Page 2 of Safety Valve Manufacturer, Pressure Relief Valves Supplier China

The Spring-loaded safety valves is the most conventional safety valve, the spring load is preset to equal the force the inlet fluid exerts on the closed disk when the system pressure is at the set pressure of the valve. As the pressure exceeds a set value, the spring opens releasing the disc and the fluid flowing through the valve.
 

The Pilot-operated safety valves are composed of a "pilot valve" and a "main valve", differently from spring-loaded safety valves, the pressure is monitored and adjusted by the pilot valve, not the main valve. Pilot-operated safety valves are widely used for large size valve and high-pressure applications, which need lager torque and high reliability.

the Principle of pilot operation:
Step 1: THE MAIN VALVE IS CLOSED
When P < 100% of Set Pressure, 
The feeding seat of the pilot is open and the main valve’s dome is pressurized.
 
Step 2: THE MAIN VALVE OPENS
When P =100% of Set Pressure, 
The drain seat of the pilot opens and drives the closing of the feeding seat.

Step 3: THE MAIN VALVE STAYS OPEN.
When 95% < P < 110% of Set Pressure, 
The drain seat of the pilot stays open and lets dome to the atmosphere.
 
Step 4: THE MAIN VALVE IS CLOSING.
When P = 95% of Set Pressure, 
The drain seat of the pilot recloses and drives the opening of the feeding seat.
 

Bellow balanced safety valves are designed to reduce the effetc of back pressure accumulation, the "Bellows" located between the bonnet and the valve body, where maintained free from the effetc of back pressure, the opposing pressure on the inlet fluid is only generated by the spring without contribution from any sort of backpressure. The space enclosed by bellows is freely vented to air, the back pressure is expetced to be 10 - 50% of the set pressure. The Bellows can also protetc the spring and other trim components from corrosive media.
 

Breather Valves, also named Pressure/Vacuum Relief Valves, available with flanged outlets or vented to atmosphere, are special types of Relief Valves which are specifically designed for tank protetcion. Breather Valves are usually installed in the in-and out breathing lines of tanks, vessels and process equipment to prevent the of excessive pressure or vacuum which can unbalance the system or damage the storage vessels.
 

Changeover valves, also called Switch valves, are designed to connetc two safety valves to a single pressurized system, acting as the three ways devices for flow seletcion in a pipeline.
Through these Changeover valves, one of the two safety valves is in operation and ensures the protetcion of the system, the other safety valve is in "stand by" ready to come into operation if the first one must be changed or removed to maintenance. Changeover valves are often required for the continuous production.