How to Measure Control Valve Deadband ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Measure Control Valve Deadband

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What is Deadband ?
Dead band can be caused by packing friction, unbalanced forces and other factors in the control valve assembly. In technical terms, control valve deadband is the range a measured signal can vary without initiating a response from the actuator. The schematics below illustrates the concept of deadband for direct and reverse acting control valve actuators:

Every control valve actuator has
a spring with a defined spring constant and hence a certain deadband associated with it. Deadband affects the control valve operation during automatic loop control. The control loop tolerance for deadband varies widely depending on loop response. However one fact must be clear, an increasing or widening deadband in a control  valve  elicits sluggish control response. Some common symptoms of the deadband being too wide include:
(a)  No movement in the control valve after control action is initiated by controller
(b) A jump movement after control action is initiated or
(c) Oscillating movements of the valve actuator during automatic loop control

How to Determine the Deadband of a Control Valve
Knowing the percent deadband can be helpful in troubleshooting problems with process control loop. As have been stated above, increasing deadband produces sluggish control response . It is therefore wise we have an idea of the level of deadband in a control valve especially one that is at the heart of a critical process operation. Connect a source of controlled pressure - pressure in the bench set range of the valve, say 0 - 60psi - to the control valve actuator as shown below and proceed as follows through steps 1 to 4.

Step 1
With your control valve firmly out of operation and supplied with a controlled source of pressure, supply a pressure near the lower benchset  pressure, slowly increase the pressure until the valve is approximately at mid-travel. Note this pressure reading (P1) on the pressure gauge (PG)

Step 2
Slowly decrease pressure until movement of the valve stem is detected and note this pressure (P2).

Step 3
Subtract pressure in step 2 above from that in step 1. The difference between these two pressures is the deadband, in psi.

Step 4
Calculate the percent of deadband by using the formula below:

Bench Set Span = Upper Bench Set, psi  - Lower Bench set, psi

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