Among the many types of safety valves, the full-lift safety valve, with its unique design and outstanding performance, has become a widely used safety device in the industrial field. The opening height of a full-lift safety valve is greater than or equal to one-quarter of the flow channel diameter. This design enables it to achieve rapid and large-volume medium discharge during operation, effectively protecting equipment from overpressure damage. It is widely applied across various industrial settings, from steam boilers to air compression systems, from chemical equipment to food processing machinery, full-lift safety valves play an indispensable role.
Today, let us take a deep look into the full-lift safety valve. We will analyze this important industrial safety device from its definition, working principle, advantages, critical pressure ratio, comparison with the low-lift safety valve, to its application scenarios and maintenance. Through this article, you will gain a comprehensive and in-depth understanding of full-lift safety valves, enabling you to better select and use this key safety equipment.
A full-lift safety valve is a special type of safety valve, with an opening height greater than or equal to one-quarter of the flow channel diameter. This design allows the valve to achieve rapid, high-volume medium discharge during operation, effectively protecting equipment from overpressure damage.
The main parameter of a safety valve is discharge capacity, which determines the amount of medium a safety valve can release per unit of time. The discharge capacity depends on the seat diameter and the opening height of the disc. Based on different opening heights, safety valves are divided into low-lift and full-lift types. The low-lift type has a relatively small opening height, 1/40–1/20 of the throat diameter, suitable for liquid media and low-pressure gas; while the full-lift type has an opening height of 1/4 of the throat diameter, suitable for steam and gases, i.e., compressible media.
The action characteristics of the full-lift safety valve belong to the two-stage type. When applied to steam, it usually exhibits a sudden full-opening action, with the pressure at which this occurs called the set pressure (pop pressure). When applied to air, the disc first opens gradually with increasing inlet pressure and then suddenly opens fully once a certain pressure is reached. This opening mode is called two-stage opening. However, full-lift safety valves are generally not suitable for liquid media.
The full-lift safety valve is widely applied in industry because of its many advantages.
The structure of the full-lift safety valve is straightforward, making manufacturing and maintenance relatively easy. At the same time, its sealing performance is excellent. The seat adopts a Laval nozzle-type design, allowing steam to reach sonic velocity at the seat outlet, thereby achieving a higher discharge coefficient. In addition, the sealing surface of the seat is hardfaced with cobalt-chromium-tungsten alloy, a material with wear and erosion resistance, which significantly extends the seat’s service life.
The opening pressure of the full-lift safety valve can be precisely set by adjusting the regulating nut. By adjusting the compression of the spring, the set pressure can be easily and accurately obtained. Moreover, because of its relatively large opening height, the discharge capacity is also correspondingly larger, enabling a rapid reduction of internal pressure within a short time to ensure safe equipment operation.
The reseating pressure difference refers to the pressure drop required for the valve to close again after discharging. The full-lift safety valve has a relatively small reseating pressure difference, meaning it can quickly reseat when the pressure drops slightly, preventing unnecessary leakage. At the same time, by adjusting the auxiliary mechanism for back pressure on the disc, an appropriate reseating pressure difference can be easily obtained. When adjusted upward, valve back pressure decreases; when adjusted downward, valve back pressure increases.
The disc of the full-lift safety valve adopts a thermo-elastic structure, allowing slight deformation under medium pressure to improve sealing ability and effectively overcome the pre-leak phenomenon that occurs when the medium pressure approaches the set pressure. The disc sealing surface is treated with advanced laser hardening technology, further improving hardness, wear resistance, and impact resistance.
The critical pressure ratio is an important parameter in the design and use of safety valves. It refers to the ratio of outlet to inlet pressure when the gas velocity at the minimum flow cross-section reaches local sonic velocity. For nozzles, the critical pressure ratio can be determined by formula. When the nozzle outlet-to-inlet pressure ratio is lower than or equal to the critical pressure ratio, the nozzle is in critical or supercritical flow, at which point the pressure at the outlet section remains unchanged, and the relative discharge also remains unchanged.
However, for spring-loaded full-lift safety valves, due to their complex structure, it is difficult to measure the gas velocity at the minimum throat area, and thus impossible to accurately determine the critical pressure ratio based on whether sonic velocity is reached. Currently, the method for determining whether a spring-loaded full-lift safety valve has reached critical flow is by measuring the discharge coefficient. When the discharge coefficient does not change with the pressure ratio, the valve is considered to have reached critical flow. Experimental results show that the discharge of spring-loaded full-lift safety valves always varies with the pressure ratio, but when the ratio is below 0.2–0.3, the variation is relatively small. Therefore, this small variation is considered measurement error, and the critical pressure ratio is judged to be about 0.2–0.3.
There are significant differences between full-lift and low-lift safety valves in terms of structure, discharge, and applicable media.
The disc opening height of a low-lift safety valve is relatively small, 1/40–1/20 of the throat diameter. Its opening height is proportional to pressure: at low pressure, the opening height is small; at high pressure, it is larger. The disc opens gradually, and reseats gradually as pressure decreases. In contrast, the full-lift safety valve has a disc opening height of 1/4 of the throat diameter, with a blowdown ring attached. When pressure reaches the set value, the blowdown ring pushes the disc to open fully and rapidly, achieving large-volume discharge.
The discharge coefficient of low-lift safety valves is relatively small. At 1/40 opening height, the discharge coefficient is 0.14–0.16; at 1/20 opening height, it is 0.07–0.08. The discharge coefficient of a full-lift safety valve is much higher, 0.7–0.8, which is 5–10 times that of a low-lift safety valve. This means a full-lift safety valve can discharge much more medium in a short time, quickly reducing equipment pressure.
Low-lift safety valves are suitable for liquid media, low-pressure gases, and steam, with relatively small discharge. Since liquids are incompressible, even small discharges can effectively reduce pressure. Full-lift safety valves, on the other hand, are more suitable for compressible media such as steam and gases, providing greater discharge and rapid depressurization to ensure equipment safety.
Full-lift safety valves are widely used in various industrial fields, especially in equipment handling compressible media such as steam and gases. For example, in boilers, pressure vessels, and piping systems, full-lift safety valves effectively prevent equipment hazards caused by excessive pressure. When the internal pressure of the equipment reaches the set value, the valve quickly opens and rapidly discharges the medium, thereby reducing internal pressure and protecting the equipment from damage.
To ensure proper operation, regular maintenance and care of full-lift safety valves are essential. The following are some recommendations.
Check the appearance of the safety valve regularly for signs of damage, corrosion, or leakage. Also, check whether the adjusting nut and back pressure adjustment sleeve are loose to ensure they remain in normal working condition.
Clean the surface and internal parts of the safety valve regularly to remove dust, impurities, and dirt. For components requiring lubrication, such as the spring and valve stem, apply suitable lubricants regularly to reduce wear and extend service life.
Periodically calibrate the safety valve to ensure its opening and reseating pressures match the set values. If performance declines, promptly adjust or repair the valve.
As a highly efficient and reliable safety device, the full-lift safety valve plays a vital role in industrial production. With advantages such as simple structure, excellent sealing, accurate opening pressure, large discharge capacity, small reseating pressure difference, and convenient adjustment, it effectively ensures the safe operation of equipment. When selecting a safety valve, the choice between full-lift and low-lift types should be based on the specific medium, pressure range, and equipment requirements, to ensure safe and stable operation. At the same time, regular maintenance and care are key to ensuring the long-term stability of safety valves.
