Open path toxic gas detectors: key to worker safety

Hazards from toxic hydrogen sulfide lurk in every upstream oil and gas production site. Open path detectors provide critical capabilities to warn personnel of dangerous conditions.

One of the challenges related to upstream oil and gas production is hydrogen sulfide (H₂S), which, along with the general potential for fire and explosions from hydrocarbons, represents a serious and persistent threat to workers in the industry. Producers do not create H₂S, it is simply a by-product of the same process that turned prehistoric vegetation into fossil fuels. Unfortunately, it is impossible to recover the fuels without the by-product, so producers must find ways to capture it safely. From time to time it escapes, putting workers in danger.

H₂S is a highly toxic compound, normally in gaseous form, capable of incapacitating and killing workers even at low levels of exposure. It has been classified as immediately dangerous to life and health at a concentration of just 100 ppm: it is a broad-spectrum poison, with its most acute effect on nervous and respiratory systems. Heavier than air, it tends to accumulate in low-lying areas like trenches and sumps. While H₂S has a strong odour of rotten eggs and is easy for humans to recognise, it rapidly destroys the sense of smell, sometimes causing individuals to believe it has dissipated. Low concentrations can be difficult to sense, requiring careful selection and implementation of detection equipment.

Two main detection methods hinge on monitoring an electrochemical reaction to ‘smell’ the gas, or by ‘seeing’ the gas due to the ability of H₂S to absorb specific wavelengths of light which electronic sensors can characterise. Equipment designers have applied these techniques in a variety of ways with the aim of optimising detection under specific circumstances as part of a larger safety system.

Immediate vicinity: point detection

Point source detectors are an appropriate choice for many applications where likely toxic gas sources are understood and localised. The escaping gas must reach the sensor in sufficient concentration to cause a specific chemical reaction to take place, typically with a metal oxide on a surface or between electrodes. Point source detectors are usually placed near potential sources, with the number and location of sensors determined by the extent and geometry of an area requiring coverage and the amount of air movement. Consequently, a large outdoor area, such as an oil and gas well site, calls for a large number of detectors since there are numerous potential gas sources and extensive air movement.

This may sound at first like an impractical approach for an outdoor installation, but point source detectors can be equipped with wireless transmitters, making such a deployment far less expensive than traditional wired methods. The individual detectors are relatively inexpensive so this can be a viable approach using careful placement by experts. Wireless transmitters also allow detector repositioning if necessary, to accommodate changes in the threat profile.

Long distance: open path detection

As mentioned, it is also possible to detect H₂S due to its absorption of specific light wavelengths using differential optical absorption spectroscopy (DOAS). A DOAS detector monitors those specific wavelengths and when they decline more than the overall light level, H₂S presence is the likely cause and the detector will trip. The degree of attenuation can provide an accurate quantitative value of gas concentration on a near real-time basis. The concept sounds simple enough, but implementation is more complex.

Since the detector is looking for a change in the intensity of specific wavelengths, there must be a consistent light source serving as the benchmark. When implemented, this approach is configured as a two-piece unit (Figure 1) with a source and detector placed some distance apart. Ideally, the detector should be oriented to only see its corresponding light source to avoid being misled by some change in ambient conditions. With this in mind, the source and detector are deployed together, looking from one to the other across the area of coverage, effectively creating a beam of light with consistent characteristics sent from the source to the detector. Both use lenses and shading to minimise 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 minimise condensation or ice formation, which can interfere with transmission.

Figure 1: An open path toxic gas detector consists of a source and detector pair that detect H₂S in the path of the beam.

The detector is programmed to recognise overall changes in light level along with the specific wavelengths of interest. High humidity, fog, airborne dust and the like can change the overall level, or some obstruction may block the beam entirely, but H₂S should be the only gas able to create the tell-tale attenuation causing the detector to trip.

Concentration versus distance

Point detectors depend on capturing a sufficient concentration of H₂S in the immediate area in order to trip. There is no indication whether the escaping gas is a small localised cloud or an enormous release. As a practical matter, large releases will likely affect more than one detector, but the concentration at each needs to be higher than the setpoint threshold.

An open path detector set-up, on the other hand, is able to realise the cumulative effect of a cloud dispersed over a wide area, even if the concentration is low, or it can recognise a less diffused release with a high concentration. The beam attenuation across the entire distance is additive, which provides some flexibility with the nature of the cloud it is able to measure — particularly a low-concentration cloud spread over a wide area. This therefore also provides flexibility with the location of a leak, so it can cover a wide area with a large population of equipment and many potential leak sources.

Optimising the light source

The critical wavelengths for H₂S fall in the middle of the ultraviolet (UV) section of the electromagnetic spectrum, between 190 and 300 nm, so the source must be rich in that radiation.

There are two primary technologies capable of producing high-intensity UV with a form factor suitable for this type of application: tunable diode laser (TDL) and xenon strobe tubes. The former can be a continuous source, while the latter is intermittent, providing individual flashes.

Table 1: Comparison of TDL versus xenon strobe tubes. For a larger image click here.

Each light technology has its advantages, but a Xenon strobe tube is generally easier to work with (Table 1).

TDL technology

Using a TDL for H₂S detection applications has two advantages:

1. The detector can have very high sensitivity for that wavelength, providing high detection sensitivity.
2. It is well suited for process monitoring in indoor applications with stable environmental conditions.

