Historically the maximum combined relief loads during a common mode failure has been the sizing basis for the flare header, stack and tip together with all associated seal and knockout drums. As plant capacities have grown, it has become necessary to examine ways of reducing expenditure on these systems whilst maintaining the desired level of safety protection for personnel and investment. Furthermore, existing plants have been debottle-necked to produce far higher production rates than the original design. This has led to detailed review of flare system capacities to determine whether they can handle the greater loads with or without plant modifications.

The design of the pressure relief system of a large petrochemical complex can be optimized using a number of sophisticated analysis techniques. The application of these techniques has two principal objectives:

1. To reduce the maximum flare load by means of process modifications and shutdown systems.
2. To estimate the resultant maximum load with a great degree of certainty.

REDUCTION OF MAXIMUM FLARE LOAD

High integrity shutdown systems can be included on major relieving sources such as:

1. Cracking Furnaces
2. Catalytic Cracking Systems
3. Process/Refrigerant Compressors
4. Slug Catchers
5. Oil/Gas/Water Separators
6. Fractionation Towers

Trident Consultants has the capability to perform such studies on the reliability and availability of such shutdown systems. These studies use fault tree techniques with failure data at the component level (sensor, transmitter, actuator, etc.). Other features include human error rate, testing and maintenance frequency. The results of such analysis can be used to compare the likely reduction in maximum loads for various shutdown systems and philosophies.

Reassurance can be given to operating companies and the authorities that the shutdown systems can provide at least equal and preferably higher and availability than conventional safety valves.