Fail Safe Valves

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. 

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. 

Many of the fail-safe valves used in chemical processes are quarter-turn valves. Generally, ball valves with spring loaded actuators are recommended so that there is positive fail-safe movement in case of power or air failure. It may not be practical for very large valves to be spring loaded. These large valves should generally have a local air supply tank to cause them to fail to the fail-safe condition. 

Of course, it would make sense in this case to use fail-safe valves, i.e. valves which close on instrument air failure. Then air failure would not cause the undesired event unless none of the two valves would adopt its rest position (closed), which might occur with a certain—even if small—probability. 

Installation of internal excess flow or fail-safe remotely operated valves for tanks at locations where piping connects... 

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. 

Fail-safe Design features that provide for the maintenanee of safe operating eonditions in the event of a malfunetioning eontrol deviee or an interruption of an energy souree (e.g., direetion of failure of a motor-operated valve on loss of motor power). 

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.)...

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. 

The principles of operation of a hydraulic actuator are like those of the pneumatic actuator. Each uses some motive force to overcome spring force to move the valve. Also, hydraulic actuators can be designed to fail-open or fail-closed to provide a fail-safe feature. 

Fail-safe is a concept used to specify the position of process instrumentation in the event of power, air pressure, or other utility failures. For instance, the valve supplying cooling water to a chemical reactor would fail in the open position ( fail open ) in the... 

Specify the proper fail-safe positions for the valves in the following equipment. Specify either fail open or fail close. 

Add fail-safe block valves with a positive indication of the valve position (limit switches). 

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... 

For ESD isolation valves (i.e., EIVs) a fail safe mode is normally defined as fail closed in order to prevent the continued flow of fuel to the incident. Blowdown or depressurization valves would be specified as fail open to allow inventories to be disposed of during an incident. Special circumstances may require the use of a foil steady valve for operational or performance reasons. These applications are usually at isolation valves at components, i.e., individual vessels, pumps, etc., where a backup EIV is provided at the battery limits that is specified as fail closed. The fail safe mode can be defined by the action that is taken when the ESD system is activated. Since the function of the ESD system is to place the facility in its safest mode, by definition the ESD activation mode is the foil safe mode. 

ESD valves should lock in the fail safe mode once activated and be manually reset once it has been confirmed the emergency has passed or has been resolved. 

Fail-safe design of valves and instrumentation, including entire cooling or heating loop... 

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 ... 

Remote automatic—based on a sensing element, logic solver, and output signal to close the valve. In most cases, these automatic valves are designed to fail safe. Automatic isolation can be associated with the automatic activation of a shutdown system for equipment or a process unit. 

In a lot of literature, the fail-safe operation of a pilot valve is questionable due to the potential accumulation of dirt, hydrates, and so on, in the pilot supply lines. 

Actuators often provide a fail-safe function. In the event of an interruption in the power source, the actuator will place the valve in a predetermined safe position, usually either full-open or full-closed. Safety systems are often designed to trigger local fail-safe action at specific valves to cause a needed action to occur, which may not be a complete process or plant shutdown. 

Relief valve lifts light (fails safe) 0.1... 

Valves can have four modes of operation during failure Fail open, fall closed, fail in the current position, and fail in any position without having an impact in the process (this is rarely a valid option). In order to fail safe,... 

Fig. 8. Polyethylene tubing on fail-safe, spring-loaded, fire-safe valve used as an emergency block valve.

Low pressure gas line is plugged. Loss of fuel for heater and Instrument gas supply Total shutdown Valves are fail safe, ESO occurs ... 

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