Failing close

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. 

Fail Closed (usually normally closed)-An instrument that will go to the closed position on loss of power (pneumatic, electric, etc.). 

Accumulation of Noncondensihles - Noncondensibles do not accumulate under normal conditions since they are released with the process vapor streams. However, with certain piping configurations, it is possible for noncondensibles to accumulate to the point that a condenser is "blocked". Such a condition could occur if an automatic vent control valve failed closed for a period of time. This effect is equal to a total loss of coolant, and thus need not be considered separately. 

Remember that the failure position of a valve refers to its failure mode if there is a utility failure. A valve can mechanically fail in any position it is possible for a fail closed valve to get stuck in the open position. When doing a process hazard analysis it is important to consider all possible failure positions of a valve, and not only the failure position resulting from utility failure. 

Note Liquid dump valves are normally fail closed to prevent giis blowby. That means that in ca.se of loss of instrument gas or air pressure, the spring will drive the valve to the closed position. However, the valve can mechanically fail because the level controller malfunctions or the seat cuts out due to solids erosion, which would cause it to fail in an open configuration. 

No No Flow Excess ammonia in reactor. Release to work area. 1. Valve A fails closed. 2. Phosphoric acid supply exhausted. 3. Plug in pipe pipe ruptures. Automatic closure of valve B on loss of flow from phosphoric acid supply... 

Opwatad ValVM 3.5.3.1 Hvdraulic. 1 Fail Close Actuator. 2 Fall Open Actuator. 3 Fail in Position Actuator. 1 AMamating (2-position Control). 2 Continuous (Throttling Control). 3 Standby Open. 4 Standby Closed 1-4 All Modes Catastrophic a. External Leakage b. Internal Leakage > 1% c. Spurious Operation d. No Change of Position on Demand... 

OTHER Unplamied Utility failure Does LICV fail closed or... 

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... 

CV3 fails closed considered low flow alarm, rejected - needs resetting at each rate... 

We refer to the valve here as a fail-closed valve, which is the preferred safety design in Fig. 5.8. 

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. 

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. 

None No flow - chlorine to chlorination loop No/low level in chlorine drum. Pressure reducing valve fails closed. No chlorine flow to tower basin. Low chlorine concentration in tower basin. Local pressure indication on chlorine injection line. Local flow indication on chlorine injection line (rotameter). 6 ... 

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

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. 

The Hopkinson-Cranz scaling law described earlier applies to scaling of reflected blast wave parameters just as well as it does to side-on waves. That is, all reflected blast data taken under the same atmospheric conditions for the same type of explosive source can be reduced to a common base for comparison and prediction. Sachs law for reflected waves fails close to high explosive blast sources but it does apply beyond about ten charge radii. 

Gulf of Mexico, Matagorda Island, Block 669. Blowout, Explosion/Fire Annular preventer failed, closed pipe rams, casing pipe failed at 7,200 psi. 

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