Process Instrumentation & Control

• Control valves make a significant contribution to pressure drop.
• Control valve location can create a need for additional pumps and compressors, and must be decided in order to size the pumps and compressors
• It is therefore necessary for the design engineer to understand the plant control philosophy even at the PFD stage and the PFD usually shows the location of control valves
• More details of process control are usually included in the piping and instrumentation diagram (P&ID) – see Ch 5
• This lecture is a very brief overview of control for design purposes – more will come in your process control class

Chemical Engineering Design

Control Loop Components

• The sensing instrument detects the measured variable and sends a signal to a controller, which signals the actuator to close or open a control valve and adjust the manipulated variable (usually a flow rate)

Process or

utility stream

Alarms

Instrument line

Actuator

Final control element

Controller

Sensing

element

Chemical Engineering Design

Control Valves

• The final control element is usually a control valve
• Exceptions: electric heaters, mixers, variable speed drives
• The actuator is either a motor or a bellows that opens or closes the valve in response to the signal

​

Source: Valve Manufacturer’s Association, www.vma.org

Chemical Engineering Design