An open flare is the simpler, lower-cost choice and is often enough when it acts as back-up to gas engines. An enclosed flare adds a higher, controlled temperature and a hidden flame, at a higher price. A quotation for a landfill energy site in southern Europe priced both, and shows how the decision looks in practice.
The challenge: a flare to protect an engine installation
The site generates electricity from landfill gas. When the engines stop, the gas has to go somewhere. The flare is what the operator relies on in that moment. The requirement was for an 800 Nm³/h skid-mounted package: knockout pot, gas booster and flare, with a booster able to deliver a head of about 300 mbar g.
The approach: a standard skid, with an enclosed alternative priced beside it
Design data
- Flow: 160 to 800 Nm³/h (5:1 turndown).
- Thermal rating: about 4 MW at 50% methane.
- Methane range: 25 to 55%, with up to 5% oxygen.
- Blower: 300 mbar g maximum head, with 30 kW EExD motor and speed control, supplying the flare at up to 50 mbar g.
- Materials: galvanised carbon steel for pipework and skid.
What the package includes
- A galvanised knockout pot with a polypropylene dewatering filter and a 2-inch drain.
- A gas booster with a 30 kW explosion-protected motor driven through an inverter, controlled by a pressure transducer on the outlet.
- A 5-micron dry gas filter, with two spares, and a gas outlet that can feed the engines.
- An electro-hydraulic slam-shut valve on the flare feed, which opens when the engines shut down so that the flare lights.
- An open flare stack of about 5 m with a 321 stainless steel tip and shroud, and manual temperature adjustment to 850°C.
- UV flame detection and automatic electric ignition under PLC control.
- An operator choice between continue running on flame-out, shut down on flame-out, or vent only.
- An orifice plate flowmeter and a gas sample point.
The equipment was priced to meet ATEX requirements and the industry codes of practice for area classification at landfill gas sites.
Open versus enclosed
| Open flare (AC range) | Enclosed flare (SC range) | |
|---|---|---|
| Combustion temperature | Manually set, around 850°C | Automatic control, 1,000°C or above |
| Flame | Visible at the stack tip | Hidden in the shroud |
| Control of air | Manual | Automatic louvre |
| Typical use | Back-up, remote or lighter requirements | Where permit or emissions demand it |
| Cost | Lower | Substantially higher |
The quotation priced the enclosed alternative as an extra, and offered a full specification for it on request. The size of the premium is the reason open flares remain common on back-up duty.
Why the open flare can be enough
When a flare operates only during engine downtime, its operating hours are limited. If the permit allows an open flare for back-up, the saving can be put into better gas treatment or monitoring. But regulators in the UK expect enclosed flares to reach at least 1,000°C, as described in LFTGN05, so confirm the requirement before choosing. See also LFTGN03.
What to consider for your own project
- Check the permit first. It may settle the choice.
- Decide the flame-out behaviour. The three options (continue, shut down, vent) each suit a different risk profile.
- Add gas analysis if needed. Methane and oxygen analysers were offered as an option, with 4 to 20 mA outputs.
- Plan the commissioning day. The commissioning allowance assumed a fused mains supply next to the flare and a live gas main.
- Allow lead time. Delivery was quoted at 14 to 16 weeks.
FAQ
Why is the engine shutdown signal connected to the flare?
So the flare valve opens and the flare ignites as the engines stop, avoiding gas venting.
What temperature does an open flare reach?
This design could be adjusted manually to around 850°C.
What is the purpose of the inverter on the booster?
It holds outlet pressure constant as gas flow changes.
Back-up flare for an engine site?
Tell us about your engines and permit and we will recommend a flare type.