Cooling water failure

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 cooling water failures ... 

Figure 5.108. A cooling water failure caused the disc to burst, as shown by the pressure profile.

Figure 1-7 presents the causes of losses for the largest chemical accidents. By far the largest cause of loss in a chemical plant is due to mechanical failure. Failures of this type are usually due to a problem with maintenance. Pumps, valves, and control equipment will fail if not properly maintained. The second largest cause is operator error. For example, valves are not opened or closed in the proper sequence or reactants are not charged to a reactor in the correct order. Process upsets caused by, for example, power or cooling water failures account for 11 % of the losses. 

Determine the consequence frequency for a cooling water failure if the system is designed with two IPLs. The IPLs are human interaction with 10-min response time and a basic process control system (BPCS). 

The frequency of a cooling water failure is taken from Table 11-3, that is, / = 1CT1. The PFDs are estimated from Tables 11-4 and 11-5. The human response PFD is 10"1 and the PFD for the BPCS is lO-1. The consequence frequency is found using Equation 11-30 ... 

Cooling water failure The loss of cooling water is one of the more commonly encountered causes of overpressurization. Different scenarios should be considered for this event, depending on whether the failure affects a single piece of equipment (or process unit) or is plantwide. 

TEMPERATURE HIGH Ambient Conditions Fouled or Failed Exchanger Tubes Fire Situation Cooling Water Failure Defective Control Valve Heater Control Failure Internal Fires Reaction Control Failures Heating Medium Leak into Process Faulty Instrumentation and Control... 

The flare header, which collects the vapors from the safety valves for safe discharge to the knockout drum and the flare stack, is sized for the largest vapor load caused by a single failure. This vapor load is obtained from a tabulation of relief loads from safety valves connected to the flare system. The loads which may occur simultaneously as a result of fire, cooling water failure, etc., are summed up. From these summations the largest load is determined. 

The word "unrelated above is key. API s RP521 (10) defines causes to be unrelated if "no process, mechanical or electrical linkages exist among them, or if the length of time that elapses between possible successive occurrences of these causes is sufficient to make their classification unrelated. When one failure causes another, the events are related. For instance, if reboiler heat is controlled by a differential pressure controller, a cooling water failm-e will cause the steam controller to open fully. In this case, cooling water failure and failure of... 

The many examples given in this book have illustrated that dynamic simulations of distillation columns can be used to develop effective control structures for a wide variety of individual columns and multiple-column systems. However, there is another use of dynamic simulations that is very important for the safe operation when process and equipment emergencies occur. The most common example is a cooling water failure, which can lead to very rapid increases in column pressure. 

Adequate backup features are required to minimize the impact of cooling water failure. The following are minimum requirements that should be considered in the system design ... 

Adequate instrumentation and alarm systems are required to reduce the impact of cooling water failure. For example, a low-pressure alarm on the makeup cooling water header is helpful to identify low makeup water rate. 

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