Vapor Combustion Unit

A vapour combustion unit (VCU) is an enclosed combustion device that destroys hydrocarbon vapours and low calorific gas streams from storage tanks, loading operations and vapour recovery systems. It is used where intermittent vapour flows must be disposed of without a visible flame or noise, which makes it suitable for terminals, distribution facilities and sites near populated areas. Typical duty covers tank breathing and working losses, truck, railcar and marine loading vapours, VRU tail gas and backup destruction when a VRU is offline. CRA builds VCUs as skid-mounted packages with the combustion hardware, ignition, safety devices and controls integrated.

A VCU and an enclosed ground flare are structurally similar and differ mainly in the duty they are designed for. A VCU handles intermittent, low-pressure, low calorific vapour streams from loading and tank breathing, is usually smaller, uses assist fuel to hold combustion temperature and is often skid-mounted. An enclosed ground flare handles continuous or larger relief flows from process units and biogas plants, uses staged multi-burner combustion and is normally a standalone installation. Both are designed for destruction efficiency above 98% on hydrocarbons. CRA supplies both and selects between them on duty cycle, flow profile and calorific value.

A properly designed VCU achieves destruction efficiency above 98%, and often above 99%, on tank loading and breathing vapours, and is designed for the requirements of US Coast Guard 33 CFR 154 and EPA 40 CFR 63 Subpart Y where marine loading applies. Performance depends on adequate residence time at combustion temperature, a stable combustion temperature held by assist fuel modulation, a multi-burner arrangement that stays stable across the vapour flow range, and a chamber sized for peak loading rates without flame quenching. For terminals handling several products, CRA sizes the VCU to the worst-case calorific value and flow profile.

A VCU includes a detonation arrestor or flame arrestor on the inlet vapour line, an automated vapour isolation valve interlocked with flame detection, continuous pilot monitoring with automatic relight, high-temperature shutdown and overpressure protection. A purge gas system prevents air ingress during low-flow periods and LEL monitoring on the vapour inlet watches the mixture entering the unit. The emergency shutdown logic is designed to the SIL level the site specifies. CRA designs the safety arrangement in line with API 2000, API 2521 and NFPA practice, and lays out ignition source separation and exclusion zones to the local fire and electrical codes for the hazardous area.

A VCU normally acts as the destruction stage for VRU tail gas and as full backup when the VRU is offline. The VRU recovers most of the vapour as condensable product and the VCU destroys the lean vapour that leaves it. In bypass mode the VCU takes the full vapour flow while the VRU is down for maintenance or upset, and in peak shaving mode it handles spikes that exceed VRU capacity during heavy loading. Where a VRU is uneconomical the VCU runs as the primary destruction device. Control logic switches between modes automatically on flow, pressure or VRU status, and CRA integrates this logic in the VCU control panel.