This worked example documents the ASME Section VIII Division 1 design of a 12 in NPS x 150 psig vertical knockout drum in lethal service (typical for chlorine or hydrogen-sulfide upstream of a process scrubber). The vessel is SA-516 Grade 70 Normalized throughout, 0.500 in nominal shell, 0.375 in nominal 2:1 SE heads, with five Class 150 RFWN weld-neck nozzles. The vessel is qualified for ASME lethal service per UW-2 and carries an L stamp on the nameplate. MAWP is 240 psig at 200 °F (flange-limited).
The teaching content is the lethal-service rule walkthrough in section 5 example 3, which is the audit trail for compliance with UW-2 and its referenced clauses: RT-1 100% radiography on every butt weld, Type 1 joints only on Categories A and B (no permanent backing, no slip-on flanges, no Figure UW-13.2 attachments), full-penetration Category D nozzle welds, no telltale holes in the shell, mandatory PWHT per UCS-79, the materials prohibited list (SA-36, SA-38W, SA-283), the hydrostatic test with visual leak inspection on the unpainted vessel per UG-99(g) and UG-99(k), the L stamp on the nameplate per UG-116(c), and the lethal-service notation on the Form U-1A data report per UG-120(d)(1). Mirror of PVEng's Lethal-Service Quick Guide article.
Five sections. Each section is a collapsible block. Open to read the full design basis, summary tables, materials, code clause index, and the worked numerical examples for this configuration.
| Parameter | Value | Reference |
|---|---|---|
| Code of construction | ASME Section VIII Division 1, 2023 Edition, with lethal-service supplementary requirements | U-1, UW-2, UCS-79 |
| Equipment type | Vertical lethal-service knockout drum (chlorine or H2S service) | |
| Service classification | Lethal substance, gaseous or liquid, dangerous when inhaled in small concentrations | UW-2(a) |
| Design pressure | 150 psig | UG-21 |
| Design temperature | 200 °F | UG-20(a) |
| Design MDMT | -20 °F | UG-20(b), UCS-66 |
| Corrosion allowance | 0.0625 in (internal surfaces, baseline; service-specific allowance may exceed if H2S service) | UG-25, UG-25(e) for lethal |
| Joint efficiency, all welds | 1.00 (RT-1, all butt welds 100% radiographed) | UW-11(a)(1), UW-12 |
| Joint types permitted, Cat A and B | Type 1 only (full-penetration butt, no permanent backing) | UW-2(a)(1)(b), Table UW-12 |
| Joint types permitted, Cat D | Full penetration (set-in or set-through nozzles), no partial-penetration attachments | UW-2(a)(1)(c) |
| Radiography | Full RT (RT-1) on every butt weld, surface NDE (MT or PT) on every nozzle attachment and the U stamp area | UW-11(a)(1), UW-50 |
| Post-weld heat treatment | Mandatory, all carbon-steel components stress-relieved per UCS-56 after fabrication | UCS-79, UCS-56 |
| Marking | Standard U stamp PLUS "L" suffix on the nameplate per UG-116(c) | UG-116(c) |
Per UW-2(a), a vessel containing a lethal substance is one in which the contents are dangerous when inhaled in small concentrations. The user (not the engineer of record) classifies the service as lethal; once classified, the supplementary requirements of UW-2 apply in addition to the rest of Section VIII Division 1:
Internal design pressure of 150 psig at 200 °F is the governing load. Weight of the vessel and the small lethal-substance inventory is trivial. The vessel is skid-mounted; wind and seismic are screened out per the small mass.
The design MDMT is -20 °F. Shell and heads are SA-516 Grade 70 Normalized, Curve D under Table UCS-66. Required MDMT at 0.500 in nominal is approximately -50 °F at no impact testing. Mandatory PWHT does not alter the no-impact MDMT for SA-516-70 N at this thickness.
