Start-up bleed valves remove air or gases from systems or pipelines during the filling process without requiring an external energy input. When a system is drained they act as venting valves; venting may be prevented by fitting a commercial check valve to the outlet.
Thanks to the large cross-sectional area of the seat these valves are capable of handling large air volumes at low pressures. They close as soon as the system is filled with fluid. When the liquid level drops these valves will only open if the system is depressurised; they will not open under working pressure