Why is the dry seal pipe screw thread designed with a
tapered external screw thread and an parallel internal screw thread?
Let us start the explanation using the National Pipe Taper (NPT)
screw thread per ANSI/ASME B1.20.1. The National Pipe Taper (NPT) screw thread has loose
control of the major and minor diameters. As a result when assembled
there is most always two helical leak paths, one along the major
diameter and one along the minor diameter. Pipe sealant fixes this
problem except in situations where the chemical in the pipe eats
In this case a mechanical seal is needed and the National Pipe Taper
Fuel (NPTF) dry seal screw thread was designed and is documented in ANSI/ASME B1.20.5. The National Pipe
Taper Fuel (NPTF) dry seal screw thread is designed to have a mismatch
at the major and minor diameters which deforms under wrench tight
condition and thus forms a mechanical seal. As you can figure it
takes a good force to deform the entire major and minor diameters
along the whole thread.
To gain a better seal a tapered male National Pipe Taper Fuel (NPTF)
dry seal screw thread is mated with a parallel female National Pipe
Straight Fuel (NPSF) dry seal screw thread. This combination gets a
better seal because of the reduced assembly force required and the
concentration of the compressive force at one point which better
deforms the threads into each other.
With the British Standard Pipe Taper (BSPT) pipe screw thread per ISO 7,
during the design process I am sure that Mr. Whitworth sought to
eliminate the helical leak path by adding the radii. Theoretically
this would work when assembling the internal Rc tapered British
Standard Pipe Taper (BSPT) pipe screw thread with the external R
tapered British Standard Pipe Taper (BSPT) screw thread, but all
screw threads are so full of manufacturing errors that the perfect
screw thread just does not result. Leaks then occur which requires
the use of pipe sealant; however; the situation of the chemical
being transmitted in the pipe eating the sealant still exists.
Now we are back to the use of the female internal Rp parallel British
Standard Pipe "Taper" (BSPT) screw thread with the male
external R tapered British Standard Pipe Taper (BSPT) screw thread
to concentrate the pressure at one point and create a mashed together
thread and a dry seal.
This data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root document: ANSI/ASME B1.20.1; ANSI/ASME B1.20.5 and ISO 7/1; which are all copyrighted documents. To purchase a copy visit an Authorized Reseller.
Original Posting: 12/18/2008
Last Revision: 6/4/2013
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