Process contingencies

Type of Estimate Process Contingency Data Available... 

Regardless of the estimating method, a process contingency should be added to the total plant cost for feasibility studies. As discussed earlier, this contingency depends on the status of the project. For most factored estimates, on a first of a kind process that is fairly well defined, the process contingency should be 30%. 

Equipment which may be exposed to vacuum by any single process contingency should be designed for vacuum. In many cases, equipment with a substantial positive design pressure is capable of withstanding full vacuum, but the design should be checked. As an alternative to designing for vacuum, means... 

Exception handling Exception handling is the procedures and/or functions that deal with plant or process contingencies and other events which occur outside the normal or desired behavior of batch control. 

Materials that have a tendency to grow r eadily on the walls of the ciytaUizer require periodic washout, and therefore an other wise continuous operation would be interrupted once or even twice a week for the removal of these deposits. The impact that this contingency may have on the processing-equipment train ahead of the crystalhzer must be considered. 

Basic Yield Data. This is a good place to start asking questions. If the process uses a catalytic reaction, do the yields represent new catalyst or catalyst regenerated a number of times For a thermal reaction like an olefin plant steam cracker, questions might be asked about on-stream time between decokings. Therefore, how much contingency is there in the specified number of crackers required ... 

Normal Individual and 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 electric power failures ... 

Other Utihties - Failure of other uOhties, such as inert gas to seals and pinge systems, or compressed air (when used by the process) may in some cases determine pressure relief requirements. These cases are evaluated on a single contingency failure basis similar to the above. 

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. 

Streams which must be diverted because of emergency shutdown of downstream equipment (e.g., compressor failure). Diversion routes should be provided where such a contingency would otherwise require the immediate shutdown of the affected process unit, resulting in appreciable economic and operational debits. 

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