Strength of Materials

[dynamo78]

We are auditing Pressure vessels in the refinery & was task to verify Safety factor used on them,
a)Is there a place in the code that mentions safety factor in the calcs?we were informed ASME is 5:1 until during world war II while API- ASME is 4:1 from 1934 until They merged in 1952.then 3:1,at the latest.are they using the wrong terminology?I'm thinking they mean the rating,at a certain temp.like FB55000,T strength used is 13750psi,SA515 60,TS used is 15000 psi(@650F)as shown in their records.
b)Where in the ASME Code can I find the Tensile strength for different materials corresponding to certain temperatures. Like;SA285C is 13800psi @ 650F(max)
Need your input Guys!! Thank you

Feati

 

[metengr]

Please review ASME B&PV Code, Section II, Part D, Appendix 1,and associated Tables.

 

[jte]

dynamo-

What is the purpose of the exercise? If you are looking for code compliance, look for a nameplate. The factor against ultimate stress at temperature for an ASME VIII (Div. 1 post 1968) vessel varied and not uniformly. First 5, then 4, then 5, then 4, then 3.5. But I would argue that it doesn't matter.

The term "safety factor" is a misnomer. It implies that a vessel could be safely operated at several multiples of the design pressure. This is not typically true. The factor you are referring to is one of the three factors which are considered when determining the allowable stress for an ASME VIII Div. 1 or old Div. 2 vessel. I'll leave the "new Div. 2" out of this discussion for clarity since you don't have any "new Div. 2" vessels anyway. The other factors which may govern are 2/3 yield at temperature and time-dependent (creep). For example, a post 1999 Div. 1 vessel at 600°F has its basic allowable tensile stress set by 2/3 yield and is not governed by ultimate at 600°F divided by 3.5. So, what is the so called "safety factor" here? While you're at it, don't forget that many places on the vessels will exceed the basic allowable tensile stress. Many vessels are designed with details which may be up to 3*S for Div. 1 or 3*Sm for Div. 2 under the design conditions. Since you will be dealing mostly with Div. 1 vessels, read UG-23(e). So, someone may say that the vessel has a "safety factor" of 4, but parts of the vessel were designed to use 3/4 of that margin while at design conditions...

Where is this refinery located? Though I'd understand if you can't be very specific. Though a city would be nice, can you at least tell us what country and state/province/region it is in? I guess I have strong feelings about this type of exercise since I've been dealing with the aftermath of a similar (though different) exercise for a client which was about 95% useless and did little more than cause confusion among the refinery folks as to the "safe" status of their equipment.

see also thread292-161762

jt

 

[TGS4]

meteng pointed you in the right direction to find the answer to b) and some of a).

However, I am going to use this opportunity to get on my soapbox briefly to discuss "Safety Factors"...

For starters, there is no such thing as a safety factor in the pressure vessel design codes. There is, however, a "Design Margin". That said, when the discussion turns to design margin, the first question I always ask is "Against what failure mechanism?" As you are no doubt aware, there are numerous failure mechanisms that can befall a pressure vessel. Burst is one, plastic collapse another, gross plastic deformation another, ratcheting to gross plastic failure another, creep deformation, creep rupture, brittle fracture, local buckling, gross section buckling, etc, etc. Now, each of these failure mechanisms has it's own "Design Margin". This design margin for each failure mechanism is decided upon by a learned group of engineers (let's call them the Code Committee - although, in fact, there are several dozen Code Committees, Sub-Committees, Sub-Groups, etc...). Also of note, is that these design margins are usually against minimum-specified material properties (strength, toughness, etc). The actual pressure is guaranteed, at the time of construction, to have met these minimum requirements. Often, the actual material values are better than the minimum-specified, but sometimes in-service degradation may bring them below these values.

OK - I'm now off my soap-box.

So, given the above, and judging solely by your questions, it seems to me that you are unqualified to be performing an audit of this type. My recommendation to you is find a qualified pressure vessel engineer (someone with a minimum of 10 years of pressure vessel design experience, including at least 5 years of ASME Section VIII, Division 2 experience), to assist you in performing this audit. This engineer should be able to know the design margins of all failure mechanisms that I listed above, and the better ones should have a fundamental understanding of WHY the design margins are what they are.

Any questions? Does this make sense to you?

 

[JStephen]

Note also that part of the reason for having a safety factor in any kind of engineering is to allow for uncertainties in the analysis. Out in the middle of a cylindrical shell, that uncertainty might be fairly small. On something like an anchor bolt chair or weld attachment, it might be 50% or more. Even if you only had one failure mechanism to consider, you still can't assume that your vessel would be good for FS x P pressure.