Instrument air failure

Normal Individual Process Unit Basis for PR Sizing Considerations - The following single contingencies should be considered as the normal basis for evaluating overpressure that can result from an instrument air failure ... 

Consideration. of Plant-wide or Refinery-wide Failures - Although not normally used as a basis for sizing pressure reheving facihties, the following general instrument air failures must be considered ... 

Instrument air failure The consequences of the loss of instrument air should be evaluated in conjunction with the failure mode of the control valve actuators. It should not be assumed that the correct air failure response will occur on these control valves, as some valves may stick in their last operating position. 

Always check what could happen in case of electrical power or instrument air failure be sure that in case of electrical power/instrument air recovery, nothing hazardous will occur ... 

Liquid sub-cooler level control valve shall "fail open" on Instrument air failure. 

Once a set of premises is available for each failure, the list of possible overpressure causes can be narrowed down. A possible cause can be eliminated from the list if it is certain that its relief requirement is lower than or identical to the relief requirement of another source. For instance, when coliunn pressure is controlled by manipulating cooling water to the condenser, failure of the pressure controller may have identical consequences to coolant failure. In this case, failure of the pressure controller can be eliminated from the list. Another example is a column whose heat-input control valve and all feed control valves fail shut, while cooling is likely to continue normally during an instrument air failure in this case, the relief load is likely to be small (if any) upon instrument air failure, and this cause can be eliminated from the list. 

Flow interruption is effected by the level gauge LSHl which activates the solenoid valve VSOLl. This in turn closes the shut-off valve VI (the shut-off valve is fail-safe on instrument air failure with an idealized failure probability of 0). For... 

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. 

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... 

New evaporation systems are designed to be controlled and supervised from modern control centers that assure safe, efficient, operation. The basic design philosophy is that all control systems be designed to fail safe in the event of instrument air failure, power failure, combination of both, or other utility service failure. Further, the control equipment should be designed to prevent release of flammable or toxic materials, and should be designed to permit safe maintenance of Instrumentation. 

There are however other situations that may produce very high contingency due to instrument air failure. In some plants, blowdown contingency... 

Instrument air failure may have a substantial impact on blowdown. The blowdown valves normally fail open (i.e., the valve will open with instrument air failure). In fact, tire blowdown valve opens when the instrument air pressure drops below a particular design pressure (approximately 400 kPaG). In case of instrument air failure, if the instrument air pressure drops below the minimum pressure required to keep the blowdown valves shut, all blowdown valves will open at the same time. This may produce a large contingency and, in some cases, more than the capacity of the plant. This situation can be improved by introducing an instrument air accumulator as shown in Figure 4.11. The instrument air accumulator will allow the blowdown valve to close for a substantial period of time to enable any delay action. With this modification, the total blowdown contingency can be reduced substantially. 

Blockage due to instrument air failure is established in an instrument air failure scenario and need not be considered in a blocked-discharge scenario. 

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