Vapor-locking on Propane reflux pumps (question).

[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. If the pressure gets any lower than 165, one of the pumps will usually lock up, and we have to "blow down" the pump to get it going again. Thank you for any assistance that any of you could provide.

 

[DubMac]

What type of pumps are you using??

 

[25362]

It seems that this pump is the only one, among others, that is suffering vapor lock, true? Reasons for that could be piping (inlet side) geometry and arrangements, pump type, pump mechanical condition, etc. My suggestion: put your query to the pump engineering forum (Mechanical Engineers).

 

[Latexman]

A PFD, P&ID, or sketch might help.

Good luck,
Latexman

 

[dcasto]

More feed means more deluxe which means higher npshr one pump is starved

 

[Latexman]

More reflux yields less subcooling, which means lower NPSH_A,
Lower pressure yields less [Δ]T in condenser which yields less subcooling, which means lower NPSH_A, and
Lower suction vessel pressure (reflux drum/column) means lower NPSH_A.

So, more NPSH_R and less NPSH_A. A double whammie!

Good luck,
Latexman

 

[smCHe]

raise the level in the pump suction level, up to the max you can sustain. this is if you did not try it already

raise the cooling of the reflux condenser, this gives more margin for the pump as well

then do the other checks on NPSH

 

[SNORGY]

Depending on the type of pump, if it is a centrifugal, for example, you might look at the possibility of running a small vent line from the high point of the volute back to the vapor space of the tower or (if there is one) reflux accumulator. It might help with degassing the pump.

 

[PlantOp]

Thanks for all the comments everyone.

@Dubmac - They are centrifugal pumps.

@25362 - Thank you. I will try that forum as well.

@Latexman - I will try to put up a P & ID as soon as I can. Also, good thoughts on NPSHA vs. NPSHR. I will bring this up to my supervisors.

@smCHe - Unfortunately, due to our limited Y-Grade storage capacity, we can only cool down the condensors to a certain degree, to avoid too much liquid collection, but I will bring up lowering the amount of reflux we are using as Latexman mentioned, as we definitely could go lower if needed.

@SNORGY - The vent line is a good idea as well. I had looked into doing something like this previously, but the company's budgeting may not allow for additional vent lines to be added (even though they would probably be fairly inexpensive comparitively to other things we've spent money on).

Thanks for all the great suggestions everyone. I will try to add a P & ID to this conversation later.

 

[maddocks]

If you can post some more details, I'm thinking that the pump may be running down the curve and the NPSH requirements will go up when that happens. The other possibility is that when the pressure drops, the fluid in the pump cavitates and then vapour locks the pump. You could try rigging up a tubing vent back to the vapour line with a small valve and just leave that cracked slightly to help degas the pump.