Detecting dangerous gases to improve safety and reduce emissions
One versatile technology helps protect people and plants, while also detecting fugitive greenhouse gases.
One of the persistent hazards around upstream, midstream, and downstream oil and gas plants and facilities is gases of various types entering the surrounding atmosphere through leaks. The hazardous part in this context is the fact that many gases are combustible, toxic, or both. Escaping steam and compressed air pose some danger and certainly waste energy, but methane or hydrogen sulfide, among others, can be fatal if inhaled, and cause fires and explosions.
Those fugitive gases, even if they don’t cause immediate harm to workers or result in other damage, are emissions that are untreated and unmeasured. Methane lost this way is a financial hit, and it is also a serious greenhouse gas (GHG), contributing to climate change. Even if a company believes it can tolerate the financial impact and the local environmental agency is unaware of the release, this flies in the face of any company claiming to make serious efforts at improving its sustainability. Every facility should detect and control such problems for the sake of environmental responsibility, but also to avoid the potential for escalation to a more serious situation.
In this article, we will examine how gases escape, and show what mechanisms can be used to determine when a leak is happening, hopefully before the loss of containment becomes a dangerous problem.
Leaks result from a variety of causes, depending on operating conditions and environment.
- Corrosion is a major source, often due to water and acid gas condensation in piping and vessels, eating away at the metal from inside.
- Vibration can cause flange bolts and threaded joints (e.g., unions) to loosen.
- Shifting equipment foundations or supports place strain on piping and joints.
- Inadequate valve maintenance reduces the sealing action of packing or a seat itself.
- A combination of factors, such as corrosion attacking the heat-affected area near a pipe weld, allows a crack to open when a physical shift or change in temperature increases strain.
Whatever the case, a leak forms and the pressurized gas inside escapes to the surrounding atmosphere. It likely makes a hissing sound as it comes through the opening, although in the noisy environment of most plants and facilities, nobody may hear it. The gas mixes with surrounding air, creating a localized cloud with the concentration declining as it diffuses.
If the cloud is not carried by ambient air currents, the gas will rise or fall depending on its weight. If the leak is inside a building where the air is relatively quiescent, it will diffuse less quickly. A heavy gas, such as propane, will sink and accumulate in low trenches and sumps. Hydrogen will do just the opposite. These considerations are important when selecting a gas detection technology since they influence the type of sensor mechanism and placement.
There are three gas leak and/or gas presence detection mechanisms commonly in use today. These are designed to mount permanently in a critical location and communicate with a larger safety system for automated response, and eventually actions by a human operator in most cases.
First, a gas leak can be detected acoustically via a listening device tuned to ultrasonic frequencies (Figure 1). Using an array of directional ultrasonic sensors, it is possible to get an indication of the general direction of the source, but an operator must pinpoint the location. This detects a leak rather than a specific gas, so it can’t determine which gas is present. For example, it can’t differentiate between compressed air and hydrogen sulfide, so operators must make the determination based on what might be present in the location.
Second, combustible hydrocarbon gases and vapors can be detected through a chemical reaction with a catalyst (Figure 2). When the gas encounters the catalyst, it reacts with atmospheric oxygen, effectively combusting without actual fire. The action creates heat detectable via a temperature sensor. This approach is only suitable for point detectors and tends to respond comparatively slowly due to thermal inertia, although the speed of detection is sufficiently quick in most applications.
Third, various gases absorb specific wavelengths of light in the ultraviolet (UV) or infrared (IR) bands, which can be measured using differential optical absorption spectroscopy. A differential optical absorption spectroscopy detector monitors those wavelengths, and when they decline more than the overall light level, presence of the subject gas is the likely cause. The degree of attenuation can provide an accurate quantitative value of gas concentration on a near real-time basis. This is the most versatile approach since the technology can be configured in a variety of ways, and it responds very quickly.
Focusing on differential optical absorption spectroscopy
The primary advantage of differential optical absorption spectroscopy is its scalability. Two elements are required: a calibrated light source tuned to emit a specific wavelength, and a receiver able to read the same wavelength. In some cases, the receiver must also read a reference source for comparison. The two elements can be within the same housing to function as a point detector, but the source and receiver can also be separated, sending a beam across an open path, looking for a cloud of the target gas to move into its field of view.
Since the detector responds to a change in the intensity of specific wavelengths, there must also be a consistent light source for comparison. If both the target and baseline wavelengths are similarly affected, say by rain or a complete beam obstruction, the sensor will typically not cause a false alarm.
The two units (Figure 3) look from one to the other across the area of coverage. Both use lenses and shading to minimize any effects from ambient light since these systems must operate equally well in full sunlight and night-time darkness. Transparent covers protecting the equipment are often heated to minimize condensation or ice formation, each of which can interfere with transmission.
