"Zero Leakage" conept and EN 1779 std... 2

[NGiLuzzu]

Dear All,
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?

Thanks and regards, 'NGL

 

[JLSeagull]

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?

 

[NGiLuzzu]

The mentioned standard is intended for general purposes and "describes criteria for the selection of the most suitable method and technique for the assessment of leak tightness by indication and measurement of a gas leakage".
Some of those methods and techniques are: vacuum hood or chamber, chemical detection with detection, tracer gas (usually helium), vacuum box, sniffing (with helium or halogen), pressure change (rise or decay), bubble tests, flow-metering, etc...

In any case, I was only trying to focus the discussion about the "ZERO LEAKAGE" expression which (if not further defined) represents only a dangerous marketing concept, with no technical and/or scientific meaning.
In addition, many other issues may be involved: such as the measurement accuracy concept (just to mention one) which seems to be ignored by everybody, Engineers and Specifiers included, in the valve world!!

;-)

But this is my opinion... and, as I said before, I wanted to hear about the other Members' ones!

Bye, 'NGL

 

[Ashereng]

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 passed the test in the lab, once.

 

[JLSeagull]

API uses the phrase zero leakage, thus I specify zero leakage for ball valve seat leakage. This is in terms of zero bubbles per minute, etc.

If people are testing flange leakage rates for toxic gas applications then bubbles per minute are not the measurement of interest. Still, some measureable acceptance criteria is required in terms of grams per fortnight or something. That is why I asked about HLD - as opposed to seat leakage. Leakage through the seat of a valve is a different than through a pourous casting or between flanges - into the atmosphere.

 

[Ashereng]

Yes, I am referring to through the valve (seat/ball/etc.).


Another fugitive emission problem is leakage through the packing. Is that included in EN 1779?

 

[NGiLuzzu]

In my opinion, the expression “zero leakage” should be avoided.
In any case, when it is used, it must not be intended in general terms, but referred to the specific test conditions.

For example, API 598 provides for well specified test media, pressures, durations and temperature (ambient), then in note b to Table 5, gives the following definition: “There shall be no leakage for the minimum specified test duration (see Table 4). For liquid test, 0 drops means no visible leakage per minimum specified test duration. For gas test, 0 bubbles means less than 1 bubble per minimum specified test duration”.
Even if the adjective "visible" may be questionable, that is a clear statement.
Nevertheless, there are important Customers and Specifiers who extend such a requirement far from its own limits, like maximum valve design temperature and pressure, helium instead of water or air, longer durations and so on... because "zero" times any number is always zero!! ;-)

Regarding fugitive emissions from packing (and body flanges), as anybody can see from the many related threads within this forum too, the most common references (ISO 15848-1 and -2 standards; Shell MESC SPE 77/312 and 77/307 specifications; etc...) provide for helium tests.
Unluckily, almost all packings are made of graphite or PTFE... materials that are intrinsecally permeable to helium!
But nobody seems to have considered this aspect... as well as outgassing problems when performing tests with vacuum, for instance.

Bye, 'NGL

 

[JimCasey]

I agree that "Zero Leakage" is inherently meaningless, as is "Bubble Tight". Unless there is a test procedure and a pass-fail criteria attached, they are non-defined.

On the other hand, if a customer orders a valve tested and certified to (for example) FCI70.2 Class IV, he knows what the leakage is at 50 psi. There is no relationship between that and what the valve will leak in service, unless it is installed on 50 psi air.

 

[JLSeagull]

I also lack the latest copy of FCI 70-2 that includes the IEC criteria. FCI and API include a testing criteria. The tightest FCI class for control valves leaks like a sieve compared to the API zero leakage criteria. Zero leakage is definitive. OK, one gram per fortnight per cm2 area at one kPa differential is more difinitive. I would be happy if that language were to exist in the standard as an acceptance criteria for zero leakage. Then if you want something with tighter shutoff you must state one gram per fortnight per cm2 at 1000 kPa differential.

So what terminology is prefered over zero leakage or bubble tight. I like to differentiate acceptable control valve leakage from the positive shutoff required for block valves in toxic or reactive situations. Zero leakage as defined by API makes this clear.

