Hydrogen Sulphide Safety PDF

Hydrogen Sulphide Safety Learning Outcome When you complete this module, you will be able to… Describe the effect of hydrogen sulphide (H2S) on the health and safety of individuals in the workplace. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Desc...

Hydrogen Sulphide Safety Learning Outcome When you complete this module, you will be able to… Describe the effect of hydrogen sulphide (H2S) on the health and safety of individuals in the workplace. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Describe the physical and chemical characteristics of hydrogen sulphide. 2. Describe the effects of H2S on human beings and the specific toxicities at various concentrations. 3. Explain the Occupational Exposure Limits (OEL) for H2S and the responsibilities of employees and employers in the workplace. 4. Describe the general procedure for responding to an emergency in the workplace, where H2S may be present. Introduction Hydrogen sulphide (H2S) is an extremely poisonous substance, normally encountered as a gas or vapour. Numerous deaths have occurred over the years as a result of exposure to H2S. Some were caused by falls sustained when workers were overcome by the gas. Other deaths resulted from suffocation when workers were briefly exposed to the gas. H2S is found in varying concentrations in many oil and gas wells. It is also found in septic tanks, sewers, manure pits and anywhere bacteria can break down organic matter in an oxygen-deficient environment. H2S is found in coal mines and is referred to by coal miners as stink damp. H2S is a by-product of many industries. For example, it can be produced when sulphuric acid is accidentally mixed with black liquor in pulp mills. Some people have the mistaken idea that H2S is "created" in gas plants; in fact, H2S is a natural component of raw gas. H2S is produced and liquefied in Western Canada for use in the nuclear power industry for the production of heavy water. It is shipped to Eastern Canada in pressurized tank cars in its liquid state. 1 SAFE 6005  H2S must be removed from crude oils and natural gases. After removal, the H2S is converted to elemental sulphur by first burning part of it in a waste heat boiler, which produces sulphur dioxide (SO2). This combustion creates heat that is used in the generation of steam for process purposes. The SO2 is then combined with the remaining H2S in a catalyst bed, where they chemically react to produce sulphur. Sulphur production has significantly contributed to Alberta's economy. Note: All jurisdictions in Canada have the same or similar legislation concerning safety and health. Since it is impossible to refer to them all, only the Alberta acts, regulations and codes may be referred to in this module. You are encouraged to refer to the appropriate documents for your province or territory. 2 SAFE 6005 OBJECTIVE ONE When you complete this objective, you will be able to… Describe the physical and chemical characteristics of hydrogen sulphide. Learning Activity Complete each of the Learning Activities listed below. 1. Read the Learning Material. 2. Demonstrate your understanding of the material by completing the Assignment/Quiz which is a gradable component of the course. Learning Material PROPERTIES OF HYDROGEN SULPHIDE Workers must thoroughly understand the physical properties of H2S so they can safely work in an environment that may become contaminated by it. Chemical Symbol The chemical symbol for hydrogen sulphide - H2S - indicates the chemical combination of two atoms of hydrogen with one atom of sulphur. It is a very simple compound to produce. For example, vinegar mixed with cigarette ashes will produce H2S. Relative Density In its pure form, H2S is 1.189 times as heavy as air. In the petroleum industry, H2S seldom exists in its pure form, but is usually associated with hydrocarbon gases. All heavier-than-air gases tend to settle in low-lying areas when the air is still and cool. This means that workers must be especially cautious in underground installations or tanks and in confined spaces where ventilation is poor. H2S does not naturally separate itself from the gas stream and will always be present with hydrocarbons. In the event of a leak of sour hydrocarbons, H2S will not separate and migrate to low areas; rather, it will follow the natural air convection currents and be present in high or low areas. The best location for H2S sensors is in the path of those normal air currents and close to the most probable source of a leak (for example, above a compressor cylinder). 