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Sulfur Pit Fires and sulfur pumps

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This topic contains 13 replies, has 4 voices, and was last updated by  Jeanne rameau 9 years ago.

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  • #2809

    deloresbx18
    Member

    Does anyone else out there have a problem with unexplained pit fires, or SO2 spikes in their incinerators?  As best we can tell with the information we have (no in-pit instrumentation) our vertical line shaft sulfur lubricated pumps are somehow causing pit fires.  We suspect it may be caused by the upper bearing of the pump running dry, but can’t explain how the pump can keep running through the fire and how snuffing steam makes the problem go away, at least temporarily. Ie, we’ll get an SO2 spike in our incinerator stack (where the offgas from the pit is sent),  initiate snuffing steam into the pit, and reduce SO2 concentration.  After ~15 minutes we take snuffing steam out and the SO2 is back to normal, and the pump keeps running.   We have seen where the pump will trip out before, during, or after a pit fire and we’ve seen a locked up pump relate to a fire.  What I can’t explain is if it is indeed caused by a hot bearing, how does it cool off such that another event doesn’t occur?
     
    Does anyone have a bad experience with iron sulfides causing repetitive fires?  Could exposed steam coil piping or old degassing boxes produce sufficient iron sulfides to cause a pit fire?  Wouldn’t that require the steel to be submerged in sulfur and then exposed to oxygen rather than at a relatively constant level?
     
    We are in the process of modifying our pumps to prevent them from being able to run dry, but I am wondering if there are other factors at play that someone else has previously identified. 
     

  • #5845

    Jeanne rameau
    Participant

    In your pits do you have any air or n2 sweep? Are your vapors routed directly to the incinerator?

    With no pit instrumentation (Temp, Pressure, level) you are a bit in the dark. If you are running your pits dry, that can trigger fire by sulfur dropping down in the pit and causing a static charge.

    Your jacketing steam? What pressure do you run at? 50#? 70#? 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?

    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.

    With a fire in the pit, it would make sense that you would have high S02 numbers at your incinerator(burning sulfur = S02) and then after introducing the snuffing steam, your numbers go back down. Your not alone though, a lot of sulfur folks have had experiences like this.

    Here is a link to what I think is a pretty good website:

    http://www.sulphuric-acid.com

  • #5844

    Pete Bisila
    Participant

    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.

  • #5837

    Anonymous

    Pete, It sounds there are quite a few variables that could be adding to your issue. First and foremost, you mentioned the addition of your degasser and rerating your pumps for 1800 to 3600 RPM. Can you work backwards and bypass the degasser for a time (probably not with environmental regs) and/or pinch back on the discharge of your pumps? Maybe your pits are running dry? Can you increase your bushing clearance a bit more? As far as the steam jacketing, do you think 35# might be a bit low? Just throwing some things out there.

  • #5836

    Pete Bisila
    Participant

    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.

  • #5809

    Pete Bisila
    Participant

    SRS- just another point of interest that adds to our confusion.  We have two separate sulfur pits with the same length and style of pump, but different discharge pipe size and different numbers of intermediate bearings.  We have one set of pumps that were originally designed for 1800rpm operation that we installed 3600rpm motors on to increase the discharge head with a 2X3″ jacketed discharge line. We have another set of pumps the exact same length, but with 3X4″ jacketed piping, designed for 3600rpm with 3600rpm motors on them.  The key difference between them is the number of intermediate bearings on the pump shaft.  The original 1800rpm pumps have one less intermediate bearing than the 3600rpm pumps.  We are having a lot of problems with the pumps designed for 3600rpm and virtually no problems with the ones converted from 1800 to 3600.  We believe the problems are related to the lubrication of the pump, but haven’t figured out how to completely resolve the problem yet. 

  • #5801

    Jeanne rameau
    Participant

    Questions – are all your problems you described in your prior notes caused by your original pump or the new one? Or both? Were modifications made by your reliability group or the vendor?

    If possible, bring the vendor out to the field and have them check it out.

  • #5798

    Pete Bisila
    Participant

    “Questions – are all your problems you described in your prior notes caused by your original pump or the new one? Or both? Were modifications made by your reliability group or the vendor?

    If possible, bring the vendor out to the field and have them check it out.”
     
    We are having the most problems with a pair of our new pumps, and vitually no problems with the pair of original pumps with faster motors.  We have a third set of new pumps that are in between. All three sets are slightly different.
    We started with the factory clearances in 2005 and immediately had problems with the pumps tripping out and locking up.  Our reliability group recommended opening up the clearances on the pumps and installing trap bypasses, and that has worked for the third set of pumps.  We had the vice president of engineering from the manufacturer onsite to look at the setup and talk about the pumps.  They didn’t have much to offer in the way of solutions. 

