Typically, the design of a pneumatic valve actuator consists of two cylinders, whose pistons are connected by a single shaft. When compressed air enters the first cylinder, a piston performs a direct movement on the shaft, whilst at the same time compressed air from the second cylinder is discharged to the atmosphere. The reverse motion is performed with an inverse logic. The output shaft of the pneumatic actuator is in turn connected to the stem of a quarter-turn valve, such as a ball or butterfly valve. Hence the torque from the pneumatic actuator is transferred via the stem to the valve’s closing member (the ball or the disk in the case of a butterfly valve), causing it to rotate around an axis.
Based on the pneumatic actuator’s operating algorithm, it can be concluded that at any moment in time one of pneumatic actuator’s cylinders contains compressed air, as this was of course required to operate the valve. It is this compressed air that prevents the valve’s closing member from spontaneously turning due to the impact of medium.
It should at this point be understood that quarter-turn valves, particularly butterfly valves, have a specific operational feature, namely that the disk can spontaneously move from an open or intermediate position under influence of the medium’s pressure in a pipeline. For actuators with a mechanical reduction unit (manual or part of an electric drive), this negative operational phenomenon is prevented by means of a reduction unit design, as due to the laws of kinetics, torque applied to an output shaft cannot cause turning of a reduction unit’s gear.
Given its design, a pneumatic actuator does not have such a feature. For a single-acting pneumatic actuator, this technical problem is not relevant, because the counteraction is achieved thanks to the resistance of the spring.
However, for a double-acting pneumatic actuator without return springs and lacking a mechanical transmission element to prevent the disk from spontaneous turning, there is only one technical option to avoid this phenomenon. That is to ensure that there is compressed air in at least one of the cylinders which prevents the pneumatic actuator from being turned under the influence of the pressure in the medium acting on the valve’s closing member.
Accordingly, it should be noted that when designing a pneumatic control system for a pneumatic actuator, a pneumatic element that blocks air in one (or both) cylinders in case of compressed air loss must be installed in the system.
The most common pneumatic element with this function is a lockup valve. This valve recognizes air pressure as a control signal and connects input and output ports; the lack of pressure is recognized as a signal to cut off an air-pressure line. During the cut-off of an air pressure line in a valve body, compressed air in the actuator cavities is safely blocked (see Figure 1).
Fig. 1. Pneumatic actuator control circuit for shut-off valves with blocking function.
If a pneumatic actuator is used as an actuating mechanism for control valves, it is possible to use a positioner with ‘fail freeze’ function. This function implies that if a control signal or compressed air pressure is lost, the positioner will automatically block its pneumatic outputs and thereby cut off air pressure lines of the pneumatic actuating mechanism, effectively safely blocking the compressed air pressure in its air cavities (see Figure 2).
Fig. 2. Pneumatic actuator control circuit for control valves with blocking function.
It must be understood that due to natural leaks in pneumatic control systems, for example, via pneumatic elements, couplings and fittings, blocking air in the cavities of a pneumatic actuator is only a temporary measure. Within a few hours, air pressure will be lost due to leaks in the pneumatic system, and the valve’s disk or control element can start to move due to the pressure of the medium in the pipe. Accordingly, during this time, plant operating staff must take measures to restore the normal instrumentation air pressure or to mechanically block the spontaneous turning of the disk. A manual override in a valve assembly enables the disk to be mechanically blocked by engaging a worm with a worm gear.
The specific technical points as described above can be summarized as follows:
• A butterfly valve with a double-acting pneumatic actuator is subject to spontaneous turning if there is no air pressure in pneumatic actuator’s cylinders
• Spontaneous turning of the disk is impossible in valve assemblies with a pneumatic single-acting actuator
• This phenomenon can be prevented by blocking air in the pneumatic actuator’s cylinders
• Air blocking in pneumatic actuator cylinders is a temporary measure
• A manual override enables disks to be mechanically blocked if there is no air pressure in the pneumatic actuator cylinders.