Internal stress in a Female NPT
Internal stress in a Female NPT
(OP)
I'm redesigning a Female NPT fitting and was wondering if anyone had any info on how to figure out internal stress or how it might be distributed throughout the fitting. Is it all hoop stress, or is there a lot of axial stress from the threads pushing against each other? What is a reasonable estimate for the hoop stress? Any help at all would be appreciated, I can't find much info anywhere.





RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
JTraeger:
If axial stress is a problem, it will concentrate at the thread root which will eventually result in circumferential failure initiating in that location.
Axial splitting is the most common failure mode for female NPT fittings and results from overtightening. Once thread engagement occurs, further tightening expands the circumference of the female socket. The stress in the area is strictly a function of the circumferential strain --- the difference between the stressed circumference minus the relaxed circumference divided by the relaxed circumference. Multiply the strain by the modulus of the material and you’ll get the average hoop stress in that cross-section.
If that stress is below your long-term design stress for the material, failure should not occur in a properly manufactured fitting.
Over –insertion of the pipe end into the fitting is a common cause of excessive circumferential stress in a female NPT fitting. Another cause is the use of PTFE thread tape. Excessive amounts of thread tape cause the thread interference to occur with less thread engagement, therefore the technician feels he must insert the pipe further to ensure tightness. Also, tightness is often assessed by the “feel” or torque required. PTFE reduces the coefficient of friction between the threads, therefore there is a tendency to overtighten.
Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com
RE: Internal stress in a Female NPT
Which way do the cracks run in the fitting, are the parallel to the fluid flow or at 90 degrees? if they are at 90 degrees the hoop stress is not responsible.Further as the previous post states what materials have you got in contact and what fluid is going through the fitting and what pressure is the system at?
regards desertfox
RE: Internal stress in a Female NPT
Rich, are you saying that I can take (Cs-Cr/Cr)*E to find the average hoop stress in that cross section?
Thanks for the help all.
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
Not had time to study this in depth but have you considered that your problem may be caused by hydrogen embrittlement due to the playing process.
regards desertfox
RE: Internal stress in a Female NPT
JTraeger:
Yes. The stress is strictly a function of the strain induced by the interference fit of the piping in an NPT. The more the piping is threaded into the fitting after thread engagement, the more the fitting must expand to accommodate the pipe. Therefore, the fitting must expand. It is the expansion of the fitting (strain) which results in the hoop stress in the fitting cross section.
Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com
RE: Internal stress in a Female NPT
Last night I came up with an idea to change the amount of thread engagement. I figured that the more engagement on the threads, the more the stress would be distributed. I then read in Machinery’s Handbook 26 Ed. “It is recognized that in special applications, such as flanges for high-pressure work, longer thread engagement is used, in which case the pitch diameter E1 is maintained and the pitch diameter E0 at the end of the pipe is proportionately smaller.” Basically what it is saying is to change the pitch between the end of the pipe and the plane of hand tight engagement. Has anyone done this, or know how much I should proportionally change the pitch?
As far as hydrogen embrittlement, I don’t think that’s the case. Everything I read about it has to do with steel, or aluminum, or electroplating. I’m sure that SCC caused the cracking in these fittings, which come from a combination of internal stress and exposure to ammonia. I’m waiting for some chrome plated fittings to come in so I can test them for SCC, but I’m concerned by the way contractors might handle the parts, which could scratch off the chrome, leaving the brass exposed.
Anyway, I’ll have to make up a bunch of different designs and test them. Thanks for the help with the stress-strain equation Rich.
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
My reason for being persistant is, I know someone who recieved life threatening damage caused by someone else fitting incorrect NPT threaded weldolets. Why dont you buy your contracter a torque wrench and give them some training. It wouldnt take five minutes, could be done on the job and save you all this time. Remember your time sorting this out is costing money.
RE: Internal stress in a Female NPT
JTraeger:
Be careful not to specify torque in your joining directions. Torque is too dependent upon the coefficient of friction between the threads. If a thread sealant is applied to the connection, inserting the pipe to the proper torque can result in excessive strain which can result in failure.
Most manufacturers will specify a number of turns beyond hand-tight. Hand-tight insertion ensures thread engagement; while the number of turns beyond hand-tight ensures a tight joint within the recommended circumferential strain for the fitting body.
Makeup is correct that you cannot alter the taper on an NPT --- it’s standard. If you change it, it will no longer be compatible.
Also, your notion of having less stress on thicker sections is incorrect. The stress is a function of the strain. No matter how thick or thin the section is, if you strain it a certain amount both the thin and thick sections will have the same stress (however, it will take more load to deflect the thicker section). If anything, when using the same deflection, your thicker section will have a higher hoop stress at the inside surface because of the smaller inside diameter of the thick section compared to a thin section.
Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com
RE: Internal stress in a Female NPT
The solution to this problem is not as easy as simply giving torque wrenches to the contractors. First off, torque is a function of friction, which changes because these fittings require Teflon tape or thread compound to seal. These affect the friction and therefore affect the amount of torque applied. Secondly, These fittings fail from SCC at relatively small amounts of torque, even before they're sealed.
Like I said, this is just an idea. I was just wondering if anyone heard of changing the angle of a NPT thread. I read that is can be done, but I assume that it needs a custom tap to make it. I'll look into line pipe specs; it sounds like it's for high-pressure pipes and might be of some use. I have to look through some books and see if I can find anything, but thanks for the help.
RE: Internal stress in a Female NPT
I was thinking if I could increase the diameter near the opening a little, it would reduce the force that is applied there. How about adding an extra thread? That would make the opening a little larger. I know I'm not an expert in this area; I'm just trying to come up with some ideas.
RE: Internal stress in a Female NPT
JTraeger:
I mentioned that thickening the cross section of the fitting socket will not affect the stress in the fitting at a given deflection or strain. However, it will take more load for the pipe fitter to effect that strain. Therefore, a thicker wall can give the assembler the “feel” that the pipe is “locked in”, thereby, averting further excessive strain that might result in failure.
Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT
RE: Internal stress in a Female NPT