A power plant is a type of industrial facility that produces electricity using primary energy. Most power plant rely on one or more generators to convert mechanical energy into electrical energy in order to offer electricity to the electrical grid for societal requirements. Solar power plants are an anomaly, as they generate energy using photovoltaic cells rather than a turbine.
Equipment Failure
Plant protection basically involves anything that plays a role in preventing damage to plant equipment or systems. In many cases, protecting equipment and systems means protecting personnel, as well. Because of the amount and complexity of equipment in a plant, minimizing the effects of equipment failure is one of the main considerations of plant protection.
There are many problems that can lead to equipment failure. Some basic causes include things such as natural disasters, improper operation and maintenance, and the gradual wearing out of the equipment. Natural disasters cannot be prevented, but in some cases you can prepare for such an event. You can also be familiar with your plant’s emergency operating procedures and follow those guidelines during emergency situations. Equipment failures that result from improper operation or maintenance can be minimized by following operating procedures and routine maintenance schedules.
Of course, even when equipment is operated and maintained properly, it is inevitable that it will age and eventually wear out. But by monitoring operating parameters, you can often detect trends that enable you to predict when major repairs are going to be needed and prevent equipment failure and unexpected down time.
Among the more common mechanical conditions that lead to equipment failure are a failure of the material that makes up the equipment, excessive vibration, overspeed, and various lubrication problems. Material failure can occur because of problems with the quality of the material. It can also occur if the material is subjected to severe conditions or if it simply wears out over time. Conditions such as overloading equipment can place considerable stress on equipment parts. Also, poor operating habits, such as starting up or shutting down equipment too quickly, can subject components to excessive thermal stress. Many of these conditions can be avoided by simply operating the equipment within the limits established by the equipment manufacturer.
Excessive vibration is another common cause of equipment failure. While a certain amount of vibration is normal in most pieces of equipment, too much vibration is often a sign of problems. Excessive vibration can loosen components, cause excessive bearing wear, and subject the equipment to excessive material stress. Many plants use continuous vibration monitoring equipment. In these systems, permanently mounted vibration detectors provide input to some type of recording device for permanent record keeping. It is also common to check vibration using a portable vibration detector. And, of course, you can always check vibration by carefully placing your hand on a bearing housing or the equipment casing. But with this method, you will have to be careful of rotating components and hot surfaces. Plus, you will need to know what the equipment’s normal vibration feels like before you can determine whether or not the vibration is excessive.
Overspeed is another cause of equipment failure. Overspeed is a mechanical condition that occurs when rotating machinery exceeds its designed operating speed. The excessive centrifugal force created by an overspeed condition increases the stress on the moving parts of the equipment. The additional stress can cause material failure, and the equipment can actually tear itself apart. A common cause of overspeed in a turbine generator is the separation of the generator from the electrical distribution system. This causes a loss of load on the turbine, which, in turn, causes an immediate increase in the speed of the turbine. This type of Overspeed condition usually occurs too rapidly for an operator to take action to correct it. So, automatic devices called Overspeed governors are commonly used to protect turbines. If Overspeed occurs, the Overspeed governor automatically energizes trip circuits that trip the turbine valves to stop steam flow to the turbine. There is normally a turbine trip alarm in the control room to alert operating personnel.
Operators are generally responsible for making sure that equipment is properly lubricated. Lubrication reduces friction between moving and non-moving parts in machinery. Improper lubrication can increase the wear on parts and cause bearings to overheat and eventually fail. Most machinery uses either grease or oil as a lubricant. Depending on the type of equipment, the considerations for proper lubrication generally include the quantity of lubricant, the grade of the lubricant, the type of lubricant, possible contamination of the lubricant, and operating temperatures. Often, you can verify the quantity of lubricant that is required by checking a level indicator of some type. If you have to add lubricant to a piece of equipment, make sure you use the proper type and grade of lubricant. You can often find lubrication information on equipment tags or in operating manuals or lubrication charts from the equipment manufacturer. Also, be sure to check the appearance of the lubricant. Different lubricants have different appearances. Your experience in knowing what the lubricant should look like is invaluable in determining if the lubricant is contaminated. Since contaminated lubricant may not lubricate properly, you should follow plant procedures for shutting down the equipment and notifying maintenance if contaminated lubricant is discovered. The source of contamination can then be determined. The lubricant’s temperature is also important, because lubricant will not work properly unless it remains within a certain temperature range. If the temperature of the lubricant is too high, its viscosity will be too low, and if the temperature is too low, the viscosity will be too high. Either condition will affect the lubricant’s ability to lubricate. Sometimes you can tell whether a lubricant is doing its job by checking the temperature of a bearing or a bearing housing. If the bearing or housing is too hot, there could be a problem with the lubricant.
