Allowable stress Vs Yield strength for bolting design 1

[EngineerRed]

I am designing a bolting for Body-Bonnet connection of my valve . It is a BPVC Section I valve . I have two material options i) A193 - B16 ii) SB637 -Inconel 718 . I'm calculating the thread shear (stripping) stress and tensile stress to valuate my bolting . If I compare these with allowable stress (S) values from Table 1A / Table 3 Section II d , my calculated values are always greater . Whereas if I compare them with the Yield strength in Table Y-1 my calculated values are within 80% of the table values . There is no way that I can design a bolt with stresses less than Table 1A / Table 3 because the bolt preload itself is higher than allowable stress (S) . Together with working load this will be much higher than Allowable stress . Is it safe to design bolting based on yield strength instead of allowable stress (S) ?

 

[r6155]

Did you checked with ASME B16.34?

Regards

 

[Vikko]

You should not do these kind of comparisons and calculations in my opinion. The allowable stress values (indicated by capital letter 'S') for bolting, reported in BPVC II part D tab. 3, shall be used only as it is prescribed by the code, for example in ASME VIII div. 1 apx. 2 (which is according to the Taylor Forge method for bolted joint design).

To understand what these allowable stress values 'S' means and how they are derived from yield and tensile strength, you should check BPVC II part. D apx. 2 and tab 2-100(b).

As far as I understood the 'S' values for bolting which are consistent with the Taylor Forge method can be overpassed in real practice by the preload; in fact these 'S' values are really really small (about 1/4 of tensile strength according to tab 2-100(b)); also it is not possible to calculate precise preload and torque by the the Taylor Forge, which would result as smaller than waht is required in practice. You should also check ASME PCC-1 2013 10 (the guideline for bolted flange joint).

 

[SnTMan]

EngineerRed, ordinary practice would be to determine the clamp load the bolting needs to provide. Select bolt area and material tensile strength (S, selected from appropriate Code) to provide same, or better. Unless bolting is mating to tapped holes do not worry about stripping and so forth, ordinary bolting design is adequate in these respects.

If tapped holes are used, either evaluate needed thread depth by methods such as found in Sec VIII Div 1, UG-43(g) or Machinery's Handbook, or just tap 1 1/2 x bolt diameter. This last method would likely work in all except the softest materials.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EngineerRed]

Mike

Thanks for that approach . I'm wondering what is the real length of engagement . B 1.1 , states that it is equal to 9 pitches . whereas my tapped hole is of 3 inches . Which one is right ?

Vikko , Thanks . I'll use the allowable stress (S) hereafter .

 

[SnTMan]

EngineerRed, I haven't read the whole thing closely, but the "9 pitches" language seems to apply to thread form tolerancing. I wouldn't necessarily trust it as an engagement length.

Which one is right? Dunno, what diameter is your fastener?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Some Curious Guy]

In my opinion and experience you should not try to mix and match difference codes. Codes are like government taxation department. An apparent rebate is collected indirectly from somewhere else.
In your case use the allowable strength per Table 3 of ASME BPVC Sec II Part D and use the engagement length per UG-43(g) of ASME Sec VIII Div 1. Also make sure that you use a bolt standard and thread type fulfilling the requirements of UG-12 , UG-13 , UCS-10 and 11. If you follow all these diligently then you may not need to worry about stripping of threads.

Having said that, good engineering practice suggest that the bolt should fail in tension before it's thread fail in shear. Recommended is to have thread engagement area twice that of both tensile stress area. You should also take into account any difference in the strength of bolts and nuts. If a bolt is weaker than the nut then its engagement length shall be increased proportionately.

 

[engineering_patrol]

consider using yield stress instead of rupture.

 

[r6155]

I insist, see ASME B16.34 – VALVES- FLANGED, THREADED AND WELDING END.
It is clear.

Regards