Bolt material 1

[mielke]

We have a flange with 3/8" bolt holes made of 304 stainless, inside this vessel hot gas is cycling through at 900F and 50psia. Gas flow will be pulsating and not continuous use. Is there any real worry if we use grade 8 c.steel bolts? The thermal expansion difference between dissimilar metals seems minimal. Any thoughts?

 

[SnTMan]

The holes are 304?

Seriously, carbon steel bolting is often preferred for SS flanges as the COTE of the bolt is generally less than that of the flange. Helps keep the bolts tight.

Regards,

Mike

 

[trottiey]

Grade 8 bolts are not permitted by the ASME boiler code. You can consider SA-325, which is similar to grade 5, or SA-354 if you really need grade 8 equivalent strength. SA-354 is usually a custom order.

 

[unclesyd]

Why not old normally used ASME A193-B16 bolts or stud bolts?
I would not use A325 for this application as you are above the temperature limit.

Any fastener you use should have an anti seize coating applied especially if exposed to the elements. We use C-5 if outdoors and FAG if indoors. Use no more than the minimum thread exposure of 1 1/2 threads. Buy your fasteners from a reputable supplier.

 

[trottiey]

Thank you unclesyd; I overlooked the temperature, which rules out both of my suggestions.

 

[desertfox]

Hi mielke

I would suggest you due a calculation assuming the flanges rise in temperature but the bolts do not increase in temperature above the ambient that the joints is operating at and see what stresses this imposes on the bolt and flange.
Also the stresses due to temperature are additive to those created when the joint is tightened, so if your at 90% of yield stress on the bolt at the assembly stage, then when service conditions are imposed on the joint the small increase in temperature may overstress your bolt.
Also the yield stress of the joint will decrease with high temperature in service.

desertfox

 

[unclesyd]

desertfox,

Why go to 90% of the yield and not use something in the order of 60,000 psi bolt stress as it is so easy to overload a fastener at these levels. Depending on the gasket type the lower value will allow you possibly restore the integrity by hot torquing if a leak occurs. We have many SS lines that cycle from ambient to 600F for thousands of cycles with no problems using CS fasteners.

milke

One thing I forgot to mention in my last post is if the flanges strong enough you need to use a spiral wound gasket with an inner ring. I would use a graphite filled one.

 

[mielke]

Thanks all, we're using a sheet gasket (the customer requested no graphite) this is not code unit and will be in some harsh environments.

 

[desertfox]

Hi unclesyd

Not a problem using CS fasteners but what I believe is the joint should be tightened to a pre-tension suitable for the service conditions, from the OP's post I can't tell what pre-load would be suitable, I agree that its easy to overtighten the joint thats why I suggested he does a calculation, I didn't suggest he use should use 90% of yield for a pre-load I was pointing out that if that was the pre-load he might have a problem.

Regards

desertfox

 

[unclesyd]

The only sheet material I know that has any recovery under your conditions is Blue African Asbestosis. I personally wouldn't use sheet gasket unless it was asbestos under your stated operating conditions unless I absolutely, positively was forced to.
I agree if that graphite wouldn't be the material of choice if the process stream is oxidizing.

Thought I would add this since they may not like graphite on the fasteners.

Thermoplate 220 in aerosol can or as grease in a jar from the
Chemola division. This is a very good antiseize that has excellant weathering properties.

http://www.socousa.com/home/socohome.htm

The only sheet material I know that has any recovery under your conditions is Blue African Asbestosis. I personally wouldn't use sheet gasket unless it was asbestos under your stated operating conditions unless I absolutely, positively was forced to.

desertfox,

Agree with you 100% as you know it's getting there that is the problem.

Anecdotal:
WE manufacturer a white polymer either in a continuous or batch polymerization equipment that has had gasket problems since the removal of Asbestosis. There was many dollars and time thrown at this problem by various groups who positively had the right answer. Our group was saying all along that that if the flange cloud support a spiral wound graphite the majority of our problems would be over. There was and uproar every time graphite was mentioned as a filler you would think we stole the outhouse. This rocks along for several several years where the only positive thing that came out of all the meetings was we got flange facers, Biach Tensioner, and Hydraulic Ratchet wrenches and a thousands of gasket samples.
I very seldom went to the yarn analytical lab but happened by chance to around a conversation about black specks the arch villain in the proposed use of graphite. The lab was seeing a certain number of black specks which they consider normal. there wasn't much duplicating there results as the number varied all of the map. I ask could they see 5 additional black specks and the answer is probably no. I went to a new Polymer Engineer to discuss the possibility of black specks if the gasket failed. In a 2500 pound batch of polymer we would see only 5 or 6 black specks if the gasket under went a catastrophic failure and all the graphite was dumped in at one is the stirred autoclave.us. When we did actually test a graphite filled gasket on an autoclave the lab reported that black specks were non discernible. One for the good guys.