Fugitive Emissions: how to detect something you can't see or smell
23 September 2015
Fugitive emissions are the release or leak of a substance to the atmosphere by equipment or piping connections in oil & gas processing units. Both valves and packing seals have been identified as the major source of this type of emissions in refineries and oil & gas plants.
In this article, Fabio Giove of IMI Critical Engineering discusses the some of the issues surrounding leaking valves and existing leak detection t
At best, accidental leaks from process equipment and the subsequent dispersal of leaked substances into the atmosphere cause “only” a bad smell in and around the facility, or create a nuisance to neighbourhoods around the plant.
However, far more dangerous are leaks of gases which have no obvious smell, which can be highly hazardous to both site personnel and local residents.
One example is the emission of gas rich in H2S (i.e. sour gas, typically upstream of gas stripping units). While such leaks may meet legal concentration limits (which differ significantly from region to region and can be contradictory), the long-term impacts of such concentrations are not well known. Even gas concentrations below 5 to 10 ppm (parts per million) can have immediate and significant effects on the human body, while it has been demonstrated that continuous exposure to H2S limits far below 1 ppm can result in serious, long-term diseases among workers and nearby residents.
Research conducted by the University of Southern California revealed that depression, anger, fatigue, tension, confusion and respiratory ailments were significantly higher in people who were exposed to H2S than in the rest of the population. The importance of limiting the emissions of pollutants to the atmosphere is therefore clear, not just for health and safety reasons, but also to mitigate against the possibility of legal claims from individuals, trade unions and individuals living nearby.
The European Sealing Association’s own investigations into the major sources of leakages in refineries discovered that up to 70% of emissions to the atmosphere come from uncontrolled or leaking valves.
Severe service control valves are the most critical in this sense as quarter turn and on-off types are subject to less stroking and wear of packing during their lifetime, making the importance of continuous leak detection absolutely vital for these types of valves.
Leak detection techniques
Current maintenance techniques for reducing emissions are based on periodic checks (“accidental maintenance”) undertaken by sniffing sensors, infrared cameras and ultrasonic leak detection. However, none of these techniques can be considered 100% reliable and objective.
Sniffing, for example, can detect the approximate concentration of a particular pollutant in the air, but the measurement of such concentration is heavily affected by factors such as wind, mass spectrometer type, sniffing sensor position and outside temperature. These factors make it practically impossible to correctly measure values below 10ppm and, in some cases, even to assess where the leak has originated. For this reason, sniffing sensors are often used in combination with infrared LDAR (Leak Detection And Repair) cameras. Ultrasonic leak detectors and infrared cameras cannot be considered as quantitative ways of measurement and can provide only a rough guide as to the origin of the leak.
The costs of such systems are very high in terms of capital investment, but it is the man-hours, with the qualification and training time for personnel, that can make them an onerous solution.
Despite these shortfalls, the fact that the intervals between two consecutive leak detection sessions can be long means any leak occurring between them may potentially create serious safety issues on site. For this reason, some plants have restricted areas with mandatory breathers and training before people can enter.
Extending packing life and monitoring status
Once the source of the leak has been identified, it is normally required to shut down the plant to safely take remedial action. It is critical to maximise the life of packing seals to reduce the number of shutdowns, or at least to help to apply a predictive maintenance regime. For this reason, many end users install both a primary and a secondary packing in series on critical valves (Figure 1).
Figure1 -------------------------- Figure 2 ----------------------- Figure 3
The primary packing prevents leakage from the valve, while the secondary packing is a back-up solution in case the primary packing fails. A lantern ring and leak detection port separate the two packings, allowing sniffing and monitoring of leakage through the vent.
There are, however, two limitations to this configuration. Firstly, if the leak detection port is unplugged (Figure 2), the valve continues to discharge unwanted Volatile Organic Components (VOCs) into the atmosphere until the problem is discovered. In case of high leakage, the port can be plugged to switch operation to the secondary packing.
Controlling emissions in this way is very expensive since it requires qualified personnel to check hundreds of valves across the plant on a regular basis. Even if the system is only partially functioning, serious safety issues persist for the personnel carrying out the leak detection manually. Moreover, the detection process is significantly affected by valve movement, speed and direction and the positioning of the sensor, meaning operators will receive different measurement results depending on whether the sniffer is inserted in the port, or used at a distance.
Usually the port is kept plugged and is unplugged only for leak checking. However, if the leak port is plugged at all times (Figure 3), the leakage from the primary packing pressurises the space between the two packings, while the pressure build-up in the lantern ring volume causes the secondary packing to operate continuously, meaning it will wear out before the primary packing. The secondary packing, which is normally sized for emergencies, is unintentionally kept in continuous operation and therefore releases more emissions to the atmosphere.
However, the main issue is that, during LDAR sessions, when the port is unplugged and the lantern ring volume is sniffed, only the status of primary packing is checked, meaning the LDAR sessions give an ineffective result.
Last, but not least, if the LDAR session is not carried out on a daily basis, pollutant volumes released can be very high and unpredictable.
The latest systems consist of a mass micro-flowmeter with intelligent logic. The system continuously and automatically monitors the performance of control valve packings by measuring the effective leakage passing through the primary packing seals of the valve. The mass flow measurement is extremely accurate and can measure in the range of 1 to 50 standard cubic millimetres per second, which is equivalent to a range of 5 to 500 ppm or to a bubble size of between 0.5mm and 5mm per second.
If the values exceed preset limits which can be set into the positioner, the system sends a command to the leak port valve to shut off the vent port, with the sealing operation automatically switched to the secondary packing.
The system is also able to relate values on the direction and speed of movement of the stem, resulting in a true understanding of data measures and avoidance of false alarms. It can also communicate the status of the packing to the operator and control room via HART, Foundation Fieldbus or Profibus communication protocols, for a simple alarm mode. This means it can play a key role in reducing the hazard of leaking packing within the plant and greatly enhance safety both in and around the plant.
About the author:
Fabio Giove is NPD director at IMI CCI, part of IMI Critical Engineering, where he has completed certification of Fugitive Emission Solutions according to ISO 15848 for extremely high pressures. Part of IMI plc, IMI Critical Engineering is a world-leading provider of critical flow control solutions which enable vital energy and process industries to operate safely and more efficiently.