Article
A pipe cap is a dome-shaped fitting welded to the end of a pipe to close it permanently. It is one of the simplest fittings, yet it carries the full line pressure, so the right thickness and material matter just as much as for an elbow or tee.
Where caps are used
- Dead ends of headers and manifolds
- Future connection points that will be cut open later
- Ends of pressure vessel nozzles and small drums
- Bottoms of drip legs and dirt pockets
Cap vs blind flange vs plug
| Closure | Joint | Can be removed? | Typical use |
|---|---|---|---|
| Buttweld cap | Welded | No (must be cut off) | Permanent closure, lowest cost |
| Blind flange | Bolted | Yes | Closures that need inspection or future access |
| Threaded plug / socket weld cap | Threaded or socket welded | Plug yes, SW cap no | Small-bore lines, usually 2" and below |
Shape and dimensions
Buttweld caps are usually elliptical (2:1) heads. ASME B16.9 gives the length (E) for each size and, for heavier walls, a longer length (E1). The cap end is bevelled to match the pipe and must have the same wall thickness, so the schedule is part of the order.
Materials
- Stainless steel – ASTM A403 WP304/304L, WP316/316L, WP321, WP347 and others.
- Carbon steel – ASTM A234 WPB for general service, ASTM A420 WPL6 for low temperature.
- Alloy steel – ASTM A234 WP11, WP22 and WP91 for high-temperature lines.
Checks before dispatch
Caps should be checked for wall thickness at the knuckle, where forming tends to thin the material, and for bevel angle and root face. A cap that is thinner than the pipe at the knuckle is a weak point even if the bevel end measures correctly.
A complete cap description
Cap 3", SCH 40S, ASTM A403 WP316L-S, ASME B16.9, bevelled end – 40 nos
Frequently asked questions
What is the difference between a pipe cap and a blind flange?
A cap is welded on and is permanent. A blind flange is bolted and can be removed for inspection or future connections.
Are pipe caps rated for full line pressure?
Yes. A B16.9 cap with the same schedule and grade as the pipe is designed for the same pressure rating as the matching straight pipe.
What shape is a buttweld cap?
Most are 2:1 elliptical heads, made by pressing or spinning.
Key takeaways
- Caps close a line permanently and cheaply.
- Use a blind flange if the end must be opened later.
- Match cap schedule and grade to the pipe and check thickness at the knuckle.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.