Flow Diagrams

Valve ANSI or ISO Schematic Symbols and Flow Diagrams

Solenoid valves are widely used to control the flow of either a gas or liquid in many types of components or equipment. These electrically operated control valves are typically employed in a two-way or three-way operation, and are specified as normally closed or normally open in a de-energized state. Most equipment requires either a 12V or 24V (volt) DC circuit.Understanding the ANSI symbols and manufacturer's flow diagrams ensures the proper valve is specified.

Normally Close (NC) or Normally Open (NO)

When designing a direct-acting solenoid valve as part of a fluid control system, the process begins with the flow in a de-energized mode. This minimizes energy draw to operate the control of the fluid or gas. As an example, a standard refrigerator water dispenser would require a two-way normally closed solenoid valve. The valve is "normally closed" while not in use. When the valve is "energized" it opens allowing water to flow at a specified flow rate say 1oz per second (see valve sizing). The valve shuts off or closes when its de-energized.

A "normally open" application is just the opposite. The media flows through the valve, until it is energized when it closes and stops the flow.

Two (2) Way or Three (3) Way Valve Operation

Every solenoid valve has a number of standard components including electrical coil, a plunger and spring, seals and a body. In a multi-valve set-up, a manifold can replace the valve's body, but provides the same function. The body has an inlet and an outlet "port" allowing media to flow through the valve, and has a external connection... often threaded to a tube or hose. The body also has an "orifice" which in simplest terms controls the flow rate of the media.

Calculating the "flow coefficient" or Cv factor of a gas or liquid and the pressure drop (differential) is required to properly size the valve.

Two-Way Operation

As noted above, a two-way valve typically functions as a shut-off (NC) valve having a single inlet and a single outlet port. Regardless of the operation (NC/NO), the media enters the inlet passes through the orifice and out the outlet.

Three-Way Operation

Three-way valves provide either directional control or an exhaust feature when controlling the gas or liquid having one inlet and two outlet ports, or an outlet and an exhaust port.

Three-way "directional control" operates with an inlet port with the media flowing through the orifice and exits the outlet port. When the plunger is energized, it seals one orifice and allows the gas/liquid to exit or escape through the second outlet or exhaust port.