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End Caps
A comprehensive engineering reference for EPC contractors, piping engineers, pressure vessel designers and procurement teams — covering pipe cap geometry, dome head pressure design, ASME B16.9 dimensional data, wall thickness design per ASME B31.3, future tie-in specifications, material grades, NACE compliance and full project documentation for all piping and pressure vessel end closure applications.
End Cap Geometry, Head Types
& Engineering Function
Hemispherical · Toriconical · Flat End · Future Tie-In
Definition and Engineering Function
A pipe end cap (also called a pipe cap or butt weld cap) is a fitting butt-welded to the open end of a pipe to provide a permanent or semi-permanent pressure-tight closure. The end cap is the simplest closed-end fitting in piping — it seals the pipe bore with a shaped head, transmitting the full end thrust of the contained pressure into the pipe wall through the circumferential butt weld. End caps are used in four primary engineering applications: (1) permanent pipe end closure — terminating a pipeline or pipe spool that has no downstream connection; (2) future tie-in point — closing a branch or pipe end that will be connected to future piping during plant expansion or phased construction; (3) hydrostatic pressure testing — temporarily capping a pipe spool or pipeline section for pressure testing before commissioning; and (4) pigging operations — end caps form the closure on pig launcher and pig receiver barrels during pipeline pigging.
A capped pipe end under internal pressure experiences an end thrust force equal to the internal pressure multiplied by the pipe bore area: F = P × π/4 × D_i². For a 12" NPS pipe at 100 bar (10 MPa), this is approximately 10 × 10³ × π/4 × (0.304)² ≈ 727 kN (74 tonnes) acting on the end cap and transmitted back into the pipe as longitudinal tensile stress. This end thrust is the primary load that the end cap, its butt weld and the adjacent pipe section must carry. The butt weld between the end cap and the pipe must be a full-penetration weld — a fillet weld is never acceptable for pressure-containing end caps. The adjacent pipe section must also be checked for longitudinal pressure stress in addition to hoop stress when end caps are fitted.
End Cap Anatomy
Head Geometry Types
Pressure Design, ASME B16.9
& Dimensional Reference
End Thrust · Wall Schedule · NPS ½"–24"
Head Thickness Design — 2:1 Ellipsoidal Head
The minimum required wall thickness of a 2:1 semi-ellipsoidal end cap head is governed by ASME VIII Div.1 UG-32(d) (for pressure vessel applications) and ASME B31.3 Clause 304.3.2 (for pipe fittings). The design formula for a 2:1 ellipsoidal head gives a minimum thickness that is identical to the required thickness of the adjacent cylindrical pipe shell for the same pressure and material — this is the key engineering advantage of the 2:1 ellipsoidal profile. Specifying a cap in the same schedule as the mating pipe is therefore always adequate for pressure design, provided the cap is a standard ASME B16.9 fitting in the correct schedule.
