I'm wondering about the criteria for choosing diaphragm seal in toxic services. by other words, which service is toxic, or how much h2s in a fluid is lead to a toxic service.

 

You can google H2S toxicity for whatever area of the world you're in. In the US, OSHA limits are listed here:

https://www.osha.gov/SLTC/hydrogensulfide/hazards.html

With regards to diaphragms, I'm not sure what you're asking, but if the topic is fugitive emissions from valves, you might want to read an overview of LDAR (Leak Detection and Repair) as it applies to valve construction in this 2009 article in Valve magazine

http://tinyurl.com/ha5625u

 

I assume you are talking about instrument diaphragm seals.

With H2S you need to refer to corrosion standard NACE MR0175. If I had any concentration of H2S in my process I would require NACE MR0175 certification. The seal manufacture should be able to help you determine the exact material. 316SS is usually sufficient.

 

exactly, I'm talking about diaphragm seal for pressure instruments. I was wondering about the criteria for using diaphragm seals.

 

See what Rosemount has to say about materials compatibility
http://s22.postimg.org/q1kz43vlt/03_H2_S_compatability_Rosemount_Pressure.jpg

 

The process owner has the responsibility to define the level of toxicity.

Where "lethal" service is involved, the owner has or must have piping guidelines that covers the instrument connections and area monitoring.

Remember H2S is more dangerous and fatal than most realize.

 

Do you have any standard, specification or recommended practice that mentions about considering diaphragm seal for pressure instruments in toxic service?

 

If you are thinking about contaminating the environment with a toxic product caused by a leak in the pressure sensor, I would consider equipping that pressure instrument with a diaphragm seal. At least that will give you a second barrier against leaks. It will also provide an early warning when the first barrier (diaphragm) is broken because the pressure reading will be different.

 

Unfortunately, such design requirements are part of the piping specifications for the project.

The pipe specs include materials of construction, pipe weight, installations requirements, and so on. There is no simple answer, but an engineered design.

The dangers of H2S are covered in:
https://www.osha.gov/SLTC/hydrogensulfide/hazards.html

 

>exactly, I'm talking about diaphragm seal for pressure
>instruments. I was wondering about the criteria for using
>diaphragm seals.

The general criteria for chemical seals (apart from H2S service) is to use them when the fluid is viscous or filled with particles that settle out, or fluids that freeze or solidify if allowed to cool.

There are other installation methods that do not use chemical seals, but they are more complicated to maintain.

 

Several standards would apply to H2S service, ASME B31.3 Category M Fluid Service piping and tubing and NACE MR0175 for material selection.