Flange MDMT 2

[tankman64]

Hello All
I have a vessel just completed with MDMT @-40F. All components are either impacted tested or exempt. However we just noticed one SA105 150# WN flange does not have any impact test results. So now I’m looking at UCS-66(b)(1)(b) for exemption which is clear, using pressure ratio, but doesn't help me. The alternative method in UCS-66((b)(1)(c) is confusing to me so could someone please help explain the process in dummy language. Appreciate any help.
Thanks

 

[chevproe]

Hi Tankman64,

UCS-66(b)(1)(c) simply states that instead of using the coincident ratio for allowable reduction in MDMT, you can use the MDMT reduction applicable to the neck or shell to which the flange is attached.
So, no need to calculate any ratio, just apply the MDMT reduction of the connected neck or shell to the Flange and see if it is exempted.
Hope this helps...

 

[racookpe1978]

The flange will have been bought from a particular supplier.

Get his spec for the material properties at that temperature.

 

[tankman64]

Thanks chevproe
Initially that’s exactly what I thought but it didn’t make sense since (b)(1)(b) gives me -17F (reduced to -20F per UG-20(f)) compared to (b)(1)(c)which gives me -155F (limited to -55F per (b)(1)(a)). The nozzle is rated at -155F because the stress ratio is <0.35. The flange has the same allowable stress as the nozzle so what you are saying is the flange can be rated for -55F. That’s great but it makes no sense to me since most nozzles have low stress ratios and would make impact testing for flanges pretty redundant. Same story if the nozzles were impact tested. I also remember years ago an instance where the AI rejected an A105 flange @ -40F without impacts. All very interesting, thanks again.

 

[chevproe]

Tankman64,
My initial post was aimed at providing you a simplified explanation for UCS-66(b)(1)(c)...i really didnt check in detail the application of this clause..
Could you provide some more data ragrading your vessel...such as Design pressure & Temp, MDMT, Size, Material etc.

 

[tankman64]

Design 180 psi @ 500F/-40F MDMT
Shell and Heads SA516-70 impact tested
Nozzle neck in question SA516-70 Curve B (no impacts). Flange 20” WN150# SA105 Curve B (no impacts). Neck thickness 1/2”.
Corrosion allow 1/16"
RT Full
PWHT None

I guess it would be easy if I had Compress but no longer have access to it. Done it the old fashion way.

 

[TomBarsh]

Assuming the flange is a standard ASME B16.5 flange, A-105, this is rated to 170 psi at 500° F and to 285 psi at ambient.

UCS-66(c)(1) rates the flange to -20° F.

UCS-66(b)(1)(b) permits the rated MDMT to be reduced by the coincident ratio given in Figure UCS-66.1. It explicitly indicates that this can be applied to flanges from UCS-66(c). The ratio is defined as "the maximum design pressure at the MDMT" to the maximum allowable pressure (MAP) of the component at the MDMT.

Assuming the design pressure is 170 psi, then the coincident ratio at the MDMT equals: 170/285 = 0.5962

This is "about" 0.60 and eyeballing Figure UCS-66.1 gives "about" a 40° F reduction. So the flange MDMT for 170 psi is found: (-20° F) - (40° F) = -60° F, but this is restricted to -55° F per the last sentence of UCS-66(b)(1)(b).

Or you can use the rule of UCS-66(b)(1)(c) where flange's ratio is taken to be that for the nozzle neck or shell to which it is attached.


The problem that I see is that the design pressure is said to be 180 psi, but the rating for the B16.5 flange is only 170 psi. So perhaps this isn't a standard flange. In any case, this is the basic method. MDMT for an Appendix 2 flange would be found by a similar method but the basic exemption temperature would be based on the weld joint thickness of the butt weld (assuming weld neck flange).

 

[tankman64]

Great explanation thank Tom. Looks like I need to read this again. Also sorry the DT is 100F not 500F.

 

[chevproe]

Hi TomBarsh...
You explained it quite nicely....thanks a lot..

To rephrase the question by OP...
Considering if UCS-66(b)(1)(c) was applied, and the ratio for nozzle neck is found to be less than 0.35.
In such a case, will the flange be exempt to -155F??
Does the -55F limitation from UCS-66(b)(1)(b) not apply in such a case??
If so, this sounds a bit illogical (my opinion only).

Could you shed some light?

 

[TomBarsh]

chevproe,

See this thread in eng-tips: Link

 

[tankman64]

Thanks again Tom
In that link (last para) for the flange you state “…..If the "headroom" is sufficiently large, then the MDMT rating can go down to -155° F.”
i’m not sure this is correct since this is restricted to -55F per UCS-66(b)(1)(b) as you state in this thread. It seems to me that you can only take advantage of -155F when not a flange or other non primary stressed component.
For my particular nozzle the neck is rated at -155F and the flange -55F. Very confusing to me (and I think chevproe)

 

[TomBarsh]

tankman64: UCS-66(b)(3) provides that when the coincident ratio is no greater than 0.35 then no impact testing is required for MDMT no colder than -155° F. There is no restriction on the type of component that this can be applied to.

So, per the rules of UCS-66 a component can have a "rated" MDMT down to -55° F, and if the coincident ratio is 0.35 or less it can be rated to -155° F. Funny thing is that there is no "rating" that will be intermediate between those two temperatures. The only rule that would provide for a rated MDMT between these two temperatures is UCS-68(c), which permits a 30° F reduction in the MDMT if PWHT is performed on the component when it is not otherwise required by the Code. So that could get the part rated to -85° F. But there would never be a rated MDMT between -85° F and -155° F.

Note that the flange bolts themselves are subject to their own analysis for MDMT rating. In the end, the warmest rated MDMT for all the vessel components will govern the vessel's MDMT.

Figure UCS-66.2 is a very useful resource, "Diagram of UCS-66 Rules for Determining Lowest Minimum Design Metal Temperature Without Impact Testing", although it doesn't address UCS-68.

 

[tankman64]

Ton thank you very much again for your precise explanation. The confusing part is the limit of -55F in UCS-66(b)(1)(b). But then I sit around all day pressing buttons hoping no one will ask me any questions. Is good to read the Code now and again, keeps you honest. So to sum it up my flange is rated at -60F not -55F which now conflicts with what you stated above but that's ok, its all good. Or if I choose to use the alternative method per (b)(1)(c) it goes down to -155F.
Cheers