F_end = P × (π/4) × D_i² // End thrust [N]; P = pressure [Pa], D_i = pipe bore [m]
// WORKED EXAMPLE: NPS 8" Sch 80 cap, A234 WPB, P=70 bar=7.0 MPa, D_i=193.7mm, S=138 MPa, E=1.0
t_min = 7.0×193.7/(2×138×1.0−0.2×7.0) = 1355.9/274.6 = 4.94 mm // Sch 80 wall = 12.7 mm >> adequate
F_end = 7.0×10⁶×(π/4)×(0.1937)² = 205.6 kN // ~21 tonnes end thrust on butt weld
| NPS | OD (mm) | Sch STD Wall (mm) | Sch XS Wall (mm) | Cap Length — STD (mm) | Cap Length — XS (mm) | Head Depth h (mm) |
|---|---|---|---|---|---|---|
| ½" | 21.3 | 2.77 | 3.73 | 25 | 25 | 5 |
| 1" | 33.4 | 3.38 | 4.55 | 38 | 38 | 8 |
| 2" | 60.3 | 3.91 | 5.54 | 51 | 51 | 14 |
| 3" | 88.9 | 5.49 | 7.62 | 64 | 64 | 21 |
| 4" | 114.3 | 6.02 | 8.56 | 76 | 76 | 27 |
| 6" | 168.3 | 7.11 | 10.97 | 102 | 102 | 41 |
| 8" | 219.1 | 8.18 | 12.70 | 127 | 127 | 53 |
| 10" | 273.0 | 9.27 | 12.70 | 152 | 152 | 67 |
| 12" | 323.8 | 9.53 | 12.70 | 178 | 178 | 79 |
| 16" | 406.4 | 9.53 | 12.70 | 216 | 216 | 100 |
| 20" | 508.0 | 9.53 | 12.70 | 254 | 254 | 124 |
| 24" | 609.6 | 9.53 | 12.70 | 305 | 305 | 149 |
ASME B16.9-2018. Cap Length = overall length from bevel end face to the outside crown of the head. Head depth h = D_i/4 for 2:1 semi-ellipsoidal profile. Wall thickness at bevel end matches the pipe schedule per ASME B36.10M. The head wall thickness (at the crown) is typically equal to or greater than the cylindrical shell wall for standard B16.9 caps. For high-pressure schedules (Sch 160 and XXS), confirm availability with manufacturer as not all sizes are standard stock. All dimensions in mm.
The most common specification error for end caps is ordering a heavier-than-needed schedule without verifying that the bevel end thickness matches the pipe schedule. When an end cap with a heavier wall (e.g. Sch 80 cap) is butt-welded to a lighter pipe (e.g. Sch 40), the thicker cap bevel creates an internal step at the weld root that acts as a stress concentration and flow restriction. ASME B16.25 and B31.3 allow a maximum internal misalignment of 1.6 mm at a butt weld — beyond this, a taper transition must be machined on the heavier component. Always specify the cap in the same schedule as the mating pipe, or verify that the bevel end can be machined to match if schedules differ.
Material Grades, NACE & HIC Compliance
& Corrosion Considerations
NACE MR0175 · HIC TM0284 · PWHT · Charpy Impact
| Material | ASTM Grade | Yield (MPa) | Temp Range (°C) | Corrosion | Key Application |
|---|---|---|---|---|---|
| CS — Standard | A234 WPB | ≥240 | −29 to +538 | Low | Process piping, utilities, oil & gas, general service |
| CS — High Strength | A234 WPC | ≥275 | −29 to +538 | Low | High-yield piping, Class 600+ end closures |
| LTCS −46°C | A420 WPL6 | ≥240 | −46 to +343 | Low | LNG, ethylene, cryogenic plant end closures |
| LTCS −73°C | A420 WPL3 | ≥205 | −73 to +343 | Low | Very low temp, LNG/ethylene, deep cryogenic |
| 1.25Cr-0.5Mo | A234 WP11 Cl.1 | ≥205 | −29 to +593 | Moderate | High-temp refinery, hydrogen service closures |
| 2.25Cr-1Mo | A234 WP22 Cl.1 | ≥205 | −29 to +621 | Moderate | Hydrocracker, reformer, high-temp closures |
| 9Cr-1Mo-V | A234 WP91 | ≥415 | −29 to +649 | Moderate | Ultra-supercritical power steam closures |
| SS 304/304L | A403 WP304/304L | ≥205 | −196 to +816 | High | Chemical, pharma, food, cryogenic end caps |
| SS 316/316L | A403 WP316/316L | ≥205 | −196 to +816 | Very High | Offshore, chloride, chemical, pharma |
| SS 321 | A403 WP321 | ≥205 | −196 to +816 | High | High-temp SS, sensitisation resistance |
| Duplex 2205 | A815 WP-S31803 | ≥448 | −50 to +315 | Very High | Offshore sour, seawater, chloride end caps |
| Super Duplex | A815 WP-S32750 | ≥550 | −50 to +300 | Extreme | Subsea, HPHT, severe sour service |
| Inconel 625 | B366 WPN-06625 | ≥276 | −196 to +980 | Extreme | Corrosive acid, high-temp, offshore |
| Titanium Grade 2 | ASTM B363 | ≥275 | −196 to +315 | Extreme | Seawater, wet chlorine, halide service |
NACE and HIC Requirements for Sour Service
End caps in sour H≶S service must comply with NACE MR0175 / ISO 15156 hardness limits — maximum 22 HRC (237 HB) throughout the full cross-section for carbon steel (A234 WPB). Standard normalised A234 WPB typically complies, but every heat must be hardness-verified on the cross-section of the finished fitting, not just the input raw material. For sour wet gas service with elevated H≶S partial pressure (above 0.0003 MPa in liquid water-containing systems), HIC-resistant plate material and the NACE TM0284 HIC test apply. For duplex 2205 end caps, ferrite content (FN 35–65%), hardness (≤28 HRC), and Charpy impact are mandatory.