  • #5678

    Mike Corron
    Participant

    When we put in new Degass units on our sulfur process we experienced pit fires for a while with the new pumps associated with the new Degass.  After a week or two of this occuring the problem disapeered?  Not sure what was causing it?

  • #5677

    Pete Bisila
    Participant

    Surfer-thanks for the input.  We’re finding that the high speed (3600rpm) pumps associated with the higher head requirements of the process change have some reliability issues.  From our work it appears to be related to loss of lubrication in the upper intermediate bearing.  Others are reporting shaft stability/ excessive vibration problems as well. The cure appears to be the same with a design change to the bearings and clearances.  We have a plan forward, but are still in the testing stage so I can’t confirm that the problem is solved.
    If your problems appeared and then seemingly went away, I would not be surprised if you find them return.  We have managed to run the pumps for a little over 12 months with no issues, and then suddenly have a rash of events, with no apparent changes in the system.  I suspect sulfurcrete/solids lodged in the upper bearing clearances tends to starve the bearing, forcing the lubricant out the lower ports. 
    Do you have cast iron/steel bushings or the carbon filled/graphite bushings?

  • #5673

    Anonymous

    To be honest I have no clue.  I am a console operator and have no control or input from the engineering/purchasing side.  If we have any more pit fires at least I will be able to give some imput on your problem and narrow down a solution.

  • #5670

    Mike Corron
    Participant

    Ooops, I wasn’t logged in [8|].  The above post was from me.

  • #5618

    Anonymous

    Sulphur hydroxide does not exists. There is a compound H2SO2 (formally S(OH)2), but it is an unstable acid known only as diluted orange-yellow water solution. This acid is called hyposulphurous acid.
    andwiki answers

    SO2 + H2O ==> H2SO3
    then
    tends toward dilute sulfuric acid
    or compounds with ions.

    What will happen, especially if the system operates, is any sulfur and thus dioxide will react with the steam until there is some fluid usually a trend toward sulfuric acid highly diluted
    and then when the system is switched down (cools) any residuals will reform and adhere films of slight acid which concentrate during evaporation and parts of sulfur, the pitting may be due to the steam, the sulfur just utilizes this as a location point as there is a hollow and grip surface where the system cannot rub it off. The sulfur water relationships require much equation arrangement with other compounds available, iron is a good one and you end up with H2S and varied gaseous extremes. Obviously it can just burn off, not good for your equipment over a long period of time. In theory lubricating fluids if feasible should clear such residues, in practice they may not and may be difficult to run with the equipment, some of those can lead to fires in unsuitable circumstances. Be careful of the combination of fluid products resulting. You may have seen fire start on lathe and metal swarf start to burn in both School, Technical College and industrial sites, smoke and everybody has to leave. Architects do not like steel, it expands concrete and the whole system will burn in high grade tube, tower fires, chimney heat as well as warp; all residues engineer’s oil rag and wastes and left over substances in buildings may catch alight in a fire damage and add to fire brigade (battalion) nightmares. Kings Cross underground. Sulfur substances are common migrated accumulated growths in waste materials, chemicals form by migration, hence igneous rocks. Wear and tear on surfaces may mean the equipment needs renewal, this can get much worse with time and no action. My understanding not having taken technology onward but from physical biological chemistry and physics is that sulfur related damage as you can observe may be severe. You may have steam pitting, the sulfur just uses this as adherence, that helps none in the wear of your equipment, on fine surfaces and particular materials it will rot the pits.
    You are dealing with something at the opposite end to Tundra freeze thaw, hot cold similar to a Tropical forest but working much faster and soils will shift forest materials in accumulative rapid exchanges, the equipment you build/ operate is after all a soil rock derivative and OH and O are its main exchange activities. Mike Stagg BScWales MSc hydrology soils Geology CBath Technical School, in the days when the institute was the University of Technology at Bath forming, before I went UCW soils hydrology, hence all the interest and materials with architectural students later.    Best wishes, the tech people will answer your resolution as best practice, in soils we keep away from sulfur sites obviously.  

  • #5369

    Pete Bisila
    Participant

    Update on the pit fire issue. Has anyone else experienced a pit fire attributed to static discharge in the pit? If so, how did you determine the cause and what did you do about it? As rare as it may be, we feel it is quite likely that one of our problematic pumps may indeed be starting fires due to static charge off the lubricating flow. (Unless someone else can postulate an ignition source that occurs within 45-60 minutes of the installation and start up of a brand new pump???) There is a fair amount of literature avaialble referring to the potential to build a charge, but I have yet to see it be associated with the pumps. Anybody?…..Hello, hello, hellooooooo……….

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