What Is a Pipe Cap? End Closure

A pipe cap is a buttweld fitting that seals the end of a pipe. It is a dome-shaped or flat closure welded directly to the pipe with a full-penetration butt weld. Pipe caps per ASME B16.9 are permanent closures; removing them requires cutting the weld.

Property	Detail
Standard	ASME B16.9
Size range	NPS 1/2 to NPS 48
Shape	Ellipsoidal (standard), hemispherical, or flat
End preparation	Beveled (37.5° per ASME B16.25)
Wall thickness	Matches pipe schedule
Key dimension	End-to-end length (E)
Materials	ASTM A234 WPB (CS), A403 WP304/316 (SS), B366 (nickel)
Connection	Full-penetration butt weld

Dimensions

The critical dimension for a pipe cap is the overall length (E), measured from the butt weld end to the crown. ASME B16.9 specifies this length based on the nominal pipe size.

NPS	OD (mm)	Length E (mm)	Length E (in)
2	60.3	48	1.88
4	114.3	64	2.50
6	168.3	76	3.00
8	219.1	89	3.50
10	273.1	102	4.00
12	323.9	114	4.50
16	406.4	140	5.50

Pipe Cap vs Blind Flange

Both close a pipe end, but they differ fundamentally in accessibility.

Feature	Pipe Cap	Blind Flange
Connection	Butt welded (permanent)	Bolted (removable)
Access for inspection	Requires cutting	Unbolt and remove
Weight	Lighter	Heavier (flange + bolts + gasket)
Cost	Lower (fitting only)	Higher (flange + stud bolts + gasket)
Use case	Dead ends with no future access needed	Dead ends requiring periodic access
Hydro test	Common for temporary closures	Common for test boundaries

When to Use a Pipe Cap

Pipe caps are specified where:

• The pipeline terminates permanently (no future extension planned)
• Weight and cost must be minimized compared to a flanged blind
• The pipe end does not require periodic cleaning or inspection access
• Hydrostatic testing requires a temporary sealed end (caps are later cut off)

For small-bore piping (NPS 2 and below), forged caps per ASME B16.11 in socket weld or threaded configurations replace buttweld caps.

Material Matching

Cap material must match the pipe. ASTM A234 WPB caps pair with ASTM A106 Gr. B carbon steel pipe. Stainless steel caps in A403 WP316L pair with A312 TP316L pipe. Using a mismatched material can create a code violation and accelerate corrosion at the dissimilar joint.

Read the full guide to pipe fittings