Mean Diameter Formula for HP pipelines 1

[fmanzanilla]

Hello Everyone,

I'm trying to calculate the minimum wall thickness for specially manufactured high pressure pipelines using the "mean diameter formula" (according to manufacturer specs), which to my knowledge is:

Bursting Pressure = 2(Ultimate Tensile Strength)(OD-ID)/(OD+ID)

The thing is, the documentation has an example which does not match at all with my calculations, it speaks of a "minimum K ratio" and the usage of a "M.F.D":

"... GIVING 52.56 TONS/IN2. USING M.F.D. THIS GIVES MINIMUM K RATIO OF 1.637. THUS MAXIMUM BORE IS 1.861 INS AND MINIMUM WALL THICKNESS 0.593 INCHES."

Any idea?

Best Regards.

 

[BigInch]

Cute formula.

If you're designing pipelines, you need to dump the mfgr's formula and use the one in the pipeline design code.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[fmanzanilla]

Oh, I'm not designing, this is for inspection purposes only. This pipelines are old and were custom made, information is scarce. The specification do say minimum wall thickness must be calculated using that "mean diameter formula". The only additional information is that weird example I can't figure out.

Thanks.

 

[BigInch]

You wouldn't want to use that formula for checking or design because there is no factor of safety. Nobody designs or operates at burst strength, so even if checking, it would not give a reasonable answer.

Why not use the simply B31.4 and 8 formulas for hoop stress.

WT_inches = 2 * SMYS * DF / OD / P
DF = Design Factor 0.72 for liquids, and somewhere between 0.4 to 0.72 for gas, see area class.
SMYS = spec min yield strength (psi)
OD = Outside Diameter of Pipe (in)
P = Pressure (psig)

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[fmanzanilla]

Hello,

Thanks for your answer BigInch, I failed to mention that these pipes have a very thick wall, so barlow's formula can't apply as it's only accurate when the wall thickness is supposed a membrane.

Actually I'm just trying to interpret the following text, so I know how these people calculate retirement limits for lines:

"H.P. pipe is sized based on achieving a factor of safety of 2 on the bursting pressure. Thus if the design rating is 2000 kg/cm2 at 350 C then the pipe is design for a burst pressure of 4000 kg/cm2 at 350 C we use the mean diameter U.T.S. and worst dimensions allowed by specifications.

To calculate the minimum wall thickness you must find the lowest ultimate tensile strength of your pipe from your test certificates then using the maximum outside diameter allowed in the specification and required burst pressure to give a factor of 2 on design(.) Use mean diameter formula to calculate the maximum bore and hence the minimum wall thickness"

That's fine; But then it follows an example which talks about M.F.D and "K" Ratio. That's where I'm lost.

 

[BigInch]

The only thick-wall stress formulas I know and use are here.
They should be used only at points away from the ends of the pipe. If its really using ultimate strength, I'd still check it against yield strength too, just to make sure there is some appropriate margin of safety there as well.

http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

Maybe this interesting (at least to me) paper about Lame equations could help.