[Pete Bisila]

Claus, one of our challenges is evaluating the risk of a rundown line failure versus the cost of preventing it or mitigating it.  We have a single 4″x6″ jacketed rundown line about 1/2 mile long between our sulfur pits and our storage tanks/rail loading unit.  Loss of this line would shut down the refinery.  Replacing the line/ adding a redundant line is ~$3,000,000.  It’s carbon steel piping with 70# steam tracing on the jacket. Theoretically, there are very few damage mechanisms in these services.  Loss of steam appears to be the main probable reason for loss of the line, although contaminants and poor steam quality have also been identified.  The problem becomes, how do you evalaute the probability of a failure?  What are other people doing for reliability strategies around rundown lines?

[Pete Bisila]

Can you elaborate on the skin effect tracing systems?  I’m not sure what you are talking about.  How do they work? Websites?
 
I prefer jacketed piping to steam tracing in sulfur service based on my experience.  It’s too easy to lose the insulation on traced systems.
 
Steam traps are an issue, even when considered ‘critical tracing’.  You can inspect them all you want, but they can fail the very next minute.  There are several trap monitoring devices being marketed that will continuously monitor trap performance and send a wireless signal back to a receiver that allows real time monitoring.  It does require setting up a wireless data network, however.

[Pete Bisila]

Well………we outsmarted ourselves, again.  We have a single air bubbler level transmitter in the pit that is setup 0-100% at something less than the actual dimensions of the pit. Neither one of our pit pumps would run (there’s another long story beyond that) so we were in scramble mode to get another pump retrofitted and in service.  We were over 100% on the level indicator, but we made some manual measurements of the pit depth, level of sulfur, and assumed height of the pit and ‘calculated’ how long we had before we needed to pull the plug on the feed.  Unfortunately, we were off on our calcs by about 12 hours.  I happened to notice sulfur oozing up through the roof of the pit around the steam coil piping and notified the operators.  It was a rather embarrasing day, to say the least. 

[Pete Bisila]

See my post in the waste heat steam generator thread.  It’s in the process of being a bad day as we have yet to figure out what we are doing wrong.
 
The other bad day I had was when the sulfur pit overflowed on start up after a TA because both sulfur pumps were found locked up with corrosion products/sulfurcrete.
 
Smaller bad days were due to lean amine leaks caused by extreme localized corrosion of the heat affected zone of butt welds in hot lean amine piping.  360 degree corrosion of the HAZ was found, almost like it was machined.
 
Also have had unplanned shutdowns due to knocking the trays out of one side of the amine regenerator tower.  Tracer scan on one side looked liked the tower was fine.  Opened up the tower and could look from the top to bottom on the other side of the tower. Lesson- scan both sides of the tower when you have split trays.
 
Lean/Rich Amine exchanger fouling also caused an unplanned shutdown (in January in Minnesota, naturally).  Fouling composition was mostly iron, sulfur, silica, and alumina/aluminum.  Still not sure where it all came from.
 
Life is good. There’s no end of lessons to learn in the yellow cokers!

[Pete Bisila]

It is understandable why refiners would want to filter on the rich amine side as that is where most of the crud is coming from.  The lean/rich exchangers tend to be the most effected equipment when deposits foul the tubes and you can no longer cool your lean amine.   As SRS mentions, rich amine has PSM issues due to the entrained H2S.  It is a lot simpler to deal with lean amine filter cake without the H2S and fresh air handling.  We run a 10% slipstream off the lean amine side through a precoat filter with diatomaceous earth and have decent results with run length in the system.  We have had issues with excessive corrosion of the carbon steel lean amine piping and equipment when it is operated above 140F, but cooling or upgrading to stainless seems to take care of the problem.

[Pete Bisila]

Thanks for the input,
We do have a level indicator in the pit which I didn’t mention. It’s an air bubbler type device that seems to work.  Our level stays fairly constant at around 45%.  We also have a flow meter at the degas tower that reads a combined flow from two separate pits, but it is also fairly steady. I don’t believe we are running the pit dry or cavitating the pumps.
 
You are correct in assuming I cannot bypass my degassing unit, at least not for long enough to make a difference.  The pumps meet their hydraulic expectations, and have at times run for up to a year with no issues.  On the opposite hand, we can have pit fires three days in a row that appear to be related to one particular pump at the time.  I’m keeping an open mind as to other potential causes of pit fires/SO2 spikes, ie iron sulfides, static discharge, globs of sulfur fuzz hitting the incinerator, etc, but with the build up in the pump bearings it seems likely the pumps are the culprits.  We’ve installed pumps with a 0.020″ clearance and taken them out with a 0.002″ clearance due to this “plating” in the bushings. 
 
We have tried several bearing clearances.  Opening the bearing up much beyond 0.020″ total clearance results in high vibration of the shaft.  Closing them too much results in starving the upper bearing.
 
35# steam is the ideal temperature for pure sulfur.  Granted our sulfur is far from pure, but we have seen problems with the pump tripping out when the jacket steam gets up around 50-55psig.

[Pete Bisila]

Our seal legs are set to lift at between 9 and 15psig, depending on the unit.
 
We have high pressure critical alarms that alert the board operator of the situation.
 
Blowing a seal leg would be considered an environmental release of sulfur and H2S.

[Pete Bisila]

SRS- thanks for the reply.  Here are the answers to your questions, along with some additional information.
 
“In your pits do you have any air or n2 sweep? Are your vapors routed directly to the incinerator?”  We have a continuous ambient air sweep pulled across the pit by a steam powered eductor with the gases routed directly to the incinerator.  We also have the offgas return stream from our out of pit degassing tower returning to the pit vapor space.  Our problems appear to have started at the same time we upgraded the pumps from 1800 rpm to 3600 rpm and moved the degassing function out of the pit. 
 
We are finding a very hard, very well adhered metallic build up on the internal clearances of our pumps in the upper intermediate bearing.  This material is closing up the bushing clearance by up to 0.018″.  We have to machine the buildup off the bushings. 

“Your jacketing steam? What pressure do you run at? 50#? 70#? ”  We have the jacket steam to the pumps set at 35psig and monitored on operator rounds 4x per day.  We also have bypass valves open around the condensate trap on the discharge to keep the steam in the jacket from superheating.
 
 
“If you had a TI, that would be helpful in checking if your pumps are seizing from running your jacketingsteam at the wrong temperature. Without a TI, take a heat gun and perform a temperature survey?”   The jacket steam is confirmed at 35psig.  The bulk temperature of the sulfur in the pit is unknown.  We have a project in the works to add instrumentation to the pit and pumps, but as projects go it’s out a ways.

“When you take your pumps out of service for maintenance, do you follow a warm up procedure so it doesn’t seize up? I would work with your sulfur pump vendor and double check if you are following the correct reliability practices. ”  Yes we do.  We don’t have a problem with the pumps starting up, these fires occur after the pumps have been running awhile.  Sometimes after a year of good service, sometimes after a week.  We have been in close contact with the pump manufacturer and still do not have a resolution. We are in the process of making some modifications to the pumps, but we have not yet found the right set up.

[Pete Bisila]

If you are able to degas your sulfur to meet EPA limits, you may be able to store it onsite. Otherwise you will definitely have an SO2 emissions exceedence by exposing tons of molten sulfur to atmosphere.  As previously mentioned, check with your local regulatory agency before changing your operation.  Elemental sulfur by itself is generally not considered an environmental problem as it is very slow to dissolve in water.  There may also be short term vs long term storage requirements to comply with that may require you to remove your emergency pile.

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