The specific wavelength selected must match the target gas, so it is necessary to select from a vendor’s offering. No single unit can handle every imaginable possibility, but then no facility has the potential to emit every imaginable kind of gas either. Hydrogen sulfide is detected using UV, whereas hydrocarbon gases affect IR. In situations where multiple gases are possible, it may be necessary to deploy multiple units. Fortunately, hydrocarbon gases have similar characteristics, so the same unit selected for methane can also detect propane and ethylene, although there may be slight differences in sensitivity.
There are countless application examples within the oil and gas industry, given the enormous volumes of products present. Here are a few application strategies where facilities are using open path differential optical absorption spectroscopy technology successfully.
Wellheads and production sites—While these sites are typically remote with few operator visits, it is still important to monitor hydrogen sulfide, methane, and vapors from crude oil to avoid fires and support environmental responsibility. Such facilities have been identified as major sources of fugitive methane emissions and proven stubborn to bring under control. Effective detection is the first step.
Offshore platforms—The dangers related to having so much equipment and people packed into such a small space makes effective detection of methane, hydrogen sulfide, and other gases paramount. Monitoring around crew quarters is particularly critical for the safety of operators.
Refineries—Many refinery production units create process safety hazards, including:
- Crude desalting.
- Crude distillation.
- Catalytic cracking.
Gas or vapor leaks from these processes are highly combustible and can create a fire or explosion if they contact an ignition source from a nearby electric motor or fired heater. Some also create sour wastewater, laden with hydrogen sulfide, in potentially lethal concentrations. Continuous combustible and toxic gas monitoring is therefore critical within a refinery.
Petrochem plants—Production processes, using petroleum- and natural gas-derived feedstocks and intermediates, manufacture a wide variety of plastics, paints, solvents, adhesives, agricultural products, specialty gases, and more. The chemicals and feedstocks involved in each process present hazards that must be monitored. Many petrochemical processes also create dangerous byproducts, including hydrogen sulfide. For example, manufacturing aromatic hydrocarbons such as benzene, toluene, and xylene produces both toxic and combustible fumes.
Within all these facilities, there are common areas where monitoring is critical, and safety system designers must place these high on a list of detector deployment locations:
- Processing units, especially those including ovens, boilers, reactors, and heaters.
- Storage tanks and vessels within production areas.
- Areas around control rooms, plant offices, labs, break rooms, etc.
- Waste collection areas and spill pits.
- Turbine enclosures.
- Clusters of pumps and compressors.
- Larger storage tanks and tank farms.
- Valve clusters on pipelines and storage tank transfer lines.
A comprehensive plan
Open-path detectors have great versatility, but they should be deployed in conjunction with point detectors and acoustic detectors (Figure 4) to improve overall coverage. There are situations where gases may accumulate but open path detectors are impractical or too costly.
But the larger safety discussion should not stop there. A fully integrated strategy for worker, plant, and community safety must include detectors for hydrogen sulfide, combustible gases, and even perhaps flame detectors. Designing such systems to be effective and economical will usually require a mix of the point and open path detectors just described. While it is difficult to rank these in importance, it could be argued that open path detectors carry the heaviest load of protection, and therefore deserve particularly careful consideration.
As a result, safety system design often begins with selection and placement of open path detectors, which are then supplemented by point detectors for areas of specific concern. When protecting workers from exposure to combustible gases and hydrogen sulfide, this is the best place to start.
Sidebar: Open path sensor placement
This diagram illustrates typical open path sensor placement practices, following several basic concepts:
- Open path technology can measure a gas cloud anywhere along its sight line between the transmitter and receiver set.
- The total concentration measured is the product of the cloud density multiplied by the cloud size. So, a large low-density cloud can have the same effect as a small high-density cloud.
- Beam location relative to specific equipment reflects a balance of sufficient diffusion to reach the beam, while retaining high enough concentration to be detectable.
- Installation areas must remain clear of obstructions, such as parked cars, bins, and so forth, as blocking the beam renders the system inoperative.
- Gas clouds tend to drift in the direction of prevailing wind, although direction can change.
Looking at the diagram, there are four detector sets at the “fence line” around the equipment cluster, providing coverage for detection in all directions. These instruments should detect any significant release within the larger space. We’re assuming here that we are only concerned with one type of gas, for example hydrocarbons, which can be detected by the same type of detectors. If hydrogen sulfide is also a factor, for example, a second set of detectors is necessary.
Although not applicable in this case, it is important to avoid situations where beams are too close, causing the spreading beam from one transmitter to hit a different receiver. As a practical matter, this is easy to avoid with a bit of analysis based on angle data.
Original content can be found at Oil and Gas Engineering.