Combined bleeding and venting valves are both start-up and continuous bleed/vent valves combined into one singular unit. They have both a large seat for the start-up operation and a small seat for the continuous operation, which are controlled by a float and system of levers. Both valve seats are open while the system is being filled and during operation the large seat is kept closed by the internal pressure. Any accumulating small air volumes are vented from the system via the small valve seat.
If the pressure drops below 0.1 BAR, the large valve seat can open again. If a vacuum situation occurs (eg. by a pump failure) then the large seat opens instantaneously and prevents damage. Subsequently, if a pressure surge runs back through the pipeline, the large seat closes causing the air volume which has entered to act as a damper, while escaping to atmosphere through the small seat.
Not sure where to start? For an overview of our complete range of bleeding and venting valves you can click here to get a basic summary on our different types and advantages.