A333 Gr 6 Minimum Temperature

[FlashPurp]

Hello,

We are planning on using A333 Gr 6 piping in our design. The MDMT in B31.3 is -50 F and we have design conditions that may reach -55 F. I was told by by using the load case referenced
in B31.3 323.2.2 (a)(3) below and by reaching a stress ratio no greater the 0.95 that A333 Gr 6 material will be suitable to use at -55 F.

Referencing (23a) thru (23d) the equations given are only sustained loads. Do I only need to check the calculated sustained stress ratio to allowable stress but use 1 for the in-plane/out-plane moments? Would I use the allowable stress at MDMT? I am using Caesar II for stress calcs.



Thanks,

 

[georgeverghese]

Typically, this piping stress ratio route to allowing some exceedence on lower design temp is not used, since the actual stresses whilst in operation may well exceed what you compute in the design office. Since this is a marginal exceedence, see if the process engineer can calculate a higher resolution MDMT with some other simulation program.

 

[LittleInch]

I'm pretty sure you can A333 grd6 tested at lower temperatures down to about -60C which makes the problem go away. But is additional testing.

 

[goutam_freelance]

As per table 323.2.2A, up to a thickness of 12.7 mm -55 deg F minimum temperature can be used without impact testing.

For higher thicknesses, both table 323.2.2A and 323.2.2B can be used, in my opinion. The most restrictive minimum temperature should be used for obviating impact tests.

in B31.3 323.2.2 (a)(3) below

For combined stress, you need to consider all stresses, like pressure and dead loads, thermal and anchor displacement loads, torsional loads, etc.
Software like Caeser can calculate this stress at all points and find the maximum. The respective I factors should be input as 1.0 for this run.

 

[FlashPurp]

Thank you everyone for your responses.

@georgeverghese what software would our process engineer use?

@LittleInch if we do additional testing such as impact testing we will be ok at designing to -55 F?

@goutam_freelance by combined stress do they mean the operating load case with Weight, Pressure, and Temperature? Or do I just analyze the all the singular cases such as sustained and expansion using the i factor as 1.0 and checking the stress ratio is not > .95?

Again, thanks everyone for their input.

 

[LittleInch]

Thank you everyone for your responses.

@georgeverghese what software would our process engineer use?

@LittleInch if we do additional testing such as impact testing we will be ok at designing to -55 F?

@goutam_freelance by combined stress do they mean the operating load case with Weight, Pressure, and Temperature? Or do I just analyze the all the singular cases such as sustained and expansion using the i factor as 1.0 and checking the stress ratio is not > .95?

Again, thanks everyone for their input.

Yes, that's what I've done recently

 

[goutam_freelance]

operating load case with Weight, Pressure, and Temperature?

You need to add anchor displacement loads also and any other possible load, if any.

 

[FlashPurp]

@LittleInch is there a provision in B31.3 referencing -60 C as a tested temp for A333 Gr6?

@goutam_freelance any other external load will need to be considered as well? As for the ratio I can use the basic allowable at min temperature? Or because there is thermal effects in this load case I would use the allowable from expansion load case below with Cold, Hot, and Longitudinal stress?

f [1.25(Sc + Sh) - SL]

Thank you both for your insight.

 

[goutam_freelance]

The factor of 1.25 allowed for cyclic thermal loads and 1.2 for occasional loads(B31.1) because these loads are transitory in nature, whereas the sustained load is continuous. As per ASME codes it is allowable to exceed the allowable stress for short periods.

But at low temperature applications, the allowable stress can not be exceeded as at low temperatures the pipe material is brittle and can fail if the maximum stress exceeds the allowable stress. So, the factor of 1.0 should be maintained.

any other external load will need to be considered as well?

You should evaluate all the load possibilities and consider all the loads. Any type of sustained loads+displacement loads should be considered.

 

[GD2]

Remember, the calculated stress ratio is the greatest of the three stress criteria given in ASME B31.3 Fig. 323.2.2B.
The 3rd criteria is essentially the equivalent Primary and Secondary load criteria in PV design. For displacement loads, you should include thermal, restraint movement and external loads like wind load (if it is connected to at tall vessel say 15 ft and up) and if there is appreciable temperature changes in the connected equipment.
I feel, the easiest option is to talk to the process engineer and fix the MDMT at -50F. They always provide a margin to the operating temperature while setting the design temp.
Setting -50F will save you tons of dollars while purchasing piping components such as pipes, valves, fittings and flanges as most low temperature components are set at minimum temp -50F.