First, the definition of the dead zone
In the field of instrumentation, it is also called the insensitive area of the instrument. A change in the amount of input does not cause a finite interval of any appreciable change in the meter's output. The cause of the dead zone is mainly the friction and intermittentness between the components inside the meter.
In the field of valves, the dead zone of the valve is also a general phenomenon, referring to the range or width of the controller output (CO) value that does not change the measured process variable (PV) when the input signal changes direction. . When a load disturbance occurs, the process variable (PV) deviates from the set point. This deviation will pass through the controller first, and then through the process to produce a corrective action. However, an initial change in the controller output may not result in a corrective change in the corresponding process variable, only if the controller's output is large enough to overcome the dead zone change, a corresponding process variable change. will happen.
Second, the cause of the dead zone
There are many reasons for the dead zone, but the regulating valve (control valve) adjusts the friction and the movement of the butterfly valve, the torsion of the rotary valve shaft, and the dead zone of the amplifier are several common forms. Since most of the regulated control actions consist of small signal changes (1% or less), a regulating valve (control valve) with a large dead zone may not even respond to so many small signal changes at all. A well-made valve should be able to respond to signals of 1% or less to effectively reduce process variation. However, it is not surprising that some valves exhibit a dead zone of 5% or greater. In a recent factory audit, 30% of valves had more than 4% dead zones. More than 65% of audited circuits have more than 2% dead zones. Figure 2-3 shows exactly how big the combination of dead zones is. This figure represents the open loop test of three different regulating valves (control valves) under normal process conditions. These valves accept a range of step inputs ranging from 0.5% to 1%. The step test must be performed under those important flow conditions, as these conditions allow evaluation of the performance of the entire regulator (control valve) assembly, rather than evaluating the performance of the valve actuator under most standard test conditions.
Friction is a major cause of the dead zone (control valve). The rotary valve is very sensitive to friction caused by high seat load. For some seal types, a high seat load is required to achieve a shut down rating. Due to the high friction and low drive strain stiffness, the valve shaft twists and does not transmit motion to the control element. As a result, a poorly designed rotary valve may exhibit a large dead zone, which has a decisive influence on the process deviation. Manufacturers typically lubricate the seal of the rotary valve during the manufacturing process, but after only a few hundred cycles, the lubricant wears away. In addition, pressure-induced loads can also cause seal wear. As a result, for some valve types, the friction of the valve may increase by 400% or more. This suggests that the performance conclusions obtained by evaluating the valve using standard types of data are misleading before the torque is stabilized. Valves B and C (Figure 2-3) indicate that these higher friction torque factors can have a devastating effect on the performance of a regulating valve (control valve). Packing friction is the main source of friction for linear stroke control valves (control valves). In these types of valves, the measured friction may vary greatly depending on the valve form and the packing structure.
The type of actuator also has a fundamental effect on the friction of the regulating valve (control valve) assembly. In general, the spring diaphragm actuator produces less friction against the regulator valve (control valve) assembly than the piston actuator. Another advantage of the spring film actuator is that its friction is relatively constant and does not change over time. The friction of the piston actuator increases significantly with wear of the guide face and O-ring, loss of the lubricating layer, and degradation of the performance of the elastomer. Thus, in order to ensure continuous optimum performance, the piston actuator requires more frequent maintenance than the spring diaphragm actuator. If maintenance is not performed, the process deviation will increase significantly, and the operator knows nothing about it.
This lost motion causes a discontinuity in motion when the device changes direction. The lost motion typically occurs in gear driven devices of various configurations. The rack and pinion actuator is particularly prone to dead zones due to the lost motion. Some valve shaft connections also show the effect of a dead zone. The spline connection generally has a smaller dead zone than the valve spool or double D-shaped structure that is connected to the keyway. Although the friction can be greatly reduced by the excellent valve design, it is a difficult problem to eliminate it all. A well-designed control valve (control valve) should eliminate dead zones due to lost motion and valve shaft torsion. In order to achieve the best results in reducing process variation, the total dead zone of the entire valve assembly should be equal to or less than 1%, ideally as low as 0.25%.
Third, the adjustment of the dead zone
The dead zone is used to adjust the sensitivity of the valve. Take the electric actuator as an example: the input 4mA valve is fully closed, 20mA is fully open. When the dead zone is adjusted to the minimum, the sensitivity is the largest. At this time, the input 4.01ma valve may follow the signal change and move to the corresponding opening degree. If the dead zone is turned up, perhaps the input signal is 4.2, the valve is still closed. Adjusting the potentiometer of the dead zone is to adjust some of the comparator's circuit. The dead zone can make the valve stable, and the valve is relatively stable when it is not sensitive. When a small disturbance signal enters the control system, the valve will oscillate near the opening when the valve sensitivity is high. The dead zone can effectively solve this problem, but at the same time Also sacrificed sensitivity.