These benefits can also prove to be limitations in real-world installations. Consider each point:

Point 1

The ability of a TDL to generate light within a band of about 2 nm creates high sensitivity in that specific wavelength. This results in high effectiveness for detecting H₂S, provided the gas is exhibiting ideal characteristics and absorbing precisely the expected wavelength. In the real world, H₂S is often mixed with various contaminants from the well or in the atmosphere, which cause the spectral fingerprint to shift. It does not take much of a shift to cause the detector to misread and possibly understate the concentration level or miss the gas mixture entirely.

Point 2

Heat haze, or heat shimmer, is common to warm, outdoor environments particularly under full sunlight. When light passes through multiple temperature gradients, a beam can be disrupted just as we see fake water in the distance on a road. This effect jeopardises stable performance for a TDL outdoors under these conditions, hence the recommendation to concentrate use in protected areas.

If operators understand these qualifications, a TDL instrument set can provide effective service, but the alternative of a xenon strobe tube generally delivers simpler operation.

UV xenon strobe tube technology

As mentioned, a xenon strobe tube produces a wider spectrum of light, including high levels of UV radiation, which can be maximised by selecting specific tube materials and controlling current density. When used in this application where a collimated beam is desirable, reflectors and optics can focus the output to provide maximum intensity across the full distance. The slightly thicker beam compared with a TDL source makes alignment easier and more forgiving of shocks and vibration in operation.

The tube flashes once per second, so there is no significant measurement lag compared to a continuous source. Different models have flash tubes of varying intensity and optical systems corresponding to the maximum measurement distance. The specific light spectrum in conjunction with the flashing operation provides detector immunity to solar radiation or light from burning hydrocarbons.

The detector has a beam splitter which directs part of the incoming light to a broader band reference signal detector and the balance to the absorbed-band signal detector. The reference signal indicates the overall conditions at the site (Figure 2). If there is fog or dust, all the light reaching the detector will be attenuated, including the absorbed band. Only when the absorbed band signal decreases differently than the reference signal will the detector determine H₂S is present. The amount of difference is proportional to the gas concentration. The detector is still able to recognise the presence of H₂S even when the total light reaching the detector is obscured up to 95%.

Figure 2: Range of ratio combinations: the critical evaluation point is the intensity relationship between the reference signal and the absorbed band signal. When these diverge, H₂S presence is the cause.

Applications in upstream oil and gas production

While this article concentrates on open path toxic gas detection, it is only part of a larger safety system approach for land-based and offshore production sites. These must be outfitted with a combination of toxic and flammable gas detectors, including both point and open path designs, deployed alongside flame detectors to determine if a flammable release has escalated into an actual fire.

Open path detectors, by their nature, are best at guarding clusters of equipment from around the perimeter where there is a clear line of sight. Since H₂S is heavier than air, it tends to diffuse close to the ground, so detectors should be close to ground level. For example, where there is a line of gas separators (Figure 3), the open path detector pairs should be placed as close to the equipment as practical, ideally on the side downwind based on normal weather patterns. Segmenting a large installation with multiple detector sets can result in faster reporting of a release, with a better indication of the source. Detector sets can be placed so the beams cross at right angles without concerns about interference.

Figure 3: A complete site safety system will include multiple open path detectors mixed with point source detectors and flame detectors. For a larger image click here.

Typical upstream placements include:

• Floating production, storage, and offloading (FPSO) vessels
• Offshore production platforms
• Land-based oil and gas well collection sites
• Lease tank transfer sites
• Production and storage area monitoring
• Facility perimeter (fence-line) monitoring
• Pipeline monitoring

Installation considerations

When determining how and where open path detectors should be installed, there are a few basic points to keep in mind:

• The appropriate detector model should be selected corresponding to the open path length to be monitored, which is influenced by typical atmospheric conditions.
• The mounting point for the source and detector must be secure and stable with little or no vibration. The longer the path, the more important it is to have a solid and vibration-free mount.
• The selected location must have a direct view between the detector and the source.
• The open path should be free of potential obstructions that could block the beam, such as pedestrian traffic or a vehicle parking place.
• For H₂S, the beam should be about 60 cm above the ground. Maintain a 15–30 cm radius around the beam’s line of sight to avoid reflective interference from nearby piping and supports.
• The open path must allow for free flow of air so escaping gas can move easily into the beam.
   

The coverage distance of each detector model is expressed as a range. Achieving the maximum range reliably depends on compatible conditions with minimal disturbances. In an area with poor weather, such as heavy fog, rain or snow, it is important to install the detector at the low end of the range and also use the highest intensity model available. If these are only intermittent problems, try to remain within 75% of the maximum. In severe weather conditions such as an offshore oil production platform, this should be reduced to 50%.

As discussed, an integrated strategy for worker safety on land or offshore must include detectors for toxic gases, including H₂S, flammable gases and flame detectors. Designing such systems to be effective and economical will usually include a mix of point and open path detectors. 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.

This importance stems from the particular combination of open path detection capabilities already discussed:

• Long distance coverage, able to protect large groups of installed equipment outdoors.
• Fast response, typically less than 10 seconds once a cloud reaches the beam.
• High immunity to false alarms caused by sunlight, fires or other types of gases.
• Low maintenance cost with no need for consumables, long flashtube life and minimal calibration.
• Low installation costs since few units are necessary to cover large areas and installation is uncomplicated.
   

As a result, safety system design often begins with selection and placement of open path detectors, which are then supplemented by point source detectors for areas of specific weakness. When protecting workers from exposure to toxic H₂S, this is the best place to start.

Top image credit: ©stock.adobe.com/au/GreenOak