Intro4u2u

For many applications, the 0.2 to 1 bar pressure in the diaphragm chamber may not be enough to cope with friction and high differential pressures. A higher control pressure and stronger springs could be used, but the practical solution is to use a positioner.

This is an additional item (see Figure 6.6.11), which is usually fitted to the yoke or pillars of the actuator, and it is linked to the spindle of the actuator by a feedback arm in order to monitor the valve position. It requires its own higher-pressure air supply, which it uses to position the valve.

Fig. 6.6.11 Basic pneumatic positioner fitted to actuator pillars (valve not shown) Fig. 6.6.11 Basic pneumatic positioner fitted to actuator pillars (valve not shown)

A valve positioner relates the input signal and the valve position, and will provide any output pressure to the actuator to satisfy this relationship, according to the requirements of the valve, and within the limitations of the maximum supply pressure.

When a positioner is fitted to an ‘air-to-open’ valve and actuator arrangement, the spring range may be increased to increase the closing force, and hence increase the maximum differential pressure a particular valve can tolerate. The air pressure will also be adjusted as required to overcome friction, therby reducing hysteresis effects.

Example: Taking a PN5400 series actuator fitted to a DN50 valve (see Table in Figure 6.6.10)

1. With a standard 0.2 to 1.0 bar spring range (PN5420), the maximum allowable differential pressure is 3.0 bar.
2. With a 1.0 to 2.0 bar spring set (PN5426), the maximum allowable differential pressure is increased to 13.3 bar.

With the second option, the 0.2 to 1.0 bar signal air pressure applied to the actuator diaphragm cannot provide sufficient force to move an actuator against the force provided by the 1.0 to 2.0 bar springs, and even less able to control it over its full operating range. In these circumstances the positioner acts as an amplifier to the control signal, and modulates the supply air pressure, to move the actuator to a position appropriate to the control signal pressure.

For example, if the control signal was 0.6 bar (50% valve lift), the positioner would need to allow approximately 1.5 bar into the actuator diaphragm chamber. Figure 6.6.12 illustrates this relationship.

Fig. 6.6.12 The positioner as a signal amplifier Fig. 6.6.12 The positioner as a signal amplifier

It should be noted that a positioner is a proportional device, and in the same way that a proportional controller will always give an offset, so does a positioner.

On a typical positioner, the proportional band may be between 3 and 6%. The positioner sensitivity can usually be adjusted. It is essential that the installation and maintenance instructions be read prior to the commissioning stage.