3 SAFE 6005  Auto-Ignition Temperature Natural gas (methane) requires a very hot ignition source, such as an electric spark, to produce the required ignition temperature of 650°C. However, H2S ignites at a relatively low temperature (260°C). Because of its low ignition temperature, workers are advised to use only approved power tools and non- sparking tools to reduce the ignition hazard when H2S may be present. Static electricity from drive belts, etc. can easily ignite H2S. Flammability H2S combines with oxygen to produce sulphur dioxide (SO2) and water vapour. H2S burns with a very light blue flame that is invisible in daylight. The combustion products are visible as a brownish-yellow gas. The SO2 produced by burning H2S is also toxic and requires the same caution as H2S. The only safe way to extinguish an H2S fire is to interrupt the supply of the gas. Explosive Limits Most people are familiar with a propane-powered barbeque. When you first turn on the gas, it does not ignite because there is too much gas and not enough oxygen mixed with it. Once the gas expands enough and mixes with oxygen, the gas will ignite. Compare this to H2S, which has a lower explosive limit (LEL) of 4.3% and an upper explosive limit (UEL) of 46% (Figure 1 illustrates the concepts of LEL and UEL). H2S is a safe and simple fuel to burn under the controlled conditions of a sulphur recovery boiler. However, its wide explosive range combined with its low auto-ignition temperature makes it a very dangerous gas as a potential source of fire or explosion. Figure 1 Explosion Limits © SAIT 4 SAFE 6005 Colour H2S is an invisible gas. A leak may become visible because of hydrocarbon or water vapour content, but the H2S component of a leak cannot be seen. Odour Rotten eggs have a pungent, repulsive smell because the gas produced in the decay process is H2S. Therefore, H2S is often referred to as rotten egg gas. It is an insidious poison because the gas will dull the sense of smell; a victim will not be able to detect high concentrations. For this reason, you cannot rely on your sense of smell to detect H2S. Vapour Pressure The vapour pressure of H2S is 17.7 atmospheres at 20°C. This means that when H2S is to be maintained in the liquid state, it must be under a pressure of at least 1800 kPa. Loading liquefied H2S into tank cars must be done within strict guidelines to ensure that the cars are not overloaded and not over-pressured. Boiling Point Because of the low boiling point of H2S (-60°C), it is normally in the gaseous state at atmospheric pressure. Melting Point The melting point of H2S is -83°C, so we do not expect it to be in the solid state. However, if water containing H2S gas is frozen, the H2S becomes part of the solid mass. When the ice melts, the H2S is liberated as a gas. Reactivity H2S reacts violently with acids and oxidizers. It tarnishes silver and reacts with lead acetate to produce lead sulphide. This reaction is indicated by a colour change from a clear lead acetate solution to black lead sulphide and is used to detect H2S in chemical reaction tubes for use in gas detectors. Solubility Gases dissolve in liquids to a greater or lesser extent, depending on the temperature of the liquid, the pressure on the gas above the liquid and the solubility of the gas (some liquids absorb some gases better than others). In a closed system, gas will be absorbed by the liquid as the liquid temperature is reduced and the pressure is increased (Figure 2). Conversely, gas is liberated from a liquid if the temperature is increased or the pressure is reduced. H2S has a solubility (at 20°C and 1 atmosphere pressure) of 3067 ppm in water and 12 350 ppm in conventional crude oil (C6+). SAFE 6005  Figure 2 Solubility of Gas in Liquid © SAIT The principle of solubility is the basis for one of the processes to remove H2S from hydrocarbon gases. Liquid amine absorbs the H2S from high-pressure gas as it comes in contact with that gas. The liquid amine (now containing H2S) is pumped to another vessel where it is heated and the pressure reduced to liberate H2S, which is then burned. Considering the solubility of H2S in water and in crude oil, the concentration of H2S in the vapour above a slightly sour liquid at low pressure is very high. In some cases, it is lethal. Assuming that the pressure above the liquid is 1 atmosphere and that the liquid is at 20°C, the concentration of hydrogen sulphide in the vapour or gas phase can be calculated by: ppm H2S (gas phase) = ppm H2S (water) x 106 3067 ppm H2S (gas phase) = ppm H2S (C6+) x 106 12350 Table 1 uses these formulas to show