Valves & Flow Control
Valve types, applications, flow characteristics, check valves, pressure relief, and control valve selection.
- Identify gate, globe, ball, and butterfly valve types and applications
- Explain check valve operation and proper installation orientation
- Describe pressure relief valve function and set-point adjustment
- Identify control valve types and their flow characteristics
- Select the correct valve type for a given service condition
Leçon 1
Gate, Globe, Ball & Butterfly Valves
Isolation vs. Throttling
Valves serve two main purposes: isolation (fully open or fully closed) and throttling (regulating flow). Using the wrong valve type for the job causes premature wear, poor control, and safety hazards.
Isolation Valves
Gate, ball, plug
Full open or full closed only
Low pressure drop when open
Not designed for flow regulation
Throttling Valves
Globe, butterfly, needle
Regulate flow at partial positions
Higher pressure drop when open
Designed for flow control
Gate Valve
The gate valve is the most common isolation valve. A wedge-shaped disc slides up and down to open or close the flow path. Gate valves provide a straight-through flow path with minimal pressure drop when fully open. They must never be used partially open - the disc vibrates and causes severe wire drawing (erosion of the seating surface).
Globe Valve
The globe valve is designed for throttling. The disc moves perpendicular to the flow path, creating a variable opening. The S-shaped internal passage provides excellent flow control but creates a higher pressure drop than a gate valve. Globe valves are installed with flow direction matching the arrow on the body - typically flow under the disc.
Ball Valve
The ball valve uses a rotating sphere with a bore through the center. A quarter-turn (90 degrees) of the handle opens or closes the valve. Ball valves provide fast operation, tight shutoff, and low pressure drop. Full-bore ball valves have the same bore as the pipe, allowing pigging and cleaning tools to pass through.
Exam Tip - Valve Selection
When the exam asks which valve is best for throttling, the answer is globe valve. When it asks which valve has the lowest pressure drop, the answer is gate valve or full-bore ball valve.
Butterfly valves:
The butterfly valve uses a rotating disc (the "butterfly") mounted on a shaft through the valve body. A quarter-turn opens or closes the valve. Butterfly valves:
- Are compact and lightweight compared to gate or globe valves of the same size
- Are used for throttling in large-diameter lines where globe valves would be impractical
- Have higher pressure drop than ball valves when fully open (the disc always remains in the flow path)
- Are commonly used on cooling water, water treatment, and HVAC systems
Plug valves:
A plug valve uses a tapered cylindrical plug with a port that aligns with the pipe bore when open. Quarter-turn operation like a ball valve, but the tapered plug can be lubricated to maintain a tight seal. Plug valves are used in services where the fluid contains suspended solids that might damage ball valve seats.
Gate valves are for isolation with low pressure drop - never throttle with a gate valve. Globe valves are the standard for throttling service. Ball valves provide fast quarter-turn operation and tight shutoff. Butterfly valves are compact and suitable for throttling large-diameter lines.