Applications, Installation Notes
& Quality Control and Documentation
Petrochemical · Offshore · LNG · Power · Chemical
Applications by Industry and Use Case
End caps are structural components in pig launcher and pig receiver barrel assemblies on oil and gas pipelines. The launcher barrel is a pipe spool larger than the pipeline bore that allows a pig (pipeline inspection gauge or cleaning device) to be loaded. The open end of the barrel that the pig enters through is closed by a large-diameter end cap — typically a 2:1 ellipsoidal or hemispherical forged or fabricated cap. This end cap is designed for the full pipeline operating pressure and must be rated for repeated opening and closing during pigging operations. The closure mechanism is typically a quick-opening closure (QOC) rather than a welded cap. For permanent closures on pipeline dead ends, A234 WPB Sch 40 or STD caps are standard.
In phased plant construction, future tie-in points are pipe ends that are capped during Phase 1 and will be connected to Phase 2 piping at a later date. Future tie-in end caps must meet: (1) the same pressure class and material specification as the adjacent pipe; (2) a vented cap with a 1/2" NPT vent plug for safe depressurisation before cutting during Phase 2; (3) the weld between the cap and pipe must be a full-penetration butt weld that can be cut out during Phase 2 construction — the cap must be long enough that cutting it off does not damage the pipe or require weld repair on the line pipe itself; (4) the cap must be tagged with the future tie-in reference number from the plot plan and piping isometric drawings.
End caps are routinely installed on pipe spool fabrications for hydrostatic pressure testing in the fabrication workshop before field erection. Test caps are temporarily butt-welded to the pipe ends and removed after the test. For economy, test caps are typically in the minimum standard pipe schedule for the applicable NPS (even if the pipe spool is in a heavier schedule), since test caps are only pressurised once to 1.5× design pressure and not subject to cyclic operating loads. All butt welds on test caps must be full penetration and must pass visual inspection before the hydrotest. Test caps are not reused once removed.
End caps in the same material as the pressure vessel (A516 Gr.70, SS 316L, duplex 2205 etc.) are used to close off nozzle connections on vessels and heat exchangers that are not in active service — spare nozzles, future connection points, and inspection nozzles opened only during turnarounds. Vessel nozzle end caps must be designed per ASME VIII Div.1 UG-32 as pressure vessel heads — not simply as pipe caps per B16.9 — and must be covered by the vessel manufacturer's Data Report and ASME U-stamp if the vessel is ASME-stamped.
Standard A234 WPB end caps are used throughout refinery and petrochemical plant piping for pipe terminations, drain and vent connections, sampling point closures and dead legs. A234 WP11 and WP22 for high-temperature hydrogen service; A403 WP316L for corrosive chemical service; A815 S31803 (duplex 2205) for sour service with chloride. NACE-compliant A234 WPB (hardness ≤22 HRC, verified per heat) required for all sour service end caps. HIC-resistant plate required for high H≶S partial pressure wet gas systems.
