Shear Stress Though Tapered Pipe Threads
Shear Stress Though Tapered Pipe Threads
(OP)
I am trying to calculate the maximum pressure a threaded pipe fitting may withstand when the fitting has a NPT thread (1/4 -18 NPT). The failure mode I am worried about is excess shear load through the thread teeth. For this calculation I cannot determine shear area though the threads (because of the taper), nor can I easily determine length of engagement for an NPT thread.
Thanks to anyone who can provide advice.
tc7





RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
http://bellsouthpwp.net/r/s/rsnmar/METhreads.zip
RE: Shear Stress Though Tapered Pipe Threads
This does not mean the problem is insurmountable. With good success, I have been applying Unified National specifications to NPTs, noting the tooth profiles are virtually identical, i.e. 60 degree vee type threads. The central problem is to "rectify" the threading inclination to an "equivalent UN profile", bastardized threads are allowed for computational purposes.
In your case, 1/4 NPT with an 18 TPI is virtually midway between 1/2 - 18 UNS-2G and 9/16 - 18 UNF-2G threading profiles. What I suggest is that a thread mid range between pin specifications behaves "somewhat like" that of a 1/4 NPT when loaded in shear.
Therefore it stands to reason that the 1/4 MNPT with major, pitch and minor maximum diameters of 0.536, 0.492, 0.447 inches respectively has the same shear area as a 17/32 UN thread, 18 TPI with thread classification tolerance near that of a coarse profile. I utilize a thread program, computations have been standardized or looked up in the Machinist Handbook, and obtain an equivalent major, pitch and minor pin diameter of 0.530, 0.494, 0.465 inches. What I am suggesting is that the 1/4 MNPT thread behaves like 17/32 - 18 UNC-2G.
Apply the threading computations for Unified National, ANSI B1.1-1982, noting that any thread pin with shear prior to the box (i.e. shear area of the screw is LESS than that of the nut) for maximum internal minor diameter of 0.484 inches, minimum external pitch diameter of 0.490 inches and find the screw shear area is 0.854 in^2 per unit length thread engagement.
When a NPT thread seals, although all threads are engaged between pin and box, typically only a few threads act to seal the joint. In the literature, computations use only one single thread. Therefore the effective length is around 0.047 inches, resulting in a screw shear area of 0.041 in^2. Naturally this is purely ficticous, when the threaded joint fails in shear we would expect all threads along the effective length of engagement to let go similtaneously. For the purpose of calculation, we recognize the single thread shear area method.
Hence, applying normal fracture mechanics, the pressure required to load a MNPT joint to failure is simply the product of pin shear area, material yield in shear divided by area of the thread seal, 0.492 inches in our case. The resulting mathematics suggests a pressure of 4700 psi assuming your material is Atlas T316 Stainless Steel. (TY = 22 ksi, Average Shear Theory)
Consulting industry specifications such as Swagelok, Parker-Hannifin, etc typical joint operating pressures are 4500 psi typical, never to exceed 5000 psi as a rule of thumb. Apparently our computation bears this out.
I hope this helps you out. My gut feeling with stainless fittings is to stay below 3000 psi.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
There are several outstanding reasons for it, the mechanics of the thread necessarily imply swaging the joint to specifications in order to provide a seal capable of carrying 10 or 20 ksi. Just gripping the pipe will necessitate external wall scaring while torsion is applied. Then there is wall allowance; obviously the joint must be able to withstand the stresses, i.e. Theory of Elasticity for Pressure Vessels. Typically the annulus are small, you need thick walls, perhaps the application cannot withstand choked or resticted flow, i.e. inlet feed to a pump. And what about shear? NPT threads cannot withstand transverse loading, they have a tendancy to bend at the last thread. The industry uses the notion of a fotbol to stabilize the joint should you need to carry weight lateral to the connection. And of course there is a material concern. Typically ferrous connections govern higher pressure ratings, stainless sometimes and brass never. Oilfield applications, my personal experience, do not use brass, yellow metal is forbidden since geological formations begin to yield sulfide bearing gases as the water tables begin to rise. Stainless is suspectable to pitting, chloride radial surface attack. I guess my judgement is one biased upon as a seasoned professional.
