ASME Section VIII, Division 1 has special design and construction requirements for vessels that will be used in “Lethal Service.” Sounds logical and easy. Logical – yes, Easy – not so much! Section VIII mentions the term “lethal service” dozens of times. Still, it is Subsection B – Requirements pertaining to Methods of Fabrication of Pressure Vessels where the code gives us the best understanding of what ASME means by “lethal service.” But the code does not define which of OSHA’s Highly Hazardous Chemicals or EPA’s Extremely Hazardous Substances would be considered “Lethal”; that is left up to us to define.
Here is what section UW-2 says …
UW-2 SERVICE RESTRICTIONS
(a) 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.
By “lethal substances” are meant 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. 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.
As you can see, this is clear as mud! How a business decides to classify their substances is up in the air, but I could make a strong argument that there are many of OSHA’s toxic HHCs and EPA’s toxic EHSs that would fall into this “lethal service” category. But there is a much better rationale to use to establish those substances which could be considered “lethal”; that is the Immediately Dangerous to Life and Health (IDLH) values. Below is the latest listing of those substances that have IDLH value of 500 ppm or less. I am providing this list merely as a point of reference for consideration in your discussions with your engineering resources. I am IN NO WAY implying that every chemical on the IDLH listing below should be considered “lethal”, just trying to provide some means to quantify this definition. Take a look at some of the really low IDLH values…
NOTE: I only compared the chemicals colored red (IDLH)
I am not sure how anyone could make a rational argument that a substance with an IDLH value of 1-10 ppm would not meet the intent of ASME Section VIII, Subsection B Substances with these extremely low IDLH values, in my opinion, MUST be managed as “lethal service” substances. But there are those who would challenge my position.
So how can I tell if my ASME-coded vessel is rated for “lethal service”?
Section UG-116 “REQUIRED MARKING” states:
(a) Each pressure vessel shall be marked with the following:
(1)(a) the official Code U Symbol shown in Fig. UG-116 sketch (a) on vessels inspected in accordance with the requirements in UG-90 through UG-97 (when inspected by a user’s Inspector as provided in UG-91, the word USER shall be marked above the Code Symbol); or (b) the official UM Symbol shown in Fig. UG-116 sketch (b) on vessels constructed in accordance with the provisions in U-1(j).
(2) name of the Manufacturer of the pressure vessel preceded by the words “certified by”;
(3) maximum allowable working pressure37,38 at temperature;
(4) maximum allowable external working pressure39  at temperature;
(5) minimum design metal temperature at maximum allowable working pressure37;
(6) Manufacturer’s serial number;
(7) year built.
(b)
(1) The type of construction used for the vessel shall be indicated directly under the Code Symbol by applying the appropriate letter(s) as follows: vessels having Category A, B, or C joints (except nozzles or other openings and their attachment) in or joining parts of the vessels: 2010 SECTION VIII – DIVISION 1 Type of Construction Letter(s) UG-115 MARKING AND REPORTS GENERAL Arc or gas welded W Pressure welded (except resistance) P Brazed B Resistance welded RES (2) Vessels embodying a combination of types of construction shall be marked to indicate all of the types of construction used.
(c) When a vessel is intended for special service and the special requirements have been complied with [see UG-120(d)], the appropriate lettering shall be applied as listed below:
Lethal Service = L
Unfired Steam Boiler = UB
Direct Firing = DF
This lettering shall be separated by a hyphen and applied after the lettering of (b) above.