A rig worker smells rotten eggs near the well head, rubs his itchy eyes as he investigates the frosty pipe connections. After a few moments, he believes all is fine as he no longer smells the tell-tale hydrogen sulphide odour. In this scenario, we can only hope the worker recognises the danger and quickly leaves the site. Oil fields, especially mature ones, can produce hydrogen sulphide gas – deadly at relatively low concentrations. On average, a person takes 12 breaths per minute, so if a hydrogen sulphide (H2S) gas release occurs, a person might have a very few seconds to reach safety. Crucial to safety, effective hydrogen sulphide gas detection is the topic of this article.
Deadliness of H2S
Although H2S is a flammable gas, its toxicity is so high that its flammable level is not reached before it begins to harm people.
People can recognise even trace amounts of H2S in the parts per billion range, well below the danger level, by a distinctive odour of rotten eggs. Although the odour threshold of H2S is very low, continued exposure above 30ppm results in an individual losing the ability to smell it; it paralyses the olfactory nerve. The rule here is that; a) If you can smell it, it may be harmful and b) If you can no longer smell it, it may still be present and dangerous.
According to the UK Health and Safety Executive, the acceptable concentration limit for exposure to H2S is 5 ppm for an 8-hour period. The maximum peak exposure is 10 ppm for 15 minutes. In the US the levels are higher at 20 ppm for an 8-hour period and 50 ppm for 10 minutes according to the Occupational Safety & Health Administration (U.S.Department of Labor). As sensor technology has improved over the years, these levels have been driven down by the health and safety departments in many countries.
H2S can cause instant death in very high concentrations while relatively short-term exposure to 500-1000ppm can be life-threatening and cause serious harm. Repeated exposure to lower concentrations can cause conjunctivitis, photophobia, corneal bullae, tearing, pain and blurred vision.
Considering the isolation of drilling locations and the length of time for first-aid to be applied, quick detection of the gas and personnel retreat from the hazardous area is imperative. The distance to the nearest hospital may simply prohibit consideration of receiving treatment in time.
Danger of H2S on Rigs
Sour gas (or methane containing hydrogen sulphide, H2S) exists in many industrial applications including chemical and oil and gas exploration and production. Specific to oil and gas production, these are some potential hazard areas:
- On HVAC air vents of accommodation buildings and personnel areas
- Driller stand, the shale shaker, and the bell nipple
- Mud return line receiver tank
- Crude oil storage tanks, pipes, flanges, and valves
- Remote well sites at the well heads, the storage tanks, and flare stacks
- In enclosed analyser buildings
Of course, there are many other potential risk areas across oil and gas production and refining as well as many other industries, especially waste water. In each situation, there should be a review of drawings to analyse the probable sources of leaks. In addition, remember that H2S is heavier than air and will sink to the lowest lying area.
It should also be understood that the specific and changing local environment to each detector can have a significant impact on the sensor detecting gas and providing the necessary protection. Both wind speed and direction along with changing humidity and temperature can all affect decisions on a detector’s placement and its ongoing effectiveness.
Issues of Climatic Challenges with Sensor Performance in Hot and Cold Climates
As more and more of the world is opened up to exploration, the areas that companies are exploiting are becoming more challenging. Desert conditions with massive variations in both temperature and humidity along with the arctic conditions found especially in Russia and Alaska are stern challenges for designers and manufacturers of toxic gas detectors. In any case, the established technologies must always be questioned to see if the industry can provide more applicable detectors and other field devices.
When it comes to toxic gas, it should also be remembered that differing climatic conditions have significant effect on the human body, so it would be of benefit to consider a combined effect. A toxic gas detector needs to continue to provide reliable protection as the climate changes through each day and each season.
H2S detectors should be primarily designed to save the lives of workers by warning them of excessive H2S concentrations in the workplace and initiating emergency procedures and precautions.
The flammability of H2S is generally not considered in safety applications because the toxic level of the gas is significantly lower. By the time the H2S level rose to the level of combustion, the toxic air levels would be far exceeded, and people would die. H2S kills at 1000ppm or 1/40th of the lowest explosive limit (LEL).
Increasingly, the use of personal/disposable portable detectors and hand-held portables, in combination with fixed gas detectors are providing better site-wide safety. Many countries require that every worker should be equipped with a personal device when in potentially hazardous areas.
As manufacturers continue to improve and advance toxic gas detection technologies, it is important to understand how each type of devicemay be applicable. Electrochemical sensors and metal oxide semiconductor (MOS) sensors have for many years, provided the platform upon which H2S safety has been built. Now newer technologies are starting to arrive and these should be considered on their merits.
Electrochemical cells combine enclosed electrodes and electrolyte. The gas/air diffuses through a permeable membrane into the cell. As the volume of H2S increases in the air, an oxidation or reduction reaction occurs at one of the electrodes. Small, linear μA current changes occur between the electrodes so that a display/amplifier device may provide a clear indication of gas level.
The low power consumption, allowing for intrinsically safe circuits and their high degree of sensitivity and repeatability make this type of sensor popular in a wide variety of applications. Electrochemical sensors are all cross sensitive to other specific species of gas and this should always be considered, although this is not usually problematic in most applications, particularly in exploration and production.
As we push further into more extreme and isolated environments, the lack of resilience in high and low temperature and humidity of the electrochemical sensor becomes more important. Some sensors may suffer from evaporation of electrolyte in dry and hot conditions, while the extreme cold will reduce the sensor output and its speed of response. You could normally expect a response to 90% of the concentration (T90) in around 30 seconds in 20°C ambient conditions. At temperatures around and below -20°C a significant reduction in speed of response and final output will be noted and at -40°C output will decrease by asmuch as 25%.
Routine calibration of electrochemical sensors is required at least every 6 months and often more frequently depending on site conditions. These sensors are not fail-safe and cannot provide fault outputs if the sensor has failed to respond to gas.