Safety valves are core safety accessories in boilers, pressure vessels, and piping systems, preventing overpressure and ensuring safe equipment operation through automatic pressure relief. They are classified by structure into spring-loaded, lever-type, and pulse-type valves, with the spring-loaded type being the most widely used. They are also classified by discharge method into fully enclosed, semi-enclosed, and open types. Safety valves must be installed vertically and regularly calibrated and maintained to ensure sealing and reliable operation. They are widely used in steam boilers, pressure vessels, petrochemical industries, and other fields, serving as crucial protective devices to prevent accidents caused by uncontrolled pressure.
Safety Valve Working Principle
Phase 1: Normal Operation (Closed State)
Under the normal operating pressure of the equipment, the downward pressure exerted by the loading mechanism (e.g., spring) on the valve disc is greater than the upward thrust of the medium on the valve disc. At this time, the valve disc is tightly pressed against the valve seat, relying on the sealing force to prevent medium leakage, and the equipment maintains normal operation.
Phase 2: Overpressure Alarm (Open State)
When the internal pressure of the system rises due to faults or other reasons, exceeding the set pressure (opening pressure):The upward thrust of the medium on the valve disc begins to exceed the downward pressure of the loading mechanism.The spring is compressed (or the lever is lifted), the valve disc leaves the valve seat, and the valve opens instantaneously.The medium is discharged through the valve at high speed, producing a loud airflow sound.
Phase 3: Restoration to Safety (Reseat Closure)
As the medium is discharged, the pressure in the system drops rapidly. When the pressure drops to a certain level (called the reseat pressure), the thrust of the medium is less than the spring force/gravity of the loading mechanism. The loading mechanism pushes the valve disc back into position, tightly fitting the valve seat, cutting off the medium discharge, and the equipment resumes safe operation.
How Safety Valves Ensure Safety
- Automatic Pressure Relief: When the pressure of the medium inside equipment or pipeline exceeds the specified value, the safety valve automatically opens, discharging the medium to the outside of the system, reducing pressure, and preventing accidents such as equipment damage and explosions caused by overpressure.
- Closing After Pressure Recovery: When the pressure drops to the normal range, the safety valve automatically closes, preventing further flow of the medium, ensuring normal system operation, and reducing media waste.
- Type Compatibility Guarantee: Different structural types of safety valves are suitable for different scenarios. For example, spring-loaded valves are suitable for most pressure vessels, while pulse-loaded valves are used for vessels with high pressure and large discharge capacity. Matching ensures safety.
- Regular Calibration and Maintenance: Safety valves require regular calibration and maintenance to ensure their sealing and operational reliability, and to promptly identify and resolve potential malfunctions.
Symptoms of Safety Valve Failure
Pressure Exceeds Design Limit and Fails to Operate
When the system pressure gauge shows that the set opening pressure has been reached or exceeded, the safety valve remains motionless, showing no signs of venting or pressure relief. As the equipment pressure continues to rise, the safety valve remains silent, as if it were “dead.”
Leakage
Gas/Vapor Leakage: You will hear a continuous hissing sound, or see steam escaping from the silencer or abnormal steam discharge from the drain pan.
Liquid Leakage: In water systems, you will see continuous water droplets seeping from the valve body outlet, or water accumulation on the ground.
Media Deterioration: For some special media, leakage may cause an unpleasant odor or corrosion in the surrounding environment.
Frequency and Vibration
The safety valve opens and closes frequently in a short period of time, or the valve body vibrates violently. It emits a piercing whistling sound or a metallic clanging sound. You can clearly feel the pipes connected to the safety valve vibrating violently, which may even cause weld cracks.
Frequency and Vibration
The safety valve opens and closes frequently in a short period of time, or the valve body vibrates violently. It emits a piercing whistling sound or a metallic clanging sound. You can clearly feel the pipes connected to the safety valve vibrating violently, which may even cause weld cracks. Jamming
Insufficient opening height: Although the safety valve jumps up, it only opens slightly, failing to reach a fully open state, resulting in insufficient venting and a slow drop in system pressure. Slow or no reseating: Even though the pressure has dropped significantly, the valve remains open, continuing to vent steam. When you pull the lifting handle (if any) for testing, the valve does not automatically reset after being lifted and released, or requires a tool to push it back.
Abnormal Physical Appearance and Condition
The valve body, vent pipe, or drain port is blocked by thick rust or sludge, or the valve stem appears to be rusted and immobile. Damaged seals (anti-counterfeiting/anti-tampering markings) indicate possible tampering, leading to inaccurate pressure setting. The valve body or drain pipe is excessively hot to the touch when not venting.
Remedial Measures
Immediately stop the operation of the relevant equipment to prevent a continuous rise in pressure and potential danger. For example, if the safety valve of a steam boiler fails, immediately shut off the boiler and stop steam production.
If a manual operating device is available, the safety valve can be manually opened to release pressure, but safety precautions must be taken.
If leakage or abnormal opening is caused by scale, desilting material, or other debris on the sealing surface, it can be manually purged or disassembled for cleaning.
Mechanical damage or corrosion to the sealing surface can be repaired by grinding or turning followed by grinding; in severe cases, replacement is necessary. If the spring fails, replace the spring. For issues such as a bent or deformed valve stem, identify the cause and reassemble or replace the valve stem and other components.
After repairing the fault, the safety valve needs to be recalibrated and tested to ensure that its opening pressure, reseating pressure, and other parameters meet the specified requirements.
Establish a comprehensive maintenance system, regularly maintain and inspect the safety valve, and promptly identify and address potential problems.
As the “last line of defense” for pressure-bearing equipment, the failure of the safety valve often has very serious consequences. Regular maintenance is necessary; contact a professional when necessary.
