Discussion on valve leakage rates 1

[passist]

I am a valve technician, my job is to assemble, and inspect valves.
Most of the valves in the company I work for are quarter turn eg. Ball, V-Ball, butterfly, and a few other ones.
For a while now I've been struggling to understand why the requirements often are somewhat odd.

For example when producing ASME #600, OR ASME #900 valves specially designed for high pressure control applications, a low pressure air test ( 3.5 - 6 barg ) is often required, with extremely tight leakage allowance. ( ANSI VI, API 598 )
Wouldnt it make more sense to order a 100barg water test such as ANSI Class V to match the real situation?

Also valves like V-ball valve, for flow control applications, leakage rates ANSI V, or ISO D are often required. Are this kind of leakage rates really required for a control valve, or do they just want them to be tighter, because nowadays everybody is offering it?

Now how long will it be, that a metal to metal seated valve will hold the ANSI VI rating, or will it soon be an ANSI IV valve when it is being used in the real world outside the testing facility?
Im especially interested in the triple offset butterfly valves, they are marketed as bubble tight, but I find it hard to believe, that the rating would be long-lasting.
Obviously the flow media will have a huge impact on the leakage ratios, but since I have no field experience, id appreciate all possible information regarding valve leakage lifetime.

 

[gerhardl]

You raise several questions, and each will require a lengthy answer to cover all details. Some of the answers can be found by searching this forum. Some brief comments, to help start the discussion:

1. Choice of test type and requirements are either given by the valve class/regulations or the factory or customers. Reasons can often seem obscure, but are mainly based on experience . What works works, but things can always be improved!

2. For some valvetypes a higher pressure will help valveparts and sealings to come in thighter/better contact and seal better. A good and precise manufactured valve will prove itself with good sealing at low pressure.

3. Valves will wear, and yes, often the test at the factory is the 'one time' that some valves will qualify. Operation conditions and fluids vary very much, and some valves will have long standing with minimum leakage. Time before wear: some hours to some ten-years, depending.

4. V-port valves are often used as combined control and shut-off valves, and need hence to close properly.

5. Double and triple eccentric BFL valves are capable of long-time tightness. The quality (non-leakage) are depending of exact and repetitive accuracy and quality of all parts, helped by the geometry of the valve, and correct operation. For larger valves of this type it is (among other operational requirements) important to avoid cavitation conditions caused by throtteling the valve down to 'haircrack' openings. Precise double or triple eccentric BFL valves in top-class (for instance for waterworks- watermains, several brands in Europe) can remain bubble thigt for a very long time (30-50 years plus). Note: There is a wide range in quality from producer to producer.