File: PVE-3856, Last updated June 6 2017 LRB / CT


Requirements for lethal service are scattered through VIII-1, the code cases and the interpretations. ASME VIII-1 section UW-2 has most of the requirements for lethal service. Two definitions of lethal service are provided in UW-2. The second definition is also duplicated in the end notes:

[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[.]

By “lethal substances” are meant poisonous gases or liquids of such a nature that a very small amount of the gas or of the vapour 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.

It is up to the user to determine if a service is lethal. We at Pressure Vessel Engineering do not determine if the vessel service is lethal (but we might have opinions based on previous jobs).

ASME VIII-1 Lethal Service Code References

UW-2 Service Restrictions is a very important section for lethal service vessels and must be read in its entirety. A few brief points from UW-2:

  • UW-2(a) and UW-11(a)(1) – All butt welds shall be 100% radiographed
  • UW-2(a) – ERW pipe (like some grades of SA-53) is not permitted but interpretation VIII-1-01-118 says it is acceptable if the long seam is fully radiographed
  • UW-2(a) – Post weld heat treatment is required for CS and Low Alloy
  • UW-2(a)(1)(a) – Category A welds shall be type 1 only (butt welded with no permanent backing strip)
  • UW-2(a)(1)(b&c) &Interpretation VIII-1 92-211 Category B & C welds shall be type 1 or 2 only (butt welded). No slip on flanges! No Figure UW-13.2 Flange or Head to Shell attachments
  • Interpretation VIII-I-98-23 – Category D welds (typically nozzles) shall be full penetration.
  • UW-2(a)(1)(c) – Category C joints for stub ends have a long list of requirements
  • UW-2(a)(2 and 3) – Heat exchangers have a long list of requirements
  • Read all of UW-2 for more restrictions…

When it has been determined that a vessel is in Lethal Service, some of the following code sections will apply. Other code sections might also be required. (List last updated June 16 2015 based on the ASME 2013 VIII-1 code edition.)

  • U-2(a)(2) – The user determines if the service is lethal.
  • UG-16(5)(a) – Air cooled and cooling tower heat exchanger tube walls to be 1/16″ min.
  • UG-24(6) – Casting RT requirements.
  • UG-25(e) – No telltale holes.
  • UG-99(g),(k) – Hydro test visual leak inspection cannot be waived. Do not paint or line prior to the hydro test.
  • UG-100(d)(4) – Pneumatic tests cannot be used for lethal service vessels, but also see code cases 2046-2, 2055-2, 2407 regarding pneumatic instead of hydrostatic testing.  
  • UG-100(e)(3) – Do not paint or line prior to the pneumatic test.
  • UG-116(c) – “L” stamping must be added to the nameplate.
  • UG-120(d)(1) – “lethal service” is added to the data report
  • UW-2 – Service Restrictions – main source of info on Lethal Service (see lots more from UW-2 below)
  • UW-11(a)(1) – All shell and head butt welds to be 100% RT
  • Fig UW-13.5 – One possible lap joint stub end configuration – see also interpretation BC-79-680 / VIII-80-111
  • UB-3 – Brazed vessels shall not be used
  • UCS-6 – Do not use SA-36, 38W or SA-283
  • UCS-79 – PWHT and extreme fiber elongation – read for rules when heat treatment is required
  • UCI-2, UCD-2 Cast iron and ductile cast iron vessels are not allowed.
  • UIG-2(c) – Metal parts for graphite vessels are to be designed to UIG requirements
  • UIG-23(b) – Factor of safety for lethal service
  • UIG-60 – Lethal service
  • UIG-99 – Lethal service pressure tests
  • ULW-1 & ULW-26(b)(4) – Layered vessels lethal restrictions apply to the inner shell and heads only
  • UHX-19.1(b) – Heat exchanger markings
  • Appendix 2-5(d) & 2-6 – Maximum Flange Bolt Spacing
  • Appendix 2-14(a) – Mandatory flange flexibility
  • Appendix 7-1, 7-5 – Steel casting examination for 100% quality factor
  • Appendix 9-8 – if the jacket does not carry lethal substances, lethal service restrictions do not apply to it
  • Appendix 17-2(a) – Dimple jackets will not contain lethal substances
  • Appendix 35-3(c) & 35-7(d) – Mass production rules
  • Appendix W Table W-3 – Filing out the U forms
  • Appendix KK (note 12) – Filling out User Design Requirements forms
  • Appendix NN Table NN-6-5 – User or designated again determines if the contents are lethal
  • End notes 65 – Definition of lethal service
Weld Restrictions - Fig UW-3

