NPT (ASME B1.20.1) vs ‘LP’ (LINE PIPE) API 5B FOR API 6A VALVES

October 26, 2023

NPT is for fittings, LP is for pipe but is also used for high pressure plain pipe nipples/swages. Yes the thread is the same, but…

The differences are:

B1.20.1 NPT V.S API 5B LINE PIPE (LP) THREAD FORM

1. NPT HAS NO RECESS – LP HAS A RECESS

2. FEMALE THREAD – PLANE OF VANISH POINT

- NPT: M & F THREADS OUT, LP DOES NOT

- LP: MALE THREAD 12°, FEMALE THREAD 35° BEFORE RECESS POINT

3. MALE THREAD – BREAK CORNER

- NPT: THREADS OUT

- LP: 65° AT BREAK CORNER (PLANE OF HAND-TIGHT)

4. NPT NEEDS SEALANT – LP DOES NOT MENTION IT IN THE STANDARD

If you are fitting a plain NPT male nipple into a female LP valve the differences of LP vs NPT female will be of no consequence. This is why some people say they are “The Same”. However, an LP female threaded valve, mated with LP male pipe or plain nipple provides a superior join. For this reason for 2” and over API 6A specifies LP must be used for 3,000 - 5,000 PSI.

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