Thickness Calculation for Water Pipe Line

[momo83]

Dear All,

I am currently computing for the required water transmission line pipe thickness. Basically, my reference is AWWA M11 Steel Pipe: A Guide for Design and Installation. The main formula used to determine the minimum thickness is t = PDo/2s.
where: P is the internal pressure
Do is the outside pressure
S is allowable design stress

My questions are:
a. regarding P, is it only design pressure or this is Pd (design pressure) + Ps (surge pressure)?
b. regarding Do, is it safe to initially assume the thickness since Do = Di (inside diameter) + 2t (thickness), then do the checking when t has been properly computed.
c. regarding S, is this Yield Strength of the Material?

Thanks

 

[CGonsalves]

I only have 6 months of engineering practice in HRSG's; however, I may be able to answer this.

In our analyses, P denotes the design pressure; however, I've never heard of Surge Pressure, so my answer may not be reliable.

Outside diameter is normally constant. ID changes depending on thickness.

In the denominator, we use 2*(SE+yP), in which:
S = Allowable Stress
E = Joint Efficiency / WSRF (normally 1 in my analyses)
y = Temperature Correction Factor (normally 1 in my analyses)
P = Design Press

Maximum Allowable Stress is found from tables in ASME 31.1.


EDIT: You know, this looks like a pretty valuable resource:

http://www.piping-designer.com/inde...ors/2000-asme-b31-3-pipe-thickness-calculator

 

[NovaStark]

a. regarding P, is it only design pressure or this is Pd (design pressure) + Ps (surge pressure)?

Your design pressure in theory should be the maximum pressure that would be experienced and then some safety factor (depending on what your client/designer says).

So you operating pressure may be 100 psig, but the design can end up being 150 or 200 psig.

b. regarding Do, is it safe to initially assume the thickness since Do = Di (inside diameter) + 2t (thickness), then do the checking when t has been properly computed.

Your thickness is sized based on what NPS you are deciding to use. You can look these up from standard piping charts. So 6 NPS will correspond to 6.625 inches so.

c. regarding S, is this Yield Strength of the Material?

Usually your allowable stress is the yield stress/safety factor. I believe your code should give you the yield stress, you may decided to use a safety factor of 2, 1.5, etc. Depends on your design.