Safety Valves–Core Protection for Pressure Equipment

1. Definition and Function

• Working logic: Auto opens to discharge medium when system pressure exceeds preset value; auto closes when pressure returns to safe range.
• Core function: Limit system pressure within allowable range, prevent explosion of pressure vessels and pipelines, the key safety fitting for pressure-bearing equipment.
• Usage restriction: Only for overpressure protection, normally closed; cannot be used as conventional isolation valves.

2. Basic Working Principle

1. System medium pressure generates upward thrust on valve disc; spring/weight supplies downward preload to seal the disc tightly.
2. Normal condition: Medium thrust ＜ spring force → valve stays sealed.
3. Overpressure state: Medium thrust exceeds spring load → disc lifts for medium discharge.
4. Pressure drops to re-seating pressure → spring pushes disc back to closed position.

3. Main Components

• Seat: Fixed sealing base & medium flow channel
• Disc: Opening/closing sealing part, forms sealing pair with seat
• Spring: Provides sealing preload, core part of spring-type safety valves
• Stem: Transmits spring load
• Adjusting screw: Calibrates set pressure
• Body & bonnet: Pressure-bearing shell
• Lifting lever: For on-site manual opening test
• Guide sleeve: Keeps disc moving vertically to avoid jamming

4. Main Classification

2.Weight-lever safety valve
Loaded by weights and levers; stable pressure performance, fit for high-temperature boilers; large size, must be vertically mounted.

3.Pilot-operated safety valve
Consists of main valve and pilot valve; small blowdown, good tightness, suitable for high-pressure & large-flow working conditions.

5. Key Performance Terminology

• Set Pressure: Calibrated pressure when the valve starts to open
• Relieving Pressure: Inlet pressure at rated discharge capacity
• Re-seating Pressure: Pressure when the valve fully closes after pressure relief
• Blowdown: Pressure difference between set pressure and re-seating pressure
• Rated Discharge Capacity: Stable medium flow rate under standard working conditions
• Seat Tightness Test Pressure: Pressure lower than set pressure for leakage inspection

6. Basic Selection Guidelines

1. Medium type: Low-lift for liquid; full-lift for steam & gas
2. Medium hazard: Closed structure for toxic, flammable media
3. Materials: Match valve body & sealing material with temperature and design pressure
4. Specification: Calculate required relief capacity to confirm valve size
5. Installation: Check space and connection standards (RF flange, NPT thread, etc.)
6. Special measures: Insulate springs from high heat; protect sealing surfaces for particle-containing medium

7. General Installation Requirements

1. Mount vertically upward as standard.
2. Shorten inlet pipelines; no isolation valves on inlet. If bypass valve exists, lock it open with lead seal.
3. Support discharge pipelines independently to avoid pipeline load on valve body.
4. Set drain pockets at discharge pipe low points to eliminate condensate backpressure.