Electrical Hazards
Because power plant is designed to produce electricity, there is the potential for any number of electrical problems, or faults, to appear in a plant. Some of the more common electrical problems include short circuits, open circuits, and ground faults.
A short circuit is basically a connection that is accidentally established in an electrical circuit. It occurs when electricity takes an undesirable path. Shorts often result from a breakdown of conductor insulation, and they are characterized by high current flow. A short circuit can generate a considerable amount of heat, which can cause a fire if there are combustible materials in the area. Personnel who come into contact with a short circuit can be seriously burned or even electrocuted. Severe short circuits that occur inside a piece of equipment will often cause immediate equipment failure. Because of these risks, short circuits should be identified and corrected as soon as possible to lessen the danger to equipment and personnel. Two protective devices that are commonly used to limit damage to equipment if an electrical fault such as a short circuit occurs are circuit breakers and fuses. These devices are designed to interrupt current flow in an electrical system if the current flow becomes too high. They are valuable because they can operate quickly and often prevent additional damage from a severe short circuit.
In some cases, short circuits can become open circuits. If current flow is not high enough to trip circuit breakers or blow fuses, the heat generated by a short circuit can burn a conductor, creating a break and causing an open circuit. If an open circuit occurs in one phase of a three-phase AC circuit that contains a motor, it will cause the motor to operate as a single-phase motor. To continue operating, the motor must draw more current. If the motor is operating at high load, it will draw more than its maximum design current, and an overcurrent condition will develop. To prevent this from happening, most three-phase equipment has overload or overcurrent devices that trip to protect the equipment from damage.
An electrical problem called a ground fault occurs when electricity takes an undesirable path to earth, or ground usually as a result of a breakdown in insulation. A ground can occur in electrical equipment, cables, and conductors. Grounds can cause safety hazards. For example, if one phase of a three-phase motor becomes grounded to the motor’s metal casing, the casing will become highly charged. If the casing is isolated from ground, the electricity has nowhere to go, and the casing will remain charged until the electricity is discharged. This type of situation is very dangerous, because anyone making contact with the casing will provide a discharge path for the electricity through his or her body to ground. To protect personnel, metal equipment casings are usually grounded so that the electricity has a path to ground. Ground straps are often used for this purpose. Detecting and identifying grounds is essential for protecting electrical equipment and personnel.
One method that is sometimes used to indicate when grounds are occurring involves connecting a light bulb to each phase of a three-phase equipment circuit. As long as no ground is occurring, each bulb will show the same brightness. However, if a ground occurs in one of the phases, the bulb that is connected to that phase will be dimmer than normal, while the other two bulbs will be brighter than normal. Regardless of how you detect a ground, you should report it to a supervisor immediately so that it can be isolated and corrected.
Abnormal Temperature
Maintaining operating temperatures within their proper ranges is an important factor in protecting equipment from damage. Operating equipment above or below the designed range can place considerable thermal stress on the equipment. Abnormal rates of temperature change can also lead to damage. For example, if equipment is heated up too rapidly during start-up, it may expand unevenly. The uneven expansion can cause a great deal of thermal stress.
Temperature detectors and indicators are used throughout the plant to provide current temperature readings. You can check these indications to make sure that equipment is operating within the proper temperature range. A type of indicator called a temperature recorder is commonly used to display current temperature readings and record those temperatures. Temperature recorders also provide a record of the rate of temperature change. For instance, a steam temperature recorder receives its input from a temperature detector that is connected to the main steam piping. The recorder itself is located in the control room and operates an alarm to alert personnel if steam temperature becomes abnormally high.
To prevent overheating, most equipment is either air-cooled or water-cooled. Air-cooling is often accomplished by fans, which force air through or around the equipment to remove heat. Cooling fins, which are basically flat plates, are attached to most air-cooled equipment to provide additional surface area for heat transfer.
For equipment that requires more cooling than can be provided by air, cooling water systems are commonly used. Many cooling water systems are designed with a greater supply of cooling water than is necessary during normal operation. This safety factor provides additional cooling water for extreme conditions. However, if too much cooling water is supplied to equipment, the operating temperature of the equipment could be affected. If the temperature of the equipment’s lubricant deviates from its normal range, components such as bearings can be damaged.
It is common for operators to have to make occasional adjustments to a cooling system in order to maintain proper temperatures. However, be aware that any adjustment in the system will affect the operation of the entire system. You will need to use your knowledge of plant equipment and systems to make sure that the plant is protected and efficient operation is maintained.
