Electro-pneumatic actuation
Options for generating the set pressure at pressure control valves
A mechanical spring or a pressurised gas spring and/or a pressurised gas cushion inside the so-called spring cap serve as setpoint adjuster for pressure control valves (for example pressure reducing valves, backpressure regulators or vacuum control valves).
In principle, setpoint adjusters feature two different modes of operation:
The purely mechanical adjustment
The set pressure is generated by the manual pre-tension of the spiral spring that is supported by the spring cap. The spring of a pressure reducing valve keeps the valve open, consequently, it closes with the rising backpressure. The pressure spring of a backpressure regulator, however, keeps the valve closed; it opens with the rising upstream pressure.
A regulating valve with simple spring cap is especially suitable for plants on which the set pressure must not be changed constantly.
The pneumatic remote control
In the gas spring cap, the set pressure is generated by the pressurised air acting on a diaphragm. An I/P converter regulates the control air pressure and, in return, receives its 4 - 20 mA signal directly from the process control system (PLS) or from a processor in the I/P converter. The regulating device then controls a pressure which is equivalent to the control air pressure. For this purpose, the version with only one diaphragm is required.
This is commonly used in cyclical batch processes, where the pressure reducing valve can be remotely opened or closed, for example to allow
flushing or cleaning with suitable media (steam or cleaning agents) without the need for manual adjustment.
The advantages of the pneumatic control are
- Small control deviation
- Remote controllability
- Utilising the superposition of the mechanical spring force and the pressurised gas spring
To control pressures higher than the supplied control air pressure, a transformation ratio can be used. The transformation ratio results from the use of two diaphragms with differently sized so-called control surfaces.
Use of I/P Converters
All sorts of I/P converters transform the electrical input signal into a pneumatic output signal. In most cases the converter receives a direct current signal from 4 through 20 mA and, depending on the level of the pressurised gas (control air pressure), it sends a pneumatic signal in order to display different control ranges.
Benefits are:
- High accuracy
- Linear, continuous characteristics
- Directly acting, good dynamics, fast
- Well adapted for rapidly changing operating pressure conditions
Some I/P converters are provided with an internal logic in order that simple processes can also be displayed directly.
An I/P converted control may also be superposed by the permanently set mechanical set point.
Typical pressure control valves with electro-pneumatic actuation:
