Alternatives to reduce or manage flare relief loads

Greenfield refinery, petrochemical or gas processing complexes development happens over different phases. The flare design starts at quite an early phase when limited information. The flare system capital cost involved and its impact on plot plan can be high. Further unmitigated flare loads are analysed with design software and it may not be desirable to increase the flare height increases beyond a limit, requiring mitigation and management of flare loads.

Several flare load mitigation measures are available to produce a practical flare design. Some of them are described briefly:

1. In a large complex, identify common mode relief loads and mitigate by installing High Integrity Protection Systems (HIPS). Typically, for flare systems design, it is assumed that the trip with the largest load fails to operate. It could also be assumed that the trips with the two largest loads fail to operate.

2. Take credit for the lower LMTD, higher temperature credit for slumping the reboiler at the relieving conditions.

Qr = Qn(LMTDr/LMTDn) Where: Qr - Relieving Duty, Qn - Normal Duty

3. Use of Steam turbine vs motor drive for pumps and compressors: Consider using motors instead of turbines on the fresh feed systems. For example, in an FCC unit, you do not want to continue feeding the reactor with a turbine during a total power outage as the main fractionator will continue to flare with hydrocarbon feed into the unit.

4. Provide one motor driver along with the turbine drive for the FCC main column bottoms and top pump-around systems as a minimum since these pump-arounds have the largest impact on the system

5. Delink Steam system failure from general power failure by considering steam driven drives for pumps, compressors and fans in boiler, cooling water pumps, de-mineralized water pumps, air compressors, induced draft / forced draft system

6. Segregation of power supply: The power supply to the facility at the source itself can be distributed in such a way that power failure does not disrupt the entire complex simultaneously. At any point, one section of the refinery will trip on power failure while the other section can be safely shutdown without additional flare load.

7. Reduce design case flare load in a new plant by re-rating the column pressure vessel above the vapor pressure at the maximum reboiler temperature particularly when thickness of the vessel is determined by other controlling reasons.

8. Apply dynamic simulation to utilize line packing (time to pressurize flare system) and study potential of staggered blow down. Staggered blow down of multiple process units is a generally acceptable practice to limit the cumulative flaring load from various flaring sources to within the flare design capacity. However, the uninterrupted power supply backup of the instrumented protective system (IPS) should be sized for 30 min or longer.

For existing plants, the problem is more difficult.

High integrity automated shutdown systems require high reliability and procedural control particularly for level sensors. The question is whether HIPS system can demonstrate the target reliability specification. Even if the equipment manufacturers can vouch for the reliability, the operators have the complete responsibility of owning such equipment which in case of an accident could become complicated legal issue.

Comment for pros and cons, better ideas......