Dead zones are the main cause of excessive process deviation and usually occur in control valves. Dead zone refers to the phenomenon that when the input signal changes direction, the controller output value fails to cause the change of the process variable under test within a certain range. When a load disturbance occurs, the process variable deviates from the set point and the controller corrects by adjusting the output signal.

However, due to the existence of dead zones, the initial adjustment of the controller may not immediately cause the corresponding change in the process variable, and the corresponding change in the process variable will occur only when the controller output changes sufficiently to overcome the dead zone. The presence of dead zones makes it necessary for the controller to increase the output amplitude in order to trigger corrective action, thus affecting the stability of the process.

Genesis of Dead Zone

Dead zones can occur for a variety of reasons, the most common of which include friction in the control valve, idle travel, twisting of the rotary valve shaft, and dead zones in the amplifier. Because regulated control actions usually consist of small signal changes (1% or less), control valves with large dead zones may not respond to these small signals. Ideally, a well-manufactured valve should be able to respond to a signal change of 1% or less to effectively reduce the degree of process deviation. However, in practice, the dead zone of some valves may reach 5% or greater. A recent factory audit showed that 30% of valves had a dead zone greater than 4%, while more than 65% of control loops had a dead zone greater than 2%.

Dead Zone test of Valve Performance

Testing of control valve performance is usually limited to comparing the input signal with the stroke of the actuator push rod, which ignores the performance of the valve itself. Therefore, it is even more critical to measure the dynamic performance of the valve under fluid conditions so that changes in the process variables can be compared with changes in the input signal of the valve assembly.

In open-loop tests of three different valves, although the actuator push rod responded well to input signal changes, there were significant differences in the valve's ability to change flow. Valve A can cause significant changes in process variables when the input signal changes by 0.5%. Valve B requires a change in the input signal of more than 5% to produce a response, while valve C performs worse, requiring a change in the input signal of more than 10% to respond effectively. As a result, the ability of valves B and C to improve process deviation is significantly inadequate.

Friction and Dead Zone

Friction is one of the main factors causing the dead zone of control valves, especially rotary valves, because of the high sealing requirements, the friction caused by the seat load can significantly affect its performance. In order to achieve a good closing class, some valves need to withstand a high seat load, causing the valve shaft to twist and not transfer motion effectively, resulting in a large dead zone. This dead zone will have a decisive effect on the degree of process deviation.

Although the manufacturer lubricates the sealing part of the rotary valve during production, after hundreds of cycles, the lubrication layer can wear out, leading to a significant increase in valve friction. This suggests that using standard test data to assess valve performance can be misleading. The test results showed that the performance of valves B and C suffered devastating effects due to the increase in friction torque.

Packing friction is the main source of friction in straight stroke control valves. The friction force varies with the valve form and packing structure. When the device changes direction, friction can cause motion discontinuities, resulting in gaps and dead zones. Although friction can be reduced by good design, it is extremely difficult to completely eliminate it. A well-designed and manufactured control valve should be able to eliminate dead zones caused by clearance to optimize process deviation control. Ideally, the total dead zone of the entire valve assembly should be controlled below 1%, and the best case is 0.25%.

Summary and Suggestion

In summary, the dead zone of the control valve has a crucial impact on the stability and precise control of the process. The existence of dead zones not only leads to increased process deviation, but also may cause the control system to fail to respond to small input signal changes in time. Therefore, when selecting and maintaining control valves, it is important to pay attention to the evaluation and management of dead zones. By using a low-friction, well-constructed valve design, as well as regular maintenance and inspection of the condition of the seals, the impact of dead zones can be effectively reduced. In order to ensure the stable operation of the process, it is recommended to give priority to those products with small dead zones and sensitive response when purchasing control valves, and conduct a comprehensive test under actual conditions to ensure that the valve performance meets the process requirements.