Cap Screws vs Stud Bolts When Installing Butterfly Valves 1

[ReliaEng2008]

The maintenance team at my site is not happy that some butterfly valves were installed with stud bolts and nuts instead of with cap screws. Nobody was able to give me any concrete information why cap screws are recommended over stud bolts, nor is that recommendation in any internal procedure.

Does anyone have any information on this subject? Also, would the bolt stress be the same for cap screws and stud bolts, considering the are the same nominal size?

 

[r6155]

Avoid threaded fastener with head, for pressure components use stud.
Tensioning is by rotation of nut: this condition may no be achieved with bolt cap screw.

Regards

 

[The Obturator]

ASME B16.5 (2017) 6.10.3 Recommends that stud bolts with a nut at each end are used for all applications, especially high temperature.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[SnTMan]

ReliaEng2008, did you specifically ask your maintenance team why they prefer capscrews over studs? What did they say?

Regards,

Mike

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

 

[ReliaEng2008]

SnTMan, no one was able to give me an answer besides "that is how we always have been doing it".

 

[SnTMan]

OK then, yeah, they really don't know

Good luck selling them on studs & nuts. They really are preferred in most industrial uses.

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

 

[EdStainless]

Stud are used for the same reason why they use to hold the heads onto race engines, more reliable torque and no risk of head-to-shank failures.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[FacEngrPE]

If the cap screws on opposite sides of the butterfly valve are just a bit too long, they bottom against each other, it is then impossible to tighten the flanges against the gasket. Using studs with nuts, or even threaded rod is preferred.

 

[bcd]

FacEngrPE has it right. The +/- tolerance of cap screw length is very large. If cap screws are used, they can bottom out and prevent proper compression of the gasket. Or, they can be short and you may not achieve the full thread engagement.

 

[jojoyohan]

The length overtolerance of a cap screw is +0.00 for every length and size so they cannot just bottom out if the length is properly determined (see ASME B18.2.1 Table 13).

Typically I've seen the butterfly vendors provide lengths for both cap screws and studs. I'm not a huge fan of lug style butterfly valves since it is difficult to ensure proper installation.

 

[bcd]

To reiterate my point so it is clearer. If you need a 2-1/8" long cap screw, and the standard size lengths are 2" and 2-1/2", then what do the installers do?

Longer 2-1/2" screws can bottom-out before compressing the flange gasket. Maybe cut the screws and fix the damaged threads before installing?

The shorter 2", including minimum tolerance, can result in less than full effective thread engagement as required by ASME B16.34.

This is why most manufacturers of butterfly valves do not recommend cap screws for installing lug body butterfly valves. There may be catalogs that show the option, and they do work if all the details are checked. But it is definitely not good standard practice.