3-way control system
The main valve is controlled by a manually, electrically or pressure actuated selector that:
- Introduces the upstream pressure or an external pressure source to the valve’s control chamber, thus pressing the valve diaphragm from top to a closed position.
- Openes the valve by relieving the pressure from the control chamber and enabling the internal pressure in the line to push the valve diaphragm up and creating an unrestricted, full flow passage.
- Shutes all passages to and from the control chamber, thus keeping the main valve in a fixed position. This position does not exist in devices used for ON/OFF control applications but is necessary in modulating valves.
2-way control system
A control system that consist of two restrictions:
||A fixed restriction at the upstream side of the control system (nozzle or needle valve)
||ON/OFF device (valve, relay or solenoid) or a modulating pilot valve with a larger passage than restriction ‘a’ that is installed at the downstream side of the control system. The main valve position is affected by the downstream pilot valve ‘b’ in the following ways:
- When the pilot valve is closed, no water can be vented to the downstream and the main valve is closed by the upstream pressure, introduced into the valve control chamber through the restriction ‘a’.
- Opening the valve is enabled when the pilot ‘b’ is fully open and allows the drainage of pressure in the control chamber to the downstream. At this position the pressure in the control chamber nearly equals that of the downstream.
- Throttling pilot valve ‘b’ so the flow running through it equals the flow entering through the restriction ‘a’, keeps a fixed volume of water in the control chamber and the main valve in a fixed position.