Q & A > Question Details
We are operating a small refinery in which the crude column has three side draws with h naphtha, kero, diesel there is a kero pa and diesel pa. There was coke in the suction of resid (crude bottoms) pumps to find the cause we opened the coloumn on inspection little coke was found however the trays below kero pumpa were displaced and crumpled no possibility of adding water was observed as stripping steam was added after through purging and was invisible(super heated) steam addition However there was hammering at kero pa on start up which stopped after increasing kero pa temperature to avoid adding subcooled liquid in a sparger on low flow (at start up) the trays above the kero draw were not affected at all
Can any one throw light what are the possibility to look for?
Running the unit again without identifying the cause of accident doesn't make sense. Besides water surge, which doesnt appear to be the cause, is there any other cause?
We were running the column at 30 psig pressure (design operating) is 15 psig can that be a cause? But diff pressure was normal though slightly higher as we were operating with high naphtha yield than normal trays were operating at 50 to 60 npct flood as calculated thru simulation.
 
Answers
12/11/2012 A: keith bowers, B and B Consulting, kebowers47@gmail.com
There are several potential (and coincident) causes for what you describe. 1) Operating at higher pressure will increase heater outlet temperature to maintain bottoms cut point--and contribute to coking in heater tubes , transfer line and flash zone wash trays. Minimizing bottoms level (to reduce residence time) may help.
'Hammering' is often caused by pulling too hard on PA/product draws. The pressure drop at the entrance to the draw piping will cause vapor formation (big bubbles) which then collapse violently in the descending piping--causing 'hammering'. This can crack or tear loose nozzels and piping flanges. Careful evaluation of nozzel sizes and draw box liquid weir heights (and level control on the liquid height) are necessary to ensure the total 'head' in the exit piping is sufficient to prevent vaporization (and subsequent hammering.)
The damaged trays were almost certainly caused by water trapped on the lower trays superheating and then violently flashing. Plugged tray drain holes are a likely causitive factor. Of course, even a small amount of water left in the diesel pump around piping (anywhere-like pumps,drains,dead legs) will eventually flash and damage the trays above the diesel pump around return. Happens in a second or two. Studies have found water trapped in a dead drain can superheat to over 700F !! before violently exploding. The 1200:1 expansion ratio (water to steam) is destructive.
12/11/2012 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There are several ways that liquid water can get into a hot tower and cause damage to trays.
1. Wet stripping steam or failure to properly purge water from the steam line after start up (sounds like you have ruled this out).
2. Failure to drain all the water out of the system on start up. This can be a very challenging step. You need to make sure that you have low point drains in place at all low points and you need to keep draining water from them during cold circulation of the unit on start up. You wait until you stop drawing water to start heating up and you heat up the circulating streams slowly to avoid explosive vaporization of residual water left in the tower or piping circuits attached to the tower. This is a common problem and sounds like it might be what got you.
3. Injection of wet feeds into the tower. Sometimes transmix or light slop streams go into the crude column either directly or through a pumparound return. A slug of water in one of these feed streams could cause a problem like you have experienced.
4. Leaking exchangers - not real likely but a leaking crude vs. kero pumparound exchanger could potentially put some water back into the Kero PA reutrn. If a slug of water in crude hit the unit while the exchanger was leaking it could possibly put enough water into the kero to cause a problem. You would also be making dark kero if this were the problem so I doubt that is what you are seeing in this case.
12/11/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
One should avoid the temptation to design to yield light naphtha overhead and heavy naphtha as a side draw. Overhead temperature is too low to allow water to exit as a vapor. Liquid water accumulates in the column and ultimately causes the problems you have experienced. Water draw trays have been used, but not always successfully.