the concentration of H2S in the gas phase above both water and C6+, for various concentrations in the liquid phase. Table 1 Concentration of H2S Gas in Liquid Phase © SAIT Agitation increases the liberation of gases. A vessel that is being cleaned and that appears to be free of H2S can suddenly become dangerously contaminated if a worker simply walks through the sludge in the bottom of the tank and liberates the gas. The addition of heat to clean a vessel can liberate H2S from sand or other solids, as well as the H2S trapped in the metal itself. Welding on a vessel that contained sour gases or liquids will liberate H2S from the metal, unless it has been heat treated first. Failure to heat treat the metal prior to welding will also result in a brittle weld that will likely fail when the vessel is put back into service. SAFE 6005  Table 2 Properties of Hydrogen Sulphide ©SAlT Chemical Symb ol H28 1.189 (air= 1.0 heavierthan Relative Density air) Auto-Ignition 260"C Temperature Very flammable, distinctive Flammability blue flame Lower Explo sive Limit 4.3% in air by volume Upper Explo sive Limit 46% in air by volume C olour Colourless, invisible O dour Strong rotten egg Vapour Pres sure 17.7 atm at 20"C Boiling Point - 60"C Melting Point - 8 3"C Dangerous with ac ids and Reactivity oxidizers Ye s in water, hydrocarb ons, Solubility alc ohol 8 SAFE 6005 H2S Accident Any routine task can put you at risk. H2S Corrosion and the Formation of Iron Sulphide If water is present in an H2S environment, it will react with the H2S to produce sulphurous acid (H2SO3). This acid will attack the metal in the steel piping or vessel, removing iron and producing a grey, paste-like material called iron sulphite (FeSO3). In a dry atmosphere, H2S can be directly corrosive to steel piping and vessels. The sulphur component of H2S reacts with the iron in the metal to produce various forms of iron sulphide (for example, FeS and Fe2S3). Iron sulphide is generally recognizable as a grey, grey/black, brown or green powder. Dry iron sulphide is pyrophoric, which means that it can spontaneously ignite in the presence of air. Since iron sulphide forms in the presence of H2S, and H2S in air has a wide flammable range and a low auto-ignition temperature, iron sulphide can become a source of ignition if allowed to dry out. Iron sulphide reacts vigorously in acidic conditions to release H2S gas, which can pose a real problem for unsuspecting workers. H2S can also cause a type of corrosion known as stress corrosion cracking (SCC) in certain steels. This is extremely serious and can create problems such as pipeline fractures. 11 SAFE 6005  OBJECTIVE TWO When you complete this objective, you will be able to… Describe the effects of H2S on human beings and the specific toxicities at various concentrations. Learning Activity Complete each of the Learning Activities listed below. 1. Read the Learning Material. 2. Demonstrate your understanding of the material by completing the Assignment/Quiz which is a gradable component of the course. Learning Material TOXICITY OF HYDROGEN SULPHIDE Table 3 indicates the toxicity of hydrogen sulphide. Due to the highly toxic effects of the gas, the results are from tests performed on animals. No two individuals are affected in the same way by the gas, so the test results are only approximate. The information in Table 3 is given in ppm, an amount that is difficult to imagine. To put the volumes into perspective, consider the following comparisons: The box of a small pickup truck has a volume of approximately 1m3. An eyedropper has a volume of 2 mL (2/1000 litre). A 1 litre milk carton has a volume of 1/1000 m3. If just half an eyedropper of H2S gas were put into the box of the pickup, a concentration of 1 ppm would result, and at that concentration the H2S can just be detected by smell. If a volume of hydrogen sulphide equal to that of a milk carton were put into the box of the pickup, one breath could be fatal. 10 SAFE 6005  Table 3 Toxicity of Hydrogen Sulphide © SAIT Parts per Percentage (%) Physiological Effects Million (ppm) 1 0.0001 Rotten egg smell is noticeable. 8-hour Occupational Exposure Limit. Maximum 10 0.001 allowable concentration for continuous exposure for 8 hours without breathing apparatus. 15-minute Occupational Exposure Limit. 