I would like to draw your attention on the EN 1779 standard (dated August 1999, as far as I know) about  Non-destructive testing  -  Leak testing  -  Criteria for method and technique selection.

         In particular, in paragraph 6 (page 4, last sentence), it is stated that:
                  "... Zero leakage rate shall not be specified.
                   The required leak tightness shall be related to the function of the object under consideration..."

         What's your opinion about that?

Clarify the scope or application of EN 1779.  I don't have a copy.  The title is "Non-destructive testing - Leak testing - Criteria for method and technique selection."  Does the leakage criteria apply to flange leakage - associated with helium leak detection?

Many vendors in my industy tout "bubble tight". In reality, after the valve is put in service, nobody really knows whether it is bubble tight or not.

In my experience, my clients would put 2 valves in series, usually different manufacturers, if they are really concerned with "bubble tight" and/or "zero leakage".

Most valves work better with lower upstream pressure.
And, you have a better chance with two valvs than one.

I have talked to my valve rep about "bubble tight" valves. They do sell them. They don't guarantee/warantee them once they leave their lab though. The guarantee/warantee is only that the valve(Globe Valves) passed the test in the lab, once.

This thread started off as an inquiry on how to interpret the EN 1779, particularly the statement, "The required leak tightness shall be related to the function of the object under consideration".

To give an example of how I arrived at this "tightness", I need to tell a story.

I used to design leak testing equipment for the automotive divisions.  I often wondered how they arrived at the specs they did; was it a value that had some meaning, or some arbitrary high mark. An engineer at the old GM Harrison Radiator told me the following, as related to an automotive air conditioning system.

The total system of main components; compressor, condensor, and evaporator were allowed 1/4 the leak rate that would have caused the A/C unit to require service in 7 years.  The A/C group identified the mass of refrigerant loss which would require service.  Once everything was assembled (the lines and fittings and miscellaneous devices were allowed another 1/4) the sysem should work for at least 7 years before re-charging was required.

Since most testing was done with helium as a convenient detector gas using mass-spec techniques, we then converted (from a chart provided by Harrison) the equivilent rate of helium that could come through a hole that would allow no more refrigerant gas than the 1/4 allowed.

Now we knew the high limit of leak rate that the equipment should look for.  On this equipment was a number of valves, and the most important was the valve that pressurized the sample with helium.  If we could not trust that it's leak rate in the off state was below a certain minimum, additional flow could be entering the test specimen, driving up the pressure, causing a higher leak flow, and rejecting parts that would have, with a perfect "zero leak" valve, may have passed.

In the end it boiled down to economics; the value of a rejected part, versus the cost and maintainance of a "zero leak" valve.

There are valve companies that publish maximum leak rates, in helium pressure equivilents.  The lower the rate, the higher the cost.  If the part was very expensive, the valve may have been a magnetic field bobbin-type with a carbon/ceramic throat, and if the part was cheap, a relative inexpensive solenoid valve with stainless and rubber seats.

The definition of engineering is to produce the desired results (reasoned-out specs as above) while achieving a practical economic return.  Specs and Dollars (Pounds, Lira, etc.)

MORE NEWS