An open flare for 1,500 m³/h of landfill gas succeeds or fails on three details: the material of the burner tip, how the flame is detected and how the operator can control the unit. This article looks at a standard skid-mounted open flare package quoted for a client in South-East Asia, and explains why each choice was made.
The challenge: a flare that must last and fail safe
An open flare sits in the weather and the heat, usually for years, with little attention. The design has to survive 850°C or more on a continuous basis and shut off gas immediately if the flame goes out.
The approach: the package
Design data
- Flow: 300 to 1,500 Nm³/h (5:1 turndown).
- Thermal rating: about 4 MW at 50% methane.
- Methane range: 25 to 55%, with oxygen up to 5%.
- Blower: 300 mbar g maximum head, one blower, supplying the flare at up to 50 mbar g.
- Connection: DN125 flange.
Gas side
Gas enters through a butterfly valve and a galvanised carbon steel knockout pot with a polypropylene demister, which removes 99% of droplets above 6 microns with less than 3 mbar pressure drop. A level switch warns of condensate build-up. A centrifugal blower with a cast aluminium impeller on a stainless steel shaft, a carbon gas seal and a V-belt drive to an explosion-protected motor follows. A direct-drive version is an extra. A stainless-steel-element flame arrester to EN 12874 and a low-pressure switch complete the line.
Why these choices
310 stainless steel tip
The burner head is made from 310 stainless steel, which is suited to continuous service above 850°C. The quotation states a minimum design life of 20 years for the tip and shroud. A cheaper grade might save money at the start and cost more on replacement.
Tip height for ground-level radiation
The height of the tip is chosen from the amount of gas being burned, so that heat radiation at ground level stays below an acceptable level. A larger flare therefore needs a taller stack, and this should be checked against site boundaries.
UV flame detection
A UV sensor reacts at once to loss of flame. A thermocouple, which has to cool or heat before it responds, lags behind. The sensor in this design is self-checking, insensitive to sunlight and housed in an IP55 enclosure. Detection triggers the main gas valve, a fail-safe electro-hydraulic slam-shut valve that closes on power loss or alarm.
Manual air-fuel adjustment
Flame temperature is set by manual adjustment of the air:fuel mix at the base of the stack, to about 850°C. It does not track gas quality automatically, which is the main difference from an enclosed flare, but it avoids the cost and complexity of actuators.
Operator modes
A selector gives the operator three modes: vent, burn or continuous run, plus auto or manual. Panel lamps show power, ignition, flame established, continuous run on flame-out and low pressure at the flare inlet. A thermistor in the blower motor detects overheating.
What to consider for your own project
- Ask for the tip material and design life. It affects whole-life cost.
- Insist on UV flame detection. Response time matters for safe shutdown.
- Check radiation. Stack height should be set against boundaries and personnel areas.
- Understand the modes. Decide in advance how the flare should behave on flame-out.
- Know what a cheap open flare will not do. It will not give the temperature control of an enclosed unit, so check permit requirements. See the Environment Agency’s LFTGN05.
FAQ
How long does an open flare tip last?
The 310 stainless steel tip and shroud in this design were specified for a minimum of 20 years.
Why not use a thermocouple to detect the flame?
It responds slowly. A UV sensor detects flame loss immediately.
Is an open flare noisy?
The specification left the sound level to be confirmed.
Specifying an open flare?
Send us your gas flow and site constraints and we will outline a package.