A control valve that has one plug that moves upward to close the port is referred to as what?

A control valve that features a design where a single plug moves upward to close the port is best described as reverse acting. In this type of valve, when there is an increase in the control signal (for example, pressure), the diaphragm will move in a manner that causes the plug to rise and eventually close the flow path. This characteristic is essential for applications where closing or reducing flow is required in response to control signals.

The definition fits well within the context of control valve operations, where the direction of actuation can have significant implications for how the system behaves. In a reverse acting configuration, the design is optimized for scenarios where a reduced flow is desired upon increased pressure or signal input, making it crucial in many control processes within refrigeration and HVAC systems.

This understanding of reverse acting illustrates why it accurately defines this particular control valve. It highlights the relationship between movement and control inputs, which is vital for anyone completing the TSSA Refrigeration Class 4A Certificate.