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Content archived on 2024-05-07

Valve stem leak-tightness testing methodologies


The overall aims of the project were:
- to establish measurement practices and procedures which improve accuracy and repeatability and, crucially, which avoid ambiguity
- to clarify why simple "conversions" are not possible between results using different measurement approaches and test gases
- to suggest, if possible, means of "translating" between the measurement possibilities and evaluate associated error and scatter.
Experimental work was concentrated on valves representative of typical volatile organic compound (VOC) duties over a range of sizes and types (but particularly 4" Class 300 gate valves) with several different packing types (but mostly graphite) over a range of leakage rate representative of this duty.
A full programme of dissemination is currently being implemented. This is targeted primarily at the valve industry but also to end-users and seal manufacturers.
There are close working links with ISO/TC153/ SC1/WG10, which is preparing a draft standard on leak-tightness testing of industrial valves. The results of this study have been summarized to this committee.

Please find the summary report of this project with a detailed description of work results in the CORDIS document library.
Standards are currently under development covering valve leak-tightness qualification and quality assurance testing. If these standards are to gain widespread acceptance, they must balance the conflicting requirements of:
- end-users' needs for PPM targets to comply with site emissions legislation;
- regulatory authorities' concerns over VOC (volatile organic compound) leakage;
- cost and safety of laboratory or factory testing.
The draft standards define:
- test methods and procedures
- leak-tightness criteria
- measurement methods.
There are a number of issues yet to be resolved concerning leakage measurement and leak-tightness criteria. This project has focussed on these issues and has provided technical information intended to help find solutions with the potential to satisfy the conflicting requirements identified above.

The issues
There are essentially two approaches to specifying leak-tightness targets:
- leakage rate
- concentration (PPM) near the leakage source using a "sniffer" device.
On-site leakage detection practice has widely led to the severity of valve stem leakage being assessed by sniffing. Whilst this does not actually measure leakage rate, it is widely misunderstood to do so, and many end-user valve test protocols specify it, because sniffing is relatively quick and simple, hence low cost.
There are also two classes of test gas: a safe gas for testing purposes (helium is accepted as the most suitable) and VOCs representative of the intended duty.
Thus there are four possible combinations as the basis of any leak-tightness test. The requirements in relation to these possibilities are conflicting: no single combination of test gas and measurement is satisfactory to all concerned.
If there were a reliable means of translating between the basic measurements, these conflicting requirements could be simply resolved by taking measurements in the most convenient manner, and "converting" the results to any other desired form. For example, a helium leak rate measurement might be taken to satisfy the ISO standard, and the result "translated" to an equivalent PPM of methane to satisfy an end-user's requirements. Unfortunately, however, a straightforward conversion (as if between units, for example) is not currently possible.

Call for proposal

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British Hydromechanics Research GroupLtd.
EU contribution
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MK43 0AJ Cranfield - Bedfordshire
United Kingdom

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Participants (6)