Schedule of piping for saturated steam 3

[JrValveEng]

I'm designing a system that has saturated steam running through pipe at 750 psig and 513 degrees F a little perplexed on what schedule pipe it should be. I thought that I read somewhere that according to ASME, the actual working pressure for steam should be around 25% of the pipings max pressure rating. Is this true? If so, I think this would lead me to believe that I would want schedule 160 grade B pipe which seems a little heavy to me. Any help would be appreciated.

 

[racookpe1978]

"Seems" a "little heavy" ??????

Hint: Do the actual calculation.

Then, when you have a calculation, report back to the group what you calculated, what pipe strengths you have assumed for a "cold pipe" and what strengths you actually can use (based on the working temperature of your assumed pipe),
and what corrosion allowances you assumed,
and what wall thicknesses you actually CALCULATE that you think you need.

You are, or claim to be, a trained and educated engineer. You don't design ANY high-pressure piping system by "feel" ......

I don't want you to go "read a Code" to see what "The Book" says.
Yet. You need to ACTUALLY DO THE CALCULATION.

 

[MJCronin]

"I thought that I read somewhere".... The brave new motto of future third world designs

 

[ApC2Kp]

JrValveEng,

The ASME B31.1 Power Plant Piping code material allowable strength had a 4:1 design factor for decades, until it was recently 'reduced' to a more economical 3.5 :1 design factor that is consistent with the same design factor in ASME Section I Fired Pressure Vessels.

The B31 Code allowable strengths are tabulated in the different Codes appendices with de-ratings for temperatures and different material grades. The B31.3 Process piping code has a 3:1 design factor. The design factor includes fatigue 7,000 cycles allowance for one complete thermal cycle per day to result in approximately 20 years of life. The piping is not designed just for the presure hoop stresses to equal the material ultimate tensile strength.

There are calculations that summarize the sustained stresses of pressure, dead weight, live loads, bending, cyclical stresses from thermal expansion, occasional loads of excess pressure or temperature variation, wind, seismic, and even wave action. Pressure design is the starting point.

 

[LittleInch]

I agree with all above with one of my concerns (among many) being the phrase "...pipings max pressure rating". There is no such thing as the pressure capability of a pipe changes with temperature and material. You may have found a table someone produced to do this for a particualr set of circumstances, but you won't find it in any ASME code for piping. The ASME flange code (B 16.5) provides max working pressures for each type and rating of flange at different temperatures and different materials, but this is not the same thing.

The B31.1 and 31.3 codes work on an allowable stress system whereby the input into your initial wall thickness calcualtion is pre-determeind by material and max temperature. This value does reduce as temperature rises, so depending ont he temperature, you could be at 25% of the ambient temperature stress level, but this is not a fixed amount. As ApC2KPquite rightly states, initial wall thickness for pressure containment is only the start point - there are many other forces to consider and check.

I truly hope you take due cognisance of the advise offered above as high pressure steam isn't something to be trifled with and if you get it wrong it could have very serious consequenses.

You asked for any help, well, Any feedback would be appreciated....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way