A valve is a control component in fluid delivery systems. Its primary function is to change the cross-section of the flow path and the flow direction of the medium, with functions including flow diversion, shut-off, regulation, throttling, check, diversion, or overflow and pressure relief. Simply put, a valve is the “switch” and “regulator” of a piping system, used to control the flow of liquids, gases, or slurry fluids. A detailed explanation of valves is as follows:
1. Core Functions
Valves do more than just “open” and “close”; their roles include:
- Shut-off (On/Off): Completely cut off fluid flow (e.g., gate valves, ball valves).
- Regulation: Adjust flow rate or pressure within the pipeline (e.g., control valves, throttle valves).
- Check: Prevent reverse flow of fluid (e.g., check valves).
- Safety Protection: Automatically discharge excess fluid when system pressure exceeds a set value to protect equipment (e.g., safety valves).
- Diversion: Change fluid direction or distribute flow (e.g., three-way ball valves).
2. Main Classifications
Valves come in a wide variety and are typically classified as follows:
By function and application
- Shut-off valves: Used to isolate or connect pipelines, such as gate valves, globe valves, ball valves, butterfly valves.
- Check valves: Prevent backflow of the medium.
- Control valves: Regulate pressure and flow of the medium.
- Safety valves: Automatically relieve pressure when equipment is overpressurized.
- Diverter valves: Distribute, separate, or mix the medium.
By structure (closure member shape)
- Gate Valve: Opens and closes by moving a gate up and down like a door.
- Ball Valve: Uses a rotating ball around the stem axis to open/close, with fast operation.
- Globe Valve: Adjusts flow via a disc moving up and down, offering good regulation performance.
- Butterfly Valve: Uses a rotating disc (≈90° rotation) to open, close, or regulate flow.
- Check Valve: Automatically opens and closes using fluid pressure to prevent backflow.
By material
- Cast iron valves: For low-pressure, non-corrosive media.
- Cast/Forged steel valves: For medium‑high pressure, high-temperature steam or oil services (e.g., WCB, A105).
- Stainless steel valves: For corrosive media (e.g., 304, 316L).
- Alloy steel valves: For special high-temperature and high-pressure conditions (e.g., Cr‑Mo steel).
3. Working Principle
The basic principle of a valve is to use an actuator (handwheel, electric actuator, pneumatic device) to drive the valve stem, moving the closure member (gate, ball, disc) relative to the seat. This changes the flow area and achieves fluid control.
4. Key Components
A complete valve generally consists of the following parts:
- Body: The outer shell of the valve, forming the flow passage for the medium.
- Bonnet: Connected to the body to form a pressure chamber, usually housing the stem and sealing components.
- Closure member (Disc / Plug / Ball): The part that directly controls fluid flow.
- Seat: Forms a sealing pair in contact with the closure member.
- Stem: Connects the closure member and actuator, transmitting motion and force.
- Gland / Packing: Seals the gap between the stem and bonnet to prevent leakage.
- Actuator: Includes handwheels, gearboxes, electric actuators, or pneumatic actuators.
5. Application Areas
Valves are used in almost all industries involving fluids:
- Oil & gas: Wellhead equipment, long-distance pipelines.
- Chemical industry: Reactors, transfer pipelines.
- Power industry: Boiler feedwater and steam systems.
- Water supply & drainage: Water plants, wastewater treatment plants.
- Construction & HVAC: Central air conditioning, fire protection systems.
Summary
Valves are indispensable control elements in piping systems. Like heart valves in the circulatory system or faucets on water pipes, they open, close, and regulate to ensure fluids flow safely and as intended.
