Every flare system starts from one number: how much fluid, of what composition, at what temperature, in the worst credible case. Get that number wrong and everything downstream is wrong with it — header diameters, tip size, stack height, the radiation footprint on the plot plan.
Techso determines relief loads scenario by scenario to API 521, identifies the governing case, and sizes the collection network that has to carry it — including the back pressure that decides which type of relief valve can be used upstream.
Scenarios we evaluate
Each protected item is examined against the credible causes of overpressure, and each is quantified independently before any combination is considered.
- External pool fire — wetted area, environmental factor, vapour generation rate
- Blocked outlet and inadvertent valve closure
- Control valve failure, in the open and closed position
- Power failure and utility failure, including instrument air
- Loss of cooling, condenser failure and reflux failure
- Heat exchanger tube rupture, with the high-pressure side governing
- Thermal expansion of blocked-in liquid lines
- Runaway reaction and abnormal heat input, where applicable
Identifying the governing case
The design load is not the sum of every scenario. It comes from a common-cause analysis: which releases can physically occur at the same time, and which are independent events that will not coincide. A system sized on the arithmetic sum is oversized, expensive and often harder to keep stable at low flow; one sized on a single item is unsafe.
- Global scenarios — fire zone, power failure, cooling water failure
- Local scenarios affecting a single item
- Fire zone definition and simultaneous relief within the same zone
- Relief load summary table, traceable back to each scenario
Sizing the collection network
- Header and sub-header hydraulics for the governing case
- Built-up and superimposed back pressure at each relief device
- Consequences for valve selection: conventional, balanced bellows or pilot operated
- Velocity and Mach number limits, noise and vibration in the header
- Two-phase flow, liquid carry-over and slope towards the knock-out drum
- Low-temperature effects from auto-refrigeration on material selection
Depressuring
Where emergency depressuring is required, the blowdown rate is set by the need to bring the equipment below a target pressure within a target time, while the vessel wall is losing strength in the fire. The two problems are coupled and we solve them together.
- Blowdown rate and restriction orifice sizing
- Transient vessel wall temperature during fire exposure
- Contribution of depressuring flows to the total flare load
What you receive
- Relief load summary with a datasheet per scenario
- Hydraulic model of the flare network and back pressure results
- Relief device sizing check to API 520
- Line list and material selection input
- The design case handed over to tip and stack sizing