B7 versus B8 Studs For A Stainless Steel Pipng System 3

[dave2109]

I heard an argument that it is necessary to use B7 studs instead of B8 studs for ANSI stainless steel flanges. This is due to the recommended torque creating a bolt stress that would exceed the yield stress of the B8 studs.

I've worked offshore previously and have always used B8 studs in stainless systems and have never had any problems with leakage.

Is it okay to continue using B8 studs?

 

[unclesyd]

You will hear his argument quite often. While is has some validity, it’s not necessarily so. Having fought this battle for many years I would stay with the S/S studs if your process is corrosive to C/S. We spent many hours and dollars changing out C/S studs and back-up flanges imposed on us by projects. Most problems come from over tightening either on the initial preload or re-tightening. The most problems I’ve seen were on 3" and 8" Class 150 flanges. If you need a stronger bolt for gasket seating then change to B8 Class 2 studs or bolts. Studs and nuts are usually cheaper. These are good up to 1" dia., it’s hard to cold work anything larger. If you are having to seat a gasket that requires a higher seating stress with large flanges or you have temperature constraints you may have to go to an alloy bolt. A quick flange calculation will confirm this. You mentioned you have worked offshore so you have seen the green and blue studs. These are coated B7 (Green and Blue) studs used in mildly corrosive environments.

Use a new gasket, square up the flanges, lubricate the stud or bolt, and go slow on the tightening

 

[tothepoint]

B8 class 1 stainless steel bolts/studs have a minimum yield strength of 30,000 psi compared to a B7 alloy steel bolt with a yield strength of 105,000 psi. I have been involved in many RCA of gasket failures where the B8 bolts were simply unable to develop the necessary preload to seat the gasket. You mention particular problems with NPS 3 and 8 flanges- these are the most under-bolted class 150 flanges, and thus are more likely to cause gasket leakage problems due to low bolt load with B8 stainless bolts. Also, B8 bolts, because of their low yield stress, are easily (and routinely) yielded during installation or service, thus causing more problems during subsequent re-use.
My rules of thumb for using low strength stainless bolting:
1) Don't, if you don't have to for external corrosion reasons. If you must use stainless bolting, use class 2 strain hardened- but be prepared to pay the price ($$).
2) Select "low stress to seal" gaskets that can be seated with the bolt load available. Do the calculations- don't let a gasket vendor simply tell you that its a "low torque" gasket.
3) Do the calcs and determine the necessary assembly torque, and then use a torque wrench so you don't inadvertantly tighten the bolts above their yield strength. For example, a 1/2 x 13 B8 bolt, lubricated, will yield at less than 30 ft-lb. What are the chances that a mechanic will yield this bolt if not using as torque wrench? Answer- 100% chance
4) Don't re-use stainless steel bolts

Finally, if you're using stainless bolts because your gaskets leak corrosive product onto the bolts- correct the leaky gasket and go back to using high strength, less expensive alloy bolting (B7).

 

[rversteg]

Current new EN standard for Europe restricts use of CS bolts on SS flange materials when exceeding 120 °C due to difference in thermal expansion.

 

[LSThill]

Team Members:

What Stud Bolts shall be use on Piping Class 150#, Piping Specification?

Cryogenic Process Temperture -325F to 400F: ASTM A320, B8M CL 2 stud, w/ 2 heavy hex nuts ASTM A194, GR8.

Leonard Thill
Lenard@thill.biz

http://www.thill.biz

 

[myron54]

I am a novice at bolt and nut selection, and just had to specify 316 ss B8 bolts for a B31.3 stainless steel piping system that has metallic ring gaskets. Fortunately, I found the para in B31.3 warning to use grade 2 bolts, which have higher tensile and hardness. My question now is which nuts do I use? Should the nuts be of similar hardness to the bolts?

thanks for any assistance

 

[unclesyd]

Under normal conditions they can be as hard as the stud or bolt, but a general rule is that a nut should be lower in hardness than the bolt or stud. In other words a standard B8 nut is ok for use with a B8 Class 2 fastener as stated in Isthill's post. There are some special occasions where different alloys are used for the nut where the nut ends up harder/stronger than the fastener, again these are the exceptions.

We use thousands of B8 Class 2 Studs with standard B8 nuts. You will find that a Cold Worked Class 2 nut is hard to find and somewhat more expensive.

 

[PAN]

Isthill,

A320-B8M Cl.2 is equivalent to stainless steel type 316. I understand that A320-B8 Cl.2 (equivalent to type 304) also can be used for cryogenic temperature. Please comment.

 

[LSThill]

PAN (Mechanical) & Team Members

Bolting: Cryogenic Process 304/304L
Cryogenic Process Temperture -325F to 400F:
ASTM A320, B8 CL 2 stud, w/ 2 heavy hex nuts ASTM A194, GR8

Pressure and temperature rating may be limited by certain components permitted by the specification. Refer to manufacturer’s recommended pressure-temperature restrictions.