Explotions at sulphur pit 2

[MDACH]

Hi,
We have a sulphur pit that we use for degassing (take out SH2 and polisulphurs), removing sulphur through centrifugal vertical pumps. It is made, nearly all, of carbon steel (sleeves, thermowelds and pumps), but also we have stainless steel in different parts (downs to heaters, and catalyst inyections).
It has an forced air circulation, that an ejector makes, and sucks the mix atmosphere to a furnace (that also works with vacuum)
Well, we are having fires inside (even explotions) the pit, at the time when we use sweep air. We detected high levels of SO2 before accident, but no high temperatures. Does anybody knows any reason in order that this phenomenon produces? SFe formation is possible, even though we are using continuesly air?. What about sulphur dust? Dead zones in our pit? Any similar expierence in your instalations?
Thanks a lot

 

[JJPellin]

Yes. We had a rash of sulfur pit fires after a project last year. We have three sulfur pits with two pumps each. We were degassing the sulfur in the pit as you are. New environmental requirements forced us to use a stop doing this. We removed the air purges through the pits. We added a sealed degassing system down stream of the pumps. This made it necessary to rerate the pumps. The pumps went from 1800 rpm up to 3600 rpm. As soon as the new systems were on line, we started having frequent fires. We also started having pump failures. At first we couldn't determine if the fires were causing the pump wrecks or if the pump wrecks were causing the fires. Our process engineers consulted with experts around the US and told us there are three possible causes of pit fires. Carbon steel in the pit can lead to iron sulfide formations that ingite the sulfur if exposed to oxygen. Static charges can build up if the sulfur is allowed to free fall into the pit or if grouding is not complete on all pumps, pipes, etc. The last cause would be a hot spot caused by a pump failure. The pits were not changed during the project other than shutting off the air purge and speeding up the pumps. We did not have any free falling sulfur and everything was well grounded. We determined that pump failures were the cause. We increased the clearance in the bushings in the pumps and the problem went away. Now we have not had a pit fire in many months. We worked with the pump manufacturer (Chas. Lewis Co.) and they helped us make some other changes, most notablly jacket steam pressure, that were also expected to reduce pump failures. I would suggest you do the following: Check all pumps, motors and piping for proper grounding. Make sure the jacket steam is 50 psi or less and saturated. Check the design of your system to make sure there is no free falling sulfur coming into the pit. Contact your pump manufacturer and make sure you are running the correct bushing clearance. I have information from at least one other plant that was having fires because of tight bushing clearance in the pumps.

 

[0707]

At least once it happens an explosion in our sulphur tank pit. It was also due to iron sulphide ignition. We also have sweeping air injection.
Our sulphur pit is under vacuum and uncondensed gases go to incinerator for burning. We use snuffing steam to extinguish sulphur pit fires.

Luis

 

[JJPellin]

I have a few updates to my earlier comments. First, I am told that the steam jacketing should be 35 psi saturated steam. A few days ago we had another pit fire. The pumps were pulled out and checked no significant problems were found. However, what they found was that a pressure regulator that lets down 70# steam to 35# for the jacketing had failed open and we were running with 70# steam in the jackets. We believe that this heated up the pumps which thickened the sulfur in the bushings. The thicker sulfur generated even more heat as it passed through the bushings and eventually, the drive motors started tripping off on high amps. The operators kept restarting the pumps. Eventually, the got a bushing hot enough to start a pit fire. We found the bad pressure regulator and replaced it. We have not had any further problems. It doesn't take much to start a fire in a sulfur pit. Hot molten sulfur likes to burn.

 

[djack77494]

JJ,
Though I like your earlier suggestion to set your steam pressure at 50 psig, I really don't think having 70# steam instead of 50# steam would create the problem you describe. I am aware of the unusual property that sulfur has to exhibit higher viscosity as temperature is increased, but don't accept this as an explanation. It would take substantially higher steam pressures to get near a problem area, in my opinion. In fact, using 35# steam would be more likely to lead to problems of not getting enough heat into your molten sulfur.

I welcome your return comments,
Doug

 

[JJPellin]

Doug,

Under normal circustances in a pipe or vessel, I agree. But in the pump we have bushing that are surrouding a sleeve turning at 3600 rpm. If the temperature of the sulfur is incresed even a relatively small amount, it becomes more viscous. At higher viscosity, there is more heat generated from shear in the bushings. This heats the sulfur up even more. At some point it becomes a runaway process that results in tripping off the motor or seizing up the pump. We had this problem even with correct jacketing temperatures. We found that the bushing clearances were too tight. But increaseing the bushing clearance from 0.014" to 0.020" the problem went away. No more pump trips and no more pit fires. I do believe that 70# steam is enough the initiate the same process.

Thanks for your reply.

Johnny

 

[MDACH]

Following my own experiences, I must agree with Johnny, we also had fire inside the pit, once our pumps were seized up (they were tight wiht sulphur in) and we tried to melt this sulphur alingning 250# steam and sulphur got fire.
Even so, we have checked steam pressure in all our jackets(that is 50#), also bushings and motor amps. Our analyser works propperly, and so our gas thermocouples.
Just aligning air and taking off snuffing steam it starts to burn (our meassure gadgets note an increase of Tª and of SO2)
I am absolutly convinced that fire is provocated by pyroforic elements we have inside. Do yo think Carbon Steel elements could cause this? Anyone have seen pits (where sulphur drops from the unit and where you degass) made of this material and whitout damages?
Thank you for your answers.
Best regards,