Gate valve tightness issues 2

[DG4]

Hello,

We recently installed a piping network in which there are several gate valves.

All gate valves are socket welded, 1500#, SA-105, metal seats, less than 2'.
Fabricated and tester as per ASME B16.34 @ API-598.
Operating conditions: 1500psi@520F

A few valves have shown some leaks during operation. My customer told that those leaks are not acceptable and asks for perfect tighness of all valves.

My question for you guys are the following.

-Are my customer request realistic?

-If it is not realistic, is there official documentation stating maximum leakage during operation?

API-598 & ASME-B16.34 indicates zero leakage during testing for those diameters, but it is only for 15 seconds, at cold temperature. Does those tests guarantee complete long term operation tightness?

Valves have been located and welded as per recommended manufacturer procedure.
We openened 1 valve, found no particles in it preventing tightness. Representative replaced internal parts but it did not solve the problem.

If you need more info please tell me.

Regards,

 

[bcd]

As soon as you limit the test time, in this case 15 sec., you are measuring a leak rate, not just a leak. Everything leaks....period. It depends upon how you test it and how long you are willing to wait during the test. In the case of the 15 second test, there is zero leak acceptance criteria, but I guarantee that if you test with helium, and wait for an hour, and use high sensitivity soap solution, you will see those gate valves leaking. For the 15 second test, if a bubble leaks past the seat on the 16th second, it passed the test. 1 bubble every 16 seconds is a leak rate.

The main thing to consider is if it really matters or not. If the resulting leakage rate is so small that it takes hours, days, weeks, or months to fill the volume of the pipe downstream, does it matter?

Ultimately, the method of leak testing and the equivalent leak rate (for example, 1 bubble every 16 seconds)needs to be carefully considered and selected so it is appropriate for the system use. If a water system for your lawn, who cares. If toxic or even lethal gas, then leakage is really important and should be checked much more carefully.

 

[DG4]

Thanks bcd,


However, even if leaks can be tolerated, my customer will not accept any unless I prove the actual leak rate is acceptable. I'm looking for a reference standard quantifying acceptable leak rated during normal operation over extended period of time, or at least stating that some leaks are inherent during operation. If leaks are too high, so be it we will replace it. If it is normal, and any gate valve will shown similar behavior, then it's different.

Maybe you or someone know about this kind of documentation, because so far, I have found nothing about it.

Thank you

 

[ET1983]

Hi DG4,

i hope the standard EN ISO 15848-1, section 6, whcih talks about tightness classes would answer to your question,.

 

[engineer2385]

If the valve nameplate specifies B16.34, your valves should pass standard shell and seat tests. As an additional requirement, if you are testing with Helium, the leakage rate should be specified by the customer based on his application. Then, the valves should pass this criteria. However, this specified leakage rate has to make sense for your valve design and should be agreed between you and the customer.

 

[bcd]

The valve rep. and the valve manufacturer need to tell you what valve standard the valves were tested to before they shipped from the factory. Was is API, ISO, FCI, MSS, AWWA, etc., etc. There are many valve standards with different test requirements that randomly selecting one is not going to help you solve this problem.

Once you find this out, then test the valve per the requirements of the standard (media, pressure, and time). If the valve passes, then the customer has no right to question unless they specifically ordered something else.

Please note that ASME B16.34, as mentioned in a message above, does have test methods for valves, but it does not have acceptance criteria. So this standard will not help you other than to define a commonly used test procedure.

 

[georgeverghese]

If you want bubble tight operation, should have gone for valves with soft seats, not gate valves.
And if you want nominal "perfect tightness", it should be double block and bleed with soft seated valves.

 

[crshears]

An observation on terminology...

Clearly, any piece of equipment being tested either passes or fails the test.

In my history within the operating world, however, I am accustomed to stating that when a valve "passes" it is allowing the controlled medium to "pass" from one side of the valve to the other WITHOUT escape of medium to the environment or ingress of air or other contaminant/adulterant into the controlled system.

When a valve "leaks" however it IS allowing escape of controlled medium to the environment [or ingress of...as stated above].

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]