water hammer in self supporting steel pipe

[bjb]

When considering the stresses in a self supporting steel pipe with water hammer, can an increased allowable stress be used? This is for the design of a self supporting steel water main designed according to AWWA M11. In other words, I have a section of water main that needs to span a distance like a bridge.

In chapter 7 of M11, it says that for the normal condition the equivalent stress is not to exceed 33 percent of the yield stress. I interpret normal condition to mean the dead load of the full pipe including insulation, lining, water, etc. and the normal operating pressure of the water main. For non-normal conditions, such as water hammer or earthquake, is it recommended to allow the equivalent stress to exceed 33 percent of the yield stress? Again, the material is steel, and we are going to use a material with a Charpy V-Notch toughness of 25 ft-lbs @ 32 degrees F.

In chapter 4 for hoop stress, M11 says that the hoop stress may rise above 50% of yield for transient conditions, but shouldn't be allowed to go over 75% - a 50% increase in allowable stress. Therefore, would it be reasonable to increase the limit on the equivalent stress by 50% for consideration of water hammer effects or earthquake?

 

[stanier]

Other codes such as ASME B31.3 allow the stress from increased pressure to be 33% above allowable pressure for events not exceeding 10hours duration and not more than for 100hours per year. Waterhammmer is not however such a pressure excursion as the pressure rise is more akin to an impact load. I understand that the pressure excursion clause in B31.3 was designed for a process plant upset rather than forces from waterhammer.

When I analyse a system for surge I take the resultant pressure time history at various nodes. Then apply that as time history forces on the same nodes in a pipe stress package. A modal analysis is carried out initially to determine the natural frequency of the pipe at up to 8 Modes. Then the software calculates the dynamic stresses in the pipe.

Various papers on fluid structure interaction recommend a factor be applied to pressure derived forces from waterhammer. The most commonly recommended factor is 2. I use this in the calculation.

Care should be taken in this sort of analysis as it is difficult to replicate a waterhammer model and a pipe stress analysis model so that the fluid and structure (pipe) are coupled accurately. I believe that ANSYS may in fact allow both analyse in the one package.

Getting back to your posting in answer to your question about the level of stresses as a % of yield stress one could summise that one third of yield stress is a criteria used in some codes for design stress and that exceeding this by one third to give 44% of yield for short durations would be sound. There are a number of criteria that also need to be satisifed. You need to get the owner's approval for a start. You need also to consider fatigue. Check out

http://www.pipingdesign.com

papers on surge that I have placed there.

As you would be aware one shouldnt mix the requirements of codes and standards as design is but one portion of a successful installation. It goes hand in glove with NDE, inspection, welder qualification, testing and maintenance. Thus mixing the provisions of MW11 with ASME B31.3 should be tempered with a conservative design.

I am involved in investigating one pipeline failure that was suspended in a bridge. The pipe was suspended from hangars. It is believed a surge event overloaded one hangar which failed. Adjacent hangars alongside then were overloaded and the whole thing collapsed in a domino effect.

 

[VBHMBG]

Dear stanier,
I agree with your thoughts concerning the allowable stress values acc. to B31.3 with respect to waterhammer loads. Also the mixing of codes often leads to endless discussions when not clearly approved by owner etc.

One question to you:
For the modal analysis of the pipng system you mentioned the determination of natural frequencies up to the 8th mode. Why not to a higher number of nodes? My experience is that time history forces acting along the centerline of the relevant pipe section often need pre-extracted natural frequencies up to 200Hz (or more) to develop there full dynamic contribution on axial stress as well as anchor loads.

Best regards,
VBHMBG

http://www.pipestressanalysis.com

http://www.surgeanalysis.com

 

[bjb]

Thanks for the replies. Another firm that we have partnered with is conducting the water hammer analysis. My part is to ensure the structural adequacy of this pipe as it free spans across a small creek. This project, which is a public water main is governed by AWWA Standards and Specs. Once the other firm gives us the results of the water hammer analysis I will incorporate this info into my design. Given the nature of this project I plan on being conservative, but I haven't seen the results of the water hammer analysis either. The particular design document that I am following is by the American Water Works Design Manual M11. Unfortunatley, it is kind of vague in some areas.