Instrumentation tubing specifications
Remember that to connect transmitters to the process the pipes must not exceed ½ inch in diameter. 75% of the time the transmitters are connected using couplings with NPT threads.
The following tables show all the types of tube connections, the diameters are given for each component according to the piping class (the class is the rating).
NPT (National Pipe Thread) is an American standard for taper threads (NPT) and NPS (Nominal Pipe Size) for straight threads for connecting piping and couplings. The ANSI/ASME B1.20.1 standard covers 60 degree NPS threads with flat thread crest for sizes from 1/16 inch to 24 inches. The taper angle for all NPT threads is 3/4 inch per foot. The 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2 and 2 inch sizes are frequently used on pipes and couplings produced by most American suppliers.
Smaller sizes are sometimes used for compressed air. Larger sizes are rarely used because other connection methods are more practical above 3 inches in most applications.
NPS threads are not tight in the internal diameter of the pipe (Schedule 40). Due to the pipe wall thickness the real thread diameter is greater than the NPS threads, and considerably greater for small NPS threads.
Other pipe schedules have a different wall thickness but the outer diameter (OD) and the thread profile remain the same and therefore the internal diameter of the pipe is different to the nominal diameter.
Polyamide tube
Polyamide tubes are increasingly used today and are replacing copper tube for instrument air or gas supply. They are obviously quicker and easier to replace when worn than copper.
This type of tube can resist a pressure of up to 14 bars and a temperature of 70°C.
The diameters most commonly used for instrumentation are 4/6 mm, 6/8mm, 8/10mm or even 10/12mm. The diameters of these tubes are internal diameter/outer diameter.