When a landfill gas flare’s modulating valve fails, the cause is often upstream of the valve itself. In a service call to a UK landfill gas site, a panel shutdown and a damaged valve were traced to two things: a faulty emergency stop pushbutton that broke the safety circuit, and the valve being operated by hand while it was still energised. This post walks through the diagnosis and the repair, and what operators can take from it.
The problem: a shutdown, a reset, and a damaged valve
The site reported two linked faults: a modulating valve that was not working, and a loss of power to the control panel. On arrival, the service engineer was told that the panel had been reset at the weekend, and that the modulating valve had been operated manually while it was still energised.
That detail turned out to matter. Both faults were present, but they did not have the same cause.
The diagnosis
The panel failure
The panel had shut down because of a faulty emergency stop pushbutton. The failed button broke the emergency stop circuit, and with the circuit open the panel shut down as designed. Resetting the panel treated the symptom without addressing the faulty button.
The valve failure
The modulating valve itself was found to be damaged. The probable cause was the manual operation while the valve was energised. Because the valve was damaged, the engineer could not check the rest of the valve circuit for other faults. A damaged component can mask problems elsewhere in the loop, which is a common reason to replace related components together rather than only the visibly failed one.
The repair
The recommendations were to replace the components in the valve circuit that could contribute to a repeat failure:
- A new PID controller.
- New relay cubes.
- A larger circuit breaker in the control panel, upgraded to 16 amps.
- A new emergency stop pushbutton rated for use in a hazardous (Ex) area.
The follow-up quotation covered an inspection visit for the valve and control circuit, an inspection of a second defective gas blower, and a visit to deliver and install the replacement parts. A related order for the site covered an electric actuator with a 4 to 20 mA control card, a digital PID controller and an emergency stop push-button/isolator switch, all fitted and commissioned in a single day. Parts lead time was two to three weeks.
Why it works: fix the loop, not just the part
A modulating valve is only as good as the loop that drives it. The controller sends a signal, the relays and breaker supply and protect the circuit, and the valve positions itself. If one of those parts is weak, replacing only the valve invites another failure. Replacing the controller, relays and breaker at the same time restores the whole loop to a known condition.
The emergency stop deserves a particular mention. It is a safety device, but a faulty one will shut down a panel without anyone pressing it. Replacing it with an Ex-rated unit means the part is suitable for the area it sits in.
What to consider for your own site
- Do not drive a valve by hand while it is energised. If manual operation is needed, isolate the supply first and follow the procedure in the instruction manual.
- Investigate before resetting. A panel that has tripped on its emergency stop circuit has told you something. Find out what.
- Replace the loop, not just the part. When one component in a control loop has failed, review the controller, relays and breaker.
- Use components suited to the area. Emergency stops in hazardous zones should be rated for them.
- Keep spares for critical items, because parts lead times can be several weeks.
FAQ
What does a modulating valve do on a flare?
It adjusts gas flow or air supply in response to a control signal, in this case through a PID controller.
Why did the panel shut down?
A faulty emergency stop pushbutton broke the emergency stop circuit, which shut the panel down.
Can a valve be damaged by manual operation?
Yes. In this case the probable cause of the valve damage was operating it manually while it was energised.
Why replace the PID controller and relays too?
Because the damaged valve meant the rest of the circuit could not be fully checked, replacing related components reduces the risk of a repeat fault.
Need help with a flare control fault?
Describe the fault and send us the panel details and our service team will advise. You may also find our post on a vibration fault found on a service visit useful.