analysis on the difference in mech strength of NPT & UNF

vendita

Newly Enlightened
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Feb 8, 2008
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HI All,
I am a new user in this blog. working as a Grad Engineer in a O&G Industry. my issue is, I have to decide wheather to use NPT plug or UNF plug in order to permenanetly seal as well as contain pressure of about 2000 psi. hydraulic fluid (oil) is used inside the tool.

this is because of the operator error caused while drilling a port of 1/2" -20UNF (tap dia.453" X deep.437") in a hollow tubular metallic tool. now we have to seal the hole with a blind plug and re-machine another port hole on another side.

bascially I have to analysis the sealing as wells the mechanical strength of the both type of threaded (NPT & UNF) plug to hold atleast of 2500 psi pressure. can u please able to help me in this regard.

ThX,
Ven
 
Hi Ven,

Dunno about the sealing strength but if you want to "seal" I would use NPT instead of UNF because it's taper. After tightening to a certain point the thread will seal.

BTW: 2500 psi is quite a bit of pressure :D

Good luck,
Jonathan
 
Never ever use NPT on hydraulic systems if other types of fittings are available. Mechanical strength with 2000-2500 PSI is nothing to worry about on either fitting in the size you are working with as long as the base metal is strong enuf to withstand the stress from the hole. Straight thread O-ring fittings are best for leak free service. NPT fittings leak like a sieve in hydraulic service.

Do not use aluminum parts or fittings on systems that see a lot of pressure cycles or shock loads. Even fatigue rated aluminum parts will fail.

BTW- I have been an industrial hydraulic systems instructor for years.
 
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So standard straight thread with compression oring? Or dynamic?

Learn something new everyday.
 
The type to use is compression.
See the drawing named "S.A.E. STRAIGHT THREAD "O-RING" TYPE END"
http://www.lenzinc.com/index.cfm?template=catalog&form_product=225
Plugs in this configuration are available from many manufacturers. Check out types; AHHP or STP below:
http://www.lenzinc.com/index.cfm?template=catalog&form_product=224

Lenz fittings are very good. Hydraulic fittings of many types and brands can be purchased at any good hydraulic supply shop. The S.A.E. STRAIGHT THREAD "O-RING" TYPE is an industrial standard configuration.

There are fittings with elastomeric seals for replacing flare fittings also.

Remember there are lots of tapered pipe thread adapters available because it was so common back in the days when people didn't know any better.:thumbsup:
 
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Hi all,
Thanks folks for your support and help.
can you guys please advice me how, to approach to prove them by theoritical calcs... Iam stuck at the start itself.

thanks,
Ven
 
Don't look to me for that, it has been WAAAAY too long since I did any of that. Perhaps a university professor could help. The difference between them would be that one was tapered and one was straight. The pipe thread is tightened to an interference fit and has a slightly different thread form. The pipe thread tightening torque also increases the hoop stress in the parent material.
 
If I understand the problem correctly, they need to permanently "remove" a hole, and plan to re-machine the correct hole on another area.

If this is in fact the case, I would go with NPT and then tig weld the plug diameter to the original part. Once cooled, grind/turn the weld down and re-drill in the new area.

This will be leak-free, permanent, and no one beside you, the machinist and the welder will ever know it was done.

And, I agree a face-seal o-ring will be superior (assuming the base metal is not porous) to a pipe thread for sealing against high-pressure oil.

And, 2500 PSI is nothing. We do oil field gate valve coatings, and these must withstand another order of magnitude of pressure...
 
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