When a boiler or vessel is hydrotested above the relief valve set pressure, the safety valve is gagged so it will not lift, or it is removed and blanked. The gag bears on the spindle through the cap and must be taken off before the system goes back in service, because a gagged safety valve gives no protection. Gagging also describes holding a valve shut during set pressure work on another valve.
Before we fire the boiler, I want every gag off those safety valves and in my hand, count them.
Also heard as
- test gag
- gagged
Related terms
Safety valve
A spring-loaded pressure relief valve for steam, gas or vapor that opens with a rapid full lift at set pressure.
Hydro
A hydrostatic pressure test: filling a valve, piping system or vessel with water and pressurizing it to prove strength.
Car seal
A numbered tamper seal fixed to a valve handle so it cannot be moved without visibly breaking the seal.
Pop
For a safety valve to lift suddenly and fully open, discharging with a loud blast at set pressure.