Fail-safe control valves

Eail-Safe Design features which provide for the maintenance of safe operating conditions in the event of a malfunction of control devices or an interruption of an energy source (e.g., direction of failure of a control valve on loss of signal). A system is fail-safe if failure of a component, signal, or utility that would create a hazard initiates an action that maintains the system in a safe condition. 

Figure 12-31. Capacity control suction valve unloaders are available in either of two designs with pneumatic operators (A) direct-acting (air to unload) (B) reverse-acting or fail safe (air to load), which automatically unloads the compressor in the event of control air failure (C) an innovation in manually operated unloaders. Here, the lever cam arrangement provides positive loading or unloading, eliminating the requirement to turn a handwheel completely in or out. (Used by permission Bui. 9-201B, 1991. Cooper-Cameron Corporation.)...

The high-high probe is completely independent of the other probes and is hardwired to shut down the system completely, independent of the computer. (In the preliminary safety review, the hazards associated with HF overfeed were identified as important thus the independent high-high shutdown probe system was installed.) All systems are designed to fail into safe conditions. The HF control valves are air operated and of a design that makes it impossible for HF to contaminate the air supply. 

An important safety feature is provided by the spring in an actuator. It can be designed to position a control valve in a safe position if a loss of supply air occurs. On a loss of supply air, the actuator in Figure 36 will fail open. This type of arrangement is referred to as "air-to-close, spring-to-open" or simply "fail-open." Some valves fail in the closed position. This type of actuator is referred to as "air-to-open, spring-to-close" or "fail-closed." This "fail-safe" concept is an important consideration in nuclear facility design. 

Simpler plants are friendlier than complex plants because they provide fewer opportunities for error and because they contain less equipment that can cause problems. Often, the reason for complexity in a plant is the need to add equipment and automation to control the hazards. Simplification reduces the opportunities for errors and misoperation. For example, (1) piping systems can be designed to minimize leaks or failures, (2) transfer systems can be designed to minimize the potential for leaks, (3) process steps and units can be separated to prevent the domino effect, (4) fail-safe valves can be added, (5) equipment and controls can be placed in a logical order, and (6) the status of the process can be made visible and clear at all times. 

Systems are designed to function normally even when a single instrument or control function fails. This is achieved with redundant controls, including two or more measurements, processing paths, and actuators that ensure that the system operates safely and reliably. The degree of redundancy depends on the hazards of the process and on the potential for economic losses. An example of a redundant temperature measurement is an additional temperature probe. An example of a redundant temperature control loop is an additional temperature probe, controller, and actuator (for example, cooling water control valve). 

The unmanned compressor building was equipped with a combustible gas detection system. However, it failed to sound an alarm because of a faulty relay in the control room. Automatic fail-safe valves functioned properly, blocking-in the flow of ethylene, but not before 450-11,000 lb of gas had already escaped. 

The filling method (See Figure 2) is essentially a fail-safe system in that controls are designed to prevent double-cycling. The filling valve and the reservoir valve are electrically interlocked so... 

The fail-safe condition is to not feed air into the regenerator. Therefore, the suction valve is air-to-open and the vent valve is air-to-close. What action should the flow controller have, direct or reverse ... 

The designer must also consider the failsafe condition of loop components control valves must fail in the safe mode. Reliability may be... 

In the most common design, a fail-passive arrangement reduces the system to its lowest energy level. The system will not operate again until corrective action is taken. Circuit breakers and fuses for protection of electrical devices are examples of this type of fail-safe device. Solenoid valves (see Figure 11-3), such as this one on a steam control valve which is configured fail close shuts off instrument air, are another example. 

Much railway equipment is based on fail-safe principles dedicated to the concept that gravity is a force that will always be there. As a result, semaphores, signal switches, and the lights to which they are connected are weighted devices. In the event of a power or system failure, a heavy arm is allowed to drop and activate the fail-safe warning signal. In the processing plants, if instrument air is lost control valves go to the more safe position— either fail open or fail closed. 

A significant issue to address is the impact of "fail safe" versus "non fail safe" actuators. Some designs such as diaphragm and piston actuators with hydraulic and pneumatic power are readily available as spring return valves. The failure or release of control pressure permits the spring to drive the actuator to its safe position. 

An easy way of improving the system consists in making the control valve RV fail in its open position in case of instrument air failure ( fail safe ). This would almost halve the expected frequency of filling level too high . If, in addition, the motor valve M is activated by the existing level measurements the expected frequency for filling level too high becomes... 

The basic level of control routinely used in the chemical process industries involves sequencing operations such as manipulating valves or starting/stopping pumps, instrument verification, data acquisition, on-line maintenance and fail-safe shut down procedures. The next level of computer control involves process control of parameters such as flow, pressure and temperature. RO/NF systems require both levels [43]. 

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