A small hydrogen stream can be flared safely in an enclosed ground flare, provided the unit has pressure-based start and shutdown logic, flame arresters on both the main and pilot lines, and automatic temperature control. This post uses a skid-mounted ground flare rated at 22 m³/h, quoted to a chemical manufacturer in North America, to show what a hydrogen flare package needs to contain and why.
The challenge: a small flow that still has to be burned safely
The enquiry came from a chemical manufacturer that needed to burn off hydrogen gas at a modest flow of about 22 m³/h. That is far smaller than a typical landfill gas flare, but the safety questions are the same. The flare must light reliably, stay lit, shut down cleanly if the gas supply is interrupted, and stop a flame travelling back up the feed pipe.
Small flows also cause a design problem of their own. The flare has to cope with a range of flows, not just the maximum. A flare that burns well at full load but cannot hold stable combustion at a tenth of that flow is not much use on a process that rises and falls.
The approach: a skid-mounted ground flare
The quoted package was an enclosed ground flare on a single galvanised skid, with the stack supplied separately and lifted into position on site. Building it as a skid means the unit can be assembled and tested at the factory before delivery, which gives a tidier installation. Total weight was around 1.5 tonnes, on a skid roughly 1 m wide and 2 m long.
Pressure switches that decide when the flare runs
Three pressure switches on the inlet line do most of the supervisory work:
- Low pressure: closes the isolation valve and shuts the burner down if a valve has been closed by mistake or water has built up in the inlet pipework.
- High pressure: gives the start signal. When gas is present at pressure, the pilot gas valve opens and the ignition sequence begins.
- High-high pressure: indicates excess flow reaching the burner, shuts the flare down and raises an alarm.
In other words, the flare lights only when there is gas to burn, and it protects itself at both ends of the pressure range.
Flame arresters on both lines
Both the main inlet and the pilot line have a flame arrester with a stainless steel element. Each arrester carries a temperature sensor and a latching switch. If a flame ever burns on the arrester element, the switch latches and the arrester must be replaced. A spare arrester for each line is part of the supply, so the flare does not have to stay out of service while a replacement is ordered.
Temperature control and turndown
The flare is designed for a maximum combustion chamber temperature of 1,000°C. A PID controller reads the flame temperature and moves a combustion-air louvre through a motorised actuator. The secondary air damper is set mechanically at start-up and then hands over to PID control to hold the right temperature.
The thermal turndown is 10:1, so the minimum flow is roughly 2 m³/h. The stack, from ground level to the top of the shroud, is 6 m high.
Ignition and flame detection
Ignition is automatic and sequenced by a PLC. It is not simply a spark: the start sequence involves the igniters, a pilot line with its own solenoid valve, flame arrester and gate valve, a UV sensor to detect the flame, and the secondary air damper. A slam-shut valve controls the flow of gas to the burner, and the UV sensor is self-checking. Flame temperature is read with an Inconel thermocouple.
The control panel is designed to sit outside any designated hazardous area (the quote specifies an ExD enclosure for the relevant items). There is also a vent-only mode, intended for commissioning and test purposes only.
Why it works
The design relies on layers rather than a single safeguard. Pressure switches decide when to light and when to stop. Flame arresters stop a flashback. Temperature control keeps the flame in its working range. A slam-shut valve provides positive isolation. The skid and lifting arrangements are practical: four corner lifting eyes on the skid, and lifting points on the stack that let crane hooks fall free once the jib is lowered.
The stack design also had to meet structural wind-loading codes, fixings that comply with the relevant British Standards, and acceptable radiation levels at ground level.
Optional flow measurement
Two options were offered for measuring the hydrogen flow. An orifice plate with a local manometer gives a simple local reading. A remote option uses an intrinsically safe circuit to send a 4 to 20 mA signal from a differential pressure switch to a digital readout in a remote panel.
What to consider for your own project
- Define the flow range, not just the maximum. With a 10:1 turndown, a 22 m³/h flare can still burn about 2 m³/h at the bottom of its range.
- Decide how the flare is told to start. Here the start signal comes from line pressure. Other sites might use a different trigger.
- Do not skip the flashback protection. Flame arresters on both main and pilot lines, with a way of knowing when one has been used, are worth having.
- Plan for crane access. The skid and stack arrive separately and need a suitable crane and a telescopic unit for the stack.
- Check the hazardous area classification with the safety team at your site. Guidance on classifying areas is available from the Health and Safety Executive (DSEAR).
FAQ
Can a ground flare burn hydrogen?
Yes. This quotation covered an enclosed ground flare designed specifically for a hydrogen stream, with the pressure switches, arresters and controls described above.
What is turndown?
Turndown is the ratio between the maximum and minimum flow a flare can burn properly. A 10:1 turndown on a 22 m³/h flare means a minimum of roughly 2 m³/h.
Why does the flare need a pilot line?
The pilot burner provides a small, controlled flame to light the main burner. The main gas valve does not open until the pilot is established.
Can the flare be commissioned without burning gas?
The unit has a vent-only mode, intended for commissioning and test purposes only.
Need to flare a small process gas stream?
Tell us about your gas, flow range and site and we will advise on a suitable flare package. For larger flows, see our worked example on ground flare versus elevated flare.