Per UG-99(b) with the lethal-service additions of UG-99(g) and UG-99(k):
The vessel is NOT painted prior to hydrotest. Every weld and nozzle is visually inspected for leakage during the hold per UG-99(g)/(k). A pneumatic test per UG-100 is not used for lethal-service vessels except by special permission of the user, because air retention of a lethal substance during a test failure is hazardous.
| Component | Material | t_req (in) | t_avail (in) | MAWP (psig) | Clause |
|---|---|---|---|---|---|
| Cylindrical shell | SA-516 Gr 70 N (PWHT) | 0.045 | 0.375 | 1,205 | UG-27(c)(1) |
| 2:1 SE heads (top and bottom) | SA-516 Gr 70 N (PWHT) | 0.045 | 0.275 | 912 | UG-32(d) |
| Cl 150 weld-neck RFWN flanges (all process) | SA-105 (PWHT) | standard | standard | 240 | UG-44, B16.5 Gp 1.1 |
| Nozzle necks (Sch 80, all weld-neck only) | SA-106 Gr B (PWHT) | < 0.02 | > 0.20 | matches shell | UG-27(c)(1) |
The Cl 150 weld-neck flanges govern the vessel MAWP at 240 psig at 200 °F. The shell and head carry significant margin to the vessel MAWP. Note that slip-on flanges are explicitly NOT permitted in lethal service per UW-2(a)(1), so all process connections use weld-neck (RFWN) flanges only.
All components meet UG-16(b) by very wide margins.
| Component | Calc Stress at 312 psig (psi) | 0.9 × S_y at 70 °F (psi) | Status |
|---|---|---|---|
| Shell, UG-99 Note 35 | 5,000 | 32,400 (SA-516-70 N) | PASS |
| Heads | 5,420 | 32,400 | PASS |
| Visual leak check per UG-99(g)/(k) | visible-leak inspection, unpainted vessel | zero leakage permitted | PASS (witnessed by AI) |
| Component | Specification | S at 200 °F (psi) | UCS-66 Curve | Form / PWHT |
|---|---|---|---|---|
| Shell | SA-516 Grade 70, normalized, lethal-service supplementary requirements | 20,000 | D | 0.500 in plate, rolled and welded, PWHT'd after fabrication per UCS-79 |
| Heads (2 ea) | SA-516 Grade 70, normalized | 20,000 | D | 2:1 ellipsoidal, one-piece formed, 0.375 in nominal, PWHT'd after fabrication |
| Nozzle necks | SA-106 Grade B, seamless | 17,100 | B | NPS 4 / 2 / 1 Schedule 80 pipe, PWHT'd |
| Flanges (weld-neck only) | SA-105 | 17,500 | B | ASME B16.5 RFWN Class 150, all process nozzles. Slip-on flanges NOT permitted per UW-2. |
| Bolting | SA-193 Gr B7 studs, SA-194 Gr 2H nuts | 25,000 | n/a | standard B16.5 stud-and-nut sets |
| Material restrictions per UW-2 | Not permitted: SA-36, SA-38W, SA-283. Brazed and cast-iron vessels prohibited. | n/a | n/a | n/a |
| Tag | Function | Size and Class | Location | Type | Weld |
|---|---|---|---|---|---|
| N1 | Process inlet | NPS 4 Cl 150 RFWN, Sch 80 neck, weld-neck only | Top head | Set-in, full penetration radial | Full pen plus fillet per UW-16(f), surface NDE per UW-50 |
| N2 | Liquid outlet (collected lethal liquid) | NPS 2 Cl 150 RFWN, Sch 80 neck, weld-neck only | Bottom head | Set-in, full penetration radial | Full pen plus fillet per UW-16(f), surface NDE per UW-50 |
| N3 | Gas outlet (to scrubber) | NPS 2 Cl 150 RFWN, Sch 80 neck, weld-neck only | Top head | Set-in, full penetration radial | Full pen plus fillet per UW-16(f), surface NDE per UW-50 |
| N4 | Pressure relief (PSV) | NPS 1.5 Cl 150 RFWN, Sch 80 neck, weld-neck only | Shell, top | Set-in, full penetration radial, exempt UG-36(c)(3)(a) | Full pen plus fillet per UW-16(f), surface NDE per UW-50 |
| N5 | Level instrument | NPS 1 Cl 150 RFWN, Sch 80 neck, weld-neck only (NO threaded coupling in lethal service) | Shell | Set-in, full penetration radial | Full pen plus fillet per UW-16(f), surface NDE per UW-50 |
Important departures from non-lethal-service practice: threaded couplings (B16.11 forged) are typically NOT used in lethal service because the threaded connection is more leak-prone than a flanged joint. All five nozzles on this vessel use Class 150 RFWN weld-neck flanges, with the level instrument as a flanged connection rather than a threaded coupling.
