Hydrogen Sulphide Safety

Transcription

1 Hydrogen Sulphide Safety Learning Outcome When you complete this module you will be able to: Describe hydrogen sulphide (H 2 S) and it s effect on humans. 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 effect on humans of various concentrations of H 2 S. 3. Briefly outline how to respond to an H 2 S emergency. 1

2 INTRODUCTION All jurisdictions in Canada have the same or similar legislation in place concerning safety and health. Since it is impossible to refer to them all, the Alberta acts, regulations, and codes are referred to in this module. You are encouraged to refer to the appropriate documents for your province or territory. Hydrogen sulphide (H 2 S) is an extremely poisonous substance, normally encountered as a gas or vapor. Numerous deaths have occurred over the years as a result of exposure to hydrogen sulphide (H 2 S). Some were caused by falls sustained when the workers were overcome by the gas. Other deaths resulted from suffocation when the workers were overcome because of brief exposure to the gas. Hydrogen sulphide is found in varying concentrations in many oil and gas wells. Hydrogen sulphide is also found in septic tanks, sewers, manure pits, or anywhere bacteria can break down organic matter in an oxygen deficient environment. H 2 S is found in coal mines, and is referred to by coal miners as stink damp. H 2 S 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. H 2 S 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. H 2 S must be removed from crude oils and natural gases. After removal, the H 2 S is converted to elemental sulphur by first burning part of it in a waste heat boiler to produce sulphur dioxide (SO 2 ). This combustion creates heat that is utilized in the generation of steam for process purposes. The SO 2 is then combined with the remaining H 2 S in a catalyst bed, where they chemically react to produce sulphur. Sulphur production has greatly contributed to Alberta's economy. Some persons have the mistaken opinion that H 2 S is made in gas plants, but H 2 S is a natural component of raw gas. PROPERTIES OF HYDROGEN SULPHIDE Workers must thoroughly understand the physical properties of H 2 S so they can safely work in an environment that may become contaminated by it. Table 1 lists the properties of H 2 S. The following discussion will help to understand the importance of the properties of H 2 S. 2

3 Chemical Symbol Relative Density H 2 S (air = 1.0 heavier than air) Autoignition Temperature 260 C Flammability Lower Explosive Limit Upper Explosive Limit Color Odor Very flammable, distinctive blue flame 4.3% in air by volume 46% in air by volume Colorless, invisible Strong rotten egg Vapor Pressure 17.7 atm. at 20 C Boiling Point - 60 C Melting Point - 83 C Reactivity Solubility Dangerous with acids and oxidizers Yes in water, hydrocarbons, alcohol Quoted from National Safety Council Data Sheet Table 1 Properties of Hydrogen Sulphide Chemical Symbol The chemical symbol for hydrogen sulphide, H 2 S, 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 H 2 S. Relative Density Hydrogen sulphide is times as heavy as air in its pure form. In the petroleum industry, H 2 S seldom exists in its pure form but is usually associated with hydrocarbon gases. Sophisticated processes are required to remove the H 2 S gas from hydrocarbon gases because they do not readily separate on their own. All heavier-than-air gases will tend to settle in low lying areas when the air is still and cool, meaning that workers must be especially cautious in underground installations or tanks, and in confined spaces where ventilation is poor. 3

4 In the petroleum and other industries, hydrogen sulphide does not naturally separate itself from the gas stream and it will always be present with hydrocarbons. That means that in the event of a leak of sour hydrocarbons, H 2 S will not separate and migrate to low areas, but will instead follow the natural air convection currents and be present in high or low areas. The best location for H 2 S 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). Autoignition 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, H 2 S ignites at a relatively low temperature (260 C). Because of its low ignition temperature, workers are advised to use only approved power tools and nonsparking tools to reduce the ignition hazard when H 2 S may be present. Also, static electricity from drive belts, etc. can easily ignite H 2 S. Flammability H 2 S combines with oxygen to produce sulphur dioxide (SO 2 ) and water vapor. In burning 1 kg of H 2 S approximately kj of heat is produced. H 2 S burns with a very light blue flame that is invisible in daylight. The combustion products are visible as a brownish-yellow gas. The SO 2 produced by burning H 2 S is also toxic and requires the same respect as H 2 S. The only safe way to extinguish an H 2 S fire is to interrupt the supply of the gas. Explosive Limits We have all experienced problems when trying to start a gasoline engine that is either starved for fuel or flooded. This is because gasoline will not ignite if the fuel/air mixture is too lean or too rich. To ignite, the air/gas mixture must be between 1.3% and 6% gasoline; outside of this range, the fuel will not burn. We could say that gasoline has a narrow explosive range. Compare this to H 2 S, which has a lower explosive limit (L.E.L.) of 4.3% and an upper explosive limit (U.E.L.) of 46%. H 2 S 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 autoignition temperature make it a very dangerous gas, from a fire and explosion standpoint. 4

