The operation of coal-fired electrical generating stations involves a series of steps which may expose workers to traumatic injury and hazardous chemical and physical agents. These hazards may be controlled through a combination of good design, knowledgeable workers and job planning. Good design will ensure that all components meet the necessary codes for integrity and safe operation. It will also ensure that equipment layout allows continuing safe operability and maintainability through easy access. Knowledgeable workers will be aware of hazards in the workplace and will be able to create plans to address the hazards they do encounter. These plans will identify hazards and apply appropriate controls, which may involve a combination of de-energization, physical barriers and personal protective equipment. Analysis of accident experience shows that modern power stations have a safety performance comparable to other heavy mechanical industries. Within the power station staff, most lost-time injuries are suffered by the maintenance staff. Injuries frequently involve sprains and strains to soft tissues of the body, with back strain injuries the most common. Industrial diseases associated with chronic exposure to noise and, occasionally, asbestos are also found.
The operation of a modern powerplant may be considered in a series of steps.
Coal Handling
This includes coal receiving (either by rail or water), storage and recovery for fuelling the turbine generator units. Heavy equipment (tractor-scrapers and bulldozers) is used to create compacted storage piles, which is necessary if spontaneous-combustion fires are to be avoided. Further handling is by conveyors to the powerhouse. Coal dust exposure (leading to possible pneumoconiosis) can be controlled by water spraying of the coal pile and the use of closed control cabs fitted with dust filters. Certain tasks associated with high coal dust levels require respirators with high efficiency particulate absorber (HEPA). Noise levels result in most workers in this work area receiving greater than 85 dBA exposure (leading to hearing loss), which should be controlled through use of ear plugs and muffs, and a hearing conservation programme.
Several conventional safety hazards are found in this area of the plant. Working near water requires careful attention to procedures and also the use of life preservers. Driving heavy equipment on uneven storage piles during the night requires large-scale area lighting, while the lifting and pushing hazards from manual clearing of the conveying coal chutes (which are prone to blockage, particularly when winter is severe) is best controlled through removable chute covers, which provide easy access. Operation and maintenance of extended conveyor systems requires guarding of drive and end pulleys, tensioners and other nip points.
Boiler-Turbine Operation
The operation of a high-pressure boiler-turbine combination should involve a rigorous set of controls to ensure safe operation. These controls include the physical integrity of the equipment and the skill, knowledge and experience of the operating staff. The integrity of the high-pressure components is ensured through a combination of appropriate specifications contained in modern engineering standards, and routine inspections of welded joints using visual and non-destructive imaging techniques (x rays and fluoroscopic methods). In addition, pressure-relief valves, which are regularly tested, ensure that over-pressurizing of the boiler does not occur. The necessary skills and knowledge of the staff may be created through an in-house process of personnel development coupled with government accreditation which extends over several years.
The environment of the powerhouse is a collection of complex engineered systems to carry fuel, combustion air, demineralized boiler water, and cooling water to the boiler. In addition to the high-pressure steam hazards, it contains a variety of other conventional and chemical/physical hazards which must be recognized and controlled. In operation, the most pervasive hazard is noise. Surveys show that all operating and maintenance staff have a time-weighted average exposure of over 85 dBA, which requires the wearing of hearing protection (plugs or muffs) in much of the powerhouse and regular audiometric testing to ensure no deterioration in hearing. Major sources of noise include the coal pulverizers, the turbine-generator unit, and station service air compressors. Dust levels in the powerhouse during operation depend on maintenance attention to the condition of thermal insulation. This is of particular concern as much older insulation contains high levels of asbestos. Careful attention to controls (primarily bonding and containment of damaged insulation) can achieve airborne asbestos concentrations which are undetectable (<0.01 fibre/cc).
