Lethal Service Criteria 3

[szoghbi]

I am designing a vessel that is supposed to collect (by gravity) condensate containing about 38 ppm of H2S. Once collected, this condensate will be displaced from the vessel to the flare using Nitrogen at 100 psi.

Would you consider this vessel to be on lethal service?

Thanks for your help.

Susana.


Ps: UW-2 defines a lethal substance as poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled.

15 ppm of Hydrogen-sulfide(H2S)is the allowable exposure limit for 15 minutes according to SHE standards.

Please excuse my Engligh... it´s not very good.

 

[deanc]

Think you have answered your own question. I would prefer not to be exposed to this.

 

[EGT01]

I can't say I have any experience with H2S but I do have a little with other lethal service chemicals and there may not be a simple answer to your question.

Look again at the UW-2 definition. I think what you need to consider is the equilibrium concentration of H2S in the vapor above the liquid at the temperature of the condensate. That would be the H2S in the "vapor" that might leak from the vessel. Then what happens to the liquid if it were to leak? Would it flash?

However, you may also need to consider single failure modes from upstream sources that might cause the vessel contents to be much different than you have indicated.

Having a full understanding of the chemical characteristics (odor, color) and physiological effects is also important. Probably good to get an Industrial Hygienist involved.

For these reasons I believe it is up to you, the user to define whether lethal service is required. Of course it would be interesting to hear if anyone has any experience directly related to the concentrations you've indicated.

I would wonder if the equipment upstream of your new collection drum is designated lethal service and whether there is already a lethal service definition for H2S at your facility. Of course this is something you should already know.

It may be more appropriate to ask how to determine when to designate lethal service rather than does the indicated concentrations require lethal service. Does anyone know of an established methodology for determining the need for lethal service?

 

[flames]

The company you are designing for and local regulations may dictate your answer. I work in a plant that has pressure >5000kPa total pressure with over 30% H2S and nothing is considered in lethal service.

 

[jte]

Susana-

I do have experience with hydrogen sulfide - in fact, I'm wearing my H2S monitor right now. We're required to have a monitor clipped to our overalls or hard hat anytime we're out and about in the operating units.

I would not define 38 ppm H2S as lethal. Getting into the 300 ppm range would be - and officially you should consider 100 ppm as the basis for your decision. That value is once you have lost containment and it is spewing out into the atmosphere. Take a look at thread794-22012, "Defining Lethal Service" at

thread794-22012 Ok, how do you link directly to another post?

 

[jte]

Hmmm... you just type in the thread and the site automatically creates the link... So easy even I can handle it!

jt

 

[EGT01]

jte - the thread you referenced above didn't seem to have much there but it did make reference to another that seemed to have some good discussion. Looks like this has been a topic of discussion several times before. Here's a couple more from the past.

Thread124-23360
Thread794-17433

I found the post by TD2K dated Feb-23-2002 in the last thread noted above especially interesting in regards to the footnote in previous editions of the CASTI guidebook which gave examples of chemicals considered lethal (hydrocyanic acid, carbonyl chloride, cyanogen, mustard gas and xylyl bromide) and those not considered lethal (chlorine, ammonia, plus others).

Though RTP-1 may not constitute a pressure vessel since it is intended for pressures <15psig, it lists a service restriction referred to as &quot;critical service&quot; which one of the criteria sounds a lot like &quot;lethal service&quot;. I mention this only because I wonder if such criteria would be reasonable to apply to the lethal service definition. Maybe this is why I've seen chlorine vaporizers and vessels in ammonia refrigeration systems not designated lethal service.

From RTP-1 1989, 1-210 Service Restrictions
&quot;substance contained is known by the User to be an insidious and extremely poisonous gas or liquid of such a nature that a very small amount of the gas or of the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled, or of such a nature that a very small amount of the substance in contact with the body may be absorbed and cause a toxic reaction that is dangerous to life. By insidious is meant the substance is of such a nature that exposure to the substance might result in a discomfort level not sufficient to warn of potential severe and irreversible damage to an individual's health.&quot;

 

[AZman]

HI SZOGHBI,
you may also have to look up NACE MR 01-75, dealing with other impacts(metallurgical) of H2S, and how to determine the concentration as effective or insignificant.There's also limits on the type of metal as well as the heat treatments/hardness of your shell material to contain H2S.
Finally, I agree with deanc in that you have much to consider - and do take your time !
hope you'll find it usefull .