The control valve is the instrument used to control or regulate the flow of fluid. For controlling of the valve one should know different parameters associated with the control valve. Key parameters that are dynamically measured
are described below.
Supply Air Pressure:
The supply air is the working medium to control the valve’s position. Depending on the valve requirements, both the supply pressure and the capability of the system to supply a sufficient volume of air are critical to ensuring proper
operation of the valve.
The supply pressure can either be straight from the compressed air system or regulated to protect some of the valve’s components. If the air supply is regulated, the term then is regulated air pressure.
This parameter is measured during the whole valve stroke to ensure that pressure is maintained. Airline restrictions would indicate normal pressure during steady-state conditions, but when a demand is placed on the supply, system pressure would drop, depending on the load.
Control Signal:
The control signal can be either a pneumatic or an electrical signal. These signals are measured and/or controlled by the Data Acquisition System. The control signal is the required position requested by the controller.
For clarification, a current signal is designated with an “I” while a voltage signal is designated with an “E.” For example, a current-to-pneumatic transducer is either called an I/P transducer or simply an I/P, and a voltage-to-pneumatic transducer, one receiving a 0 to 10-volt signal, is called an E/P transducer or simply an E/P.
The control signal is typically controlled by the Data Acquisition System to allow a fully controlled stroke of the valve. This allows verification of the valve’s operation with a known controlling signal.
Actuator Pressure:
The positioner or solenoid provides a pressure/volume of air to the actuator, depending on the demand requirements of the system. By comparing the actuator pressure with the positioner output, actuator problems such as diaphragm or seal leaks can be detected. The actuator pressure enables the valve analyst to calculate stem forces, and when compared to stem position, the spring rate can also be calculated.
Valve Stem Position:
This parameter is typically measured with a linear variable differential transformer (LVDT), a laser or a linear potentiometer (sometimes called a string potentiometer). The position of the piston rod provides a positive verification that the valve is moving with the pressure applied from the positioner. The total stroke time of the valve can also be determined from this measurement.
Strain:
In the stem or yoke, stress can be used to obtain the valve body internal forces, such as the packing friction and seating force. This measurement can be used to differentiate actuator forces from valve forces or actuator problems from valve internal problems.
Solenoid-Operated Valve (SOV) Activation:
When using current, voltage, or induction measurement methods, the exact time an SOV was activated can be useful in determining timing of the blow, an adhesive SOV, or a pilot that is not transferring (navigating) completely.
Auxiliary Pneumatic Components:
Measuring the input and output of auxiliary components, such as volume boosters or lock-up valves, can be useful in determining the component’s health or its effect on the valve’s operation
Some of the AOV Data Acquisition Systems have the ability to measure a wide range of other parameters that could be useful in determining a problem related to the system instead of a pure valve problem. For example, pressure or system flow can be useful in determining if the system, and not the valve, is causing the problem.
