ASME 16.5 Pressure/Temp FS 3

[enrjdean]

Hi, can anyone tell me what Factor of safety is used within the pressure temp ratings in ASME b16.5? For example, a cold rated CL300 carbon steel flange is rated at 51.1bar, but what is the attached factor of safety involved?

I have seen pressure tests with hoses using these flanges at well above 200bar with such flanges and no damage has occured. How are these figures derived, it seems very conservative to me?

Can anyone shed some light?

Regards

Jim

 

[TGS4]

Before you can define a factor of safety, you have to define the failure mode.

What failure mode (damage) are you talking about?
- Leakage
- Plastic deformation of the flange
- Ultimate failure of the flange
- Ratcheting

In the piping and pressure vessel world, each failure mode has a different design margin...

 

[bcd]

Hello Jim,

The pressure temp ratings are based on several factors which are included in the allowable stresses listed in the ASME pressure vessel codes. For example, material creep can occur at less than the listed yield strength. So you could temporarily pressurize to a higher pressure, but if held for an extended period of time, dimensions critical for sealing could distort and unexpectedly release pressure. So the pressure temperature rating needs to be low enough to avoid creep. The pressure temperature curve in B16.5 is derived in several ways depending upon the temperature range. If you can get a hold of an old copy from the 1950's, there is a curve inside and some text which explains the method and formulas used. They may be conservative, but they work. Nobody has been killed due to the code requirements being minimal, which is why the codes were developed in the first place. Pressure vessels were blowing-up.

To provide a factor of safety number alone is misleading and could be misapplied by having someone think they can operate at higher pressure. The factor of safety needs to be stated as being relative to an allowable value. In the case of pressure vessels, material yield strength is not a safe value to use. Do not be misled that a quick pressure test is proof that the pipeline and it's components can withstand the high pressure forever.

bcd

 

[enrjdean]

Hi guys

Heres the background to the problem.
I have a hose connected to a rigid pipe by two 20" CL300 flanges. There is a 310kNm bending moment on the hose with also an axial load of 600kN. I have worked this into an 'equivalent pressure' using the formulae:

Peq = 16*M/Pi*G^3 + 4*F/Pi*G^2

This gives me a pressure of around 136bar for the equivalent plus the 19bar for the design pressure. B16.5 states that for cold rated carbon steel CL300 I can only go to 51.1 bar.

So, I guess I'm going to have to go up to CL900, but it still doen't feel right when I look at one of those chunky flanges, I imagined them to have alot more in them.

Jim

 

[TGS4]

Please note that the equivalent pressure method is not something that is required per B16.5 - only that it is a convenient method that engineers like to use.

In fact, B16.5 is mostly silent on the topic of external loads. Use your engineering judgment (probably good to back that up with some calculations, though).

 

[enrjdean]

Thanks TGS4

I did perform some calculations to look at the bending moment, axial stress and also a surge presure seperately, but only on the pipe section of the flange (welded kneck type). Under the conditions I specified this gave me a 1.75 FS, but only on the pipe section.

To be honest, I'm a little unsure of how to go about performing a detailed analysis on the flange itself, thats why I converted the loads into an equivalent presure. Do you know of any resources on the web I can have a look at or maybe a decent book I can get hold off to help me get a firmer grasp of this problem.

Regards

Jim