A purge cycle makes a burner run its air blower for a set time after every shutdown or failed ignition, so that no gas is left in the combustion chamber at the next start. In a proposal for a landfill ammonia stripping plant, that meant a two-minute air purge of the thermal destructor unit (TDU), added to the control program on four plant lines. A restart cannot happen until the purge has finished.
The challenge: a restart on top of leftover gas
The plant treats leachate from a landfill. Its thermal destructor burns landfill gas, and the stripping air blower is one of the last units to stop in a normal controlled shutdown. That is helpful, because the blower clears the TDU of landfill gas as it runs down.
The weakness appears when a start is aborted. If the burner fails to light, the TDU may still contain landfill gas for a period of time, and restarting straight away could be unsafe. Gas that has not been burned or cleared is exactly what a burner start-up sequence is designed to avoid.
The approach: re-program the controls
The proposal makes no change to equipment. It is a program change:
- After every shutdown, the air blower runs for two minutes.
- If a start is attempted and ignition does not take place as planned, the air blower starts, if it is not already running, and vents the TDU for two minutes.
- During those two minutes a restart is impossible.
- Only when the purge is complete can the operator try again.
The same logic already happens in normal running. The change forces it to happen when something goes wrong, such as an ignition failure or other fault at start-up.
Delivery of the change
The work comprises preparing program changes for all four lines and attending site for three days to install them. Two options were offered, depending on whether an engineer was already travelling to the region for other work. Combining visits is cheaper than mobilising an engineer specially, but it depends on the schedule.
Why it works
Purging before a restart is a long-established principle in burner safety. Any gas that remains in the chamber is cleared before a spark is applied, so there is nothing to ignite unexpectedly. Making it automatic removes dependence on the operator remembering. The two-minute period is a design choice for this plant, and a different plant may need a different time, depending on the volume of its chamber and the flow of its blower.
In the UK, the Health and Safety Executive explains the duties for managing flammable atmospheres in its guidance on the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR). A purge cycle is one of the measures an operator can use to reduce the chance of an explosive mixture forming.
What to consider for your own project
- Look at the failure path, not only normal running. A plant can purge well on a normal stop and still be exposed after an aborted start.
- Make the purge automatic and non-skippable, with the restart locked out until it completes.
- Set the purge time from the plant, using the chamber volume and blower flow, and confirm it at commissioning.
- Apply the change across all similar lines, so that operators meet the same behaviour everywhere.
- Plan the site visit. Program changes must be installed and tested on site.
For the design of the plant itself, see our post on ammonia stripping waste heat.
FAQ
What is a purge cycle?
It is a period during which air is blown through a combustion chamber to clear any remaining fuel gas before the burner is started or after it is stopped.
Why not simply wait before restarting?
Relying on an operator to wait is less reliable than a control program that enforces the wait.
Does a purge cycle need new equipment?
Not in this case. The air blower already existed. The change was to the control program.
How long should the purge last?
It depends on chamber volume and air flow. This plant used two minutes.
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