American standard NPT (National Pipe Taper) threads are the default pipe thread used in the USA. The standard is a conical (tapered) design internal and external screw thread. There are a set of codified sizes correlating to standard pipe inch sizes. The NPT are reasonably simple to use and understand, but then enter alternative uses and assembly variations.
One alternate use is “Dry-Seal”; which must seal without the benefit of pipe goop/sealant/Teflon-tape. Designated NPTF (National Pipe Taper Fuel) and NPSF (National Pipe Taper Fuel); these threads are used where the liquid transmitted in the pipe would dissolve the sealant.
Another set of pipe threads is designated by the CGA (Compressed Gas Association) as NGT (National Gas Taper) and NGO (National Gas Outlet). These are used for vaporous gases with special combinations assigned to prohibit unintentional cross connection.
Still associated with pipe screw threads are oilfield screw threads. The API (American Petroleum Institute) has codified many variations with specific uses in the oilfield industry. In some cases, oilfield screw threads are like NPT, specifically with the case of LP (Line Pipe). Other API variations are completely different from NPT; and a few are not even codified by API but are kept proprietary to specific companies.
Alternate Assembly Variations
It seems simple, a conical thread screws into a conical mate, but this is not always true. Some conditions require a parallel screw thread to be screwed into a tapered mate.
Gage Crib Worldwide Inc. specializes in screw thread measurement tools. The best screw thread measurement tool for pipe threads is often screw thread gages. These screw thread gauges are specifically designed for each type of pipe thread. When you contact us we can guide you to the correct screw thread gages to meet your quality assurance requirements.