BSP / NPT Adapters & Nipples

Stainless adapters and hydraulic nipples are used to connect equipment with different thread standards, to change port sizes and to plug unused ports. The BSP family has a 55° thread profile and the NPT family a 60° profile; although they look similar, they are not interchangeable. On parallel (BSPP) threads, sealing is achieved with a sealing washer or a conical surface; on tapered (BSPT and NPT) threads, it is made on the threads themselves with a suitable sealant.

Types of stainless nipples and adapters

• Hex nipples and reducing adapters: connect two female ports or fittings over a short distance, with a size change where needed.
• BSP adapters with 60° cone seats: provide the cone seat to which BSP female swivel hose ends are connected.
• Adapters with bonded seals: sealing at the BSPP port with a bonded seal; for ports that are disconnected frequently.
• BSP to NPT adapters: join equipment built to European and American thread standards.
• NPT reducing bushings and hex plugs: size reduction and plugging unused ports.

BSPP, BSPT and NPT: three threads, three sealing principles

Choosing the right thread: sizes that are often confused

The most commonly confused sizes are 1/2 and 3/4 inch: in both sizes, BSP and NPT alike have 14 threads per inch, so the wrong part threads easily into the port for a few turns and only binds when you tighten it. On a parallel thread the diameter is constant along its length; on a tapered thread, a difference in diameter can be measured between the first and last threads. At points that are disconnected and reconnected often, a parallel thread with a sealing washer gives more repeatable sealing than a tapered thread; a tapered thread seats a little deeper every time it is tightened. When selecting reducing adapters and bushings, the size and standard of each end should be specified separately.

Assembly and sealing

On tapered threads, sealant is applied leaving the first one or two threads bare; this keeps particles from entering the line. To counter the risk of galling on stainless-to-stainless threads, clean threads and a suitable anti-seize compound are used. On bonded seal connections, the seal must sit fully on the port face; a damaged or previously tightened seal must not be reused. If a connection runs in freely for a few turns and then suddenly tightens, stop: the standard is most likely wrong.

Which industries use them?

Points on ships and in shipyards where pumps and valves of different origins meet on the same line; gauge, sampling and drain connections in chemical and food plants; pressure gauge and test point outlets on fire pump skids; and imported machine ports in rolling mills and board mills.