A vapour flare for a chemical storage tank works best when the tank’s own pressure is enough to drive the gas to the burner. In a proposal for a 25 m³/h butadiene vapour flare at a UK chemical site, the unit was designed to operate on a gauge pressure of roughly 5 to 30 mbar, with no booster and no back-pressure regulator. The design relied instead on three pressure switches, a slam-shut valve and a flame arrester to keep it safe. This post explains why that arrangement was chosen and what it leaves for the buyer to check.

The challenge: a modest flow, with a hazardous vapour

The client was a chemical manufacturer that needed to flare butadiene vapour from a storage tank. The flow was small, 25 m³/h, and the vapour arrived at low pressure from the tank. The proposal went through two revisions, the later one adding a revised scope and a process and instrument diagram (P&ID) so the client could review exactly what was being supplied.

For a small flow of hazardous vapour the priorities are straightforward. The flare must light only when vapour is available, it must shut itself down if the supply fails or surges, and it must not allow a flame to travel back towards the tank.

The approach: three pressure switches and a slam-shut valve

The proposal used an AC Range ground flare on an integral galvanised skid. The control philosophy rests on pressure switches placed either side of the main slam-shut valve:

• High-pressure switch before the slam-shut valve: detects that gas is available to flare. If pressure is low, the main valve will not open.
• High-pressure switch after the valve: detects excessive flow and triggers a control action.
• Low-pressure switch after the valve: detects a low-flow situation and also triggers a control action.

A manual trim valve allows the flow to be adjusted. A flame arrester with a stainless steel element sits on the feed line after the slam-shut valve, and the slam-shut valve itself controls the flow to the burner.

Ignition and detection

Ignition is automatic at start-up. A PLC runs the sequence, which can be adjusted to suit local requirements. The start sequence involves the igniters, a transformer and a UV sensor to detect the flame, so it is more than a spark. All pipework is 304 stainless steel, and the outdoor control panel is protected to IP65 against water and dust.

The design limits

Two exclusions say a lot about how the unit is meant to be used:

• No back-pressure regulator. The design assumes that pressure from the storage tank will not be excessive.
• No boost-pressure blower. The burner can operate from roughly 5 to 30 mbar gauge, so a blower should not be needed if the tank pressure stays in that window.

That is a clear statement of where the unit’s responsibility ends. If the tank pressure can rise above the burner’s working range, the buyer needs to add a regulator. If it can fall below, they need a blower.

Why it works

Keeping the package simple removes components that could fail. Without a blower there is no rotating equipment to maintain, and the three pressure switches give the controller everything it needs to decide when to light, when to hold and when to stop. The trade-off is that the system depends on stable upstream pressure, which is why it is worth defining that pressure carefully at the enquiry stage.

Installation, commissioning and operator training were not part of the equipment price and were priced separately, which is common for packages like this. The supply included three copies of the instruction manual and a 12-month warranty, and the warranty could be invalidated if someone other than an approved engineer carried out commissioning.

What to consider for your own project

1. Know the pressure window. Measure the range of pressures your tank or vent line can produce, including during filling, before choosing a burner.
2. Decide whether you need a regulator or blower. If pressure can go outside the burner’s range, specify them.
3. Ask for a P&ID early. It is the quickest way to see what is and is not in the supply.
4. Clarify who commissions. If the warranty depends on approved commissioning, it affects who you can use.
5. Check the safety assessment. The HSE’s DSEAR guidance covers dangerous substances and explosive atmospheres.

FAQ

Does a small vapour flare need a blower?

Not if the vapour arrives at a pressure within the burner’s range. This unit worked from about 5 to 30 mbar gauge.

What stops a flame travelling back to the tank?

A flame arrester with a stainless steel element on the feed line, together with the slam-shut valve.

How does the flare know when to light?

A pressure switch ahead of the slam-shut valve detects that gas is available. If pressure is low, the valve will not open.

Can the control sequence be changed?

The ignition sequence is run by a PLC and can be adjusted to suit local requirements.

Need to flare vapour from a tank or process vent?

Tell us the vapour, flow range and pressure window and we will check whether a simple unit like this will do. For a related small-flow example, see flaring hydrogen with a small ground flare.