All Category A and Category B butt welds (long seam, head-to-shell circumferentials) are fully radiographed (RT-1) per UW-11(a)(1), achieving E = 1.00. All Category D nozzle attachment welds receive full surface NDE (MT or PT) per UW-50. The "L" stamp area on the nameplate receives a final MT or PT examination.
| Item | Value |
|---|---|
| Shell OD | 12.750 in (NPS 12) |
| Shell wall, nominal | 0.500 in |
| Shell wall, mill-min (-12.5%) | 0.438 in |
| Shell wall, corroded (mill-min less CA) | 0.375 in |
| Shell tangent-to-tangent length | 36 in (3 ft) |
| Head nominal thickness, 2:1 SE | 0.375 in |
| Head minimum corroded | 0.275 in |
| Internal volume, new (shell + 2 heads) | 4,360 in³ (18.9 US gal) |
| Empty weight, new | 375 lb |
| Test weight, water-filled | 540 lb |
Every clause applied. The lethal-service supplementary requirements (UW-2 cluster) are listed first.
| Clause | Use |
|---|---|
| UW-2(a) | Lethal-service classification (user-determined), supplementary fabrication, examination, marking, and testing requirements |
| UW-2(a)(1)(b) | Type 1 joints only on Cat. A and B (no permanent backing, no slip-on flanges, no Figure UW-13.2 sketches) |
| UW-2(a)(1)(c) | Cat. D nozzle attachments full penetration, no telltale holes permitted in shell |
| UCS-79 | Mandatory PWHT of carbon and low-alloy steel components |
| UCS-56 | PWHT schedule (temperature, hold time, cooling rate) for SA-516 Grade 70 |
| UG-99(g) and UG-99(k) | Visual leak inspection during hydrostatic test, vessel unpainted |
| UG-116(c) | "L" stamp suffix on nameplate |
| UG-120(d)(1) | Lethal-service notation on the U-1A Manufacturer's Data Report |
| UCS-6 and material-not-permitted list | SA-36, SA-38W, SA-283 prohibited in lethal service |
| Clause | Component | Use |
|---|---|---|
| UG-16(b) | All | Minimum 0.0625 in |
| UG-23 | All | Allowable stresses |
| UG-25(e) | All | Corrosion allowance with lethal-service review (baseline 0.0625 in here; H2S service may need more) |
| UG-27(c)(1) | Shell, nozzles | Internal pressure, circumferential stress |
| UG-32(d) | 2:1 SE heads | Internal pressure on 2:1 SE heads |
| Clause | Use |
|---|---|
| UG-36(c)(3)(a) | Exempts the NPS 2, NPS 1.5, NPS 1 openings; NPS 4 (4.151 in) is above the 3.5 in limit and requires UG-37 |
| UG-37 | Area replacement on NPS 4 inlet (PASS without repad) |
| UG-44 | ASME B16.5 RFWN Class 150 flanges accepted; weld-neck only per UW-2 (slip-on prohibited) |
| UG-45 | Nozzle minimum thickness check on each weld-neck neck |
| Clause | Use |
|---|---|
| UW-9 / UW-11(a)(1) / Table UW-12 | RT-1 full radiography, E = 1.00 everywhere |
| UW-15 / UW-16 / UW-50 | Full-penetration nozzle welds, surface NDE on every Cat. D weld |
| UCS-66 / UCS-66.1 | MDMT qualification at -20 F (no credit needed; no-credit MDMT of -50 F at 0.500 in shell is colder than required) |
| UG-99(b) / UG-99(g) / UG-99(k) / Note 35 | Hydrotest at 312 psig with visual leak inspection on unpainted vessel |
| UG-116(c) / UG-118 / UG-120(d)(1) | "L" stamp on nameplate, lethal-service notation on Form U-1A |
Five worked examples. The pressure-containment calcs (examples 1 and 2) are routine for this small vessel; the central content is the lethal-service rule checklist (example 3) and the flange selection that excludes slip-on (example 4).
| Symbol | Value | Source |
|---|---|---|
| P | 150 psig | Design pressure |
| R | 6.000 in | Inside radius, corroded: (12.75 - 2 × 0.375) / 2 |
| S | 20,000 psi | SA-516 Gr 70 N at 200 °F |
| E | 1.00 | RT-1, full radiography, Table UW-12 |
Required thickness with CA is 0.108 in. Mill-min supplied 0.438, corroded 0.375 in. PASS with 0.267 in margin. Shell MAWP at corroded thickness 0.375 in is 1,205 psig.
