Issues regarding sudden power outages across multiple units and the use of flare systems

We also have just one flare line; it depends on the amount of gas to be flared. We need to check the design specifications to see if it’s possible to flare all the devices simultaneously. May I ask whether your flares are ignited automatically or require manual ignition?

The general principle for the basic data used in the design calculations for torches is to take the maximum emission volume of one set of units in the entire plant under a full plant power outage condition, plus one-third of the emission volumes of the other units, as the design condition for the calculations. Just look at the basis for your torch design and you’ll understand.

I think if there is a sequence, my principle of experience is to prioritize high pressure first； Safety hazards come first and foremost

For the purpose of ensuring device safety, design specifications are established; under normal conditions these specifications are met. However, in the event of a power outage across the entire plant, pressure buildup will occur, and measures must be put in place to address this issue.

The design of the plant’s flare emissions is based on the maximum emission levels under single-incident conditions, including situations such as water or power outages. Therefore, the issue mentioned by the poster should have been taken into account during the flare design, so no problems should arise.

A sudden shutdown of the entire plant, accompanied by flare emission, was taken into consideration during the design phase; however, in actual operation, there are cases where the flares cannot be discharged. This is mainly due to the inconsistent pressure levels of various devices, which causes backflow. In such situations, our company’s contingency plan generally involves starting with high pressure and then moving to low pressure; devices that cannot maintain pressure are released first, followed by those that can do so. We try to minimize the use of flare systems, or avoid using them altogether, in order to prevent cross-contamination that could occur if flares are used simultaneously, thereby avoiding secondary hazards.

Why are there two lines here: a high-pressure flare line and a low-pressure flare line?

The design principles for flares do not allow for the addition of the emissions from all devices together; otherwise, the flare pipelines would have to be very large. While this would ensure safety, it would also result in high costs. In general, when designing flares, the actual conditions of the enterprise are taken into account, and design principles are established to ensure the safe operation of the devices. Furthermore, the flare system serves as a safety measure and safeguard for the facility; if overpressure occurs in the facility, gas is vented through devices such as safety valves, so it is not possible to vent the gas in accordance with the sequence of the facilities~~~

Firstly, the flare system is designed based on the emission volume of each unit, so pipelines should not be a problem. There are high-altitude flares and ground flares, which burn off the gases through their respective diversion systems; thus, as gases are burned, new gases continue to be introduced, so there is no issue of pressure buildup in any particular unit. If pressure relief is necessary, it will definitely be carried out according to the actual conditions of each unit, and it’s impossible for the pressure to be relieved too quickly. However, as the original poster mentioned, when several units lose power at the same time, the rate of pressure relief becomes quite high, which can cause liquid to accumulate in the flare water seal tanks. This is especially true in units involved in coking, hydrogenation, and hydrogen production, where excessive emissions can lead to liquid accumulation and result in a rain of liquid falling from the flares, which is extremely dangerous. We have not yet encountered situations where several devices lose power at the same time, so it’s difficult to say anything definite about such cases. However, in an emergency, unified command is essential; otherwise, rapid pressure release could trigger other accidents, which would be counterproductive. Another factor affecting the pressure relief flow rate is the length of the flare pipeline; it is important to ensure that there is not too much liquid stored within such pipelines, especially in long flare pipelines, in order to prevent the simultaneous pressure release from the various devices mentioned, which could lead to water hammer effects in the flare pipeline and increase the risk. So everyone must remember that the torch-bearing role is also very important.

The flare line of the unit is a lifeline; under normal circumstances, the design will certainly meet the requirements, as long as the valves are not restricted in their operation and there is no pressure buildup in the condensate tank.