Piping design pressure and temperature.

[07071975]

What should be the design pressure and design temperature in pressure piping system with hydrocarbon services? I went through ANSI 31.3 B, but it does not clearly say anything about it.Is there any thumb rule if operating pressure and temperature is known?

 

[CRG]

The design pressure and temperature should be equal to or greater than that which the piping system will be exposed to in normal service. Occasional loads need to be considered in the analysis as well to ensure compliance with the allowable stress. Too often I see design temperature and pressure stated to match the flange class that is to be used. This is not helpful for the piping engineer when evaluating the flange loading with consideration to pipe stress.

 

[RajuChander]

System Design Conditions (pressure, Temperature etc.)are very important information. Normally all the operating conditions of the system should be below the limits specified by Design Conditions. All the components Valves, Flanges, Equipment should have their respective design conditions higher than the specified System Design Conditions. All the Over Pressure Protections for the system are based on the System Design Conditions. Regulators give their approval based on design conditions. Test conditions are based on Design Conditions.These design conditions are normally specified clearly in System Design Flow Sheets. However there is no clear cut guidline for margin between the operating and design condition, this is for the designer to decide.

Raju

 

[PAN]

07071975,
The shut off pressure of pump should be a basis to specify piping design pressure.

 

[07071975]

Dear all,

Thanks for your valuable suggestions. Usually I prefer design Pressure and Temperature 10%-20% above operating conditions considering the criticality of service fluids.

PRESSURE VESSEL CODE ASME SECTION -VIII DIV-1 suggest factor of saftey as 4. I think same can also be applicable in pressure piping designing.

 

[PAN]

07071975,

The factor of safety depends on your piping code. In ASME B31.3-1990 Edition, it should be 3 (not 4).

I use the value "3" from the following basis :-
- Material SA 106 Gr. B
- Tensile Strength 60 ksi
- Allowable Stress 20 ksi
- Factor of Safety = 60/20 = 3.0

However, there is no relation between the design pressure/temperature and the above factor of safety.

 

[abbver98]

PRESSURE VESSEL CODE ASME SECTION -VIII DIV-1 refer to Appendix 2 of ASME II Part D for safety factor. It's a combination of YS and TS.

 

[TGS4]

ALL

There seems to be a consistent misunderstanding of the Code (both piping and pressure vessel) amongst a large number of individuals regarding the design margins. Let me summarize:

The minimum design margin with respect to yielding is 1.5. The Code calculations are all based on materials that exhibit elastic-perfectly-plastic behavior. That is, once the general membrane stress has exceeded yeild, and the pipe or vessel cannot localize that stress, the strain (and hence deflection) will become very very large (theoretically infinite). For most REAL materials, this is obviously not the case. However, consider mild carbon steel - say SA-106 Gr. B. If you were to look at a stress-strain curve for this material, you would see the very large "yield plateau" aftert the yied stress has been reached where the slope of the stress-strain curve is essentially zero. This continues on until approximately 5-10% strain. For all practical piping and pressure vessels made from this material, this level of strain woudl be "failure". Therefore, this limit would govern many materials.

The minimum Code design margin against the material reaching its ultimate stress is indeed 3 (or 3.5 in Div. 1). However, for most materials to achieve that level of stress, they have undergone extreme amounts of strain - necking has probably occured. Any piping or pressure vessel that has experienced a pressure near that which would cause stresses approaching ultimate stress would have to be scrapped immediately afterwards, because they would be significcantly dimensionally unlike the vessel or piping that was designed.

So, in summary - IF you are ever asked what the design margin (note how I haven't used the term safety factor - that's a deliberate choice) is for piping or pressure vessels - the answer is 1.5.

And for the process engineering reading this forum - please leave the design of the piping to the pipign engineers and do not optimize the temperature /pressure of you process system to a perceived design of the piping system. Choose a pressure/temperature that is appropriate for your process (taking into account rotating equipment, etc) with reasonable margins - and leave the design of the piping to the piping engineers.

Rant over.