Failing open

IB High 1. Control valve fails open 1. Reactor cools, reactant cone 1. Instruct operators and update JFL 1/93... 

Fig. 9. Control valve and actuator (a) nomenclature (b) fail-open (F/O) or air-to-close (A/C) action and (c) fail-closed (F/C) or air-to-open (A/O)...

Trip Valves The trip valve is part of a system that is used where a specific valve action (i.e., fail up, Fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls befow a preset level. Trip systems are used primarily on springless piston ac tuators requiring fail-open or fail-closed acrion. An air storage or Volume tank and a check v ve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instabihty when the trip pressure and the reset pressure settings are too close to the same value. 

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

Valve Failure Positions In the event of instrument air or electrical power failure, valves either Fail Closed (FC), Fail Open (FO), or Fail in the last position (FL). The position of failure must be carefully selected so as to bring the system to, or leave the system in a safe operating state. 

Proteetion of the load and of the power supply from failures in the load should be an important eonsideration within all switehing power supply designs. It is important to know what failures are likely to oeeur within the switehing power supply and the load. An exereise that is frequently required in military designs is what is ealled a FMEA (failure modes and effeets analysis), where eaeh eom-ponent is hypothetieally assumed to fail open-eireuited and then short-eireuited. With sueh failures, how does eaeh failure affeet the other seetions of the eireuit This antieipation of failures ean make a power supply design robust. It is the responsibility of the power supply designer to provide proteetion to the load eireuitry from anomalies eneountered from the input line and failures within the supply and the load eireuitry. Often, proteetion sehemes ean be easeaded to provide redundant proteetion in the event of a failure in a proteetion eireuit. A fuse or a eireuit-breaker usually provides sueh a baek-up funetion. 

Fail Open (usually normally open)—An instrument that will go to the open position on loss of power (pneumatic, electric, etc.). 

A. MDP-IA AC Power A Breaker Al 131 Fail open Concurrent failure to start or run CFSR At pump test Pump operability only Treat as pfiri pumping... 

Inadvrttcnt opening of a sal ety/relief valve (stuck) 12. Turbine bypass fails open... 

GRBVB/GRBVA GC refrigerant bypass control valves B/A fail open... 

Diaphragm Operated Control Valve FC = Fail Closed, FO = Fail Open... 

AS—Air Supply BD—Blowdown BF—Blind Flange CBD—Continuous Blowdown CD—Closed Drain CH-O—Chain Operated CSO—Car Seal Open CSC—Car Seal Closed DC—Drain Connection EBD—Emerg. Blowdown Valve ESD—Emerg. Shutdown FC—Fail Closed FO—Fail Open HC—Hose Connection IBD—Intermittent Blowdown LO—Lock Open ML—Manual Loading NC—Normally Closed NO—Normally Open OD—Open Drain... 

This point can easily get lost in the long explanation An air-to-open valve has a positive gain and is failed-closed. An air-to-close valve has a negative gain (-Kv) and is failed-open. 

If we use a controller with positive gain (+KC), the controller output increases as the liquid level drops. We can only reduce the flow if we use an air-to-close valve (-Kv). In the case of a power outage, the valve will stay open. This fail-open valve can drain the entire tank, an event that we may not want to happen. 

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. 

Too much flow through valve Both internal pressure valves fail open Operation Excessive chlorine flow to Tower Water Basin - high chlorine level in cooling water - potential for excessive corrosion in cooling water system Rotameter Daily testing of cooling water chemistry Relief valve on Pressure check valve outlet III None... 

Chlorine flow to environment Internal relief valve sticks open Both internal pressure valves fail open and relief valve opens Operation Potential low chlorine flow to Tower Water Basin -see above Chlorine released to environment - potential personnel injury due to exposure Distinctive odor Pressure check valve located outdoors -unlikely to accumulate significant concentration III Action Item Consider venting relief valve above ground level... 

A Cooling Flow No 1. Control valve fails closed 1. Loss of cooling, possible 1. Select valve to fail open DAC 1/93 ... 

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. 

Probable causes of this accident include (1) installation of a fail-open valve instead of a fail-closed valve, (2) lack of vapor detectors, (3) lack of a block installed as a mitigating device, and (4) failure to eliminate ignition sources in this operating region. 

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. 

The utilization of a fail steady - fail safe mode may allow an undetected failure to occur unless additional instrumentation is provided on the ESD system components or unless the system is constantly fully function tested. The prime feature of a full fail close or fail open failure mode is that it will immediately indicate if the component is functioning properly. 

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