Defining Pressure Class and/or Material Grade for Steel Flanges

[RiniEIT]

Why do we have to specify both the pressure class and the material grade of a butt-welded flange (in particular, a WNRF flange)? It seems redundant to me to call out the pressure class (ex. PN 100) if I'm already specifying the material grade of a flange as well as the matching grade & wall thickness of the mating pipe; especially after confirming that the flange hub meets the applicable design pressure.

I realize that industry standards such as B16.5 and CSA Z245.12 make calling out the pressure class a standard requirement, but I don't see why.

Please fill me in.

"The important thing is to never stop questioning" - Einstein

 

[Gator]

You seem to have managed to overlook the key feature of flanged connections. At least your username is appropriate.

Piping Design Central

 

[pennpiper]

Hay! New2Pipe,
Call up your local Flange vendor and ask them.

Sometimes its possible to do all the right things and still get bad results

 

[LittleInch]

The matching grade and wt of pipe does not equate to design pressure or flange class. If you didn't call out the flange rating, how is Vendor supposed to know what you want? He doesn't know the design issues which might mean you have a certain class of flange and can't back calculate the minimal data you're giving him.

This is like going to a tyre seller and saying "give me a tyre for a Ford."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RiniEIT]

I recently reviewed a drawing that contained a flange callout in the BOM and the description for it read as follows:

FLANGE, WNRF, NPS 12, PN 100, TO MATCH: 12.7mm W.T. x Gr. 359, CSA Z245.12, CAT II, M45C

The application was for a meter station with a D.P. of 9930 kPa and a location factor (L) of 0.625.

I believe that the above flange description wasn't complete because the material grade of the flange was also required. Without it, the flange material would default to Grade 290 as per the CSA Z245.12 spec. Since the hub of the flange is the thinnest and thus the weakest part, a material grade of 290 would result in not meeting the minimum required wall thickness at the flange hub, given the above design conditions.

So am I understanding this correctly? Or is simply specifying the pressure class on its own take care of it?

Thanks guys!

"The important thing is to never stop questioning" - Einstein

 

[MickMc]

New2Pipe,

I am unfamiliar with Canadian standards but ASME B16.5 tabulates a range of flange materials into classes and lists the maximum pressure/temperature ratings per material and class. From your project line list you take your design pressure and temperature and select appropriate material and class as per ASME B16.5, most probably you will already have a piping material specification which has already assigned material to each service type and if WNRF wall thickness will match corresponding piping.

 

[SNORGY]

New2Pipe,

I just did a quick check on this using a calculation tool I developed for this very purpose.

I am assuming that this service is sour and you are using a liquid hydrostatic test medium, since these conditions give rise to F=0.8 and L=0.625. Your DOS is then based on the hoop stress at MOP, whereas it otherwise might be (indeed, in this case, it is) based on the hoop stress at test pressure if testing with a gaseous medium. No matter; I ran both cases and the same minimum required wall thickness was computed.

Remember that CSA Z662 does permit you to set your mechanical allowances to zero towards the computation of minimum wall thickness. That said, I evaluated 12" XS Grade 290 pipe with the CSA Z245.1 / Z245.11 / Z245.12 default mill tolerance of 12.5% and CA=0. If you then automatically impose the constraints on combined hoop and longitudinal stresses in Clause 4, then these considerations govern in the computation of minimum wall thickness and your findings are correct: a Grade 290 flange is not adequate at the thin wall end. A Grade 290 flange does appear to be adequate, however, when you set the mill tolerance to zero, provided your pipe wall installation temperature at the time of initial restraint is -14.4 C or higher.

As an aside, my calculation does not automatically invoke constraints on combined hoop and longitudinal stresses towards the determination of minimum wall thickness until after a stress analysis is performed to evaluate what the combined stresses actually are.

So, you could specify the flange as "machine bored" or "specified minimum wall" to 12.7 mm (SCH XS), but I agree with your original finding, namely, you need a higher grade than 290.

In short, you are correct, you need to specify the grade of the flange as well as the other data that was specified.

 

[RiniEIT]

Thanks for the input Snorgy and MickMc!

"The important thing is to never stop questioning" - Einstein