A420 WPL6 (impact tested at −46°C) and A420 WPL3 (−73°C) end caps are specified for LNG plant piping and cryogenic process systems. Charpy V-notch impact testing at MDMT mandatory — minimum 27 J (20 ft-lb) average per heat. A403 WP304L end caps for cryogenic liquid piping where the austenitic microstructure provides inherent toughness without separate Charpy testing. All cryogenic end caps must be tested and reported in the EN 10204 3.2 MTC.
Quality Control
QC for butt weld end caps per ASME B16.9 covers: (1) OD and length — outside diameter at bevel end (must match pipe OD ± B16.9 tolerance) and cap length (from bevel face to outside crown); (2) Wall thickness at bevel end — must match the specified schedule per B36.10M; (3) Head profile and depth — crown depth h ≥ D_i/4 for 2:1 ellipsoidal profile; (4) Bevel geometry — bevel angle, land and root face per ASME B16.25; (5) Head wall thickness at crown — UT measurement to verify crown wall ≥ cylindrical shell minimum; (6) PMI — 100% on all SS, duplex and alloy caps; (7) NDE — MT or PT on all seam welds (if fabricated from plate) and on the knuckle transition zone.
Export Packaging and Preservation
- End caps nested in VCI poly bags (CS/alloy) or clean poly bags (SS/duplex) and packed in compartmented cartons for small sizes (NPS ½" to 6"); large caps individually wrapped and wooden-crated
- Bevel ends protected with plastic bevel protectors; vent port threads (if applicable) plugged with plastic thread protectors not metal
- Each cap tagged or stencilled with: NPS, schedule, material grade, heat/lot number, vent port size (if applicable) and PO reference
- MTC (EN 10204 3.1/3.2), dimensional inspection report (OD, length, wall at bevel, head depth), bevel geometry report, PMI (alloy grades), Charpy impact (LTCS/duplex), NACE hardness (sour service), HIC report (sour with high H₂S), ferrite content (duplex), PWHT certificate (alloy steel) in waterproof sealed envelope with each consignment
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 (TPI co-signed) for offshore / NACE / alloy / safety-critical |
| 02 | Dimensional Inspection Report | ASME B16.9 | OD, length, wall at bevel end, head depth — all mandatory |
| 03 | Bevel End Geometry Report | ASME B16.25 | Bevel angle, land, root face confirmed per B16.25 |
| 04 | Crown UT Wall Thickness Report | ASME B16.9 / project spec. | Crown wall ≥ shell minimum; UT measurement mandatory for all caps |
| 05 | MT / PT Surface Exam Report | ASME V Art.7 | MT for CS/alloy; PT for SS/duplex; all seam welds and knuckle zone |
| 06 | PMI Report (XRF / OES) | Project specification | 100% SS, duplex, Inconel, alloy and all exotic grade caps |
| 07 | Charpy Impact Report | ASTM A370 / EN ISO 148 | Mandatory for A420 WPL6/WPL3 and duplex; ≥27 J at MDMT |
| 08 | NACE Hardness Report | ASTM E10 / NACE MR0175 | Sour service CS: ≤22 HRC full cross-section; duplex: ≤28 HRC |
| 09 | HIC Test Report | NACE TM0284 | Required for high H≶S partial pressure sour service caps |
| 10 | Ferrite Content Report (Duplex) | ASTM E562 / image analysis | Mandatory for duplex 2205 and super duplex; FN 35–65% |
| 11 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports butt weld end caps in all head profiles (2:1 semi-ellipsoidal, hemispherical, toriconical) per ASME B16.9, in A234 WPB/WPC, A420 WPL6/WPL3, A234 WP11/WP22/WP91, A403 WP304L/316L/321, A815 WP-S31803/S32750, Inconel 625, Hastelloy C276 and Titanium Grade 2. NPS ½"–48", schedules Sch 10S to XXS. Vented future tie-in caps (1/2" NPT vent) available as standard. EN 10204 3.1/3.2 MTC, crown UT, PMI, Charpy impact, NACE hardness, HIC testing (TM0284), ferrite content (duplex), PWHT certificates, TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.