Emergency conditions

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

Part 3 Building and installation requirements for functional safety of process control systems under emergency conditions (December 1998)... 

One of the most sensitive situations for the TPG string is during emergency conditions. Because of the lower inertia of the three-body... 

Plants, situations, and causes of overpressure tend to be dissimilar enough to discourage preparation of generalized calculation procedures for the rate of discharge. In lieu of a set procedure most of these problems can be solved satisfactorily by conservative simplification and analysis. It should be noted also that, by general assumption, two unrelated emergency conditions will not occur simultaneously. 

Contingency - An abnormal event which is the cause of an emergency condition. 

While the above permitted variations are not used for selecting the pressure setting of pressure relief devices, these variations may be used for piping considerations such as startup, shutdown and emergency conditions, provided that the pressure vessel limits are not exceeded and that such variations are permitted by local codes. 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

In addition to failure as a result of their utihty supply, items of equipment are subject to individual failure through mechanical malfunction. Such items include pumps, fans, compressors, mixers, instruments and control valves. The process upset resulting from such malfunctions (e.g., loss of a reflux pump) may in turn result in emergency conditions and the potential for overpressure. These contingencies should be examined and evaluated. 

We can consider a range of typical plant emergency situations which may result from utility failures, equipment malfunctions, or plant upsets, and which may result in equipment overpressure along with some guidelines for the evaluation of these emergency conditions and determination of reheving rates. 

Emergency Conditions in Integrated Plants - In integrated plants, a process upset in one unit may have an effect on other units (e.g., loss of flow of a pumparound which is used as a source of heat for reboiling other towers). All possibilities such as these must be carefiiUy considered and the potential for resulting overpressure evaluated. 

PR valve capacity should be equal to the compressor capacity at the emergency conditions. For centrifugal compressors, the combination of PR valve set point and relieving capacity should be such as to avoid surge conditions over... 

For critical, high consequence systems, simulators are useful to practice diagnosis and correction of errors and abnormal conditions in emergency conditions (CCPS, 1994a). 

The AP600 passive safety system includes subsystems for safety injection, residual heat removal, containment cooling, and control room habitability under emergency conditions. Several of these aspects are in existing nuclear plants such as accumulators, isolation condensers as natural-circulation closed loop heat removal systems (in early BWRs), automatic depressurization systems (ADS - in BWRs) and spargers (in BWRs). 

Under some circumstances it may not be appropriate to allow air to enter (inbreathe) into the tank, then the use of some other inert gas, such as nitrogen or natural gas, is acceptable on a pressure control basis however, this cannot take the place of a vacuum relieving device to allow air to enter under a Final emergency condition. 

For a flare stack to function properly and to handle the capacity that may be required, the flows under emergency conditions from each of the potential sources must be carefully evaluated. These include, but may not be limited to, pressure relief valves and rupture disks, process blowdown for startup, shutdown, upset conditions, and plant... 

Once the dispersion calculations are completed, the question arises What concentration is considered dangerous Concentrations based on TLV-TWA values, discussed in chapter 2, are overly conservative and are designed for worker exposures, not short-term exposures under emergency conditions. 

Materials that, under emergency conditions, can be lethal Materials that will rapidly or completely vaporize at atmospheric pressure and normal ambient temperature or that are readily dispersed in air and will bum readily. Materials that in themselves are readily capable of detonation or explosive decomposition or explosive reaction at normal temperatures and pressures. 

Materials that, under emergency conditions, can cause significant irritation Materials that must be preheated before ignition can occur. Materials in this degree require considerable preheating, under all ambient temperature conditions, before ignition and combustion can occur. Materials that in themselves are normally stable, but that can become unstable at elevated temperatures and pressures. 

Materials that, under emergency conditions, would offer no hazard beyond that of ordinary combustible materials Materials that will not bum under typical fire conditions, including intrinsically noncombustible materials such as concrete, stone, and sand. Materials that in themselves are normally stable, even under fire conditions. 

Pressure relief valves are provided to cater to two main conditions of the process -normal conditions and emergency conditions. Normal conditions relate to the designed operation of the process which emergency conditions can be caused by either (1) external fire conditions, (2) failure of reflux or cooling, (3) Failure of the power supply, (4) failure of steam supply, (5) heat exchanger failure (6) introduction of incompatible materials, (7) thermal expansion with outlets closed. 

Data adequacy The key study was well designed, conducted, and documented. Exercise takes into consideration some of the stress that humans might experience under emergency conditions. Animal studies addressed both acute and chronic exposure durations as well as neurotoxicity, genotoxicity, carcinogenicity, and cardiac sensitization. In animal studies, concentrations up to 11,000 ppm for up to 6 h did not produce adverse effects. Adjustment of the 11,000-ppm concentration by interspecies and intraspecies uncertainty factors of 3 each, for a total of 10, results in essentially the same concentration (1,100 ppm) as that derived from the human data. 

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