Valve Stem Leak-tightness Test Methodologies
It has been reported widely that the majority of fugitive emissions on petrochemical or chemical sites come from leaking valves. The ESA has focused much attention on this issue, with the ultimate aim to assist valve manufacturers and users in reducing valve leakage.
Consequently, the ESA commissioned a valve emission test programme, awarded jointly to two independent test institutes, the initial results of which were reported at the Conference in Antwerp. The ESA is now collaborating with the valve industry and end users on the second stage in this work programme, supported by the European Commission under the Standards, Measurements and Testing (SMT) Protocol, as project SMT4-CT97-2158.
Rationale: There is strong economic and environmental pressure to reduce stem seal leakage in process industry valves. Consequently, users have applied pressure on suppliers to demonstrate that new products meet target emission levels. However, there are currently no cross-European standards for valve stem seal leakage. To make any such standards practicable, manufacturers will be required to conduct valve qualification and quality assurance measurements economically, and demonstrate that the results are sufficiently accurate and representative of in-service performance. The focus of this project is two-fold:
1. To improve and refine practicable testing techniques which have acceptable levels of scatter (and which are not uneconomically time consuming)
2. To provide a basis for relating leakage of a non-hazardous test gas to those of VOC’s (especially methane) under representative conditions
The 500kEcu project runs for 30 months and involves the collaboration of organisations across the EU:
- independent testing institutes
- petrochemical end-users
- a valve manufacturer
- a sealing material manufacturer
- leakage detection equipment manufacturers
- a national valve organisation
- the European Sealing Association
Project focus (the area in the green box!):
Guidelines in Germany, accepted in the majority of Europe, require that valves are supplied according to a leak tightness, which is quantified as a mass leakage rate over time. Equally, the development of the ISO standard on valve leak tightness is based upon the mass leakage rate. In general terms, this demands capture of the total leak (some form of “bagging”) to provide adequate quantification (often reported in g.s-1), which can be a complex and relatively slow process.
A more practicable, faster process is required in the field, and here the most common solution is “sniffing”. This is based upon measuring the concentration of the leak (in ppm). Sniffing usually relies upon the test methodology as established by the US Environmental Protection Agency (US EPA) and known as EPA Method 21. In reality, this is the only practicable method for use in the field, and so is the method of choice for users world-wide.
In the field, the “standard test gas” is usually methane (or an alternative VOC), and the performance of the valve in sealing such a VOC is required by the user as part of the quality assurance of the valve. On the other hand, for safety and practicality reasons, it is preferable to conduct both qualification and quality assurance tests in a safe fluid, such as helium.
Measurement methods in the SMT Project
Three main measurement methods are being investigated, in order to identify the most appropriate method for use in the quality assurance laboratory, which at the same time will provide results representative of “in the field” service:
The last two measurement methods are variants of “bagging”, which is employed when a more definitive answer is required.
Fluids examined in the SMT Project
To establish an empirical relationship between the leakage rate of helium and ppm of methane (and other representative VOC’s), four fluids are being investigated. These have been chosen in order to test hypotheses about interpretation of leakage rate according to viscosity ratio (for laminar flow) and molecular mass ratio (for molecular flow).
Other considerations are that the fluids should be in common use (methane is a “must”), that they should be straightforward to detect, and that the saturated vapour pressure will allow study under elevated pressures at room temperature (to ensure they remain in the gas phase). Subsequent Work Packages will investigate elevated temperatures and pressures (to reflect service in the field). The four fluids selected are:
- propene (propylene)
Packing types examined in the SMT Project
These have been selected from those in common use:
- expanded graphite, braided, then die formed
- lubricated, braided PTFE
Initial results of Work Package 1 have indicated that graphite may provide better consistency of leakage, and that the range of leakage rates may be changed by differential gland loading. For PTFE, improved sealing performance may be attainable, although the material may be more susceptible to fluctuations of internal pressure.
Leakage rates examined in the SMT Project
A range of leakage rates is under investigation, reflecting the levels monitored commonly in the field and within the performance range anticipated in environmental legislation. For the initial parts of Work Package 2, approximate leakage rates will be set by differential gland loading and measured for methane, using an OVA (organic vapour analyser):
- ~1000 ppm
- ~500 ppm
- ~100 ppm