Balanced Bellow PRV;
Balanced Bellow Pressure Relief Valve is The Conventional PRV with Bellow for reducing the impact of Backpressure from the Outlet of PRV makes PRV can be turned off as the Set pressure setting.
Refer to: What is Backpressure testing of PRV?
To use Balance bellow PRV, open a vent hole on the Bonnet to monitor the leak of the Bellow and to reduce the impact of Backpressure on Set pressure in case of Bellow leak:
How Balanced Bellow Work;
Balanced Bellow is able to eliminate the impact of the Backpressure because the pressure from the Backpressure is applied to both sides of the Bellow and is offset.