5 Color H 2 S is an invisible gas. A leak may become visible because of hydrocarbon or water vapor content, but the H 2 S component of a leak cannot be seen. Odor Rotten eggs have a pungent, repulsive smell because the gas produced in the decay process is H 2 S. Therefore H 2 S is often referred to as rotten egg gas. It is an insidious poison since 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 H 2 S! Vapor Pressure The vapor pressure of H 2 S is 17.7 atmospheres at 20 C. This means that when H 2 S is to be maintained in the liquid state, it must be under a pressure of at least 1800 kpa. Loading liquefied H 2 S into tank cars must be done within strict guidelines to ensure the cars are not overloaded, and thus become over pressured. Boiling Point Because of the low boiling point of H 2 S (-60 C), it is normally in the gaseous state at atmospheric pressure. Melting Point The melting point of H 2 S is -83 C, so we do not expect it to be in the solid state. However, if water containing H 2 S gas is frozen, the H 2 S becomes part of the solid mass. When the ice melts, the H 2 S will be liberated as a gas. Reactivity H 2 S reacts violently with acids and oxidizers. It will tarnish silver and it 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 H 2 S in chemical reaction tubes for use in gas detectors. (Detection of hydrogen sulphide will be covered in another module.) 5

6 Solubility Gases will 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 liquid (some liquids absorb some gases better than others). Referring to Fig. 1 in a closed system, gas will be absorbed by the liquid as the liquid temperature is reduced and the pressure is increased and conversely, gas is liberated from a liquid if the temperature is increased or the pressure is reduced. Hydrogen sulphide has a solubility (at 20 C and 1 atmosphere pressure) of 3067 ppm in water and ppm in conventional crude oil (C 6 +). Gas Dissolved in Liquid Gas Liberated from Liquid Gas Phase Gas Phase Liquid Phase Liquid Phase High Pressure Low Temperature Low Pressure High Temperature Figure 1 Solubility of Gas in Liquid The principle of solubility is the basis for one of the processes to remove H 2 S from hydrocarbon gases. Liquid amine absorbs the H 2 S from high-pressure gas as it comes in contact with that gas. The liquid amine (now containing H 2 S) is pumped to another vessel where it is heated and the pressure reduced to liberate H 2 S, which is then burned. Considering the solubility of H 2 S in water and in crude oil. The concentration of H 2 S in the vapour above a slightly sour liquid at low pressure is very high and, in some cases, 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 H 2 S (gas phase) = ppm H 2 S (water) x

7 ppm H 2 S (gas phase) = ppm H 2 S (C 6 +) x Table 2 uses these formulae to show the concentration of H 2 S in the gas phase above both water and C 6 + for various concentrations in the liquid phase. ppm H 2 S ppm H 2 S ppm H 2 S in liquid above H 2 O above C Table 2 Concentration of H 2 S Gas in Liquid Phase Agitation will increase the liberation of gases. A vessel that is being cleaned, which appears to be free of H 2 S, 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 H 2 S from sand or other solids, as well as the H 2 S trapped in the metal itself. Welding on a vessel that contained sour gases or liquids will liberate H 2 S 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 placed back into service. H 2 S CORROSION and the formation of IRON SULPHIDE If water is present in an H 2 S environment, it will react with the H 2 S to produce sulphurous acid (H 2 SO 3 ). This acid will then attack the metal in the steel piping or vessel, removing iron and producing a grey, paste-like material called iron sulphite (FeSO 3 ). In a dry atmosphere, H 2 S can be directly corrosive to steel piping and vessels. The sulphur component of H 2 S reacts with the iron in the metal to produce various forms of iron sulphide (FeS, Fe 2 S 3, and so on). Iron sulphide is generally recognizable as a grey, grey/black, brown, or green powder. 7

