(15d) Safe Facility Depressurization By Blowdown Sequencing Logic Based on Pressure Measurement

An emergency depressuring is a safeguarding measure to protect pressure vessels from failure due to high thermal stresses resulting from a fire event. During a facility blowdown, the plant is isolated into number of plant areas by emergency shut-down valves and the equipment are depressurized to flare. In general, the flare system may not be designed to accommodate simultaneous relief from all blowdown valves (BDVs) for capital cost savings reasons, which is allowed according to API 521. In those cases, the flare system is required to have adequate safeguard in place to prevent an operator from accidentally opening too many BDVs and overwhelming the flare design capacity. Therefore, the industry uses a staggered blowdown approach in which BDVs are staggered to open in a sequence with a time interval in between them; thus, all BDVs are not allowed to open simultaneously. However, as each of the BDVs might produce different depressuring loads, determining the time intervals between BDVs for staggering while effectively utilizing the available flare capacity to minimize the overall duration it takes to depressure the facility can become a very complex exercise when a facility has many BDVs of different rated capacity. This work describes a different approach for staggering of the blowdown valves in which the logic for sequencing of BDVs’ opening is programmed based on a common pressure measurement in the flare header. This approach eliminates the need for tailoring time intervals for blowdown of each BDV or multiple BDVs and allows effective utilization of flare capacity to optimize overall duration for facility wide blowdown. This pressure based sequencing of BDVs is demonstrated by modeling the flare system of a large LNG facility using Dynamic simulation.