Control Valves & Final Elements
Valve types, characteristics, Cv, rangeability, positioners, fail-safe actions, cavitation, flashing, hysteresis, and actuators.
- Identify common control valve types and their flow characteristics
- Define Cv, rangeability, and valve sizing principles
- Explain positioner function, actuator types, and fail-safe actions
- Describe cavitation, flashing, and hysteresis in control valves
Lesson 1
Valve Types & Flow Characteristics
Common Valve Types
Control valves are the final control elements in a process loop. The most common types are globe valves (single and double seat), butterfly valves, ball valves, and rotary plug valves. Globe valves provide the best throttling control and are the most widely used for modulating service.
Flow Characteristics
The flow characteristic describes the relationship between valve stem position and flow through the valve. The three inherent characteristics are:
- Linear - flow is directly proportional to stem position. Best for constant pressure drop systems.
- Equal percentage - each increment of stem travel produces an equal percentage change in flow. Best for systems with varying pressure drop.
- Quick opening - large flow change at initial opening. Used for on-off service.
Linear
Flow vs Travel: Straight line
Best for: Constant DP systems
Example: Liquid level control
Equal Percentage
Flow vs Travel: Exponential curve
Best for: Varying DP systems
Example: Pressure and temperature control
Quick Opening
Flow vs Travel: Steep initial, flat later
Best for: On-off service
Example: Relief/bypass valves
An equal percentage control valve is best suited for systems with varying pressure drop. A linear valve is best for constant pressure drop. These characteristics define how flow changes with valve position.