Valve classification is a crucial aspect in the selection and application of valves in various industries. Valves are categorized based on their purpose, structural design, and operational parameters to ensure they meet specific requirements for fluid control, pressure regulation, and system safety. Below is an expanded and improved version of the original content, written in natural English with added details to enhance clarity and completeness.
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### **1. Classification by Purpose and Function**
Valves serve different roles in fluid systems, and understanding these roles helps in selecting the right type of valve for a particular application.
- **Shut-off Valves**: These are used to either cut off or allow the flow of media. Common types include gate valves, globe valves, diaphragm valves, ball valves, plug valves, butterfly valves, plunger valves, needle valves, and others. They are ideal for applications where complete isolation of the medium is required.
- **Regulating Valves**: Designed to control the flow rate, pressure, or temperature of the medium. Examples include control valves, throttling valves, and pressure-reducing valves. These are essential in systems requiring precise regulation of process parameters.
- **Check Valves**: Prevent backflow of the medium, ensuring that it flows only in one direction. Types include swing check valves, lift check valves, and dual-disc check valves. They are commonly used in pipelines to prevent reverse flow and protect equipment from damage.
- **Diverter Valves**: Used to separate, distribute, or mix media. These include distribution valves, traps, and other specialized structures. They are vital in systems where media needs to be redirected or blended.
- **Safety Valves**: Protect the system against overpressure by releasing excess pressure when it exceeds a set limit. Safety valves come in various forms, such as spring-loaded, pilot-operated, and weight-loaded types.
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### **2. Classification by Key Parameters**
Valves can also be classified based on their operating conditions, including pressure, temperature, and material.
#### **(a) Pressure Classification**
- **Vacuum Valves**: Operate at pressures below atmospheric.
- **Low-pressure Valves**: PN < 1.6 MPa.
- **Medium-pressure Valves**: PN between 2.5 MPa and 6.4 MPa.
- **High-pressure Valves**: PN between 10.0 MPa and 80.0 MPa.
- **Ultra-high-pressure (UHP) Valves**: PN > 100 MPa.
#### **(b) Temperature Classification**
- **High-temperature Valves**: T > 450°C.
- **Medium-temperature Valves**: 120°C < T < 450°C.
- **Room-temperature Valves**: -40°C < T < 120°C.
- **Cryogenic Valves**: -100°C < T < -40°C.
- **Ultra-cryogenic Valves**: T < -100°C.
#### **(c) Material Classification**
- **Non-metallic Valves**: Such as ceramic, glass fiber, and plastic valves.
- **Metallic Valves**: Including copper alloys, aluminum alloys, lead alloys, titanium alloys, Monel, cast iron, carbon steel, cast steel, low-alloy steel, and high-alloy steel.
- **Lined Metal Valves**: Such as lead-lined, plastic-lined, and enamel-lined valves.
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### **3. General Taxonomy**
This classification is based on the valve’s working principle and structure, and it is widely used both internationally and domestically. Common types include:
- Gate valves
- Globe valves
- Throttle valves
- Instrumentation valves
- Plunger valves
- Diaphragm valves
- Plug valves
- Ball valves
- Butterfly valves
- Check valves
- Safety valves
- Control valves
- Drain valves
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### **4. How to Choose a Valve**
Selecting the right valve involves considering several factors, including the nature of the medium, system requirements, and operational conditions.
- **Pressure-reducing valves** and **counterbalance valves** should always have bypasses.
- **Ball valves** and **gate valves** are best for full open and full close operations.
- **Globe valves** are often preferred for flow regulation.
- **Valve resistance calculations** must be taken into account during selection.
- **Electric valves** are suitable for automation and remote operation.
#### **Butterfly Valves**
Butterfly valves are ideal for large-diameter pipes due to their compact design and quick operation. They operate by rotating a disc within the valve body, allowing for rapid opening and closing. The disc provides minimal resistance when fully open, resulting in low pressure drop. They can be made with flexible or metal seals, depending on the application. Butterfly valves are commonly used in water treatment, oil, gas, and chemical industries.
#### **Ball Valves**
Ball valves are known for their tight shut-off capabilities and durability. They feature a spherical closure element with a bore that aligns with the flow path when the valve is open. A 90-degree rotation allows for quick operation. Ball valves are suitable for a wide range of media, including water, solvents, acids, and gases like oxygen and methane.
#### **Globe Valves**
Globe valves are excellent for throttling and regulating flow. Their stem moves perpendicular to the seat, creating a reliable seal when closed. They are commonly used in systems where frequent adjustments are needed.
#### **Check Valves**
These valves automatically prevent backflow. Swing check valves use a hinged flap that opens under forward flow and closes when flow reverses. Lift check valves use a disk that lifts off the seat under pressure and falls back when flow stops.
#### **Control Valves**
Used to regulate flow, pressure, or temperature, control valves can be actuated by electricity, air, or hydraulic power. They are essential in automated and process control systems.
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### **5. Main Technical Performance of Valves**
- **Strength Performance**: Refers to the valve's ability to withstand internal pressure without failure. It ensures long-term reliability under operational stresses.
- **Sealing Performance**: Measures the valve’s ability to prevent leakage. There are three key sealing areas: the closing element and seat, the packing, and the connection between the body and cover.
- **Flow Resistance**: The pressure drop across the valve affects energy efficiency. Minimizing this is important for system performance.
- **Action Performance**: Includes sensitivity, reliability, opening/closing force, and speed. These factors influence how effectively the valve responds to changes in the system.
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### **6. Valve Identification and Painting**
Valve identification involves using color codes to indicate material and function:
- **Body Material Colors**:
- Gray iron, malleable iron: Black
- Ductile iron: Medium gray
- Carbon steel: Silver
- Acid-resistant and stainless steel: Blue
- Alloy steel: Blue
- **Sealing Surface Colors**:
- Copper alloy: Red
- Pasteurized steel: Light yellow
- Acid-resistant steel: Sky blue
- Stainless steel: Sky blue
- Nitrided steel: Sky blue
- Boron steel: Dark yellow
- Hardened alloy: Fuchsia
- Monel: Green
- Plastic/Rubber: Black
- Cast iron: Black
- **Actuators**:
- Electric actuators: Gray for standard, blue for triple-rated (outdoor, anti-corrosion, explosion-proof).
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### **7. General Principles for Water Supply Systems**
In water supply systems, the following guidelines are commonly followed:
1. For pipe diameters ≤ 50 mm, use globe valves; for larger diameters, use gate or butterfly valves.
2. For flow and pressure adjustment, use control or globe valves.
3. For low-resistance sections (e.g., pump suction lines), use gate valves.
4. For bidirectional flow, use gate or butterfly valves, not globe valves.
5. For small installations, use butterfly or ball valves.
6. For frequent opening and closing, use globe valves.
7. For pump discharge lines, use multi-functional valves.
Additionally, valves should be installed at key points such as:
- Introduction pipes from municipal water supplies.
- Ring network nodes in residential areas.
- Pump suction and discharge lines.
- Water meters and branch pipes.
- Equipment fill and drain lines.
- Plumbing fixtures and accessories like pressure relief valves and backflow preventers.
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By understanding the classification, technical performance, and application principles of valves, engineers and technicians can make informed decisions that enhance system efficiency, safety, and longevity.
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