8 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 H 2 S, and H 2 S in air has a wide flammable range and a low auto-ignition temperature, the iron sulphide can become a source of ignition if allowed to dry out. Iron sulphide reacts vigorously in acidic conditions to release H 2 S gas, which can pose a real problem for unsuspecting workers. Hydrogen sulphide 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. TOXICITY OF HYDROGEN SULPHIDE Table 3 indicates the toxicity of hydrogen sulphide. Due to the highly toxic effects of the gas, the results are of tests performed on animals. No two individuals are affected in the same way by the gas, so the test results are only approximate. When information is given in ppm, it is often deceptive due to the fact that the average individual cannot imagine the number 1,000,000. To put the information in Table 3 into perspective, consider the following comparisons: 1.The box of a small pickup truck has a volume of approximately 1m 3. 2.An eyedropper has a volume of 2 ml (2/1000 litre). 3. A 1 litre milk carton has a volume of 1/1000 m 3. If just half an eyedropper of H 2 S gas were put into the box of the pickup, a concentration of 1 ppm would result, and at that concentration the H 2 S 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. 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. A person could work in a 10 ppm concentration of H 2 S 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. 8

9 Parts Per Million (ppm) Percentage (%) Physiological Effects Rotten egg smell is noticeable hour Occupational Exposure Limit. Maximum allowable concentration for continuous exposure for 8 hours without breathing apparatus Minute Occupational Exposure Limit. Maximum allowable concentration for exposure for 15 minutes without breathing apparatus Ceiling Occupational Exposure Limit. This level of exposure cannot be exceeded at any time without respiratory protection Dulls sense of smell, causes burning sensation in eyes and throat. Chronic exposure produces headaches, insomnia, irritability, anorexia, and nausea Attacks respiratory centre of the brain, causes loss of reasoning and balance, persistent cough, sore throat, fogged vision, chest tightening, pulmonary edema Causes nausea, vomiting, cyanosis, vertigo, amnesia, and convulsions. Casualty quickly loses consciousness; breathing will stop and death will result if not rescued promptly Unconsciousness immediately, permanent brain damage or death occurs if casualty is not rescued and resuscitated immediately. Table 3 Toxicity of Hydrogen Sulphide If the concentration of H 2 S 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 If exposed to a concentration of 100 ppm (1/100 of 1%), the sense of smell will be lost or become ineffective within in 2 to 15 minutes. The H 2 S might cause a burning sensation to the eyes, throat and lungs, and could cause headache or nausea. 9

10 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.) At a concentration of 500 ppm, the victim will appear to be intoxicated, and will lose his sense of balance and reasoning. In this state, the victim may attempt to continue with the job he was doing when he encountered the gas. For this reason, a person MUST know the people he works with, and be able to detect any unusual behavior of a coworker. 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. 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. At a concentration of 1000 ppm (1/10 of 1%) the victim will be rendered unconscious immediately. This victim will not begin breathing voluntarily if brought to fresh air. ARTIFICIAL RESUSCITATION MUST BE COMMENCED WITHIN THREE MINUTES OF BECOMING UNCONSCIOUS DUE TO HYDROGEN SULPHIDE! Effect of H 2 S 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 H 2 S (10 ppm) can be easily smelled, but once the odor is gone, the victim believes that the danger has passed. It may well be that the gas has dulled his sense of smell and the danger is greater than when it could be smelled. Also, it must be remembered that aromatic hydrocarbons can mask the smell of hydrogen sulphide. H 2 S is an INSIDIOUS POISON Another problem with H 2 S 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 H 2 S is relatively slight and the victim does not try to leave the area because he does not feel any pain. 10

11 Effect of H 2 S on the Nervous System In small amounts hydrogen sulphide is a depressant. In larger amounts it is a stimulant. In high concentrations the respiratory centre is paralyzed, causing suffocation. Special Note: There is much controversy regarding H 2 S entering the respiratory system through a perforated eardrum. Company policies vary widely on the subject, but the Journal of Occupational Medicine (May 1985) makes the following statement: We conclude that workers with perforated eardrums should NOT be excluded from working in atmospheres containing concentrations of H 2 S. Effects of Repeated, Chronic Exposure to H 2 S Some of the health problems associated with repeated exposures to hydrogen sulphide are: 1. Pulmonary Edema: an abnormal excess accumulation of fluids in the lungs. 2. Corneal Bullae: blisters on the transparent membrane covering the iris and pupil of the eye. 3. Conjunctivitis: inflammation of the mucous membrane lining the eyelids. 4. Photophobia: a high intolerance to light, which can make driving at night hazardous. 5. Rhinitis: inflammation of the mucous membrane lining the nose. 6. Bronchitis: chronic inflammation of the bronchial tubes. 7. Headaches. 8. Digestive disturbances. 9. Weight loss. 10. General weakness. 11

