ok, apparently a short primer on threaded connections is in order
tapered threads, of which the NPT is an example, when made up will impart a radial compressive stress on the pin and a tensile stress on the box end (or top in our application)
- note that "V" threads are utilized which seal by (plastic) thread deformation of the thread itself
-> so lesson #1 is that if the materials will not deform plastically, but fail instead by brittle fracture; then these are NOT well suited for this thread form
in an effort to enable the sealing to occur at lower stress levels several types of 'thread sealants' are commonly used; pipe 'dopes' of various types, Teflon tape, and conventional adhesives
- note that, unless an adhesive is used, some amount of connection compression is still required for sealing - which will have a radial tensile component in the box end
so why then are NPT connections utilized ?
well, IF a threaded connection is desired, in this country they are fairly cheap to make and fairly reliable
unless:
- there are repeated make-and-breaks
- the components fail by brittle fracture
- a metal pin end is put into a plastic box
are the reasons for the continuing failures becoming apparent ?
there are other types of threaded connections which are much more suitable for plastics, and European mfgrs use them routinely:
parallel coarse (non-sealing) threads and a captured o-ring under the integral flange nut
and of course the best solution is to eliminate the threaded connection in any form,
an example of such is the MCW5000 with integral push-in connections
so for those WCers stuck with cheap poorly designed US made crap, some suggestions:
NEVER use a brass barb in a plastic top
do NOT use Teflon tape with plastic tops, use pipe dope or, better, an adhesive sealant (difficult with pp or pe)
if reliability is a paramount issue, go with a metal top
(but if Al don't forget the corrosion inhibitor)
Ben
in the interest of accuracy, why don’t you compose your posts
THEN go back and insert “NOT” in front of the whole thing ?
your accuracy would go up by 75%+ at least
- alternatively you could stop posting about things with which you have no hands-on experience
do you know what a plug gauge is ?
what will happen when you heat those threads ? (or solvent wipe them ?)
you persist in parroting ‘forum knowledge’, which is more often than not completely wrong
have you no pride ?
how many times do you have to get your nose stuck back into your ignorant statements
- give us a break, stick to what you have some actual knowledge of
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