A proportional valve provides a change in output pressure or flow in the same ratio as the change in the input; for example if the input doubles then the output will also double. Proportional valves can be interconnected with the output from one being used as the input to another.
They are also known as…
The terms I/P and E/P valves are also used, this relates to the current, I (4 to 20mA) or voltage, E (0 to 5V or 0 to 10V) .
They are also sometimes referred to as electrically controlled regulators.
How does a proportional valve work?
We can probably recall the school experiment, where applying a current through a coil creates a magnetic field, in turn attracting (or repelling) magnetic material. This is the basic solenoid principle.
Increasing the current increases the strength of the magnetic field and, in turn, the force of attraction (or repulsion) on the magnetic material. A proportional valve has a geometry to ensure a constant field strength, at a given current, over the stroke of the magnet.
Also, a spring is introduced to counter the force of the magnet; as the spring is compressed by movement of the magnet, the spring reaction force increases until it balances the magnetic force and movement ceases. If the current is increased, the magnet (solenoid) moves further against the spring, until the forces are balanced again. Consider that the magnet is attached to a spool in a valve, the pressure or flow can be controlled in proportion to the current applied to the solenoid.
How do I select a proportional valve?
- There are some basic parameters to consider:
- What is being controlled? Pressure, flow or another characteristic?
- What range will be required and at what range of flow rates?
- What is the likely operating temperature the proportional valve will experience?
- Are there other environmental considerations, such as wet, dirty or explosive applications?
- Is the process static, step-by-step or dynamic one? This very well dictate the response time and hysteresis performance.
- Finally, cost may be a consideration.
Typical applications of a proportional valve
Proportional valves are particularly suited to applications where the output pressure or flow is required to vary, either during a process or from one process to the next. They are dependable, cost-effective and give fast response and consistent control. Proportional valves can help reduce cycle times by quickly achieving a stable pressure or flow.
Examples of typical applications are:
- Bottle filling – the proportional valve maintains a constant pressure in the filling reservoir, irrespective of the liquid level. A pressure sensor in the reservoir provides feedback to the proportional valve, ensuring that a constant volume of liquid is transferred to a bottle in a given time.
- Furnace control – the gas supply is regulated by the proportional valve; a differential temperature signal is used as feedback to shut down gas flow when the desired temperature is reached.
- Leak testing – the proportional valve can apply increasing pressure, at a defined profile, to a given test piece.