Valves are indispensable core components of piping systems. They undertake critical functions including regulating medium flow rate, cutting off or switching pipeline circulation, and ensuring the safe operation of pipeline equipment. There is a wide variety of valves, which can be classified by pipeline functions or body materials, including stainless steel valves, cast iron valves, cast steel valves, plastic valves, ceramic valves, and lined valves. Each type of valve features distinct structural characteristics. Rational valve selection based on working conditions and medium properties is essential to guarantee the safe, stable and efficient operation of piping systems.

Gate Valve

Gate valves are mainly used for full opening and full closing of pipelines, featuring low fluid resistance and bidirectional medium flow capability, making them the mainstream choice for general process pipelines.

Gate valves are not suitable for flow regulation. When the valve is partially open, the medium continuously scours the sealing surface, causing easy damage and difficult subsequent repair. Valve stems are divided into rising stems and non-rising stems. Rising stem gate valves allow intuitive observation of valve opening and prevent corrosion of stem threads by corrosive media. Non-rising stem gate valves involve no vertical stem movement during opening and closing, making them suitable for indoor environments with limited installation space.

Gate plate structures include wedge single gate plates and parallel double gate plates. Wedge single gate plates feature complex manufacturing processes and can be used for slightly corrosive media but have poor resistance to temperature changes. Parallel double gate plates are easy to process and maintain with excellent sealing performance and good temperature adaptability. They are mostly applied in water supply and steam pipelines, as well as working conditions requiring frequent opening and closing or conveying high-viscosity media.

Globe Valve

Globe valves feature fast opening and closing speed, wide applicable pressure range, and easy grinding and repair of worn sealing surfaces, with the capability to regulate medium pressure and flow rate. Under the same nominal diameter, the fluid resistance of a globe valve is approximately 5 to 10 times that of a gate valve. Small-diameter globe valves mostly adopt non-rising stem structures, while large-diameter globe valves for high-temperature and strongly corrosive working conditions prefer rising stems.

Globe valves have inferior sealing performance compared with gate valves. They are not recommended for conveying high-viscosity, precipitable media or media containing solid impurities, and are generally not used in drainage or vacuum pipelines. Their nominal diameter is mostly within 200mm, and their installation length is longer than that of gate valves of the same specification.

Check Valve (Non-return Valve)

Check valves automatically open and close relying on the force of fluid medium without manual operation, with the core function of preventing medium backflow. They are mainly categorized into lift check valves and swing check valves.

Lift check valves are conventionally installed on horizontal pipelines, with special models applicable to vertical pipelines. Swing check valves can be installed on horizontal, inclined and vertical pipelines, provided that the medium flows upward in vertical pipelines. Water pump suction foot valves are special types of lift check valves.

Check valves are only applicable for clean and low-viscosity media and cannot be used for high-viscosity media or media containing solid particles. In terms of performance comparison, lift check valves have high fluid resistance but good sealing effect and low operating noise; swing check valves feature lower fluid resistance, relatively poor sealing performance and higher noise, and are commonly used for large-diameter pipelines.

Safety Valve

Safety valves are safety protection components for pressure-bearing equipment, boilers and pressure pipelines, serving to limit the system pressure within the allowable design value. When the internal medium pressure exceeds the set value, the valve automatically opens to release pressure; when the pressure drops back to the safe range, the valve disc closes under the force of springs or weights.

Safety valves are divided into weight lever type and spring type. Weight lever safety valves are reliable in operation but bulky and heavy, and have been mostly replaced by spring safety valves. Spring safety valves are further classified into full-lift type and low-lift type. Full-lift safety valves have a valve disc opening height no less than 1/4 of the valve seat diameter with large discharge capacity, suitable for gaseous media. Low-lift safety valves have an opening height of 1/40 to 1/20 of the valve seat diameter with small discharge capacity, mostly used for liquid media.

Based on sealing forms, spring safety valves can be divided into enclosed type and non-enclosed type. Enclosed safety valves are adopted for flammable, explosive and toxic media, while non-enclosed ones are applicable for steam and common gases. In principle, no isolating valve shall be installed between the safety valve and pressure vessel; if installation is necessary, a rising stem gate valve must be adopted and kept fully open during operation. Safety valves shall be installed vertically and as close as possible to the protected equipment.

Pressure Reducing Valve

Pressure reducing valves automatically reduce high-pressure media to the working pressure required by processes, and are widely used in pipelines for non-corrosive fluids such as steam, compressed air and water. They are mainly classified into piston type, diaphragm type and bellows type.

Piston type pressure reducing valves are the most commonly used. The output pressure is adjusted by changing the pre-tightening force of the spring through the top adjusting screw, with an adjustment rotation direction opposite to that of ordinary valves. Diaphragm type pressure reducing valves have a maximum operating temperature of 70℃, only suitable for normal-temperature media. They feature simple structure but vulnerable aging and damage of diaphragm components. Bellows type pressure reducing valves are less commonly used, with the typical model Y44-10. They support a maximum medium temperature of 200℃ and a reduced pressure range of 0.05-0.4MPa. For steam and water delivery working conditions, the bellows faces downward; for compressed air pipelines, the bellows faces upward, and the medium follows the “low-in and high-out” flow direction.

Plug Valve (Cock Valve)

Plug valves realize pipeline opening and closing through the rotation of tapered plugs with through holes. They feature compact structure, small size, fast switching speed and low fluid resistance, and serve as medium distribution and switching valves for three-way and four-way pipelines.

Their disadvantages include large sealing contact area, laborious operation, easy wear of sealing surfaces, and inapplicability for precise flow regulation, though they deliver excellent pipeline cut-off performance. They are suitable for fluid pipelines with low pressure and temperature, and can convey media containing solid particles, but not applicable for high-pressure and high-temperature working conditions.

Ball Valve

Ball valves are a fast-developing valve type, achieving opening and closing through the rotation of a centrally perforated sphere. Operating on a principle similar to plug valves, they feature smaller sealing contact area, more compact structure and easier switching operation.

With the improvement of manufacturing technology, ball valves are widely applied not only in medium and low-pressure pipelines but also in high-pressure working conditions. Restricted by the temperature resistance limit of sealing materials, ball valves are generally not used in high-temperature pipelines.

Diaphragm Valve

Diaphragm valves take special rubber diaphragms as the core sealing components. The diaphragm is clamped between the valve body and bonnet, and the valve stem drives the diaphragm to compress for sealing. They feature simple structure, reliable sealing, convenient maintenance and low fluid resistance.

They are suitable for conveying pipelines of acidic media and fluids with suspended solid particles. The medium temperature shall not exceed 60℃, and they are not applicable for organic solvents, strong oxidants or high-pressure working conditions.

Butterfly Valve

Butterfly valves realize opening and closing through a rotatable disc inside the pipeline, featuring simple structure and small overall dimensions. Restricted by sealing structure and materials, their overall sealing performance is average. They are mostly used for low-pressure, large-diameter pipelines, especially common in water supply and drainage, air and gas transmission pipelines.

Lined Valve

Lined valves are manufactured by lining corrosion-resistant materials such as lead, rubber and enamel on the inner walls of valve bodies and heads for strongly corrosive working conditions. Lining materials shall be selected according to the chemical properties of conveyed media. To facilitate lining construction, lined valves are mostly designed with right-angle or straight-flow structures.

Steam Trap

Steam traps are specially used for steam pipelines and steam heating equipment. They automatically discharge condensed water generated in pipelines while preventing steam leakage, improving steam thermal efficiency and reducing corrosion of pipeline equipment caused by condensed water.

According to working principles, steam traps are divided into mechanical type, thermodynamic type and thermostatic type.

Mechanical steam traps operate based on the density difference between condensed water and steam, featuring large volume and weight but stable and reliable performance. Float type, inverted bucket type and free float type all belong to this category, among which float type and inverted bucket type are the most widely used.

Thermodynamic steam traps work by utilizing the pressure and static pressure changes of flowing steam and condensed water to block steam and drain water. Pulse steam traps fall into this type, operating based on the secondary evaporation principle of condensed water pressure reduction, with small size and large drainage capacity as an advanced structural design.

Thermostatic steam traps are driven by the temperature difference between steam and condensed water, represented by thermal expansion steam traps. They are mostly used for low-pressure steam pipelines and widely applied in low-pressure steam heating systems.

Key Points of Valve Selection

Valve selection for piping systems requires comprehensive consideration of multiple factors, including physical and chemical properties of media (corrosiveness, viscosity, solid particle content), system pressure, medium temperature, pipeline nominal diameter, valve functions (cut-off, regulation, backflow prevention, pressure relief, steam drainage), and installation space.

Gate valves are suitable for full opening and full closing; globe valves are preferred for pressure and flow regulation; check valves are used to prevent medium backflow; safety valves must be equipped for pressure-bearing systems; steam pipelines are matched with pressure reducing valves and steam traps; lined valves are prioritized for corrosive media; butterfly valves apply to large-diameter low-pressure pipelines; valves with wear-resistant sealing structures are required for particle-containing media. Rational valve selection is the foundation for safe and stable operation of the entire process piping system.