You are correct however, in noting prominent companies using elevated pressures. However, almost all note the maximum fitting working pressure is limited to the lowest NPT rating! Female pipe end ratings are typically much lower than their male counterparts of a given size, this is due to the geometry, inner/outer diameters for female threads are larger than those for male threads, i.e. higher stresses.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
I initially went down the path you are suggesting, but I could not find all the particulars on thread geometry for NPT's. The ASME B1.20.1 does not so thoroughly identify thread features as does say the H28 spec or B1.1 for UNC and UNF. So I was forced into some assumptions that I was not comfortable with for the calculation of shear area.
Cockroach -
I like the idea of your approach very much - it seems practical and no one can deny your experience with it, although I am not clear on some of your terminology. What do you mean by "pin" and "box"? More importantly, you stated the NPT thread was "mid range between pin specifications" of the 1/2-18 UNS-2G and 9/16-18 UNF-2G. Please elaborate on this. Also I am not clear on the "G" modifyer on your thread designations. I am only used to seeing "A" or "B" indicating external or internal thread.
Thanks again in advance.
tc7
RE: Shear Stress Though Tapered Pipe Threads
b) Recall that our methodology is to find an equivalent Unified National thread that is representative of your 1/4 NPT profile. Since the most important dimension of a thread is the pitch diameter, I suggest finding a UN thread closely matched by PD. The problem is that the pipe thread is inclined, therefore I use the handtight engagement as the "mean" pitch diameter for the NPT, 0.49163 inches in our situation. This is slightly larger than a 1/2 - 18 UNS-2G, but slightly smaller than a 9/16 - 18 UNF-2G thread. Suppose we split the difference and use the average between the two threads. This means I am searching for a thread with pitch diameter (0.463+0.525)/2 = 0.494 inches, i.e. mid range specification. This leads to a 17/32 bastardized thread which must have the same pitch as the NPT, 18 TPI in our case. Finally, pick the UN thread class according to NPT thread height, 0.04444 inches. The one I picked was 0.03250 inches, there may be one that is closer, but I ran with 17/32 - 18 UNC-2G.
c)Yeah, sorry about that! "G" references the thread fit, in our case class 2, which tells me something about the thread tolerancing applied in the calculations. You are absolutely correct in noting that "A" is the pin (screw-external) and "B" as the box (nut-internal). For UN threads we are permitted up to five (5) classes, 2 is what you would find as a commercial available size for example, i.e. off-the-shelf.
Just as a quick note, NPT profiles are specified by ANSI B1.20.3-1976, which is a revised designation of ANSI B2.2-1968. This same specification was reaffirmed in 1998. The layout is almost exact to that for UN threads, ANSI B1.1-1982 EXCEPT there is absolutely no appendix discussing fracture mechanics on thread performance. I find this curious, have been perplexed by it, inquired at several committee meetings to no avail. I remain perplexed and since, developed this method. You may find an old inquiry I floated in this website regarding this issue, but I have never seen, heard or otherwise given information on a solution to this problem. What is clear to me now is that others have these same problems!
Certainly, my pleasure. I hope this works out for you.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Shear Stress Though Tapered Pipe Threads
Or are you refering about the shear stress resulting from the pipe and fitting joint under either tension or compression?
RE: Shear Stress Though Tapered Pipe Threads
RE: Shear Stress Though Tapered Pipe Threads
The point of my question was not in regard to hoop stresses or external tension/compression loads or loads applied transverse to the thread axis. Although these are valid concerns and I will address them in my design. My concern was with regard to the thread strength, i.e., due to shear loads through the threads which would be associated with pressue loads applied to a closed end fitting such as an instrumentation or a shut off valve.
tc7
RE: Shear Stress Though Tapered Pipe Threads
As I stated in a prior thread we have pressure transducers and gauges that have operated at 20,000 psi and you can buy these off shelf. Yes they need to be used with 60,000 psi minimum yield materials. In the testing we have done you will typically burst the tube before the thread will fail.