15 0.0015 Maximum allowable concentration for exposure for 15 minutes without breathing apparatus. Ceiling Occupational Exposure Limit. This level Greater than 15 >0.0015 of exposure cannot be exceeded at any time without respiratory protection. Dulls sense of smell, causes burning sensation in 100-150 0.01-0.015 eyes and throat, Chronic exposure produces headaches, insomnia, anorexia and nausea. Attacks respiratory centre of the brain, causes loss of reasoning and balance, persistent cough, 250-500 0.025-0.05 sore throat, fogged vision, chest tightening and pulmonary edema. Causes nausea and vomiting, cyanosis, vertigo, amnesia and convulsions. Casualty quickly loses 700 0.07 consciousness; breathing will stop and death will result if not rescued promptly. Unconsciousness immediately, permanent brain 1000 0.1 damage or death occurs if casualty is not rescued and resuscitated immediately. 13 SAFE 6005 The Danger and Consequences of Exposure to Hydrogen Sulphide At 1 ppm, most people can smell the gas. A strong smell does not necessarily mean a high concentration and a slight smell does not mean a low concentration. Recognition threshold (0.0047 ppm) – most humans can detect “rotten egg” smell. A person could work in a 10 ppm concentration of H2S for 8 hours. If the concentration exceeds 10 ppm for a short period of time, then the exposure time must be reduced. A concentration of 15 ppm can be tolerated for a period of time not exceeding 15 minutes. There can be no more than 4 exposures of 15 ppm in an 8-hour shift, with 1 hour between exposures.  12 Borderline concentration for eye irritation (10 – 20 ppm). At this point you should turn around or walk cross-wind. \ If the concentration of H2S exceeds 20 ppm, a worker must wear approved breathing apparatus. If the concentration is not known, a worker must wear breathing apparatus until the concentration is determined. 50 - 100 ppm leads to eye damage. If exposed to a concentration of 100 ppm (1/100 of 1%), the sense of smell will be lost or become ineffective within 2 to 15 minutes. The H2S might cause a burning sensation to the eyes, throat and lungs, and could cause headache or nausea. A 200 ppm concentration will cause immediate loss of smell and a burning sensation in the eyes, throat, nose and lungs. The hydrogen sulphide combines with alkali in body fluids to form caustic sodium sulphide. 15 SAFE 6005 150 - 220 ppm – Olfactory nerve is paralyzed and sense of smell disappears often along with awareness of danger. At a concentration of 500 ppm, the victim will appear to be intoxicated, and will lose balance and reasoning. In this state, the victim may attempt to continue with the job they were doing when they encountered the gas. For this reason, workers must know the people they work with, and be able to detect any unusual behaviour in a co-worker. A victim must be watched very closely and may require resuscitation. A victim should be taken for medical attention and not allowed to return to work for at least 8 hours. 320-530 ppm – pulmonary edema with possibly of death. At 700 ppm, the victim will be rendered unconscious very quickly, and may develop seizures similar to those caused by epilepsy. Loss of bladder and bowel control can be expected. Breathing will stop, and death will result, if not rescued and resuscitated promptly.  14 530 – 1000 ppm – causes strong stimulation of central nervous system and rapid breathing, leading to loss of breathing. At a concentration of 1000 ppm (1/10 of 1%), the victim will be rendered unconscious immediately, and will not begin breathing voluntarily if brought to fresh air. 800 ppm – lethal concentration for 50%of humans for 5 minutes of exposure. Over 1000 ppm – immediate collapse with loss of breathing, even after inhalation of a single breath. WARNING: ARTIFICIAL RESUSCITATION MUST BE COMMENCED WITHIN THREE MINUTES OF BECOMING UNCONSCIOUS FROM HYDROGEN SULPHIDE. Figure 3 © SAI T 17 SAFE 6005  Effect of H2S on the Sense of Smell This effect cannot be emphasized strongly enough. Our sense of smell is usually the first indication of a dangerous airborne substance. A safe concentration of H2S (10 ppm) can be easily smelled, but once the odour is gone, the victim believes that the danger has passed. In fact, it may be that the gas has dulled the victim's sense of smell and the danger is actually greater than when the H2S could be smelled. Also, it must be remembered that aromatic hydrocarbons can mask the smell of hydrogen sulphide. H2S is an Insidious Poison Another problem with H2S is that pain, which is a common sign of most gas dangers, does not exist. Gases such as chlorine, ammonia and sulphur dioxide cause a great deal of pain to the eyes, nose and throat, so the victim will try to get away from the hazard. However, the irritation produced by H2S is relatively slight and the victim does not try