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Analysis of Breather Valves and Safety Valves
Created on 09.10
In-Depth Analysis of Breather Valves and Safety Valves in Chemical Production: How to Avoid Confusion?
In chemical production, valves are critical fluid control devices, and their correct selection and use directly affect production safety and efficiency. Among them, breather valves and safety valves are often confused due to similarities in function. This article provides a comprehensive analysis of the definitions, working principles, structural features, and key differences between these two types of valves, helping chemical industry personnel understand and apply them accurately.
I. Detailed Explanation of Breather Valves
1. Definition and Function
A breather valve is a two-way automatic regulating valve primarily used on atmospheric storage tanks for liquid media (such as oil tanks and chemical storage tanks). Its core functions are:
- When the medium in the tank increases and pressure rises, it discharges excess gas (exhalation function).
- When the medium decreases and negative pressure forms, it draws in air (inhalation function).
- Maintains pressure balance between the inside and outside of the tank, preventing tank deformation or damage caused by pressure changes.
2. Working Principle
The breather valve operates automatically based on pressure differential:
- Normal State:
When the tank pressure is within the set range, the valve remains closed, ensuring tank sealing integrity.
- Positive Pressure State:
When pressure exceeds the set positive pressure value (typically around +2 kPa), the pressure disc opens and discharges gas.
- Negative Pressure State:
When vacuum exceeds the set negative pressure value (typically around -0.5 kPa), the vacuum disc opens and draws in air.
3. Structural Features
A typical breather valve consists of the following components:
- Valve Body:
Usually made of cast iron, cast steel, or stainless steel, with corrosion-resistant properties.
- Dual Disc System:
Includes a pressure disc (exhalation) and a vacuum disc (inhalation).
- Flame Arrester:
Built-in flame-arresting element to prevent external ignition sources from entering the tank.
- Protective Structure:
All-weather breather valves are also equipped with rain, dust, and freeze protection features.
II. Detailed Explanation of Safety Valves
1. Definition and Function
A safety valve is a one-way overpressure protection device primarily used on boilers, pressure vessels, and piping systems. Its core functions are:
- Automatically opens to relieve pressure when system pressure exceeds a preset safety value.
- Automatically closes after pressure returns to normal.
- Provides discharge capability only; it does not have inhalation capability.
2. Working Principle
The safety valve operates on the principle of force balance:
- Closed State:
During normal system operation, the force exerted by the medium pressure on the disc is less than the sealing force provided by the spring/lever.
- Open State:
When system pressure exceeds the set value (set pressure), the medium force overcomes the sealing force, and the valve opens to relieve pressure.
- Reseating State:
When pressure drops to the reseating pressure (typically 5–10% below the set pressure), the valve closes again.
3. Main Types and Structures
According to the loading mechanism, safety valves are divided into three main types:
Type | Working Principle | Advantages | Disadvantages | Applications |
Spring-loaded | Uses compressed spring force to balance medium pressure | Compact structure, high sensitivity, flexible installation | Spring force significantly affected by temperature | Mobile pressure vessels, medium- and low-pressure systems |
Lever-loaded | Balances medium pressure via weight and lever | Simple structure, accurate adjustment, high-temperature resistant | Bulky, prone to vibration and leakage | Boilers and high-temperature pressure vessels |
Pilot-operated (Pulse Type) | Consists of main valve and pilot valve; opens via pilot action | Suitable for large discharge capacity systems | Complex structure, high cost | Large boilers and high-pressure systems |
III. Key Differences Between Breather Valves and Safety Valves
Although both are automatic valves, they differ fundamentally in several aspects:
Comparison Dimension | Breather Valve | Safety Valve |
Function | Two-way regulation (both exhalation and inhalation) | One-way action (discharge only) |
Working Pressure | Low pressure range (typically ±2 kPa) | Medium to high pressure range (from a few kPa to tens of MPa) |
Application | Atmospheric storage tanks (oil tanks, chemical storage tanks) | Pressure vessels, boilers, piping systems |
Structural Design | Dual disc structure (pressure + vacuum) | Single disc structure (overpressure discharge) |
Action Frequency | Frequent action (with liquid level changes) | Occasional action (only during overpressure) |
Sealing Requirements | Relatively low (minor leakage permitted) | Extremely high (zero leakage required) |
Analysis of Reasons for Confusion:
- Both belong to the automatic valve category and have pressure regulation functions.
- Conceptual overlap exists in some literature, e.g., referring to safety valves as "single-acting breather valves."
- Appearance may be similar, making it difficult for non-professionals to distinguish visually.
- Some specially designed breather valves may incorporate safety valve functions.
IV. Selection and Application Recommendations
1. Breather Valve Selection Points
- Select suitable materials based on medium characteristics (use stainless steel for corrosive media).
- Consider ambient temperature and choose freeze-protected models (all-weather breather valves required in cold regions).
- Determine appropriate pressure set points (positive/negative pressure).
- Flame arresters must be provided for flammable and explosive media.
2. Safety Valve Selection Points
- Accurately calculate the required discharge capacity.
- Determine set pressure based on system working pressure.
- Consider medium characteristics (corrosivity, viscosity, cleanliness, etc.).
- Select the appropriate structural type (spring-loaded, lever-loaded, or pilot-operated).
3. Common Mistakes and How to Avoid Them
Mistake 1: Using a breather valve instead of a safety valve on a pressure vessel.
Consequence: Insufficient overpressure protection.
Solution: Strictly select dedicated safety valves in accordance with pressure vessel codes.
Mistake 2: Using a safety valve instead of a breather valve on an atmospheric storage tank.
Consequence: Cannot prevent negative pressure damage and may be damaged by frequent action.
Solution: Select breather valves specifically designed for storage tanks.
Mistake 3: Neglecting regular inspection and maintenance.
Consequence: Valve freezing, clogging, or sticking leads to functional failure.
Solution: Establish a regular inspection schedule (recommended every 6 months).
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