12 SWEET vs. SOUR One problem in attempting to convince workers of the dangers of H 2 S is probably the result of misleading distinctions made between low and high concentrations. This may be due, in part, to the previous definitions that were adopted by producers to distinguish between 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 far less confusing to workers since it is more in line with the OELs for H 2 S. A product with a concentration of H 2 S not exceeding 0.001% or 10 ppm, is now considered sweet. (Compare the old standard of 1% H 2 S!!) The basis for the AEUB definition is the production of petroleum products and the guidelines are only concerned with the production steps that must be taken to remove H 2 S 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. 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. EXPOSURE LIMITS for H 2 S The following exposure limit definitions and workplace responsibilities are taken from the Occupational Health And Safety Act - Chemical Hazards Regulation Definitions 1. 8 Hour Occupational Exposure Limit (O. E. L.) This means the time weighted average concentration of an airborne substance for an eight hour period. For H 2 S, the limit is 10 ppm. It has been determined that a worker could work continuously for eight hours in a 10 ppm concentration Minute Occupational Exposure Limit 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 H 2 S. 3. Ceiling Occupational Exposure Limit 12 This means the maximum concentration of an airborne substance to which a worker may be exposed by inhalation. For H 2 S, the concentration is 20 ppm.

13 EMPLOYER RESPONSIBILITY The employer shall ensure that a worker is not exposed by inhalation to concentrations of H 2 S in excess of the prescribed O. E. L. Where a worker is or may be exposed to H 2 S, the employer shall ensure that each worker: (a) Is trained, and utilizes this training to minimize the worker s exposure to H 2 S. (b) Is instructed in the purpose, proper use, and limitations of any protective equipment provided. (c) Is instructed regarding health hazards associated with exposure to H 2 S. Where a respiratory device is used to control worker exposure to H 2 S the employer shall: (a) Provide, and ensure that workers wear an appropriate, correctly fitting device. (b) Ensure that each worker is instructed in the purpose, proper use and limitations of the device. (c) Ensure that the device provided is stored in a manner that prevents contamination. (d) Ensure that each device is properly maintained and is regularly cleaned. EMPLOYEE RESPONSIBILITY It is the employee s responsibility to: (a) (b) (c) Wear respiratory protective equipment when it is provided by an employer and ensure that the equipment fits correctly. Follow the procedures set by the employer. Participate in any instruction provided by the employer. 13

14 The OCCUPATIONAL HEALTH AND SAFETY ACT - GENERAL SAFETY REGULATIONS All workers should become familiar with this regulation. The following information is condensed from the regulation, and is directed to persons working with H 2 S. Hazardous work must be performed by competent workers following procedures developed by the employer. The worker must receive adequate training and know the limitations of equipment required to allow the work to be done safely. TRAINING SESSIONS ARE REQUIRED BY LAW. The laws generally require that all persons working around H 2 S 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. RESPONDING TO AN H 2 S EMERGENCY Even when a worker collapses in an area where H 2 S is a known hazard, it does not automatically mean that person has been overcome by the H 2 S. Various other conditions may have caused this collapse; for example, heart attack, stroke, or head trauma. However, from a safety standpoint, always ASSUME H 2 S IS PRESENT and wear your breathing apparatus before entering the area. AG_7_0_1.jpg P-S BEFORE ATTEMPTING TO RESCUE A VICTIM OF H 2 S POISONING YOU MUST PUT ON YOUR BREATHING APPARATUS! PROTECT YOURSELF FIRST!!! 14

15 When a co-worker has been knocked down in a known H 2 S environment, there are certain steps that must be followed to ensure your own safety and survival. Remember that a rescuer who does not take the time to properly assess the situation and get suited up and call for help before attempting the rescue, and who is subsequently overcome, is no good to the victim and creates problems for other rescuers by becoming an unnecessary victim. 1. Get out of the area to properly assess the situation 2. Sound the alarm 3. Put on breathing apparatus 4. Remove casualty immediately 5. Initiate resuscitation (artificial respiration, CPR) 6. Evacuate to medical attention immediately FATAL HYDROGEN SULPHIDE POISONING MAY OCCUR EVEN MORE RAPIDLY THAN THAT FOLLOWING EXPOSURE TO A SIMILAR CONCENTRATION OF HYDROGEN CYANIDE... (from Dangerous Properties of Industrial Materials.) (Hydrogen cyanide has been used in gas chambers for the execution of criminals.) 15

16 Notes: 16