to leave the area because he does not feel any pain. Effect of H2S on the Nervous System: In small amounts, H2S is a depressant. In large amounts, H2S is a stimulant. In high concentrations, H2S paralyzes the respiratory centre, causing suffocation. There is controversy regarding H2S entering the respiratory system through a perforated eardrum. Company policies vary on the subject, but the Journal of Occupational Medicine (May 1985) concludes that workers with perforated eardrums should not be excluded from working in atmospheres containing concentrations of H2S. 16 SAFE 6005 Effects of Repeated, Chronic Exposure to H2S Some of the health problems associated with repeated exposures to hydrogen sulphide are: Pulmonary Edema: an abnormal excess accumulation of fluids in the lungs. Corneal Bullae: blisters on the transparent membrane covering the iris and pupil of the eye. Conjunctivitis: inflammation of the mucous membrane lining the eyelids. Photophobia: a high intolerance to light, which can make driving at night hazardous. Rhinitis: inflammation of the mucous membrane lining the nose. Bronchitis: chronic inflammation of the bronchial tubes. Headaches. Digestive disturbances. Weight loss. General weakness. SAFE 6005 17  Table 4 Toxicity of Hydrogen Sulphide © SAIT Exercise Two 1. Identify the different levels of H2S at the workplace and take the appropriate action. Be sure you are aware of how the different levels of H2S may affect you before beginning this activity. 18 SAFE 6005 OBJECTIVE TWO When you complete this objective, you will be able to… Explain the Occupational Exposure Limits (OEL) for H2S and the responsibilities of employees and employers in the workplace. Learning Activity Complete each of the Learning Activities listed below. 1. Read the Learning Material. 2. Demonstrate your understanding of the material by completing the Assignment/Quiz which is a gradable component of the course. Learning Material “SWEET” VERSUS “SOUR” One problem in attempting to convince workers of the dangers of H2S is the misleading distinctions made between low and high concentrations. This may be partly due to outdated definitions of sweet and sour oil and gas. The new cutoff concentration to distinguish between sweet and sour products is based on the need for processing to make the product saleable. This new distinction is less confusing since it is more in line with the OELs for H2S. A product with a concentration of H2S not exceeding 0.001% or 10 ppm, is now considered sweet. The basis for the Energy Resources Conservation Board (ERCB) definition is the production of petroleum products, and the guidelines are only concerned with the production steps that must be taken to remove H2S from the petroleum. The extensive use of the "sweet" and "sour" terminology has perhaps resulted in workers not understanding that the terms relate to production, and not to toxicity. WARNING: HYDROGEN SULPHIDE IS A LETHAL GAS. DO NOT ENTER AN AREA WHERE IT MAY BE PRESENT UNLESS YOU HAVE RECEIVED THOROUGH TRAINING ON ITS HAZARDS. Figure 4 Class 2.3 TDG Symbol © SAIT 19 SAFE 6005  Exposure Limits for H2s in Alberta The following exposure limit definitions are taken from the "Occupational Health and Safety Act - Occupational Health and Safety Code 2009." Definitions 1. 8-Hour OEL This means the time-weighted average concentration of an airborne substance for an eight-hour period. For H2S, the limit is 10 ppm. It has been determined that a worker could work continuously for eight hours in a 10 ppm concentration. 2. 15-Minute OEL This means the time-weighted average concentration of an airborne substance for a fifteen-minute period. A worker could work for fifteen minutes in a 15 ppm concentration of H2S. 3. Ceiling OEL This means the maximum concentration at any point in time of an airborne substance to which a worker may be exposed. For H2S, the concentration is 20 ppm. Employer Responsibility The employer must ensure that a worker is not exposed by inhalation to concentrations of H2S in excess of the prescribed OEL. Where a worker is or may be exposed to H2S, the employer must ensure that each worker: 1. Is trained, and uses this training to minimize the worker's exposure to H2S. 2. Is instructed in the purpose, proper use, and limitations of any protective equipment provided. 3. Is instructed regarding health hazards associated with exposure to H2S. Where a respiratory device is used to control worker exposure to H2S, the employer must: 1. Provide and ensure that workers wear an appropriate, correctly fitting device. 