Vapor locking on Propane reflux pumps.

[PlantOp]

Hello, I'm an operator at a natural gas processing facility, and I would like to pick some of the brains of the engineers on these forums, if I could.

We are having issues with our propane reflux pumps vapor-locking when the overhead pressure on our de-propanizer tower gets too low. We are trying to reduce the pressure so that we run a higher Y-Grade feed to the tower, and thereby make more product (propane and butane). I would like to know if there is a way to reduce the pressure on the tower without vapor locking the reflux pumps. Are there any other pieces of equipment/piping we could add in order to alleviate this issue? Currently, we run about 165 psi of overhead pressure, with reflux temperature being at 88 degrees, and reflux flow at 245 gpm. The pumps are typically running at about 200 psi with these setpoints. Thank you for any assistance that any of you could provide.

 

[PlantOp]

Anyone have any ideas?

 

[Pumpsonly]

May be you should post it to the Chemical Plant design or Chemical Process Engineering forum.

 

[PlantOp]

Okay, I'll try that, thanks.

 

[JJPellin]

It could be that the pressure change is occurring too quickly. As a pump engineer, I am concerned about the Net Positive Suction Head available to the pump. With a fluid at bubble point, the NPSH should not change as the pressure is reduced. The product gets lighter, but the elevation head is unchanged. But, if the pressure is dropped too quickly, the fluid in the line and in the pump will boil and try to give up the additional vapor. All pressure changes need to happen very slowly. This is most important when the pressure is being reduced.

Johnny Pellin