Large pipe potentially subjected to vacuum 1

[pipe15]

Once the required wall thickness is calculated and a pipe schedule or thickness is selected based on Temp, Pressure, and allowances is there a criteria to check for collapse if the piping may be subject to a vacuum? I am guessing this may be an issue for lines above 30" with wall <=.5" or so, and especially large lines for circ water (>72").

Some of the lines are buried with appropriate trench and backfill requirements, but even there, should underground lines be checked too?

thanks
Mark

 

[BigInch]

Yes it should be checked esp where D/t ratios become large at around 100-120, but exactly where you start checking that ratio does depend on soil and/or water depth.

The formula is for pipe with an external pressure, but that simply means use it when absolute pressure differences between outside - inside > 0

http://www.roscoemoss.com/waterwell-calculations.html

There are other formulas which claim to be more accurate for buried pipe, considering more realistic soil pressure distributions, rather than the uniform pressure load in that formula, but it would be totally appropriate for water pressure.

 

[kacarrol]

For vacuum calculations in piping I have always used ASME Section VIII Div 1 - UG-28 Thickness of Shells and Tubes Under External Pressure.

Regards,
K

 

[rconner]

[fyi also see thread at

http://www.eng-tips.com/viewthread.cfm?qid=224536&page=90

(and even thread and references therein Mr. BigInch also provided)]

 

[stanier]

For an above ground pipeline then the piping code and Theory f Plates and Shells by Timoshenko is what you need. ASME V111 is referenced in the piping code.

For a buried Pipeline you also need to consider the behaviour of the soil in terms of deflection, stress, strain, combined loading and buckling. Australian Standard AS 2566 & AWWA M11 are a good start point.

Books :

1) Structural Mechanics of Buried Pipelines by Watkins
2) Buried Pipe Design by Moser

Standards & references:

AWWA M45, American Water Works Association Manual of Water Supply
Practices—Fiberglass Pipe Design, American Water Works Association,
Denver: 1996.

Guidelines for static calculation of drainage conduits and pipelines, Worksheet
A127, Wastewater Engineering Society of Germany (ATV) Germany: 1988

JANSON L., and MOLIN J., Design and installation of buried plastic pipes,Wavin, Stockholm: 1991.

MOORE I.D., Performance of a buckling theory for metal culverts . . . . . . . . . . .
Sydney: 1988. Fifth Australian/New Zealand conference on geomechanics.

ASTM STP 1093, Buried Plastic Pipe Technology - Chapter - Use of continuum buckling theory for evaluation of buried plastic pipe stability. MOORE I.D. and SELIG E.T., Pages 334-362. Symposium-Dallas, Texas 10-13 September 1990

CARLSTROM B., Calculations of circumferential deflections and flexural strains in underground GRP pipes used for non-pressure applications , Europipe ’82 Conference, Paper 3.

ZEMAN P., Critical evaluation of the AWWA (1981) and ATV (1984) methods, Pipetech, Conference on recent developments in the design, construction and operation of pipelines. Sydney: 1988.

Attached presentation has been delivered to Engineers Australia, IMechE and ASME.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[SNORGY]

I had a similar assignment recently. In my case it wasn't due to soil loading; it was simply a matter of vacuum in the pipe.

I compared results obtained from Timoshenko's method and the method in ASME Section VIII Division 1 UG-28. They differ by about a factor of 3.0, with ASME being the more conservative.

I didn't look at it in much detail, but I suspect the difference probably shows up in the safety factors applied in the Code.

Regards,

SNORGY.