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Analysis of the Differences and Applications Between Partial Lift and Full Lift Safety Valves

Release time:

Jan 19,2026

Safety valves are employed in boilers, pipelines, or pressure vessels to discharge the medium when pressure exceeds the set value, thereby reducing pressure and safeguarding equipment integrity. Once pressure returns to normal, the valve automatically closes.

Safety valves are employed in boilers, pipelines, or pressure vessels to discharge the medium when pressure exceeds the set value, thereby reducing pressure and safeguarding equipment integrity. Once pressure returns to normal, the valve automatically closes.

一、Low-Lift Safety Valve

The valve disc of a low-lift safety valve opens to a height of 1/40 to 1/20 of the seat throat diameter. The disc opening height is directly proportional to pressure: lower at low pressure and higher at high pressure. The disc opens progressively and gradually reseats as pressure decreases. This type of safety valve is suitable for liquid media, low-pressure gases, and steam, with a relatively small discharge capacity. Generally, liquids are incompressible, and a small discharge volume can effectively reduce pressure.

Discharge Area: Equals the cross-sectional area of the cylindrical or conical passage formed when the valve seat and disc open.

Discharge Coefficient: At an opening height of 1/40, the discharge coefficient is 0.14–0.16; at 1/20, it is 0.07–0.08.

 

二、Full-Lift Safety Valves

The disc lift height of a full-lift safety valve is one-quarter of the seat throat diameter, with the discharge area equalling the flow passage area. The disc incorporates a counter-thrust disc; when pressure reaches the set value, the counter-thrust disc forces the disc to rapidly open fully, enabling high-volume discharge. This design allows full-lift safety valves to rapidly reduce pressure, ensuring equipment safety.

Discharge coefficient: 0.7–0.8, representing 5–10 times that of a pop-up type safety valve.

Seat bore diameter: Two nominal sizes smaller than the valve bore. For example, for PN16 valves with a bore diameter DN50, the seat diameter is d=32, while the outlet bore diameter is DN65—one size larger than the inlet bore.

Structural characteristics: The pressure of the discharged medium acts upon the counter-pressure disc, prompting the valve disc to open rapidly. The valve disc employs a pop-up opening mechanism with instantaneous reseating, delivering high discharge efficiency.

 

 

三、Principal Differences

Disc Lift Height: The lift height of a low-lift safety valve is relatively small (1/40–1/20), whereas that of a full-lift safety valve is considerably greater (1/4).

Capacity Coefficient: Full-lift safety valves possess a larger capacity coefficient, making them suitable for compressible media such as steam and gases; low-lift safety valves have a smaller capacity coefficient and are appropriate for incompressible media like liquids.

Structural Design: Full-lift safety valves incorporate a counter-flush disc enabling rapid full opening, whereas lift-type valves feature progressive disc opening with lower discharge capacity.

Lift-type and full-lift safety valves exhibit distinct differences in structure, discharge capacity, and suitable media. Lift-type valves are appropriate for low-pressure liquids and steam with limited discharge; full-lift valves serve high-pressure gases and steam, providing greater discharge capacity and rapid pressure reduction.

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