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Alloy Steel Pipe Cap
Alloy Steel Pipe Cap Manufacturer & Exporter in India
A cap and a plug both close a line, and the difference is which one carries the thread. A buttweld cap has no thread at all, it is welded permanently over the end of a pipe. A plug screws into a female port. So the question is not which closure you prefer, it is whether the opening is a pipe end or a threaded port, and only one answer will physically fit. Solitaire Overseas manufactures and exports alloy steel pipe caps from India, buttweld to ASME B16.9 in ASTM A234 WP grades, NPS 1/2 inch to 48 inch. Below: cap versus plug, wall matching, and the grade table.
Part of our alloy steel fittings range. For threaded closures see alloy steel plugs
What Is an Alloy Steel Pipe Cap?
A buttweld cap is a domed fitting welded over the end of a pipe to seal it against pressure. The bevelled end matches the pipe wall so the joint is a full penetration butt weld, and the dome shape carries pressure efficiently, the same reason pressure vessel heads are dished rather than flat.
It is a permanent closure. Once welded, a cap can only be removed by cutting. That is the right answer for a line end that will not be reopened, or for the terminal end of a header. Where the closure needs to come off for maintenance or future tie-in, a blind flange or a threaded plug is the better choice, since either can be removed without hot work.
Matching the Cap to the Pipe
- Schedule must match. The cap wall is matched to the connected pipe schedule so the weld joins equal thicknesses. A mismatch creates a stress concentration at the joint and complicates the weld preparation.
- Grade must match. A WP11 cap on P22 pipe is a metallurgically weaker point at the end of a line designed for P22 duty. Match the grade, and state the class.
- Bevel to ASME B16.25. Weld end preparation follows the same standard as every other buttweld fitting in the line.
- Size range. NPS 1/2 inch to 48 inch covers most requirements, matching the buttweld fitting range.
Technical Data
Alloy Steel Pipe Cap Specifications
| Parameter | Specification |
|---|---|
| Material standard | ASTM A234 / ASME SA234 WP grades |
| Dimensional standard | ASME B16.9 |
| Grades | WP1, WP5, WP9, WP11, WP12, WP22, WP91 (state class or type) |
| Size range | NPS 1/2 inch to 48 inch |
| Wall thickness | SCH 10, 20, 40/STD, 80/XS, 120, 160, XXS |
| End type | Bevelled to ASME B16.25 |
| Construction | Seamless and welded |
| Certification | EN 10204 3.1 mill test certificate. PMI, NDE and third-party inspection on request |
Pipe and Cap Grade Correlation
Matched to the pipe grade it welds into, so fitting and pipe share compatible chemistry and high-temperature behaviour:
| Pipe Grade (A335) | Buttweld (A234) | Forged (A182) | Chromium | Service |
|---|---|---|---|---|
| P1 | WP1 | F1 | Not specified (carbon-moly) | Moderate temperature steam |
| P5 | WP5 | F5 | 4.0 - 6.0% | Refinery hydrogen and sulphur service |
| P9 | WP9 | F9 | 8.0 - 10.0% | Higher temperature refinery piping |
| P11 | WP11 | F11 | 1.00 - 1.50% | The workhorse grade |
| P12 | WP12 | F12 | 0.80 - 1.25% | General elevated temperature service |
| P22 | WP22 | F22 | 1.90 - 2.60% | Higher temperature and pressure than P11 |
| P91 | WP91 | F91 | 8.0 - 9.5% | Modern high-temperature power plant piping |
A234 Classes: State the Class, Not Just the Grade
Most chrome-moly A234 grades are divided into classes with different mechanical properties. A purchase order naming only the grade has not fully specified the fitting:
| Grade | Classes Available | What Changes |
|---|---|---|
| WP11 | Class 1, Class 2, Class 3 | Carbon and manganese limits differ; Class 3 is the strongest |
| WP22 | Class 1, Class 3 | Class 3 carries substantially higher yield and tensile |
| WP5 / WP9 | Class 1, Class 3 | As above |
| WP12 | Class 1, Class 2 | Two classes only |
| WP91 | Type 1, Type 2 | Types rather than classes. Tempering temperature is the variable |
Cap or Plug?
Both close off a line. Which one fits depends on the opening, not on preference:
| Buttweld Cap | Threaded Plug | |
|---|---|---|
| Thread | None. Welded | Male, screws in |
| Fits | OVER a male pipe end | INTO a female port |
| Closes | The end of a pipe run | An opening in a fitting, valve or vessel |
| Standard | ASME B16.9 | ASME B16.11 |
| Permanence | Permanent, must be cut off to reopen | Removable |
| Ask yourself | Is it an open pipe end? Use a cap | Is it a threaded port? Use a plug |
Threaded closures are covered on our alloy steel plugs page, which includes hex, square and round head types.
Applications
- Header and manifold ends, permanently closing the terminal end of a distribution header.
- Power generation, steam and feedwater line closures in WP22 and WP91.
- Refineries, process line ends in WP5, WP9 and WP11.
- Pressure testing, temporarily capping a spool for hydrostatic test before final tie-in.
- Future tie-in points, capping a stub left for a planned expansion, though a blind flange is often preferred where the tie-in is near term.
Quality Control & Testing
- Positive Material Identification, grade verified before dispatch, essential where several WP grades look identical.
- Hardness testing, required per batch on A234 alloy grades and the practical check that heat treatment was done correctly.
- Heat treatment records, the class of an A234 alloy fitting depends on heat treatment, so the records are part of the specification.
- Dimensional inspection, verified to ASME B16.9 or the applicable standard.
- NDE on request, radiography or ultrasonic examination per code or client specification.
- Certification, EN 10204 3.1 as standard, 3.2 with third-party inspection on request.
Frequently Asked Questions:
A buttweld cap has no thread and is welded permanently over the end of a pipe. A plug is male threaded and screws into a female port on a fitting, valve or vessel. They close from opposite sides and are not interchangeable, so check whether the opening is an open pipe end or a threaded port before ordering.
Only by cutting it off. A buttweld cap is a permanent closure, which makes it the right choice for a line end that will not be reopened. Where the closure may need to come off for maintenance or a future tie-in, a blind flange or a threaded plug is the better option, since neither requires hot work to remove.
ASTM A234 WP91. Matching the fitting grade to the pipe grade keeps chemistry and creep behaviour consistent through the weld. On P91 in particular, post-weld heat treatment and hardness verification matter, since an incorrectly tempered joint leaves the cap outside specification.
NPS 1/2 inch to 48 inch, matching the buttweld fitting range, in schedules from SCH 10 through XXS. The cap wall is matched to the connected pipe schedule so the butt weld joins equal thicknesses.
Request a Quote
Send your size, schedule, grade and class, and we will confirm availability, certification and lead time.