B31.3 Water hammer anlysis 1

[MEjmr]

Im doing a B31.3 analysis on a pipeline and I'm trying to calculate the maximum allowable water hammer within the pipe. I am taking a section [302.3.6 a] from B31.3 which says that the sum of sustained and occasional loads may be as much as 1.33 times the basic allowable.

So can I just take 1.33*basic allowable - sustained loads to find my maximum allowable water hammer (psi), then divide by cross sectional area to find max force?

 

[DSB123]

Yes you can but you would be wrong!!! Water hammer loads cause bending moments in a system so you cannot simplify the case as you infer.

 

[MEjmr]

Even if the pipe I am analyzing is a straight 2" section 4.5' long?

 

[BigInch]

Can't be 1.33 unless you have a number of conditions meet. Read the fine print. Meanwhile use 1.10 x MAOP, because you can generally meet the conditions that go with that.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[BigInch]

If your straight pipe is continuously supported, you won't have bending moments, if you have a span, you will have some bending moments with, or without, internal pressure of any kind.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[MEjmr]

the pipe is supported at both ends and one support in the middle. I am ignoring the support in the middle and treating it as a span. So then can I still take 1.1 x MAOP and subtract out my longitudinal stresses (sustained load), and my bending stresses to find max. allowable?

 

[BigInch]

Assuming you now have set a maximum design pressure and allowable overpressure in accordance with 302.2.4, including the maximum time allowances for that highest pressure and set pressure reliefs within that allowance, you must meet 2 criteria defined in 302.3.5 Limits of calculated stresses due to sustained loads and displacement strains, but you don't seem to have thermal displacement strains.

1.) Internal Pressure Stress Limit of 302.3.5 (b)
which must meet the minimum wall thickness requirements of 304
specifically the minimum thickness equations found in 304.1.2 (3a) & (3b)

2.) Longitudinal Stress S_L of 302.3.5 (c)
Sum of Longitudinal Stresses must not exceed Sh, the basic allowed material stress.

Since (at least for me) its more complicated to solve directly for Allowable hoop stress under internal pressure, subtracting it from those two plus equations there and then find the maximum allowed pressure, I prefer just to solve for hoop stress and longitudinal stress and simply compare those to their respective limits exactly as is done in the code.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[MEjmr]

I have done that, however the design I am working on is used for multiple situations, moved around from one place to the next. So rather than redo the calc for every situation I was asked to find a max value so further calcs we can just compare to the max rather than redo it. Its for a DOE complex so its quite the process to redo a calc and get it submitted! Thanks for the help, I got it now.

 

[stanier]

Having done your waterhammer analysis you can take the dynamic behaviour and use a pipe stress package to model the time history. AFT's Impulse develops a file that can be used in Caesar II.

The other matter you need to assess is the fatigue.

ASME B31.3 has quite a fe w conditions to meet in terms of unsustained loading, not least of which is getting the Owner's approval.

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

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

 

[Duwe6]

And the added NDT of "Severe Cyclic" service may apply to this stretch of pipe.