Using Jam Nut for Assembling DBB valve to Piping Flange 2

[McDermott1711]

Hi everybody,
Due to vendor fault in one of oil and gas project, several valves were manufactured and when they were going to be assembled at pressure gauge conjunction there was not enough space for heavy hex nuts therefore those guys in the field decided to use jam nuts (3/4" ASTM A194, Gr. 2H) to assemble the studs (3/4", ASTM A193 Gr. B7) for ASME B16.5 flange class 900. This is strict violation of project specification which says all nuts shall be heavy hex, but those line are already under operating pressure of 90 bar and no leakage happened. Suppose we are going to investigate acceptance of these nuts in this situation, the questions are:
1- As it is obvious that shearing stress is increased in the mating threads of bolt and nuts, how these shearing stress shall be calculated (considering the nuts was not tightened with torque metered wrench). I know there is a formula in appendix B of ASME B1.1 for shearing area. The problem is that I don't know how to calculate LE in it).
2- Why ASME B31.3 gave such a low design stress for ASTM A193 bolting? If we are not allowed to pass beyond this design stress, even heavy hex can not be satisfactory. On the other hand, if we are allowed to pass this design stress value, how far can we go (please answer with reference to the code).
3- As I got from appendix S of ASME SEC. VIII, DIV 1, We shall tighten the nuts so that there will be no leakage in hydrotest, without breakage of the bolt and nut, if so, this is our case.

Thanks in advance.

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[SandCounter]

Unfortunately I can't speak to the ASME standard at this time, but I might point out a few items that may help as clues to the solution. According to Fastenal about a 3rd of the load is applied to the first thread and then 23% on the second, 16% on the third. . . and so on. The rule of thumb for thread engagement is about one nominal diameter (for steel). The Machinery's Handbook give some first principals formulas for minimum thread engagement, starting page 1415 in my copy.

I used to count sand. Now I don't count at all.

 

[SPDL310]

The length of engagement calculation may be found here. Keep in mind that the actual length of engagement for your nut will be slightly less than your nut thickness. The ASME design limit is on the bolt and not the nut.

At the end of the day this is a manufacturer design flaw. They should remedy / replace the valve with one that should accept the appropriate nut. I understand that this line is already running, but it should have never been pressured up in my opinion. The fact that it has implies that your field technicians are either ready and willing to break piping specs without approval or that someone has approved and put into service an unknown piping element. The local authority needs to sign off on the jam nut and bolt combo with proof that it is acceptable or you need to replace the valve.

 

[McDermott1711]

Is there anybody out there who can show me an example where one of those design stress values for bolting from table A-2 (of ASME B31.3) is used (e.g. ASTM A193, Gr. B7).

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[McDermott1711]

The length of engagement calculation may be found here(

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement_fed-std-h28.htm).

As I got parameters for 3/4" bolting from ASME B1.1, reached to the following parameters:

A_t= 0.466 in²

n= 10

E_s min = 0.6773 in

K_n max = 0.663 in

Therefore Le calculates to 0.76818 in, which is even greater than heavy hex nut thickness (H) which is maximum 0.758 in (for 3/4" bolt/nut). It means even heavy hex nut cannot satisfy that formula.



Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes

 

[SPDL310]

Your At=0.466 in^2 is wrong. At for 3/4" 10 = 0.334 in^2. Using that number you should get Le=0.55052 which is less than the heavy hex. Look here for quick reference of At.

http://www.engineersedge.com/fastener_thread_stress_area.htm

 

[SandCounter]

According to Fastenal's Technical Reference Guide, your Heavy Hex Jam Nut will be between 0.398" and 0.446".

I used to count sand. Now I don't count at all.

 

[McDermott1711]

Thanks SPDL310 and SandCounte for correction.

Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has. Rene Descartes