Topic last reviewed: 10 April 2013
Sectors: Downstream, Midstream, Upstream
Valves that leak internally can lead to major losses of valuable product, or unintended transfer of process constituents, in some cases seriously elevating process risks. Surveys are carried out to check valves that are expected to be closed during normal operating conditions. The primary purpose of such surveys is to determine whether the valves are in a fully closed and sealed position, and thus not passing product to the flare systems or other lines due to a seal failure.
Technology options that are typically deployed for this type of component integrity survey include acoustic and infraredtechnology. Either may be used, as follows:
- Acoustic survey techniques combined with an empirical correlative database may be used to estimate the mass flow passing through a partially closed valve based on the monitored acoustic signal.
- Acoustic and infrared techniques may be used to determine whether a valve is passing or not. The net outcome of this survey approach is a pass/no-pass survey result.
The U.S. Environmental Protection Agency (U.S. EPA) under the Greenhouse Gas Mandatory Reporting Rule for Petroleum and Natural Gas Systems has required that subject facilities perform leak detection(s) of equipment leaks and through-valve leakage using any one of the following methods (Reference 7):
- Use of an optical gas imaging instrument
- U.S. EPA’s Method 21 (see Reference 8)
- Use of an infrared laser beam illuminated instrument
- Use of an acoustic leak detection device
Passing valve identification without dismantling or isolation
Several methods exist and can be used in combination with each other.
- Use of an acoustic detection and quantification device: A study on refineries showed that a small percentage of valves are passing, and that a smaller part of them is responsible for the majority of leaks. In order to identify the large contributors, it is necessary to quantify those leaks. A portable tool available on the market allows for quantification of the internal leaks in valves for single phase fluids (gas, liquid and recently, steam), based on acoustic detection. The measurement is quick (two minutes per valve) and non-intrusive. The noise of surrounding rotating machines generally does not affect the measurement but, in some specific conditions, the nearby ΔP of fluids can disturb the measurement. This device also has the added benefit of allowing an analysis of the integrity of a isolation valve (since some are exposed to sand erosion) for valves connected to the flare, in order to improve performance with respect to safety, production and environment (References 2 and 3).
- Detection of a passing valve using the Joule-Thomson effect principle through the valve: This principle consists of identifying a lower temperature downstream of the valve compared to the temperature upstream of the valve. This method can be applied with a thermal gun. The advantage of this method is that it requires standard equipment (‘thermo gun’). However, this method does not accurately detect passing valves in all cases (References 3 and 4). The effectiveness of the temperature differential approach can be limited by the following factors:
- Presence of ice or condensation in case of high ΔP through the valve. In the case of high temperature upstream of the valve and/or low ΔP, the temperature downstream of the valve can be higher or equal to ambient even though it is passing.
- When the section upstream of the valve is hot, it may indicate that gas passes through the valve. But for large diameter pipes, natural convection within the pipe can make the pipe upstream of the valve be hotter than downstream, even though there is no gas passing through the valve.
- Radiation from sunlight can affect the measurement, but can be overcome by taking the measurement in the evening or on a cloudy day.
- Listening for audible noises caused by the energy dissipated in the passing valve. This method can work in some cases, but can be disturbed by nearby machine noise.
- Use of an ultrasound stethoscope: this tool is different from the device mentioned above for the acoustic detection of a passing valve. It sends an ultrasound wave into the valve which bounces back and is converted into sound for the surveyor to hear. The operating frequency has to be selected carefully so that nearby noises do not disturb the measurement. Efficacy of this method is still to be confirmed.
Passing valve identification by dismantling or isolation
- On-off ball valves [emergency shut-down valve (ESDV), shut-down valve (SDV), blow-down valve (BDV)] can be verified for full closure on the outside (the stem position will be clearly visible), whilst limit switches attached to the valves would detect a false closure (>3 degrees open or closed). ESDV valves should be equipped with ‘partial stroking test’ capabilities to test the integrity of the valve control panel and the physical condition of the valve. However, this partial stroke feature is not installed on BDVs. For BDVs (to the flare), operations will frequently close the manual valves upstream and downstream of the BDV and then open and close the valve via the ESD system to ensure that the valve is properly functioning. Ball-and-seat alignment verification could also be carried out, but this requires dismantling the valve.
- Pressure safety valves (PSVs) are a well-known source of leaking/emissions. However, leak detection can be challenging. Most PSV leakage comes from improper re-setting of the disc on the nozzle, causing gas passage. Current practice is to dismantle and overhaul a PSV whenever it has been opened (even once) as a safety precaution.