Weld restrictions by category

Fig UW-13.5

Special lap joint stub end for lethal service – see also Fig UW-13.3 and Fig 2-4(5) and (6)

Fig UW-16.1

All partial penetration nozzles are prohibited

Fig UW-13.2 - restricted

All figure UW-13.2 corner welds are prohibited

Fig 2-4 - restricted

No slip on flanges

Code Cases

The following Code Cases refer to lethal service for VIII-1 vessels (2013 edition):

  • 1750-20(g) – A126 not to be used for PRVs, or rupture disks.
  • 2249 – Furnace Brazing
  • 2318 – slip on flanges in lethal service
  • 2321-1 PWHT of P4 and P5A materials
  • 2324 – Ultrasonic inspection
  • 2334 – single fillet lap joint tubesheet to shell connection
  • 2346-1 & 2537 – Alternative rules for SE and F&D head to shell attachment
  • 2369 – Testing of covers separate from vessels
  • 2377 – RT of SA-612 steel
  • 2421 – Single fillet joints in Heat exchanger tubes
  • 2437 – Diffusion bonded heat exchangers
  • 2527 – Pneumatic testing of UM vessels
  • 2537 – alternative F&D and SE head to shell attachment is not for lethal service
  • 2621 – Diffusion bonding
  • 2751 – alternative spherical head to shell attachment is not for lethal service


With interpretations now available on line at it is now easier to find relevant and up to date interpretations. A search for “BPV Section VIII Div 1” keyword: “lethal” returns many records. The comments on the most interesting lethal service interpretations are listed below. Refer to the actual interpretations for the full text.

  • VIII-1-01-118 and VIII-1-83-77R – ERW pipe may only be used if the long seam is radiographed
  • VIII-82-65 – volumetric examination of category D joints is not always required
  • VIII-1-01-42 and VIII-1-04-48   – cone >30° and and corner joints not permitted
  • VIII-1-86-84 and VIII-1-95-138 – prohibited flange attachments – Fig UW-13.2(m) and Fig 2-4 sketches (7), (8), (8a), (8b), and (9)
  • VIII-1-92-112 – full radiography of category C and D butt welds is required except for UW-11(a)(4)
  • VIII-1-92-194 – full penetration angle joints are not permitted
  • VIII-1-92-211 – Fig UW-13.2 attachments are not permissible
  • VIII-1-98-113 – permissible repad and flange pad arrangements
  • VIII-1-98-23 – Fig UW-16.1 (a) and (c) are permissible nozzle attachments, others are not discussed
  • VIII-77-62“It is the intent of Section VIII, Division 1 that Category B and C butt welds in nozzles and communicating chambers that neither exceed 10 in. nominal pipe size or 1-1/8 wall thickness are excluded from the provisions of radiography, even though the vessel is in lethal service. This overrides the provisions of UW-2(a).”
  • VIII-80-02 – do not use corner joints from Fig UW-13.2 – redesign to create butt joints that can be radiographed
  • VIII-80-111 – acceptable lap joint flanges include Fig. 2-4(1) and Fig. UW-13.3 and Fig. 2-4(5) and (6)
  • rec 15-634 – PWHT and cladding, see also 14-1910
  • rec 15-1154 – Full RT does not include the reinforcing rings per UG-29
  • rec 15-1264 – welding of non-round vessels
  • rec 15-1288 – lethal and non-lethal sections of heat exchangers per UW-2(a)(3)
  • rec 16-32 – UG-25(e) telltale holes in the shell are not the same as UG-37(g) telltale holes in repads
  • rec 16-39  – Fig 1-6(d) is category C
  • rec 16-72  – clarification on RT-4 and UG-116(e)(4)
  • rec 17-11 – UG-82(b) and attachments after hydrotest per UG-99(g)


“ASME Section VIII – Division 1 Example Problem Manual” PTB-4-2013 provides illustrative example for the use of VIII-1 rules.

  • E7.1  – RT-1 radiography on a vessel to meet the requirements of lethal service.  Required radiography to meet RT-1 and the appropriate weld efficiencies are illustrated and commented on.  The sample illustrates items like sumps and nozzles that can be exempt from radiography based on size and still maintain the RT-1 rating for the vessel.
  • E4.16.1 and E4.16.2  – maximum allowed flange bolt spacing for lethal service per App 2-5(d)

Disclaimer: This material is provided for educational uses only. Only ASME can make code interpretations.