B16.5 Flange Pressure Rating

[men5muw]

Hi,

I have two questions regarding B16.5 flange rating

1. If, for example, at 950F the maximum flange rating is 35psi, does it mean that piping is accepatble at 35psi? What about the margin for bending moment on flange due to piping?

If piping is at a pressure near to pressure rating of flange, the flanges would fail at very small moment as equivalent pressure is already equal to pressure rating

2. I have to rerate a line at 950F which was previoulsy at 800F. It was built in 1971. The pressure rating for 150# A105 flange in B16.5 1966 is 55 psi, in 1973 it is 40psi and in all later codes it is 35psi. Which rating should I use for rerate?
In my opinion, the rating was reduced based on experience and more insight in creep at elevated temperature and new values should be used. Please let me know your opinion.

Thanks

 

[ApC2Kp]

men5muw,

The CL150 A105 flange would be marginal at 800 deg F.

Service at 950 deg F would be a long term problem for CL150 A105, and I would prefer to use a welded joint at 950 deg F. as well as use an upgrade of material for the piping.
What is the fluid? What is the flange size? Is the 950 deg F. constant, or an excursion from much lower temperature?

 

[StevenHPerry]

ASME Section VIII Div 1 has rules for flange design that may give you more wiggle room.

Consider converting your forces and/or moments at the flange into an equivalent internal pressure for use with SecVII method or B16.5 tables. See the "Kellogg equation."

 

[vesselfab]

if you are rerating...you are rerating to the newest code and specifications.....normally

 

[C2it]

You could also use the 2007 edition of ASME VIII Div. 2. The new flange design method therein allows for direct input of external loads. Thus you can avoid all the usual questions about the 'Kellogg' method.

 

[StevenHPerry]

C2It: "You could also use the 2007 edition of ASME VIII Div. 2. ... "

Check with your primary piping code (B31.x). The way I read B31.3-2008, the Div2 method is not acceptable for B31.3 piping.

 

[men5muw]

Thanks for your replies,

The 950F is not excursion but constant temperature and fluid is steam. The pipe sizes are 10" to 4".

I still have confusion over "pressure rating" to be same as line design pressure. Suppose pressure rating for flange is 260psi. Is it acceptable for a line with design pressure 260psi? If we convert bending moment and axial force on flanges into pressure and add it to design pressure to calculate equivalent pressure as reccommended by experienced people such as from COADE, the flange will fail at very small loading.

What margin we should have between flange rating pressure and design pressure?

 

[C2it]

Steven,

I guess it looks like that in the Code ... but I don't believe it's intended that way. Much work went into the new method for flange design in Div. 2, and I think that B31.3 is currently out of step.

Either way, it's ultimately the owner's responsibility to allow or not, the use of Div 2 rules.

 

[stanweld]

If the pipe system is designed per B31.1, the max temp permitted for A-105 is 800F per Table A-1. Unless you like to see/hear things go BOOM, rerating a carbon steel, steam carrying, piping system to 950F actual operating temperature is not the best idea.

 

[MJCronin]

men...

Stanweld is correct....your A105 carbon steel simply cannot be used or "rerated" at 950F. It will fail...

Regardless of the code, standard or rule set selected, a process known as "graphitization" occurs when carbon steel is exposed to temperatures above 800F for long periods of time.

http://www.hghouston.com/x/37_carbon.html

http://www.eng-tips.com/viewthread.cfm?qid=120971&page=10

http://books.google.com/books?id=DF...MHyDw&sa=X&oi=book_result&resnum=17&ct=result

This phenomena is the reason why alloy steel pipe was developed !

Tell your MBA boss that the existing system will have to be ripped out and replaced with one that meets the current piping codes and material requirements therein. (he will not be happy)

-MJC

 

[uniqueflange]

You could also use the 2007 edition of ASME VIII Div.
we now use this .