I have the problem to understand ASME Code - Flange with ring type gaskets

[Jadalbert]

I have the problem to understand why if I have ASME/ANSI B16.5-1988 flange 18'' 150#, ASME bolts for this flange, ASME gasket B16.20-1998 18'' 150#, the pressure "very high" 0,1 MPa (14,5 psi), temperature 260oC (500F) the ST calculated tangential stress in flange is so high. Using the formula (4) W=0.5*(Am+ab)*Sa I get large values of W. In my project ex. Am = 11,1 in2, Ab=5.56 in2, consequently the value of Mo and ST are high and the condition ST < Sf is not valid. For me it is crazy: the bolts are 2 times better then should be. Sa and Sb values (allowable bolt stresses) must be very low 130MPa (18800psi), if not, it gets worse. What is wrong with my thinking? What am I not understand?

 

[prex]

B16.5 flanges are known for being over bolted and for often not passing App.2 checks. This comes from the criterion of Div.1 of relating W to the actual bolt area, to prevent overstressing during tightening.
However you are not required to calculate a B16.5 flange under Div.1 coverage.

prex
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[Jadalbert]

Wow! I'm surprised. I do not have much experience in the calculating of ASME. Normally I use EN 1591-1. Thank you for answer. But is there some way to resolve the problem? I must calculate the project by ASME Code. Now Ive changed the position of the gasket and ST is minimal less than Sf.

 

[prex]

If you are required to conform to ASME VIII Div.1 you don't need to calculate that flange, as Div.1 itself states that B16.5 flanges may be used within their rating limits without further justification.
However, by changing the gasket diameter you won't conform anymore to B16.5, but if you succeed with the calculation, then everything is fine.

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[Fizza453]

Please see below clause of Section VIII Divsion 1

UG-11(c)(2) Flanges and flanged fittings may be used
at the pressure–temperature ratings specified in the appropriate
standard listed in this Division.

So you may use B16.5 flanges by just selecting proper pressure-temperayure ratings.

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[jtseng123]

Standard flanges were developed more than 50 years ago and well established by rating table. You shall never use ASME code to justify the design because most of the time it won't pass.
These standard flanges are based on end users' millions of proved records, that is why it is used until now without any changes.
It is unwise to mess around the standard flange. Your flange is only 18", and you shall use standard gasket that fits to the code, service and your flange type. Do not waste your time running ANY calculation.

 

[Jadalbert]

Thanks all for advice. The problem with my calculation is I am from Poland and we have a lot of installation builded with ASME elements. Often flanges are old (30-40 years), strange materials etc. But main problem is that Polish Office of Technical Inspection demand calculations acc. ASME, no matter if element is normative or not. On the one hand it is logical, on the other hand, no. Also I have no extensive experience in the design according to ASME.

 

[jtseng123]

The way you can do is, check the size and material to make sure it is the same dimensions as B16.5, and the material property that fall into the category of rating table. Then tell your boss there is no need to run any calculation regardless it is 30-40 years old. I have done several revamp projects for 40- 50 years old plant , we never worry about how old the flange is, unless it is corroded badly not fit for service.
You will be struggling on this ridiculous calculations that inspector is asking you to do. Most of them will fail by seating stress. So, what are you going to do ? change bolting load or gasket ?? That is unwise. Tell your inspector no need to run calculations as long as you are using the right type of bolting and gasket, and size and material property ( just property only, not material spec such as A-105. Different country has different spec) fit to B16.5.

 

[Mamacint]

Is a B16.5 flange still acceptable with calculation if if there is an opening in the flange? I'm currently looking to calculating reinforcement area for the added opening and but there is insufficent bolt area - this does not seem that added opening should effect this...

 

[Mamacint]

nm,

I answered my own question (my opening is too large)