Analysis of the "Deadband"In Control Valves

Deadbands are the main cause of deviations in oversized processes. Control valves are a major source of deadband in an instrumentation loop for a variety of reasons such as friction, air travel, spool twist, deadband in amplifiers or slide valves.

Deadband is a common phenomenon and refers to the range or width of the controller output value that does not allow the process variable under test to change when the input signal changes direction. When a load disturbance occurs, the process variable deviates from the set point. This deviation is then corrected by a corrective action generated by the controller and reverted to the process. However, an initial change in the controller output may not produce a corresponding corrective change in the process variable. A change in a corresponding process variable will only occur if the controller output changes by an amount large enough to overcome the change in deadband.

If the controller output changes direction, the controller signal must overcome the deadband in order to produce a corrective change in the process variable. The presence of a dead band in the process means that the controller output must be increased to an amount large enough to overcome the dead band and only then will a corrective action take place.

● Causes of deadbands

There are many causes of deadbands, but friction and air travel in control valves, twisting of the spindle of rotary valves and deadbands in amplifiers are a few common forms. As most modulating control action is made up of small signal changes (1% or less), a control valve with a large dead band may not respond to so many small signal changes at all. A well manufactured valve should be able to respond to signals of 1% or less to effectively reduce the degree of process deviation. However, it is not uncommon for valves to have deadbands of 5% or greater. In a recent plant audit, 30% of valves were found to have more than 4% deadband. Over 65% of the control loops audited had deadbands greater than 2%.

● The impact of deadbands

This graph represents an open loop loop test of three different control valves under normal process conditions. These valves receive a range of step inputs from 0.5% to 10%. Step tests under fluid conditions are necessary because these conditions allow the performance of the entire control valve assembly to be assessed, rather than just the valve actuator as is the case with most standard tests.

● Performance tests

Some tests of control valve performance are limited to comparing the input signal with the stroke of the actuator pushrod. This is misleading as it ignores the performance of the valve itself.

What is critical is to measure the dynamic performance of the valve under fluid conditions so that changes in process variables can be compared with changes in the input signal to the valve assembly. If only the valve stem responds to a change in the valve input signal, then this test is of little relevance as there is no correction for process deviations without a corresponding change in the control variable.

In all three valve tests the movement of the actuator push rod responded well to changes in the input signal. On the other hand, the valves differed considerably in their ability to change the flow rate in response to a change in the input signal.

Valve A, the process variable (flow rate) responds well to an input signal as small as 0.5%.

Valve B, requires a change in input signal of greater than 5% before it starts to respond well to each input signal step.

Valve C, significantly worse, requires a change in signal of greater than 10% before it starts to respond well to each input signal step.

Overall, the ability of valves B or C to improve process deviation is very poor.

● Friction

Friction is a major cause of deadbands in control valves. Rotary valves are very sensitive to friction caused by the high seat load required for sealing. For some seal types, high seat loads are necessary to obtain a closing rating. Due to the high frictional forces and low drive strain stiffness, the valve shaft twists and cannot transmit motion to the control element. As a result, a poorly designed rotary valve may exhibit a large deadband which clearly has a decisive influence on the degree of process deviation.

Manufacturers usually lubricate the seals of rotary valves during the manufacturing process, but after only a few hundred cycles, the lubrication layer wears off. In addition, pressure-induced loads can also cause seal wear. The result is that for some valve types, the valve friction may increase by 400% or more. This makes it clear that conclusions drawn about performance by using data from standard types to evaluate valves before the torque has stabilised are misleading. Valves B and C show that these higher frictional torque factors can have a devastating effect on the performance of a control valve.

Packing friction is the main source of friction in direct stroke control valves. In these types of valves, the measured friction may vary considerably depending on the valve form and packing configuration.

This gap can cause discontinuities in movement when the device changes direction. Gaps usually occur in devices with various configurations of gear drives. Rack and pinion actuators are particularly susceptible to deadbands due to clearance. Some valve spindle connections also have problems with deadbands.

Although friction can be significantly reduced by good valve design, it is a difficult problem to eliminate completely. A well-designed and manufactured control valve should be able to eliminate deadbands due to clearances. To achieve optimum results in reducing process deviations, the total dead space of the entire valve assembly should be less than or equal to 1%, with the ideal result being as low as 0.25%.