The final stage of the operation process which creates potential hazards is ash collection and handling. Usually located outside the powerhouse, ash collection is typically done with large electrostatic precipitators, although there is increasing use of fabric filters in recent years. In both cases the ash is extracted from the flue gas and retained in storage silos. Any subsequent handling processes are inherently dusty despite engineered efforts to control levels. This type of ash (fly ash, as opposed to the bottom ash that has accumulated at the bottom of the boiler) contains a significant fraction (30 to 50%) of respirable material and is therefore a potential concern for possible health effects to exposed workers. Two components of the ash are of potential significance: crystalline silica, associated with silicosis and possibly subsequent lung cancer, and arsenic, associated with skin and lung cancer. In both cases it is necessary to carry out exposure assessments to determine if regulated limits are exceeded and whether specific control programmes are required. These assessments, involving surveys with personal samplers, should include all potentially affected workers, including those who may be exposed during inspections of the dust collection systems and of the grinding and heating surfaces in the boiler, where arsenic is known to deposit. Control programmes, if necessary, should include providing information to the workers about the importance of avoiding ingestion of ash (no eating, drinking or smoking in ash-handling areas), and the need for careful washing after coming in contact with ash. Dust levels encountered in these surveys are usually such that good safety practice indicates a respiratory control programme for exposure to total nuisance dust. The industrial mortality database maintained by the US National Institute for Occupational Safety and Health, for example, contains no entries for deaths attributable to silica or arsenic exposure in the US electrical utility industry.
Maintenance
It is during the maintenance phase that the highest exposure occurs to conventional and chemical/physical agents. Given the complexity of the modern generating station, it is critically important that there be an effective process for isolating equipment so that it cannot be energized while repairs are being carried out. This is typically achieved through a controlled system of locks and tags.
A broad range of conventional hazards are encountered during maintenance. They involve:
- working at heights (fall protection )
- heat stress
- rigging and craning (load security)
- work in confined spaces (atmospheric and conventional hazards)
- excavating (trench collapse)
- working/lifting in cramped environments (sprains and strains).
In all cases, the hazards may be managed by a stepwise process of analysis which identifies hazards and corresponding controls.
A large variety of hazardous commercial products are used and encountered in routine maintenance activities. Asbestos is common, as it has been used widely as thermal insulation and is a component of many commercial products. Control processes should be in place to ensure that all asbestos-containing material is correctly identified by microscopic analysis (on-site capability greatly improves response time). The actual control methods used for the task depend on the scale of the activity. For large-scale jobs, this will involve constructing enclosures that operate under slightly reduced pressure (to prevent leaks) and ensuring that workers are equipped with respiratory protection following careful procedures to avoid external contamination. In all cases the asbestos-containing material should be completely wetted, and bagged and labelled for disposal. Careful examination is necessary to ensure that all asbestos is removed before proceeding. Workers’ exposures should be recorded and periodic chest x rays coupled with pulmonary function testing will determine the onset of any disease. Positive results of these examinations should result in the worker being immediately removed form further exposures. Current practices reflect a high level of concern for asbestos exposures in the electrical utility industry.
For the great majority of other hazardous materials used in the workplace, the quantities involved are small, and the use infrequent, so that the overall impact is insignificant. The most significant class of exposures to hazardous materials are those associated with particular operations rather than particular products.
For example, welding is a common activity that can give rise to a series of possible adverse health outcomes. Exposure to ultraviolet light from the arc causes temporary blindness and severe eye irritation (“arc eye”); inhaled metal oxide fumes may cause “metal fume fever”; and nitrogen oxides and ozone formed at the high temperatures in the arc may cause chemical pneumonia and possible chronic respiratory problems. The controls to be applied include eye shields to protect nearby workers from scattered light, local exhaust ventilation or respiratory protection (through an air-purifying respirator).
A similar common activity is grinding and abrasive blasting, where the concern is for inhalation of the respirable metal oxide and abrasive particles. In this case, the control is usually through choice of abrasive agent (sand has now been abandoned in favour of more benign agents such as vegetable husks) coupled with appropriately high local exhaust ventilation.
The other activity leading to significant exposures is the application of protective coatings to metal surfaces. The coatings may contain a variety of solvents which are released into the working atmosphere. Worker exposures can be controlled either by local exhaust ventilation or, if that is impractical, by respiratory protection.