for all rpm < 100% design rpm.

In the worst, case, the pump is pumping liquid up to a tank at a fixed, higher elevation, such that the required developed head is nearly cosntant irrespective of flow. In such a case, a constant speed pump serves well.

In the best case, the pump is feeding a piping system which has a developed head which is exactly proportional to the square of the flowrate, and all developed head is related to frictional loss in the piping system. In that case, a VS drive makes sense.

Of course, the economics of what sort of VS drive to use is another matter- VFB seems to be most economical below about 200 HP, while hydraulic VS couplings become competitive above 200 HP.