Flare-gas and vent-gas recovery can capture selected gas streams for reuse or treatment when they cannot be consumed directly. During normal production, these streams may originate from control valves, vessel vents, compressor seals, storage systems, process purges, pressure-control operations and equipment depressurization.
Traditionally, many of these gases are collected in a flare header and burned. A flare-gas recovery system intercepts suitable gas before it reaches the flare and redirects it to a fuel-gas header, process unit, gas-treatment system or storage facility.
A gas-tight roots blower can be used as the gas-moving machine where the recovery duty requires high volumetric flow at a relatively low or moderate pressure differential. The machine is not installed to provide combustion air to the flare. Its purpose is to withdraw recoverable gas from the flare or vent header and boost it to the pressure required by the selected recovery destination.
What Is Flare-Gas Recovery?
A flare-gas recovery system reduces routine flaring by capturing gas that would otherwise be destroyed. The recovered stream may be:
- Returned to the plant fuel-gas system
- Recycled as process feed
- Sent to a gas-treatment unit
- Delivered to a vapour-recovery system
- Stored in a gas holder
- Used by a boiler or heater
- Sent to another compression stage
Recovery can reduce hydrocarbon loss, lower routine flare emissions and recover part of the energy or feedstock value contained in the gas.
The flare remains an essential safety system. Emergency relief, major process upset or recovery-equipment failure may produce gas flow far beyond the capacity of the recovery blower. The system must automatically allow excess gas to continue toward the flare without obstructing the plant relief network.
Flare Gas and Vent Gas Are Not Fixed-Composition Fuels
Flare-header gas can change continuously. Its composition depends on which process units are venting and on the operating condition of those units.
A flare or vent stream may contain:
- Methane
- Hydrogen
- Ethane, propane and heavier hydrocarbons
- Olefins
- Hydrogen sulphide
- Carbon dioxide
- Carbon monoxide
- Nitrogen
- Water vapour
- Solvent vapours
- Inert purge gas
- Fine particles
- Entrained liquid droplets
The molecular weight, heating value, density, dew point, corrosiveness and flammability of the gas may therefore vary considerably.
A roots blower should not be selected from one nominal gas composition if the actual system can experience substantially lighter, heavier, wetter or more corrosive gas mixtures.
Suitable Operating Range for a Roots Blower
Roots blowers are positive-displacement machines. They transfer an approximately fixed volume per revolution, making them suitable for duties in which the gas flow must remain stable despite moderate changes in header or downstream pressure.
They are most applicable when the flare-gas recovery system requires:
- Low or moderate differential pressure
- High volumetric flow
- Oil-free gas handling
- Rapid response to header-pressure changes
- Variable-speed capacity control
- Continuous operation
- Gas-tight construction
- Tolerance of changing gas composition within defined limits
A roots blower may be unsuitable when the destination requires a high discharge pressure or when the compression ratio would cause excessive discharge temperature. In such cases, screw compressors, liquid-ring compressors, reciprocating compressors or staged compression may be more appropriate.
Flare-gas recovery technology must be selected from the process duty rather than from the assumption that all flare gases require the same machine.
Typical System Arrangement
A flare-gas recovery system generally includes:
- Flare or vent collection header
- Suction knock-out vessel
- Demister or entrainment separator
- Roots process gas blower
- Gas cooler where required
- Discharge separator
- Non-return valve
- Isolation valves
- Pressure-control system
- Recycle or bypass line
- Gas-detection and shutdown instruments
- Connection to the recovery destination
- Automatic route to the flare
The recovery connection is arranged so that the blower withdraws gas without compromising the pressure-relief function of the flare header.
Liquid seals, molecular seals, check valves, flame arresters and other isolation devices may also be present, depending on the flare design and process risk assessment.
Flare-Header Pressure Control
One of the most important control objectives is maintaining the flare header within a safe pressure range.
If suction is insufficient, recoverable gas continues toward the flare. If the blower produces excessive suction, header pressure can fall too low and allow atmospheric air to enter through seals, drains, flare tips or other openings.
Air ingress into a hydrocarbon gas system can form a flammable or explosive mixture. The blower should therefore be controlled to maintain the specified positive or minimum safe header pressure rather than simply operate at maximum speed.
A simplified control relationship is:
Controlled Flare-Header Pressure
- Pheader,set = controlled flare-header pressure
- Psafe,min = minimum permissible header pressure
- Pcontrol_margin = allowance for measurement and control stability
The actual setpoint must be determined by the flare and process-system design.
Capacity Control
Variable-speed control is particularly useful because flare-gas flow may vary widely during normal operation. The blower speed can be adjusted in response to flare-header pressure.
A typical control philosophy may include:
- Speed increase when header pressure rises
- Speed reduction when header pressure approaches the lower control limit
- Automatic recycle at minimum blower speed
- Startup of an additional blower during high gas flow
- Shutdown of excess machines during low demand
- Automatic bypass of surplus gas to the flare
- Immediate isolation or trip during unsafe conditions
Multiple smaller blower units can provide better turndown and redundancy than one large machine. A lead-lag arrangement allows additional capacity to start when the normal gas load exceeds the operating range of the lead blower.
roots blowers do not experience aerodynamic surge in the same manner as centrifugal compressors. They must nevertheless be protected against excessive pressure ratio, temperature, torque and recycle heating.
Normal Recovery Versus Emergency Flaring
A flare-gas recovery blower is normally designed to capture continuous or predictable base-load gas. It should not be assumed that the blower can absorb the full emergency relief capacity of the flare network.
During a major plant upset:
Gas flow into the flare header can increase rapidly.
Header pressure rises above the normal recovery-control range.
The recovery system continues operating if conditions permit.
Gas exceeding recovery capacity passes through the flare route.
The flare safely burns the excess gas.
The recovery equipment, control valves and check valves must not restrict the emergency path.
A flare-gas recovery system supplements the flare; it does not eliminate the flare’s safety function.
Glossary
Emergency bypass: Flow path used to route gas safely when recovery equipment is unavailable.
Flare header: Pipe network that collects relief and vent streams for safe disposal.
Flare-gas recovery rate: Gas flow captured for reuse or treatment during a defined operating case.
Flare-header pressure: Pressure in the common pipe carrying gas toward the flare system.
Header pressure control: Control strategy that maintains the required pressure in a shared gas header.
Knock-out drum: Vessel that separates liquid droplets from a gas stream.
Recovery suction pressure: Absolute pressure at the gas-recovery machine inlet during a stated operating case.
Vent gas: Gas discharged from process equipment through a vent system.