FOS on High Temp 80psia Metal Piping 2

[JRW261]

T=1500F
P=80psia
Combustion Products

I dont have access to the B33.1 code and am currently doing preliminary work study feasibility of a rig that may be constructed in the future.

The Pipe will be a Nickel alloy, probably Hastelloy X and I have a great deal of material test information at elevated temperatures. I just want an idea of what FOS to use on a system like this. I was planning on using FOS=4. Just want to know your thoughts.

Any Help is greatly appreciated.

Thanks

 

[metengr]

Could you provide more specific information regarding your application? The Piping code that you mention B33.1 is not an ASME/ANSI Piping code.

Once you describe the application, we may be able to better help you in your selection of allowable stress values. Using a factor of safety means nothing unless you understand the effects of elevated temperature on material properties.

 

[JRW261]

I apologize, ASME B31.3

This is a Natural Gas Combustion Process. As mentioned, it will be operating at a max temp of 1500F, 80psia, and a 140Ft/s

Im looking for 10,000hrs of life at a minimum.

 

[metengr]

If ASME B31.3 Process Piping is the design basis for this component, I would STRONGLY recommend that you purchase this Code. You need access to Appendix A and other Chapters in this Code that contain formulas to assure a safe and reliable component design. The allowable stress values for ASME code approved materials are derived from accepted formulas that contain suitable design margins to evaluate tensile strength, yield strength, creep rupture strength, and creep deformation rate. Any one or combination of these properties is determined as limiting, and the allowable stress value is defined.

Given your stated conditions, Hastelloy X would be suitable material. ASME B31.3 Appendix A lists the maximum design temperature as 1500 deg F. However, it goes well beyond just identifying the appropriate material. You need to order the Hastelloy X pipe to a particular ASME material specification (SB-619), you need to establish methods for fabrication, inspection and testing.

Here is my professional advice; if you feel you need help with B31.3 or don’t have access to this Code, hire the expertise.

 

[JRW261]

As I said in the first place, I am not designing anything. I am exploring the feasibility of this project and part of that is exploring its cost. All I wanted was to know what FOS is used on this type of application so I can *estimate* a schedule pipe.

Quite honestly I do not see where this warrants any hardened expertise. If my calculations are wrong, the cost of the project increases or deceases during its Design phase. I understand your concern but I also think you are looking way to far into my question.

Once again, is a FOS=4 reasonable for this type of application (high temp pressurized piping)?

I appreciate your concern

 

[metengr]

Sure. You still need to have B31.3 to determine wall thickness (aka pipe schedule). A FOS of 3.5 to 4 is inherent in Code allowable stress calculations. But, why go thru all this work independent of B31.3 when you can plug and chug with the applicable design formulas to estimate with reasonable accuracy your required pipe schedule.

 

[CRG]

I have not seen FOS (factor of safety?) tabled in the code. There are safety factors built into the code; however the allowable stress is dependent on varying load cases that have different allowable stress ranges.

I also would agree with metengr about getting a copy of the code if you are going to use it as a guideline. Even if it is for estimating materials or complexity of design.

If you would like to get an idea how the code works without purchasing the code, you may want to look at this handbook:

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-1-4008/entire.pdf

It is free.

 

[TGS4]

I will interject one thought into this thread - that is that a factor of safety (FOS) - or as I prefer to refer to it - a design margin - is always compared to a failure mode.

In B31.3, the allowable stress is equal to the lesser of the minimum specified ultimate stress divided by 3 or the specified minimum yield stress divided by 1.5. Therefore, for pressure alone, there is a design margin of 1.5 against plastic collapse and 3 against ultimate strength.

I think that it does a great dis-service to the pressure vessel community when we state simply that pressure vessels (or piping) have design margins of 3, 3.5, or 4. In fact, ASME pressure vessels have a design margin of 1.5 (that's right - only 50%) against plastic collapse. For the majority of steels commonly used - carbon steels - that exhibit a relatively sharp yield point and a very shallow tangent modulus, plastic collapse is a very real failure mode.

Ok - now that I'm back down off my high horse - remember that that design margins (or FOS for the uninitiated) are ALWAYS referenced to a failure mode. In JRW's case, this failure mode may be creep damage, and I don't know what the design amrgins are against that failure mode.