A common query from our customers is: “What is the maximum pressure that the different sizes of NPT threads support?”.
This is really a question, thus formulated, without an answer. The ability to seal an NPT thread depends on many factors, so there is no single answer…
Tapered threads, such as NPT, are the most widely used unions in the industry since they provide watertight unions, are partially orientable, do not require gaskets or sealing rings (only PTFE tape) and withstand high pressures.
However, its tightness depends to a large extent on the machining quality of both parts (the female and the male) and the thickness and quality of the materials involved. This is especially important for high pressure applications.
Also in materials that are likely to produce “galling” (mesh), such as austenitic stainless steels, the skill of the assembler is very important to avoid this phenomenon and ruin both pieces.
ABAC produces tapered NPT threads up to 1″, according to ASME B1.20.1, as connection threads in the vast majority of its products, rolled or machined on CNC lathes. This provides them with high quality such as dimensional uniformity, allowing reliable sealing up to high pressures.
The ASME B1.20.1 standard defines dimensions and controls, but not minimum pressures or thicknesses. However, there are other rules that are taken into account:
The ASME B16-11 – FORGED FITTINGS, SOCKET-WELDING AND THREADED standard establishes 6000 psi (413 bar) at room temperature as a limit of its scope for threaded fittings 1/8″ to 4″ NPT.
For these threads, this standard defines thicknesses, materials and general dimensions for the different maximum pressures.
The API 6A – SPECIFICATION FOR WELLHEAD AND CHRISTMAS TREE EQUIPMENT standard, for its part, accepts the use of Line Pipe threads (the equivalent of API NPT) from 1/2″ up to 10,000 psi, from 3/4″ to 2″ up to 5,000 psi, and 2 1/2″ to 6″ up to 3,000 psi.
Threads smaller than 1/2″ Line Pipe are not accepted.
On the other hand, ABAC mainly uses the standard MSS SP99 – Instrument Valves for the design and validation of its Valves and Manifolds. This standard is applicable for connections up to 1″ and maximum operating pressures of 15,000 psi (approx. 1000 bar) at room temperature.
Through this standard, ABAC has validated the NPT threads for its accessories, obtaining the following results (to SS 316):
1/4″ NPT = 15,000 psi (1034 bar)
1/2″ NPT = 15,000 psi (1034 bar)
3/4″ NPT = 10,000 psi (690 bar)
1″ NPT = 10,000 psi (690 bar)
As a final conclusion, we can say that there is no international consensus in this regard, but as a manufacturer, ABAC recommends and guarantees these pressures.
Another point to take into account, especially for use in the Oil & Gas industry, is that these maximum pressures are considerably reduced in products suitable for Sour Gas service, according to NACE MR 0103 or MR 0175. This is because, while these high pressures can only be obtained by materials hardened by cold work, for NACE service it is necessary that the materials do not exceed 22 HRC and, consequently, the maximum allowable pressures are reduced proportionally (consult with the Engineering Department of ABAC).