NPT stands for National Pipe Tapered threads, also known as ANSI/ASME B1.20.1 pipe threads. It is a U.S. standard for measuring tapered threads on threaded pipes and fittings. Unlike straight threads found on a bolt, a taper thread will pull tight, making a closer seal. A sealant compound or PTFE (Polytetrafluoroethylene) tape must be used to ensure a leak-free seal. What is NPT? By Michelle Difrangia, www.hoseassemblytips.com
Kyle Fetherston, who is based in the Product Development team at Oasis Engineering Limited, has just released a report about Best Practice for NPT Sealing. A customer identified that they use a different thread sealing method to Oasis’s instruction manual, so he investigated and produced a report on best practice for sealing Stainless Steel NPT threads.
After discussion with other companies and investigating websites/online forums, Fetherston discovered that most fitters just used the methods they were initially taught which resulted in quite a variety of thread sealing methods (some only use tape and others suggest anaerobic sealant, dope, or a combination of these). The general consensus was, “we do it this way because it works and it is how we’ve always done it”.
Articles from Plant Engineering, Australian Mining and Luoke Adhesives state that PTFE tape is designed for the lubrication of threads (not sealing) and that for NPT threads carrying high pressure gas, anaerobic thread sealant is recommended.
Fetherston’s own investigation and testing has determined that a correctly tightened NPT fitting with correctly applied PTFE thread tape provides adequate sealing at temperatures from -40°C to +85°C, at pressures up to 410 bar. It can also be repositioned slightly without leakage and pressurized immediately after installation.
PTFE thread tape also provides suitable lubrication to prevent galling in stainless steel threads, provided it is applied in such a way that it covers all the threads of the male fitting. If galling is still a concern or tape will not be applied to all threads, a thread sealant with PTFE such as Loctite 567 can be applied over the top of the tape.
Fetherston’s findings indicate that where practical, yellow gas rated PTFE tape should be used in CNG systems. The higher density of gas rated PTFE tape will provide peace of mind that the threads are adequately sealed. The yellow colour also provides a recognizable indication the fitting is carrying natural gas.
“It is great to be part of the Kiwi team at Oasis Engineering with a global focus on setting the standard for Best Practice in the CNG industry.” Kyle Fetherston.
For a copy of the full report check the company’s website (www.oasisngv.com).