limiting factor on a facility piping for hydrostatic testing 1

[engr2GW]

Hi all,

for B31.3, the hydrostatic test pressure is 1.5xDesign pressure, well, the pipes are joined sometimes by flanges and these flanges usually have pressure ratings lower than the design pressure of the pipe body, so should
1. the test pressure be 1.5xpressure of lowest flange class in the section to be tested?
2. or 1.5 x the weakest component of fitting in the piping or section to be tested, since there are some valves, etc with even lower pressure rating.

Second question is:
most of the valves (ball, choke, wing, check,etc), may have been tested by the manufacturer. If a valve is rated for 2000# and all the other fittings and components are rated for 2000#, but the flange 600 class which equals 1480# meaning that test pressure is 1.5x1480 = 2220# which is higher than the pressure rating of the valves, fittings, etc. Since those components may have been tested to more than 2220# after being manufactured, is it okay to subject them to a hydrotest pressure of 2220# or should the test pressure (1.5xflange class) always be equal to or lower than the operating pressure rating of the other components.

Thank you.


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[zdas04]

engr2GW,
You are totally missing the point. The code says that test pressure is some factor (1.5 in your example) times design pressure. Design pressure or Maximum Allowable Working Pressure (MAWP) is set based on engineering judgement. I once had a system that was all ANSI 600 flanges XS piping and fittings, and ONE ANSI 150 flange with a blind--design pressure of the system was 280 psig and test pressure was 420 psig. Every single component in that system was rated for at least system MAWP, so it was an adequate design. The rating of the strongest components in the system was irrelevant to setting the design pressure, you have to look at the weakest component.

Any MAWP is going to be a LONG way from a failure point. I once had a fabricator ignore my specification to use ANSI 300 valves on a fabrication and he put ANSI 150 flanges on it. He called me and said that the fabrication passed the 900 psig test. I went to get the chart and saw the fabrication and my knees got weak--the fabrication had 10 ANSI 150 flanges that had been tested to 900 psig or 3.2 times a reasonable MAWP. They held up fine, but I cut them out anyway. If you have valves rated at 2,000 psig MAWP, then they'll be just fine for a 2,220 psig test.

David

 

[engr2GW]

Thanks David, what you just explain was the I understood it until the confusion set in.

Design; meaning what has been determined as what the piping will see, the pipe wall and fittings, flanges, valves may be over rated but the test pressure is limited by the weakest component; my understanding of yoru explaination.

ALSO;
does that mean that in case of any unforseen event of over pressure, either from well head or downstream valve plugging, etc, that the only way to ensure that (for the case you stated above) the pressure does not exceed 280psig is by overpressure production e.g. relief valve, etc. Or do you just go ahead and make all components able to handle the max. anticipated overpressure and tested to the same...

Thanks

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[zdas04]

On the case where I had a single ANSI 150 flange, I set and sized the PSV to keep the pressure on the entire system below 308 psig in all credible scenarios (pool fire was not credible).

David

 

[blacksmith37]

Depending on Spec, the pipe may have already passed a high pressure test at the mill (certainly for API pipe).

 

[engr2GW]

@ blacksmith37;
does that exempt the pipe from being tested afer production fabrication, besides, isn't the test for the pipe, joints, fiting, and leak test for the entire facility?

@zdas04;
you set the psv higher than the mawp of the lowest flange (of the system)? how does that work?

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[zdas04]

That is simply not what I said. The code requires that pressure transients be limited to some multiple of MAWP (in most cases the limit is 110% of MAWP). This requirement is intended to handle scenarios where pressure is increasing rapidly to the set point and might take some brief period before the PSV flow can reverse the trend. For example, if pressure is increasing at 25 psig/second at the PSV set point, and it takes 3 seconds for the PSV to fully open, then when you reach your full capacity the pressure is 75 psig above the set point. In the example above, a set point of 280 psig would result in unacceptable overpressure (280 psig + 25 psig/sec*3 sec = 355 psig which is greater than 280*1.1=308 psig). So, for a 280 psig MAWP and a valve that takes 3 seconds to reach full flow, a credible scenario that results in 25 psig/sec increase would require a set point of 280*1.1-25 psi/sec*3 sec = 233 psig.

You really have to read all the words.

David

 

[engr2GW]

@zdas04; understood. Thank.

As much as possible, do it right the first time...

 

[engr2GW]

@zdas04; what if you system was designed to not go beyond 280psig and you rightly set your PSV to 309psig, but your lowest flange (just for extra care) was a class 300 (740psig), in such a case, will your test pressure remain 1.5x280psig or 1.5x740psig???

Thank you.

As much as possible, do it right the first time...

 

[zdas04]

It is an analysis. You look at every component and find the one that is the weakest. At that point you either change it out for something stronger of set your MAWP based on that component.

The set point of the PSV is also an analysis. It is based on: (1) MAWP; (2) allowable overpressure; (3) credible scenarios; and (4) rate of pressure change in the credible scenario coupled with operating time of the relief device.

Test pressure is based on MAWP times a multiplier. An engineer determines what the test pressure needs to be. In certain location classes B31.8 calls for 1.1 times MAWP. For other classes it calls for 1.5 times MAWP. You set the MAWP, the code can give you guidance on the maximum MAWP that you can set, but it is still engineering judgement that is used to set the exact value. For example, the code may say that you can set the MAWP of a given piece of pipe at 1,400 psig--you can set the MAWP of that pipe at any value below 1,400 psig.

One thing you seem to be confusing is that PSV and test pressure are independent analysis. PSV should not be in service during the test.

David

 

[engr2GW]

Thanks a lot zdas04, all clear now.

As much as possible, do it right the first time...

 

[engr2GW]

Hi all,

on a second though, I found a system where the component with the lowest pressure rating is a valve. I understand that those valves may have been pressure tested by the manufacturer, but not sure if it's upto 1.5xp. Also, I was told that the manufacturers only test the shell of the valves, but that the test multiplier for when mechanisms is installed in the valve is less.
SO
If 2000psig ball valve is the component with the lowest pressure, does that mean the test pressure for the system is 1.5x2000psig??? is that just an assumption that this valve with it's internal mechanisms would have been tested as such.

Isolating these valves may be an option, but that means 3 to 5 test in one facility where there may have been only one test and hence, 3 - 5 time the cost.

Any suggestions, thoughts?

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[zdas04]

Is this a threaded valve? If so you need to drop a zero from the class (a 2000 lb valve has an MAWP of 200 psig).

Again, you set the MAWP. You do that by evaluating the system and applying appropriate risk analysis. If a manufactured component has a nameplate with an MAWP, then you can apply an appropriate multiplier for your test pressure. I would have no problem testing a 2000 psig MAWP valve to 3000 psig. The valve manufacturers I've worked with use a higher multiplier than the codes because they never know what field tests their valves will be subjected to.

David

 

[engr2GW]

@ zdas04;

isn't that a drastic drop for pressure in a valve that was manufactured to handle 2000Psig. Also, don't they included whatever the valves joining method is in the pressure rating? meaning, the thread, or the flange is part of what is rated as 2000psig, i.e. the valve comes with the thread or flange as a whole rated for 2000psig, why the drastic de-rate?

As much as possible, do it right the first time...

 

[zdas04]

It is not a "drastic rerate" it is a STUPID naming convention that has been around for too long. 2000 lb class threaded components are rated to an MAWP of 200 psig. I don't know where the naming convention came from, but that is what we have.

David