2. Ensure that each worker is instructed in the purpose, proper use and limitations of the device. 3. Ensure that the device provided is stored in a manner that prevents contamination. 4. Ensure that each device is properly maintained and is regularly cleaned. 20 SAFE 6005 Employee Responsibility It is the employee's responsibility to: 1. Wear respiratory protective equipment when it is provided by an employer and ensure that the equipment fits correctly. 2. Follow the procedures set by the employer. 3. Participate in any instruction provided by the employer. Occupational Health and Safety Act − General Safety Regulation All workers should become familiar with this regulation. The following information is condensed from the regulation, and is directed to persons working with H2S. Hazardous work must be performed by competent workers, following procedures developed by the employer. The workers must receive adequate training and know the limitations of equipment required to allow the work to be done safely. The law generally requires that all persons working around H2S receive adequate training including: 1. Actual practice with breathing apparatus, plus simple maintenance and cleaning of the equipment 2. Training in First Aid, including artificial resuscitation 3. Training in the use of gas detectors (see Figure 5). Figure 5 Gas Detector © SAIT 21 SAFE 6005  OBJECTIVE THREE When you complete this objective, you will be able to… Describe the general procedure for responding to an emergency in the workplace, in which H2S may be present. Learning Activity Complete each of the Learning Activities listed below. 1. Read the Learning Material. 2. Complete Exercise Four and compare it to Exercise Four Answers at the end of the module. 3. Demonstrate your understanding of the material by completing the Assignment/Quiz which is a gradable component of the course. Learning Material RESPONDING TO AN H2S EMERGENCY Even if a worker collapses in an area where H2S is a known hazard, it does not automatically mean that person has been overcome by the H2S. Various other conditions may have caused this collapse (for example, heart attack, stroke or head trauma). However, from a safety standpoint, always assume that H2S is present and wear your breathing apparatus before entering the area. BEFORE ATTEMPTING TO RESCUE A VICTIM OF H2S POISONING, PROTECT YOURSELF FIRST BY PUTTING ON YOUR BREATHING APPARATUS. 22 SAFE 6005  Figure 6 Self-Contained Breathing Apparatus © SAIT When a co-worker has been knocked down in a known H2S environment, certain steps must be followed to ensure your own safety and survival. As a rescuer, you must take the time to properly assess the situation, get suited up and call for help before attempting the rescue; otherwise, you may be overcome and create problems for other rescuers by becoming an unnecessary victim. Get out of the area to properly assess the situation. Fatal hydrogen sulphide poisoning may occur even more rapidly than that following exposure to a similar concentration of hydrogen cyanide (hydrogen cyanide has been used in gas chambers for the execution of criminals). Responding to an H2S Emergency Knowing how to respond to a hydrogen sulphide emergency may make the difference between life and death 23 SAFE 6005  Exercise Four Put the steps into the correct order to save someone's life (including your own). 1. Sound the alarm. 2. Assess the situation. 3. Put on breathing apparatus. 4. Remove casualty immediately. 5. Get out o the area to properly assess the situation. 6. Evacuate to obtain medical attention immediately. Sample Theory Questions 1. Tests indicate that the air in an oil storage tank is free of H2S. What precautions should be taken before entering the tank to remove water and sludge from the bottom of the tank? Why are these precautions necessary? 2. Why is it necessary to use non-sparking tools when making repairs to a flange from which H2S is leaking? 3. a) Describe how iron sulphide is formed in relation to the corrosion process. b) In what ways can iron sulphide be a problem in gas processing? 24 SAFE 6005 Exercise Two Answers 1. 0 ppm - rotten egg smell. 2. 50 – 100 ppm – eye damage. 3. 150 - 220 ppm – Olfactory nerve paralyzed. 4. 400 ppm – pulmonary edema. You’ve gone too far. You are toast. Exercise Four Answers 1. Remove casualty immediately. 2. Sound the alarm. 3. Put on breathing apparatus. 4. Get out o the area to properly assess the situation. 5. Initiate resuscitation (artificial respiration, CPR). 6. Evacuate to obtain medical attention immediately. 25 SAFE 6005 Printed in Canada on Recycled Paper