Process Control Systems Overview

A device that converts one type of signal into another. For example, a converter may change a current into a voltage or an analog signal.

A device that converts a reading from a sensor or transducer into a standard signal and transmits it to a monitor or controller.

A device that physically adjusts the process conditions based on commands from the controller. Examples include valves, pumps, and heaters.

Components that display visual representations of process conditions, like pressure, temperature, and flow rate. Often found in control rooms to monitor and control a process system.

A device that translates a mechanical signal (like pressure or temperature) into an electrical signal. For example, a transducer in a pressure gauge converts pressure into an electrical signal.

A device that measures a specific variable in a process, such as temperature, pressure, or flow rate. Examples include thermocouples, pressure gauges, and flow meters.

The lines or pathways used to transmit signals between various devices in a process control system.

The process of determining the necessary size and specifications of a control valve to ensure it can effectively manage the flow of fluids or gases in a process.

The time it takes for a measuring element to partially respond to a change in the system.

A type of final control element that uses a variable orifice to adjust the flow rate. They consist of a valve body, stem, seat, and actuator.

The position of the stem in a control valve determines the opening and consequently the flow rate.

A protective material surrounding the orifice in a control valve. It helps ensure a tight seal and prolongs the life of the valve.

A device that provides the force for opening and closing the valve.

A component of a control valve that creates pressure to open or close the valve.

A spring inside a control valve that helps regulate the opening and closing force.

The difference between the highest and lowest values a sensor can measure.

How well a sensor performs and how reliable it is. It also relates to the cost of the device.

The materials used to build the sensor, transmitter, and transducer. They should be chosen to withstand the environment where they are being used.

Whether a sensor needs to be physically in contact with the measured element (invasive) or if it can measure from a distance (non-invasive).

The output signal from a sensor must match the input range of the controller receiving the signal. This ensures they can communicate correctly.

The relationship between the input and output signals of a transmitter. It describes how the transmitter converts the measured variable into an output signal.

The gain of a transmitter (Km) changes with the span but remains the same when the zero point changes.

The transmitter's output should increase proportionally as the measured variable increases. Most commercial transmitters have adjustable input ranges.

A factor used in the calculation of a control valve's size, accounting for differences in units of measurement.

The degree to which a control valve can adjust the flow rate over a range of values.

A characteristic describing how a control valve's flow rate changes in relation to the valve's position.

A numerical value representing the control valve's size and capacity. It determines the maximum flow rate the valve can handle.

A specific type of valve design where each equal increment of valve stem movement results in a consistent percentage change in flow.

The difference in pressure across the control valve when it is fully open, used to calculate the valve coefficient.

The flow rate the valve is intended to manage at the design conditions.

This is the pressure drop across the valve when it is half open at the design flow rate.

A control valve's position is directly controlled by pneumatic pressure, using a signal from the controller. When the valve is open, air flows through the valve's actuator, moving the stem to allow fluid passage. When the valve is closed, the air flow is reduced or stopped, causing the stem to move and block flow. This is commonly used in industrial process control.

A fail-close valve is designed to automatically close if there is a loss of air pressure or signal from the controller. This ensures that the valve will shut off flow in case of failure, preventing uncontrolled fluid release and potentially dangerous situations.

A fail-open valve is designed to automatically open if there is a loss of air pressure or signal from the controller. This is often used in situations where it's safer to keep the flow open than to shut it off during an emergency or malfunction.

A valve positioner is a device that senses the actual position of a valve stem and compares it to the desired position set by the controller. It then automatically adjusts the air signal to the valve to ensure the stem reaches and maintains the correct position. This helps ensure accurate control over the flow rate.

The dynamic behavior of a control valve refers to how quickly it responds to changes in the control signal. It's often described by a first-order transfer function, which models the relationship between the control signal and the resulting flow rate.

The valve time constant (τv) indicates how quickly a valve responds to a control signal. It's typically much smaller than the time constant of the process (τp) it is controlling, indicating fast response and precise control.

The dynamic behavior of a control valve can be influenced by the valve's size. Larger valves tend to have slower dynamics, meaning they respond more slowly to changes in the control signal. This is because larger valves have more inertia and mass to move.

This equation helps determine the size of a control valve required to manage a specific fluid flow rate. It takes into account factors like pressure drop, specific gravity, and the valve's stem position, ensuring the valve can handle the desired flow rate without causing excessive pressure loss.