Process connection types and its details
You have noticed the available process connection thread types when choosing a pressure sensor. Choices include NPT, BSP and UNF to name a few. Which is the proper one to use? The answer usually comes down to where you live.
In the 1800’s there were many types of screw threads that were available and virtually none were compatible. Many manufacturers had their own fastening systems. Fortunately there were some people that sought to standardize process threads.
British Standard Pipe
Joseph Whitworth was an English mechanical engineer who in 1841 devised a uniform threading system to combat the threading incompatibility problem. His system was called the British Standard Whitworth system. It later evolved into what is now known as the British Standard Pipe (BSP). This standard has been accepted internationally from Europe to Asia (except for the USA), and has been adopted as a standard scale used in pipe fittings.
BSP is a type of parallel thread and the pressure tight seal is enabled with a sealing washer, which is made of different materials depending on the pressure and the medium being used. The most common sizes are ¼ or ½ inch BSP.
National Pipe Taper
Meanwhile in the USA, a mechanical engineer, inventor, businessperson and president of the Franklin Institute in Philadelphia, William Sellers proposed the adoption of a screw thread system. It was considered easier to cut and use than the British design by Joseph Whitworth. It is said that this new standard helped begin the American Industrial Revolution. The thread design was presented in 1864 to the Franklin Institute and became the US standard known as National Pipe Tapered (NPT).
Compared with BSP, NPT is easier to cut and user-friendly, and is considered a booster to American Industrial Revolution. Being a kind of screw thread system, NPT is tapered thread with the pressure tight seal being made on the thread itself. The most common sizes are 1/4, or 1/2 inch NPT
Unified Thread Standard
William Sellers also developed what became the Unified Thread Standard. Straight thread connections known as SAE are now called UNF (Unified National Fine) under the Unified Thread Standard.
A common variation of this process connection is the M/F-250, or the autoclave fitting. This fitting has a cone at the end of it for pressure ranges above 10,000 psi. It is also commonly referred to as a Sno Trik® fitting as well though that is a brand name.
The most common types of UN (Unified National) thread are:
- UNC — Unified National Coarse Thread, comparable with the ISO metric thread.
- UNF — Unified National Fine Thread.
Japanese Industrial Standards
The present day Japanese Industrial Standard (JIS) has its beginnings in the days after World War II. It came to be after the Industrial Standardization Law of 1949. Many types of screw threads are available worldwide under the JIS standard.
ISO Metric Screw Thread
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.
The “M” designation for metric screws indicates the nominal outer diameter of the screw, in millimeters (e.g., an M6 screw has a nominal outer diameter of 6 millimeters).
Sealing Your Threads
It’s important to note that your process connection must match the pipe fittings where the sensor will be installed. Some threads may fit in an incompatible location. For example, a male NPT can fit into a tapered female BSP, but they will not form a proper seal.
There is a difference in how straight and tapered threads seal. A straight thread will rely on either a seal ring or a gasket. However, a tapered thread requires thread sealant — either a compound or thread tape.
Making The Right Choice
There really isn’t a process connection that is better than the others. All were developed over time in different parts of the world for different reasons. Your choice of process connections isn’t driven by which is a better system, but by the pipe fitting where your pressure transducer needs to go.
Though there might be unique official standards used in different parts of the world, the good news is that any thread standard can be procured anywhere in the world.
A well-chosen and designed pipe-to-pipe connection can lead to longer life of the piping network and efficient fluid flow.
