Short answer
A gas governor holds outlet pressure steady as inlet pressure and flow change. Common faults are failure to lock up (outlet pressure creeps up with no flow), drop-off (outlet pressure falls too far as flow increases), hunting or instability, and complete failure from a ruptured diaphragm or blocked breather. Each is found by measuring outlet pressure at no flow, low flow and full flow.
Lock-up failure
With no demand the valve should close and outlet pressure stabilise slightly above set point. If it keeps rising, debris or a damaged seat is letting gas past. This can over-pressurise appliances and must be fixed.
Excessive drop-off
At full flow outlet pressure falls well below set point. Causes include an undersized governor, a weak spring, low inlet pressure or a partly blocked inlet filter.
Instability and breather faults
Hunting is often caused by oversized governors working near closed, long impulse lines, or blocked breather vents. A breather vent that is blocked or taped over stops the diaphragm moving correctly; one that leaks gas indicates a ruptured diaphragm.
Testing
Connect a gauge to the outlet test point, check standing (lock-up), then working pressure at minimum and maximum demand. Compare with the set point and the governor's published performance.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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