In this first, of a two-part article, Gemma Finnegan discusses the dangers posed by hydrogen sulphide to people and detection limits normally needed for toxic gas detection. In the next edition of Gulf Fire Gemma discusses how 3D fire and gas modelling techniques can be used, in combination with good engineering practice, to improve safety and aid compliance to End Users standards.
Inherent within the very nature of the energy industry, the hazards associated in refining oil and gas present a number of risks. One of the prominent and well documented risks is the potential for fire or explosions as a result of a break in containment. To combat this, facilities incorporate a combination of detection devices to mitigate events before they develop into catastrophic circumstances.
Fire however is not the only subsequent hazard we must monitor from loss of containment. Flammable and toxic gas hazards should also be addressed in terms of producing an effective plus holistic design. The oil and gas industry covers a large number of upstream, midstream and downstream activities from the on and offshore exploration and production of oil and gas to its transportation, storage and refining. The large amount of highly flammable hydrocarbon gases involved are a serious explosive risk and additionally toxic gases such as Hydrogen Sulphide are often present.
Many oil and gas facilities utilize fixed gas detection systems as a safeguard against uncontrolled release of hazardous process materials. The effectiveness of fixed gas detection systems is most often limited by the ability of the system to detect that a release has occurred. Obviously, the number of detectors and geographic placement of those detectors play a critical role in the capabilities of the system to detect a release. Traditionally, the number and placement of gas detectors has been based on rules-of-thumb and expert judgment, leading to inconsistent and sometimes ineffective systems. With the focus firmly on safety within the industry and hence an increase in prevalence of an effective F&G detection system design coupled with recent developments in detection technologies, deciding where to locate these detectors based on the hazard they are intended to mitigate has become more widely discussed.
It is important to note when toxic gases are present this directly affects the performance target of an effective system, each target gas has their own set of properties and hence require separate/specific consideration. Some gases are poisonous and can be dangerous to life at very low concentrations. This in turn adds complexity to the design, more so if the performance target of the system is not well understood. This brief technical perspective details the nuances of Hydrogen Sulphide (H2S) detection placement only.
Hydrogen Sulphide (H2S)
The primary and essential difference between regular crude oil and sour crude oil is the presence of Hydrogen Sulphide (H2S). H2S gas, also known as sour gas or sulfureted hydrogen, can be one of the most vicious and deadly hazards in the oil and gas industry. It is a highly toxic, colourless, combustible/flammable and corrosive gas that is heavier than air and has the characteristic odour of rotten eggs. H2S is a natural product of decay, and in a petrochemical setting, is most commonly a result of decomposition of organic matter containing sulphur. Within the industry it is typically encountered during hydrocarbon exploration (drilling) and production (processing).
H2S is toxic to humans at extremely low concentrations. At higher concentrations it is flammable, as well as corrosive to metals. A release of this gas, if not identified and controlled immediately, could result in injuries and/or fatalities as well as potentially a fire or explosion.
Physiological Effects
Hydrogen sulphide does not accumulate in the body therefore the acute health effects do not occur until the exposure to H2S is greater than the body’s ability to excrete the excess sulphur. While extremely high levels of hydrogen sulphide can indeed be harmful, even deadly, H2S can be detected by the nose at an extremely low level. In fact, it can be detected by the human nose at a concentration several orders of magnitude lower than the threshold for harmful human health effects. However, the sense of smell is not a reliable warning because exposure to this gas quickly deadens the sense of smell; relying on being able to detect its odour can provide a false sense of security. Loss of consciousness can occur within seconds of exposure to a high concentration of this gas. Table A shows the effects and symptoms of the varying concentrations of H2S.
More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas. (It should be noted that there is a large group of gases which are both combustible and toxic, so that even detectors of toxic gases sometimes have to carry hazardous area approval). The main reason for treating flammable and toxic gases separately is that the hazards and regulations involved and the types of detection required are quite different.
With toxic substances, (apart from the clear environmental problems) the main concern is the effect on workers of exposure to even very low concentrations, which could be inhaled, ingested, or absorbed through the skin. Since adverse effects can often result from additive, long-term exposure, it is important to not only be aware of the concentration of gas, but also the total time of exposure.
Toxic Gas Exposure Limits
Occupational Exposure Limit values (OELs) are set by competent national authorities or other relevant national institutions as limits for concentrations of hazardous compounds in workplace air. OELs for hazardous substances represent an important tool for risk assessment and management. For some substances, a brief exposure is considered so critical that they are set only a STEL, which should not be exceeded even for a shorter time. The Health & Safety Executive (HSE) for the UK define H2S occupational limits as follows: OEL – Time Weighted Average (TWA) for an 8 hour day, 40 hour week as 5 mL/m3, STEL – Short Term Exposure Limit ((4) x 10 minutes exposures per day) as 10 mL/m3[2]. It is important to note H2S threshold limits shall reflect legislation but the emphasis should be on controlling exposure.
The Occupational Safety systems in the United States vary from state to state. It is recommended that local thresholds are thoroughly checked and understood. If the local standards clash with onsite standards, then it is up to the operator to outline which should take precedence. As limits frequently change and can vary by country, the reader should consult the relevant national authorities to ensure that they have the latest information. Please note these limits protect workers against health effects, but do not address safety issues such as explosive risk.
How do we relate exposure limits to detection? Detection setpoints. As discussed above H2S is extremely toxic to humans at extremely low concentrations and exposure to which can cause rapid health deterioration (Table A). As such it is considered good practice to use the short-term exposure limits (STEL) value as maximum value for either single alarm or low set point for dual alarm equipment, unless stated otherwise by local legislation. It is common practice on affected offshore assets to set the low and high alarms of H2S area monitoring systems of 10 ppm and 15 ppm.
The continuation of this article will focus on the design and mapping of Toxic Gas Detection to meet the specified performance targets and protect personnel against such hazards.
For more information go to www.micropackfireandgas.com
References
- HSE Offshore Information Sheet No. 6/2009 Managing Hydrogen Sulphide Detection Offshore
- HSE EH40/2005 Workplace Exposure Limits
About the Author
Written by Gemma Finnegan MEng MSc CFSP MIET Senior Safety Consultant Micropack (Engineering) Ltd.