Required thickness with CA is 0.107 in. Head supplied 0.375 nom, formed-corroded 0.275 in. PASS. Head MAWP at 0.275 in is 912 psig.
The vessel is qualified for lethal service by satisfying every requirement of UW-2 and the associated clauses. The checklist below is the audit trail.
| Requirement | Clause | Compliance |
|---|---|---|
| Lethal-service classification recorded by user | UW-2(a) | PASS per process datasheet |
| RT-1, every butt weld 100% radiographed | UW-11(a)(1) | PASS per RT report log |
| Cat. A and B joints: Type 1 only (no permanent backing) | UW-2(a)(1)(b), Table UW-12 | PASS per fabrication drawing |
| No slip-on flanges | UW-2(a)(1)(b) | PASS (all five flanges are RFWN weld-neck) |
| No Fig UW-13.2 welded-flat-head attachments | UW-2(a)(1)(b) | PASS (heads are formed 2:1 SE, not flat covers) |
| Cat. D nozzle attachments full penetration | UW-2(a)(1)(c) | PASS (no partial-penetration nozzle welds) |
| No telltale holes in shell | UW-2(a)(1)(c) | PASS per fabrication drawing |
| Mandatory PWHT, carbon-steel components | UCS-79, UCS-56 | PASS per PWHT chart records |
| Materials permitted (not SA-36, SA-38W, or SA-283) | UCS-6, UW-2(a) | PASS (SA-516 Gr 70, SA-105, SA-106 Gr B all permitted) |
| Hydrotest with visual leak inspection on unpainted vessel | UG-99(g), UG-99(k) | PASS per AI witness sheet |
| "L" stamp on nameplate | UG-116(c) | PASS per stamping log |
| Lethal-service notation on U-1A data report | UG-120(d)(1) | PASS per Form U-1A |
The vessel passes every UW-2 requirement. PASS for lethal service.
Per UG-44, ASME B16.5 flanges are accepted at their P-T rating. Per UW-2(a)(1)(b), slip-on flanges are NOT permitted in lethal service. Every process connection on this vessel uses a Class 150 raised-face weld-neck (RFWN) flange.
| Item | Value | Source |
|---|---|---|
| Flange standard | ASME B16.5, Material Group 1.1 | UG-44 reference |
| Class | 150 | Selected for the 150 psig design pressure |
| Type | Raised-face weld-neck (RFWN) only | UW-2 prohibits slip-on |
| P-T rating at 200 °F | 240 psig | B16.5 Table 2-1.1 |
| P-T rating at 70 °F | 285 psig | B16.5 Table 2-1.1 |
| Gasket | Spiral-wound, SS inner + outer ring, flexible-graphite filler, suited to lethal-service tightness | ASME B16.20 |
The Class 150 weld-neck flange rating governs the vessel MAWP at 240 psig. PASS against the 150 psig design pressure.
The vessel is filled with water at 70 °F, vented to atmosphere, pressurized to 312 psig in three steps (50%, 80%, 100%), held at full test pressure for a minimum of 30 minutes, and visually inspected at every weld, nozzle, and flange per UG-99(g) and UG-99(k). The vessel is unpainted during the test, so any weep or weld leak is immediately visible. The Authorized Inspector witnesses the test and records the result on the U-1A data report.
| Component | Stress at test (psi) | 0.9 × S_y (psi) | Status |
|---|---|---|---|
| Shell (hoop) | 5,000 | 32,400 | PASS |
| Heads | 5,420 | 32,400 | PASS |
| Visual leak check, all welds and joints | zero leakage permitted | n/a | PASS (witnessed by AI) |
Test pressure stresses are an order of magnitude below the 0.9 S_y limit of UG-99(b) Note 35. The visual leak inspection is the controlling acceptance criterion in lethal service.
12 in NPS x 150 psig vertical lethal-service vessels are common in upstream and midstream gas processing as chlorine knockouts upstream of scrubbers and as H2S knockouts in sweetening unit overheads. The teaching value of the example is the UW-2 rule walkthrough, which translates a published process classification (the user marks the service as lethal) into a fabricated vessel with a long checklist of supplementary requirements: 100% radiography, weld-type restrictions, PWHT, no slip-on flanges, the visual-leak hydrostatic test, and the L stamp. Most of the rules are about reducing the likelihood and consequence of a weld leak; the math of the pressure-containment calc is the same as a non-lethal vessel.