Full Flange Rating Hydrostatic Pressure

[FDS2008]

Can someone clarify what this concept is? I am working for an engineering firm who is working for a client in the oil and gas and they indicate that for new oilfield projects, they typically determine the Hydrostatic test pressure by the "full flange rating". I understand this as the maximum test pressure will be the maximum pressure the flange can take.

I am familiar with the Working pressure tables for flanges under ASME B16.5, but it is not even close to my client's suggested "industry" full flange rating of 450 psi for a 150# and 1,125 psi for a 300# flange.

Does anyone know what this refers to and where I can find reference to this.

Thanks in advance!

 

[JohnGP]

It means that the hydrostatic test pressure is based on the full rating of the flange - i.e. the minimum test pressure for B31.3 piping will be 1.5 times the design pressure. If you take the design pressure as the full rating of the flanges in the system, then you will have test pressures around what you have stated.

Cheers,
John

 

[jmiles]

An ANSI 150# flange is rated to a pressure of 285 PSI @ a temperature of up to 100°F. that means your design pressure can be a maximum of 285 PSI using 150# flanges, you then multiply this by 1.5 to get 472.5 PSI as the test pressure for a 150 ANSI system.

this is typically done at least in oil and gas because it makes reusing piping or pipelines, or equipment much easier if your process conditions change down the road, you dont ahve to go rehydro because someone said the design pressuire was 150 PSI, and therefore only tested to 150 PSI, and now that you have a new pump that discharges at 220 PSI... what do you do?

Oh and for a 300# flange the test pressure would be 1110 PSI (740 x 1.5) any higher than that and well your exceeding what the flange was designed for and what the code requires for testing.

 

[JohnGP]

Jmiles, I think you meant 427.5 psi, but in any case the actual numbers will depend on the flange material, and whether temperature plays a part.

 

[FDS2008]

Thanks jmiles. I see the light. I was just wondering where it is wrotten that you can multiply the maximum working pressure of a flange by 1.5 and I fouind it in ASME B16.5 sub-section 2.5.

John GP, you are correct that it is 427.5 psi, however ASME B16.5 allows you to go to round off to the next 25 psi.

Thanks for the speedy replies commarades!

 

[jmiles]

my fingers always get ahead of themselves when i type, thanks for the correction there John GP

 

[dcasto]

even tough a lot of engineers like to design pipe to match full flnage ratings, I do not go that far. To get a 6" line to 1480 psi, you have to go to sch 80. This makes no sence if all the presure vessels are rated to 100 psi, an acceptable value for schedule 40 6". The same thing about 2", rated at 1459 psi, why go to sch 80 for 21 psi.

On pipelines, it's even worse. I had 1 pipeline with ANSI 900 flanges. The pipe was rated to 2280, so what, why add more steel in the pipe if you don't need it.

 

[zdas04]

dcasto,
I'm with you. I look at the process needs for pressure and temperature and set a Design Pressure a reasonable amount above that value. Then I pick pipe and flanges that equal or exceed that Design point. Finally I set the Test Pressure at some ASME B31.8 value (depending on the factors in the code) above the design point.

So if the process needs 400 psig and 6-inch pipe, I'll set my Design Pressure at 600 psig and the test pressure at usually either 750 or 900 psig, flange rating at ANSI 300 (Material Group 1.1 at 50C is 726 psig) and the pipe schedule at standard or S40 (1210 psig at 100F). Then I do the ASME calcs to verify that the stresses at either the design or test pressure. This results in a reasonable design without going crazy over exotic materials or really thick walls.

Testing every line to the max you can get out of the flanges can get you some extra pressure rating for future changes in process, but my experience has always been that I don't want to go from 600 psig MAWP to 726 psig, I want to go to 1,400 psig and have to change flanges, valves, and probably piping anyway.

David

 

[jmiles]

For myself, i will say that there are arguments for both cases, pipelines are a whole matter entirely, but for process piping in a plant or such, ive seen it too many times where someone has gone no further than looking at the flanges in a system to decide what the max pressure of that system is. thats why i will size the pipe to meet the maximum allowable of the ANSI flange rating.

Of course that being said i also tend to do most design with a client supplied piping specification, so that pretty much dictates the wall thickness i use.

One other thing of note though i almost always use 2" sch 80 simply because the pipe does not bend as much under its own weight...