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Abnormal Pressure
An abnormal pressure condition occurs when pressure becomes either too high or too low. Equipment is generally designed to operate safely within a specific range of pressures. Pressures outside of the specified range can cause problems unless steps are taken to protect equipment and personnel. Many plant systems have devices to protect them from the effects of excessive pressure.
Among the most commonly used devices in steam or gas systems are safety valves, which open quickly to relieve excess pressure before it can cause damage. Under normal conditions, a safety valve is closed. But when system pressure exceeds the safety valve’s set point, the valve pops open, allowing a large volume of steam or gas to escape. The valve remains open until the pressure in the system is lower than the set point pressure at which the valve opened. The difference between the pressure at which a safety valve opens and the pressure at which it closes is called “blowback” or “blowdown.” The discharge piping for safety valves is usually located well away from personnel to protect them from injury.
For systems that carry liquids such as water or oil, pressure relief valves are typically used. As with safety valves, pressure relief valves are used to relieve excess pressure. But rather than popping fully open when system pressure exceeds a predetermined value, a pressure relief valve opens slowly to allow a small volume of liquid to escape. The amount that the valve opens is proportional to the amount of excess pressure. During normal operation, a relief valve is closed. But when system pressure exceeds the valve’s set point, the valve begins to open. If pressure continues to rise, the valve continues to open until it reaches its fully open position. When pressure decreases, the valve begins to close. It closes completely when system pressure drops back to the set point pressure. Since relief valves do not pop open, they do not have blowback. The discharge piping for a relief valve usually leads to a drain area or sump to protect personnel from the discharged liquid.
Like abnormally high pressures, abnormally low pressures in a system can lead to potentially dangerous conditions. One example of where a low-pressure condition can create a problem is in a storage tank. Storage tanks are usually designed to withstand internal pressures that are above atmospheric, and they can be damaged if the internal pressure drops below atmospheric. Unless precautions are taken, the internal pressure can drop below atmospheric that is, a vacuum can form when the tank is being drained. To prevent a vacuum from developing, a vacuum breaker is often installed on a storage tank. A vacuum breaker is essentially a valve that allows air into the tank to equalize pressure as the tank is being drained.
Abnormally low-pressure conditions can also cause problems in forced feed lubrication systems. The flow of lubricant in these systems must be maintained at a constant rate. Because the flow though the system is dependent on pressure, changes in pressure affect the flow. If pressure decreases, flow decreases. If the decrease in flow is great enough, equipment may not receive adequate lubrication. To help prevent these types of problems, a pressure switch is often installed in the system. The switch senses discharge pressure in the system. If the discharge pressure drops below a certain set point, the switch activates an alarm in the control room and either trips the piece of equipment being lubricated or starts a standby pump. Starting a standby pump usually brings the pressure in the system back to normal. Depending on the system, the pressure switch may also send a signal to stop the main pump.
Chemical Hazards
Chemicals are used in many applications in a plant. For that reason, you need to be familiar with the potential hazards associated with some of these chemicals.
For example, hydrogen is often used to cool generator windings. In order for this cooling method to be effective, the generator must be completely filled with pure hydrogen. Pure hydrogen is relatively safe, but when hydrogen is mixed with air, the mixture is potentially explosive. The explosive range is between 4% and 75% hydrogen in air. To prevent an explosive mixture from developing, specific procedures for filling and purging a generator must be followed. These procedures usually include filling the generator first with carbon dioxide (CO2). When a proper percentage of CO2 is in the generator, hydrogen is added while the CO2 is removed. The procedure is complete when the generator is completely filled with pure hydrogen. The process is reversed when hydrogen is removed from the generator. The hydrogen that is removed is vented to a safe area outside the plant. After all the hydrogen has been removed, the CO2 is then removed by filling the generator casing with air.
Like most plant systems, systems that contain hydrogen can sometimes develop leaks. To help minimize the possibility of an explosion, you should always heed “No Smoking” signs in areas where hydrogen may be present. As further protection, alarms in the control room notify personnel if a problem such as a drop in pressure occurs in a hydrogen system.
The accumulation of oil vapors is a chemical hazard that has the potential for causing an explosion or a fire. To prevent oil vapors from accumulating, devices called vapor extractors are often installed on equipment with oil reservoirs. A vapor extractor removes oil vapors by maintaining a slight vacuum in the reservoir. Oil is usually stored in oil storage rooms. These rooms must be well ventilated to prevent vapors from accumulating. Because oil vapors are highly flammable, smoking and open flames are prohibited in oil storage areas.
The acids and caustics used in water treatment are extremely corrosive. The systems and equipment used to handle these chemicals must be specifically designed for them. As an example, for a tank that contains concentrated sulphuric acid, the containment wall surrounding the tank forms an enclosed area that is large enough to hold the entire contents of the tank if a leak occurs. If a leak does occur, the drop in the level in the tank will trigger an alarm to sound to notify personnel. A specially designed relief valve on top of the tank protects the tank from rupturing because of excess pressure. Piping runs from the relief valve to the containment area so that acid is not discharged to the environment.
