4 replies to this topic

[Pingue2008]

All,

 

Last week we have an explosion(the explosion part will not be addressed). now it is time to determine what happened. There are ideas running around as to what happened. As we all know, everyone has an opinion. i am seeking for practical explanation and calculations that can support my idea. First i think it is a vacuum event. the vessel is jacketed. dowtherm is used in the jacket to heat up the oligomer. we had to spin out the plant a few weeks ago, but we could not flush/empty this vessel.we thought that during start up we will just melt the oligomer and push it through the process. the followings  are my questions and comments.

 

1- the shell and jacket are rated for full vacuum. How can a rated FV fail under vacuum? the actual nameplate shows FV @ 600F. Can temperature affect the vacuum rating of a vessel. i still can't understand how a FV rated vessel failed under vacuum. how can one explain it? if this a possibility how to you determine this pressure?

 

2- during the outage i think there was an exothermic reaction taking place in the vessel (thermal degradation of Oligomer/PET). at some point last week we realized that the temperature and pressure in the line was building up we decided to stop the dow supply to this vessel. i think the exothermic reaction heat was enough to evaporize the dow that was in the jacket. after about 20 min of the dow pump being down, a decision was made to rapidly cool off of the system.(this is what i think caused the failure. condensing the dow on the jacket side)

 

3- My main goal is to come up with calculations that can explain the vacuum failure on the jacket. if you see anything the attached picture please share.

 

Please see the attached vessel for visual failure

 

Attached Files

•  Rx Vaccum failure.xlsx   193.38KB   26 downloads

[PingPong]

The photo is not very clear, but indeed seems to indicate that the jacket imploded between its baffles.

 

Is that all that happened? Or is the vessel wall itself also damaged or deformed?

 

a decision was made to rapidly cool off of the system

How was this cooling done?

 

If the jacket was designed for FV at 600 F then jacket implosion should not have happened provided the metal temperature was below 600 F (315 ^oC). And provided the vendor did a good job.

 

What is the normal operating temperature of the dowtherm in the jacket?

[Pingue2008]

The vessel wall was damaged (exploded) as well but not as mush as the jacket (not visible) we tie measuring type around a see a little increase in diameter.some people think there were two events pressure and vacuum, but i am just worrying about the vacuum. depending of the pressure in the line we can either run dow through the cooler or the heater. in the particular instance we have water sprayed on the vessel. the normal operating temperature is 260C. the main problem is that there is not thermal couple on this vessel if we knew the exact temperature in that vessel it would tell the whole story.

 

Thank you

[PingPong]

You should ask a qualified mechanical engineer to use the vessel drawings to verify whether the wall material, wall thickness, baffle spacing, et cetera, were indeed such that the jacket can resist a FV @ 600 ^oF.

 

If not, the vendor could be held responsable for the damage, simply because the design never met the specification.

 

Otherwise, however, you have a problem, because without recorded DCS data of temperatures (and pressures) you can not prove that the temperature never exceeded 600 ^oF.

[Pingue2008]

We have plenty of data to show that the temperature never exceeded 600F back two years.