3 replies to this topic

[jrtailor09]

Dear All,

 

What is criteria for the pressure vessel is subjected to lethal service or not ?

 

Please see below application , operating /design parameter . 

 

1 ) Pressure vessel 1 

Service - Pure Ammonia 

Operating temp /pressure = -33 °C/ 40 kpa(g)

Design temp /pressure = -46°C / 350 kpa(g0

 

1 ) Pressure vessel 2 

Service - Aq. Ammonia ( only 6%) 

Operating temp /pressure = 10 °C/ 40 kpa(g)

Design temp /pressure = -29°C / 350 kpa(g)

 

On what basis we can say that vessel has to be designed for lethal service or not ?

 

Kindly provide back up for same.

 

Thanks & regards,

Jatin Tailor

[latexman]

This is going back about 10 years for me, so there may have been an ASME change since then.  The ASME Code specifically places the responsibility of proper classification on the user of the vessel.  They only give a definition of what is considered lethal. According to the Code, a lethal substance is a "poisonous gas or liquid of such a nature that a very small amount of the gas or the vapor of the liquid mixed or unmixed with air is dangerous to life when inhaled".

 

The MSDS for anhydrous ammonia I just looked at has ammonia classified by NFPA as a 3-Health, 1-Flammability, and 0-Reactivity.

 

Ammonia has such a strong, pungent odor, people will try to escape the vapors to safety.  It takes much more than one whiff of ammonia to be dangerous to life (thankfully for me).

 

I personally don't think ammonia qualifies as a poison, but it's up to you.

[Atttyub194]

Dear All

 

Good day

 

It  is good question and really a good reply.

 

However, availing the opportunity, I would like to extend the discussions so that we may try to simplify a procedure  to avoid hazardous situation.

 

By book , "Service utilizing poisonous gases or liquids of such a nature that a very small amount of the gas or the vapor of the liquid, mixed or  unmixed with air, is dangerous to life when inhaled are classified as Lethal Services"

 

Footnote # 1 of ASME VIII UW-2, defines  “lethal substances” are meant poisonous gases or liquids of such a nature that a very small amount of the gas or liquid mixed or unmixed with air is dangerous to life when inhaled.

 

For purposes of this Division, this class includes substances of this nature which are stored under pressure or may generate
a pressure if stored in a closed vessel"

 

Additionally, when vessels are to contain lethal substances, either liquid or gaseous, all butt welded joints shall be fully radiographed, except under the provisions of UW-2(a)(2) and UW-2(a)(3) below, and UW-11(a)(4). ERW pipe or tube (i.e. Electric Resistance Welded) is not permitted to be used as a shell or nozzle in lethal service applications.

 

When fabricated of carbon or low alloy steel, such vessels shall be postweld heat treated. When a vessel is to contain fluids of such a nature that a very small amount mixed or unmixed with air is dangerous to life when inhaled, it shall be the responsibility of the user and/or his designated agent to determine if it is lethal.

 

As a layman this is not a very clear definition and technically  it is not appropriate that the decision for toxicity/ lethal service should be left on user.

 

You may find that there are many of the chemical which as per OSHA are rated "Highly Hazardous Chemicals" , however, may not be classified purely as "lethal Service". Similarly there are many chemical which as per EPA are rated Extremely Hazardous Substances however, may not be classified purely as "lethal Service"

 

There is a much better rationale to establish those substances which could be considered "lethal"; that is the concept of  Immediately Dangerous to Life and Health (IDLH) values. 

 

We as experienced Engineer and Designer can not emphasize that  every chemical having IDLH  value should be classified as lethal.

 

However, at least I am not  aware that how  could we make a rational argument that a substance with an IDLH value of 1-30 ppm would  not meet the intent of ASME Section VIII with respect to lethal service

 

  Eventually, we as Engineer using our own judgement and only using first part of definition without any rationale basis classifies a substance as lethal  As an example please refer Bromine  and Arsine  with IDLH = 3 ppm.

Even with H2S having IDLH of 100 ppm, equipment designed for H2S service are some times classified as non lethal. However, some companies are know which  classifies it as lethal service. ( it is well noted that a sour service can be non lethal)

 

This difference is found as safety conscious companies , have devised a mechanism to cater for the scenario , using the concept of LD50 / LC 50.

 

LC50"( LD stands for "Lethal Concentration". LC₅₀ is the amount of a material, given all at once, which causes the death of 50% (one half) of a group of test animals.) . Same is the case with Ammonia manufacturing facilities.

 

If in case of leakage due to any reason in the referred equipment will result in  concentration of the component  in air  to a level beyond LC50 value then the Ammonia vessel  under consideration to be designed characterizing it as "lethal service" otherwise not. It means if the hold up  volume is sufficient to achieve the concentrations beyond LC50

 

However, this is a general practice followed by few companies  and not a standard which every one must be forced to adopt.  Please do evaluate if found appropriate on these guidelines

 

As you may be aware that most of the process licensor recommends for pure ammonia vessels :

1. All butt welded joints shall be fully radio graphed

2. ERW pipe or tube (i.e. Electric Resistance Welded) is not permitted to be used as a shell or nozzle or in piping

3. When fabricated of carbon or killed  carbon steel, all vessels shall be post weld heat treated as per requirement of ASME Sec VIII, Div. 1, Part UCS-56 & UCS-79

For sure Aqua Ammonia vessels can not be characterized as lethal

 

In view of the consequences , I personally feel that the methodology of defining lethal service should not be left on user and probably some one have to approach for help and guidance from "AIChe" and "ASME" for standardization.Please correct me where I am wrong

 

Best regards and God Bless you.

 

 

Ahmed Attyub

[jrtailor09]

Latexman /Atyub ,

 

Thank you very much for your very well explanation.

 

For our project, We came across both kind of the requirement of vessel (One for Aq. Ammonia & other for pure Ammonia).

 

This question client has raised for the aqueous ammonia vessel while proceeding for FAT. I am surprised that they didn't ask same thing for pure ammonia vessel. One more thing both vessel datasheet has been approved considering that lethal service is not applicable.

 

Pure Ammonia vessel already reached st site & installed. So, I don't think we can do anything now. 

 

I have informed already that vessel for aq. ammonia not to be required consider for lethal service.

 

Yes , We should not left to user for the requirement of lethal service. 

 

This requirement is missed by the reputed organization. They are in this field from long time.

 

Thank you very much once again.

 

Regards,

Jatin