The pumps, piping, and valves in an acid or caustic system are specifically designed to withstand corrosive chemicals. For example, stainless steel piping is often used to carry concentrated sulphuric acid. In many cases, concentrated acids can be less corrosive than dilute acids. Because dilute acids are often extremely corrosive, they usually require storage tanks, pipes, and valves with protective linings. The type of lining material used in a particular application depends on conditions such as temperature, pressure, and the percentage and type of acid in the solution. Some of the most common types of linings in dilute acid systems are rubber, lead, and Teflon.
Like acids, caustic solutions are potentially hazardous to equipment and systems. However, the caustic effects of concentrated chemicals can be minimized by storing them in a dry, granular form. While caustics are in this form, precautions must be taken to prevent them from coming into contact with liquids. If dry caustics become wet, they become very corrosive.
Because acids and caustics are so corrosive, acid or caustic spills can be very dangerous. Anyone who has to work in an area where a spill has occurred must wear the proper protective clothing and follow all applicable safety precautions.
Fire Protection
The fire protection equipment in a power plant can range from relatively simple devices, such as fire extinguishers, sprinklers, and fire hose stations, to more complex equipment, such as deluge systems, foam and dry chemical systems, Halon systems, and CO2 systems. But for the most part, power plant use water as their primary means of dealing with fires. To ensure that there is an adequate supply of water in case of an emergency, most fire protection systems have several sources. Typically, the primary source is a fire tank that is supplied by a branch of the service water system or some other source. Secondary sources of water are generally large, accessible water supplies such as rivers, lakes, or a community’s water main.
The primary and secondary water sources usually supply fire pumps. Fire protection systems generally have built-in redundancy, so there are usually two or more pumps in a system. The main fire pump is usually driven by an electric motor, while the backup pumps are usually gasoline, diesel, or propane driven. Some fire protection systems use a pump called a jockey pump to maintain a constant pressure in the system and to keep the system filled with water. Keeping the system filled with water ensures that air pockets do not develop. If air pockets develop, piping can be damaged by water hammer when either of the other pumps is started. The design of fire pumps allows them to deliver large volumes of water at high pressures from a water source to the fire main in the plant. The fire main is a system of pipes that carry water to individual fire hose stations and to the other parts of the fire protection system.
A fire hose station typically consists of a fire hose and nozzle that are stored on a rack. A shutoff valve is usually located near the rack and is normally closed. In the event of a fire, the hose should be completely unrolled before the shutoff valve is opened. Then, the shutoff valve should be opened so that the hose is filled at a controlled rate. This prevents the hose from whipping around and possibly injuring personnel or becoming damaged.
In a deluge-type fire protection system, flow is controlled by deluge valves. Deluge valves are normally closed. They can be opened manually, but they are usually operated automatically by heat sensing devices. When a fire occurs, excessive temperatures will cause the deluge valves to open, allowing a large amount of water to be sprayed into the area to drown the fire.
In addition to basic fire protection systems, many plants also have foam, dry chemical, Halon, or CO2 systems. Foam and dry chemical systems are often located near oil storage tanks. These systems are designed to operate automatically to flood an area with foam or dry chemicals that extinguish a fire by smothering it. Halon equipment is usually located in control rooms or in areas that contain sophisticated equipment such as computers and solid-state circuitry. Unlike water, foam, and dry chemicals, Halon can extinguish a fire without damaging equipment. The use of a CO2 system is similar to that of a Halon system. A CO2 system can be supplied by CO2 bottles or by a CO2 generator. Before CO2 can be applied to an area to extinguish a fire, the area must be evacuated of all personnel. If personnel were allowed to stay in the area, they could be suffocated by the CO2. After the fire has been extinguished, the area must be completely re-ventilated, and oxygen must be restored before personnel can return to the area.
Fire extinguishers are located at various spots throughout the plant. Operators are responsible for knowing where fire extinguishers are located and how to use them. The types of fires that an extinguisher is designed to extinguish are usually listed on the extinguisher’s nameplate. Types, or classes, of fires are designated by the letters “A,” “B,” “C,” and “D.” Class A fires involve wood, paper, rags, or other similar materials. Class B fires are caused by chemicals or flammable liquids. Class C fires are electrical fires. Class D fires involve flammable metals such as magnesium and phosphorous. Some fire extinguishers are designed to fight all three of the most common types of fires – Class A, Class B, and Class C. These fire extinguishers have the indication “ABC” on their nameplates.
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