BTW, after we run rusty hydro-static testing fluid through the valve and place it in operation this is all moot.

 

[NGiLuzzu]

JimCasey,
I essentially agree with you, except for the sentence "... if a customer orders... he knows what ..." !!!
;-)
There's a lot of ignorance and incompetence around, even where you don't expect to find that...

I am facing, for example, some people in charge of writing the Specifications for one of the world biggest Oil & Gas Companies, who want to see "zero leakage" (or ISO-EN "Rate B", which is not much different...) on valves up to 36"/900 even at maximum design pressure and temperature, with dry nitrogen, after a hundred of mechanical cycles or so...

How can I bring them to more reasonable terms?

Many thanks, 'NGL

 

[Ashereng]

There are a couple of ways to go with this.

1) Apply for an exemption.
2) Show the project manager that:
- you have tried, but could not find a vendor who would certify that their valve is still bubble tight at MAWP and temperature, with dry N2 after 100 cycles
- the cost of the alternative solution, which is multiple valves in series, along with the associate on-going testing and maintenance costs.

Then again, there are some processes that do require this type of certainty. In those cases, the cost and time effort will be borne.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[NGiLuzzu]

Ashereng,
I do appreciate your hints, which are wise... but start from the point of view of the Engineering/Contractor Company.
In the case I mentioned above, we are the valve Manufacturer/Vendor and the... "fool Specifiers" are the Engineering!

However, I agree with you: such test requirement are in many cases far from the actual service needs (who will notice some bubbles or a few ml/min in the line? How?).
In the cases where a really tight shut-off is needed, in stead, other plant design solutions (like two valves in series, possibly of different construction types) must be considered.

Thanks, 'NGL

 

[Ashereng]

who will notice some bubbles or a few ml/min in the line? How?

I have not seen the actual lab test myself. However, a friend works for a leading valve manufacturer, and their lab does actually certify valves to the various ANSI leakage classes. I guess they either have a machine (or person) count/measure? I really should go and take a tour of the lab - hmmmm, lunch too?

In the cases where a really tight shut-off is needed, in stead, other plant design solutions (like two valves in series, possibly of different construction types) must be considered.

I agree with you completely. If you need "shutoff", as in "0" flow, you should use more than 1 valve. Two valves in series, different make and different technology (if possible) is the best way to go.

The first valve takes the brunt of the pressure drop. The second valve only sees a fraction of the pressure (essentially, the leakage of the first valve), and thus have a much much higher probability of shutting the flow off totally. A bleed in between, that is piped away, and monitored, is a great idea.

I have only seen one site with double block and bleed, piped away, parallel valve stations (they needed availability and reliability), so far in my career. When I saw that set-up, they got my attention in an instant. They were serious about their safety, and production. Their money was where their mouth was (is that the saying?).

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[NGiLuzzu]

Ashereng,
when I was wondering about the detectability of "some bubbles or a few ml/min" I was just meaning in the plant, not at the Manufacturer's laboratory.

How many processes are so "sensitive" respect to such leak rates?
When will you feel that an installed valve is leaking?

Thanks and regards, 'NGL

 

[Ashereng]

anegri,

Oh. Well, in the plant is way tougher. The only way is to pull the valve and lab test it for leakage. I don't know of very many people that do that. It has to be part of the equipment health program.

How many processes are so "sensitive" respect to such leak rates?

I don't know. I would think very little. The problem is, usually, it is not discovered until AFTER an incident.

Here is an example - not to bubbles or ppm, but the analogy is apt.

http://www.csb.gov/completed_investigations/docs/CSB_ASCO_Safety_Bulletin.pdf

They didn't think the block valve, or the check valve was an issue until the forensics.

I guess to answer your OP, I agree. I think one needs to specify the volume passed (leakage,) at a temperature, delta pressure and duration.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[JLSeagull]

The double block and bleed arrangement with three valves is common. This is the standard arrangement for heater burner gas piping. Also, single valves are manufacturered in a double block and bleed (seats on both ends with the body cavity vented.

 

[Ashereng]

Parallel double block and bleed, piped away, is 6 valves. Have you seen a valve station like this?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[FarAndNear]

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.)