Strength to maximum pressure rating 1

[dsdtdemon]

Hello,
I am questioning a "standard" one group in my company uses for determining strength test pressure. When installing new pipe or assemblies, we will determine the pressure range for the test by first determining the maximum allowable test pressure and then subtracting some tolerance to determine the minimum pressure to specify, regardless of the MAOP. For example:
I am installing a assembly of 12.750" X 0.312" X-52 pipe with an ANSI 300 valve in class 3 location with an MAOP of 500psig. We custom order the ball valves to be shell test at 1.8 X MAOP, so the maximum allowable test pressure for the valve is 1296psig. So, we would specify the test to be something like 1296/1196 psig, when the code would require a minimum test of 750psig.

Better yet, if there was no valve, the limit would be the 2160 max test nipples, so we would spec a 2160/2060 psig test for a 500psig segment.

These seems completely stupid to me, am I missing something?

 

[GHartmann]

I am missing something here. 1.8 x 500 psig = 900 psig (Not 1296 psig). How was the 1296 determined?

I am also not understanding the term "strength test pressure". There are pressure tests for systems according to ASME codes for process plant piping. What specific code are you utilizing? I do not know what you mean by strength test.

The systems are also limited by the flanges and design temperatures. I assume you are talking about "cold" equivalent pressure tests.

 

[dsdtdemon]

Sorry, to clarify, I'm talking about natural gas T&D in the United States. So with that in mind, I'm considering "cold" pressure. ANSI 300 valves are rated for 720 cold working pressure; we order the valves to be shell tested to 1.8*720=1296.
The code I'm referring to is 49 CFR 192.619.

 

[BigInch]

Title 40 Part 192 Gas Pipelines Regulation.
For a design to 500 psig for temperatures between -20F to 100F.

Choose valves rated to ANSI#300, they have a normal ambient temperature pressure rating of 750 psig, as long as temperatures are between -20 and 100F inclusive, see ASME B16.5 (not 720 psig BTW).

http://www.engineeringtoolbox.com/ansi-flanges-pressure-temperature-d_342.html

Order the valves to Pipeline Valve Spec API 6D and ASME B16.34 as the regulation requires.
They will automatically be shell tested to 1.5 x 750 psig = 1125 psig as per those specifications.

That's more than enough then you have to do to meet the regulation, as you are correct, the maximum test pressure that is required by the CFR is 1.5 x MAOP(500) = 750 psig, if installed in a class 3 or 4 location, but only 1.25X in other class locations (625 psig).

You should have some other design condition in order justify testing to 1.8X, or you're pretty much probably wasting everybody's money and time. Are there high tensile stresses, are they going into 4000 ft of water??? In any case, it would be totally illegal to cause the valves, or any pressure containing component in the system, to reach internal pressures above 500 psig, in steady state, transient conditions, or otherwise, at any temperature.






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https://www.dropbox.com/s/r9mygqm70sm4hx5/OMG something else.png?dl=0

 

[BigInch]

You might be better served by extending the time duration of the hydrotest, rather than raising the pressure. At As I remember at Enron we required a 2 hour test for critical valves (tie-in and station isolation valves, etc.)

 

[LittleInch]

Test pressures in any code or regulation are the "minimum" and occasionally state a maximum (usually 90 or 100% of SYMS).

Testing to a higher pressure can be argued to "blunt" cracks and reduce fatigue cracking risk, but most people just try and do the minimum. It only really matters if by increasing the pressure or getting a special test / design e.g. of your valve casing, your costs increase for little or no benefit.

If the theory is that the same test applies regardless and hence there is less chance of applying a low test pressure accidentally to a higher MOP rated system then I can see the point, but I guess this is now lost in the mists of time inside your company.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

Right. Costs increase for little or no illegal benefit. It just doesn't make any sense to do it.