Distillation Tower Feed Hammers

[sshep]

Hello Friends,

I have had this problem possibly a dozen times in a 25 year career, but this time I will ask for some engtips advice.

A small distillation column was converted to a new service. The feed point is a 6" inlet with an impingement plate at the inlet of the feed tray. The flow in the new service is only 4cum/hr (2" line ties into existing 6" feed line), much smaller than original design. The feed line routes vertically down (as 6") before elbow into the column feed nozzle. The feed temperature is ambient. The tower runs approx 200C. The service is short campaigns, and does not justify feed preheat. The hammers can be pretty severe, my theory is that vapor backs into the feed line and collapses against cold feed causing hammers, but it could be happening inside the tower.

Fixes in past projects have included injecting a small amount of N2 with the feed (quick crappy fix), or redesign of the feed entry point in the case of feed elbowing down inside the tower. I fully expected the situation to be inviting a hammer problem, and it does. The designers did not address it, and now the plant must sort out the problem.

Does anyone have any suggestions on this subject? I would really favor ideas that do not involve welding on the vessel pressure envelop.

best wishes,
sshep

 

[CJKruger]

sshep, I agree with you. This is a reasonably well known problem, especially for reflux return lines.

In your case it is worse, as the liquid feed flow down before the feed point, and the velocity is below self venting flow.

Propably the easiest solution is to reduce the feed pipe inside the tower. For example, reduce from 6" in steps down to a 2" or 1.5" tee. Try to keep exit velocity above 2-3 fps (to prevent backflow) but below a rho*v^2 of 1000-2000 (imperial units) to reduce turbulence in tower.

This assumes the feed location is rough 60% of tray spacing above tray and not into the downcomer.

Cilliers

 

[StoneCold]

sshep
Maybe you could get away with putting a goose neck (P-trap) in the piping right before the entrance to keep the incoming pipe liquid full.

Regards
StoneCold

 

[sshep]

Thanks for the advice from both of you.

After some consideration, we have designed a p-trap type seal just before the tower inlet. The main competing option was a check valve which was favored by some as a cheaper fix. We didn't want to raise th inlet velocity because the tower is sometimes used as a back-up for a higher flow (original) service. I have made the p-trap flanged just in case there is some unforseen problem so that it can be revised without hot work.

I will know in January if this was the right call and give an update (and maybe a star).

best wishes,
sshep