Abstract
Flare system in the oil and gas industry is an essential requirement for the continuous safe operation of any processing plant. Flares burn off the flammable gases released from the different pressure sources across the plant via a pressure relief valve returning the system back to operate within its safe pressure limits. In cases where the required flare stack or flare knock-out drum required size and height are big, it can be split into two flare systems operating in parallel. This method of flare design is not common.
In one of the fields, parallel flare design is implemented to operate in 2×50% configuration. Each flare system had its dedicated flare knock out drum. Nitrogen purging only is used as a method to prevent air ingress into the flare. The current configuration has the flares operating at the same time whenever there's any relief to the system. The dependency is on the symmetrical piping to distribute the flow equally to the two flare systems. Here, the positive flow towards both flare systems continuously prevents any air to enter. However, the change in the pressure drop across the flare systems during the relief results in the preferential flow of the relief towards one flare system over the other. This preferential flow results in a vacuum effect occurring through the other flare system. The vacuum effect results in air being sucked in from the flare stack causing deflagration to occur and in some cases detonation.
There are different sealing methods for the flare system and the most preferred method is using liquid seal drum downstream of the flare knock out drum. When designing the flare system, one of the parallel flares should be considered the preferred route for relieving with little or no liquid seal in the liquid seal drum and it's called the primary flare. The other one with a liquid seal height corresponding to the maximum emergency relief is called secondary flare and will only operate simultaneously at any point with the primary flare. The purpose of having a liquid seal drum filled with liquid in the secondary flare is to avoid air ingress. In normal scenarios when relief is channelized through the primary flare preferably, it may induce a suction effect drawing air through the secondary flare which may cause deflagration to occur.
This paper will emphasize challenges of designing flare systems in parallel configuration and the different aspects and considerations of such design.
