Fugitive Emissions How to Find Something You Can't See or Smell?
Fugitive emissions are the release of a leaked substance into the atmosphere by equipment or piping connections in oil and gas processing units. Valves, and in particular packing seals have been identified as the major source of emissions in Refineries and Oil and Gas Plants.
Sometimes accidental leaks from the process and subsequent spreading into atmosphere “only” cause bad smells and issues to neighbourhoods around the plant. On other occasions such leaks can be more hazardous to the health and safety for the site’s personnel and people living around it.
A serious issue is, for example, the emission of gas rich in H2S (i.e. upstream of gas stripping units). Even if such leaks meet the legal concentration limits (which are very contradictory and differ from region to region), the long term impacts of such concentrations are not very well known. In fact, gas concentrations below 5 to 10 ppm’s (parts per million) can have immediate effects to the human body and it has been demonstrated that continuous exposure to H2S limits far below 1 ppm, can create serious diseases to residential populations and workers.
In research made by Dr. Kilburn from the University of Southern California, it was discovered that depression, anger, fatigue, tension, confusion and respiratory diseases were significantly higher in people who were exposed to H2S than in the rest of the population, so it is very important to limit the emissions of pollutants to the atmosphere – primarily for health and safety reasons, but also to avoid claims for damages and legal actions from citizen organizations, Employee Unions, etc.
The European Sealing Association has made investigations about the major sources of leakages in refineries, and it has been discovered that up to 70 % of emissions to the atmosphere come from leaking valves. Severe service valves are the most critical in this sense as quarter turn and on-off types are less subject to stroking and wear of packing during their lifetime.
Figure 1. Valve with two packings in series.
Figure 2. Leaking by vent port. The monitoring is left to random measurements during LDAR sessions.
Figure 3. The leak port is kept plugged. Secondary packing is energized and leakage is not controlled.
Leak Detection Techniques
Current maintenance techniques for reducing fugitive emissions are based on periodic checks (“accidental maintenance”) with sniffing sensors, infrared cameras, ultrasonic leak detection etc. but these techniques can not be considered 100 % reliable and objective. Sniffing, for example, can approximately detect the concentration of a particular pollutant in the air, but the measurement of concentration is heavily affected by several factors (wind, type of mass spectrometer, sensor position, outside temperature, etc…). Due to these factors it can be difficult even to assess where the leak has originated, therefore sniffing is sometimes associated with LDAR (Leak Detection And Repair) infrared cameras. Ultrasonic leak detectors and infrared cameras can be considered qualitative measures and provide only a rough idea of the origin of the leak.
The investment costs for such systems are very high in terms of capital and human investment. The cost of equipment and the qualification and training time of personnel does not make these an attractive solution. Despite all that has been explained above, the fact that the intervals between two consecutive leak detection sessions can be long, any leak occurring between them may potentially create serious issue for safety on site. For this reason, some plants have areas with mandatory breathers and training for people to enter.
Figure 4. System diagram.
Extending Packing Life and Monitoring Status
Once the source of the leak has been identified, it is normally required to shut down the plant so that it is possible to safely remove the source of the leak. It is critical to extend the life of packings to reduce the number of shutdowns, or at least to help to apply a predictive maintenance campaign. This is why end users have developed the idea of installing a primary and a secondary packing in series on critical valves (Figure 1)
- the primary packing is to prevent the main leakage from the valve
- the secondary packing is a back-up solution in case of failure of the first packing
- a lantern ring and a leak detection port separate the two packings allowing sniffing and monitoring the leakage through the vent.
There are however two limitations to this configuration:
- If the leak detection port is kept unplugged, (Figure 2) the valve continues to discharge unwanted Volatile Organic Components (VOC) in the atmosphere until the problem is discovered. In case of high leakage, the port can be plugged in order to switch the operation to the secondary packing. Controlling emission in this way is very expensive since it requires that qualified personnel check hundreds of valves inside the plant on a regular basis. In this case, even if the system is partially working, serious safety issues still persist for personnel carrying out the leak detection. Moreover, the detection is heavily affected by valve movement speed and direction and positioning of the sensor and operators will have different measurement results if the sniffer is inserted in the port, or is kept outside at a certain distance
- In the most frequent case, the port is normally kept plugged and is unplugged only for leak checking. However, keeping the leak port plugged at all times (Figure 3), the leakage from the first packing pressurizes the volume between the two packings and the built-up pressure in the lantern ring volume causes that the secondary packing operates all the time and wears out because of the valve cycling. The secondary packing, which is normally sized for emergencies, is unintentionally kept in continuous operation and it is therefore releasing more emissions into the atmosphere than in the previous case. Nevertheless, the main issue in this case 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 and of course the result of the LDAR sessions are totally ineffective. Last, but not least, if the LDAR session is not carried out on a daily basis, the emissions of pollutants can be very high and unpredictable.
CCI has developed and patented (US 20130061954) a system consisting of a mass micro-flowmeter and an intelligent logic (Figure 4). The system monitors continuously and automatically 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 milimetres per second, which equals 5 to 500 ppm’s or to a bubble size of approx. 0.5 to 5 mm per second.
If the values exceed the preset limits, the system automatically closes the leak port valve to shut off the vent port and the sealing operation is switched to the secondary packing.
The stem movement speed and direction are sent to the logic unit with the position feedback signal to help to understand the measurement and to avoid false alarms. The alarm signal is a contact and the communication of the packing status to the control room is carried out via HART, Foundation Fieldbus or Profibus protocols.
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