- Control valves should be properly specified according to their purpose, with cost as a secondary consideration only. Full valve stroke testing is not always possible during operations: control valves are typically the most important pressure control valves in the plant, and stroking can seriously disrupt production (and cause flaring) if no bypass or other backup is available.
|Years experience in the industry:||11-20|
Project examples in the industry
Range of application:
|Capacity of up to a few MWth (0.5 to 4 MW) per heat pump (Ref. 12, Table 6). Maximum heating temperature: typically <100°C (however as high as 150°C has been demonstrated) (Ref. 3 and Ref. 12, Table)
|Efficiency:||Increase in gas recovery dependent on the leakage rate
|Guideline capital costs:||Relatively low cost of equipment (< € 100,000 for purchasing acoustical detection equipment).
|Guideline operational costs:||Passing valve surveys can pinpoint valves in need of repair or replacement. Quantitative surveys can provide estimates in lost revenue.
|Typical scope of work description:|| Leak integrity testing surveys are typically performed at the following points:
|Technical:||Acoustic monitoring has few technical limitations, but surrounding noise and/or vibration may be a consideration.
Limitations of other techniques are addressed above (see section on 'Passing valve identification without dismantling or isolation’, above).
|Operational:||Improvement in operational integrity and safety through reduction in leakage from valves.
|Commercial:||In gas exporting fields, repairing or routing the passing valves to flare (valves with internal leaks, connected to the flare system) can have a high return on investment with cost recovery within weeks or months.
For gas injecting fields, the replacement will be done for environmental and energy-efficiency reasons.
|Environmental:||Reduce greenhouse gas (GHG) footprint through increased resource recovery.|
The following technologies provide similar benefits and may be considered as alternatives to acoustic detection for identifying passing valves (see section on 'Passing valve identification without dismantling or isolation’, above).
- Joule-Thomson effect for thermal differential detection
- Ultrasound stethoscope for leak flow detection
All work performed in and around process equipment should be performed by properly trained technicians familiar with the hazards present. In particular, instruments should be intrinsically safe and all work areas checked for fire and explosion hazards prior to the survey.
Cost savings from repair or replacement of passing valves can be rapid and, given the survey alternatives presented here, not at a high cost.
Industry case studies
In this industry case study, acoustic leak detection equipment was purchased for around €50,000 to perform a survey of valves in a downstream facility. The objective was to test about 150 valves, but the crew was not able to test them all due to (i) lack of time and (ii) inaccessibility of some valves (located in areas which required the installation of scaffolding). The following is a summary of the work performed and the findings:
- To date, a total of 87 valves (mainly PSV and PCV) have been tested.
- The valves that were tested range between 2 and 6 inches. Most large diameter valves have not been tested to date.
- 16 valves were identified with high leakages: Of the 16 valves, four valves had significant leakage flow (flow rate passing through the valve on the order of 6–15 kSm3 / day gas).
- Interventions took place and/or are planned for these valves to mitigate the gas losses (Reference 6).
- Labeyrie, Hadrien an Labeyrie, H. and Rocher, A. (2010). ‘Reducing Flaring and Improving Energy Efficiency: An Operator’s View’. Society of Professional Engineers (SPE) Paper 126644
- Score Group PLC (website): ‘Valve Leak Detection Using Acoustic Emissions (AE)’.
- Wagner, H. (2005). ‘Ultrasonic Spots Leaky Valves’. International Society of Automation. (Hans Wagner is President of ClampOn Inc.; this article comes from his ISA EXPO 2004 paper and presentation, ‘Innovative techniques to deal with leaking valves’.)
- Fluke Corporation (website): ‘Fluke Thermal Imagers for Electrical/Mechanical’.
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- Mistras Group (website): ‘Acoustic Leak Detection and Quantification for Through Valve Leakage as Mandated by 40 CFR, Part 98’. Mistras Products & Systems Division.
- U.S. Federal Regulation: 'Mandatory Greenhouse Gas Reporting for Petroleum and Natural Gas Systems’ (Regulation 40, Part 98, Subpart W). Electronic Code of Federal Regulations (e-CFR data), 2013.
- U.S. Federal Regulation: 'Determination of Volatile Organic Compound Leaks' (Regulation 40, Appendix A-7 to Part 60, Test Method 21). Electronic Code of Federal Regulations (e-CFR data), 2013.