stiffeners for large pipe subject to vacuum

[pipe15]

pipe with a D/T ratio of ~90 may be subject to vacuum conditions. The thickness is determined based on a selected stiffener distance.

I plan to select B16.9 fittings which will have the typical under tolerance on thickness of 12.5%.

In one instance the required min thickness for pressure is 0.318" and the min thickness for vacuum is 0.445". The selected pipe is 0.500" NW / 0.490" MW (plate has a 0.01" under tolerance). If I spec the fitting at 0.5"NW the min would be 0.438" (.875*.5)

first question...can a fitting, elbow for instance, 'act' as a stiffener or would the min thickness of the fitting also have to meet the thickness required for vacuum service?

thanks in advance.
Mark

 

[BigInch]

Sorry, I can't get past my first thought.
Are you sure you need stiffeners with a D/t of 90 for vacuum alone? I don't believe that a vacuum alone is enough to collapse that pipe. If you don't have additional loads on the exterior of the pipe, hydrostatic, etc., I'd have a look at the calculation again.

What would you be doing, if you knew that you could not fail?

 

[pipe15]

the pipe in question is
48" OD, 0.500" Nominal WALL / 0.490" Min. wall, ASTM A-691, GR 1.25% CR, CLASS 22

I did not do the vacuum calc, but I hear that the stiffener distance used was 100 feet. Perhaps we are too conservative using that distance but that is the value.

so, the question still stands, can an elbow or other fitting act as a stiffener due to their difference in geometry when compared to the pipe?

thanks

 

[BigInch]

Probably, theoretically, but so what. Surely you won't install an elbow every 100 feet. I'd say 1.) its not a problem, given normal burial depths and only soil loads. But then you didn't say it is a dirt pipeline either. 2.) If you do need it, 100 feet is probably too far. apart to keep it from crushing, so that was set as a stop for keeping it from progressing forever, once it did.

I'd urge you to get another opinion on the calculation. There's pleanty of 48" out there without vacuum stiffeners.

What's the product? What's the temperature. What's the rest of the story? Maybe that's got something to do with it.

What would you be doing, if you knew that you could not fail?

 

[Duwe6]

At 100-ft spacing, the stiffener is immaterial for about 50-ft of that span [SWAG] Use the external pressure calc's in Sect VIII or in the Pressure Vessel Handbook - same formulas. The times I have had to hand-calc for pressure and vacuum, the vessel tends to have about half the external strength as internal. That is why there were raised eyebrows on your required thicknesses, ID vs. OD.

The calc's are a pain, as you take intermediate results to a nomograph, then take the nomograph results to a 2nd nomograph. But do them anyway, "numbers don't lie".

SWAG = Scientific Wild-A$$ed Guess.

 

[BigInch]

50 ft? I was thinking maybe 8 to 16 ft, but that guess also came from where your's did.

Anyway, don't matter, put um where you like um, because IMO stiffeners arn't needed. I'll bet that 100 psi external diff pressure would do ... absolutely nothing, never mind 1 barg.

What would you be doing, if you knew that you could not fail?

 

[stanier]

If this is a buried pipeline then you need to consider that the soil will be part of the structure. Design should be to AWWA, AS 2566.1 or DNV requirements.

Is the vacuum a process condition or an upset condition from a waterhammer study? This affects how you approach the design.

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