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In fluid piping systems, valves are control elements whose main functions include opening and closing, throttling, regulating flow rate, isolating equipment and piping systems, preventing backflow of the medium, and regulating and releasing pressure. There are many types of valves with complex varieties; common ones include gate valves, globe valves, throttle valves, plug valves, butterfly valves, ball valves, check valves, diaphragm valves, etc., while special valves include steam traps, safety valves, and pressure relief valves. With the rapid development of the petrochemical industry, most of the media used in petrochemical production facilities are highly toxic, flammable, explosive, and corrosive. The operating conditions are complex and harsh, with high operating temperatures and pressures, as well as long startup periods. Should valves malfunction, it may lead to medium leakage in minor cases—causing both environmental pollution and economic losses; in severe cases, it can result in shutdowns of the entire facility or even serious accidents. Choosing the right valve is crucial; the selection of special valves is particularly important in pipeline design. 1 Safety valves and their selection methods A safety valve is an automatic valve used for safety protection, and it is related to personal safety. It discharges a certain amount of fluid without relying on any external force, instead using the force of the medium itself to prevent the pressure within the system from exceeding a predetermined safe level. When the pressure drops to a specified value, the valve closes promptly, thereby ensuring the safe operation of equipment or piping. Safety valves are designed with adverse factors in mind. The following situations may lead to explosions due to equipment or pipelines exceeding their design pressure as a result of fires, operational failures, or disruptions in water or power supply; safety valves (or other safety measures) should be installed: the outlet pipelines of positive-displacement pumps and compressors ； On the liquefied hydrocarbon pipelines that may be closed at both ends, thereby causing pressure increase ； Pipelines in which the thermal expansion of combustible gases and flammable liquids may exceed the design pressure ； Pressure vessels and pipelines with a design pressure lower than that at the source of pressure ； Steam outlet pipe of the condensing turbine ； All pressure vessels need to be equipped with pressure relief devices. 1.1 Classification of safety valves Safety valves can be classified using different methods as follows: ① Based on their overall structure and loading mechanism, they can be divided into lever-pendulum type (usually used for lower pressures and generally installed on equipment), spring type, and control type ； ② Based on the ratio of the valve disc opening height to the valve flow diameter, safety valves can be classified into slightly opened, moderately opened, and fully opened types ； ③ Based on the method of gas emission, they can be classified as fully enclosed, semi-enclosed, and open type ； ④ It is classified into backpressure-balanced type and conventional type based on the presence of a backpressure balancing mechanism ； ⑤ Based on the principle of operation, they are classified into directly acting and non-directly acting types (including pilot-operated and power-assisted types). 1.2 Selection Guidelines for Safety Valves There are many different types and designs of safety valves; for various applications, the following types of safety valves are recommended: ① For discharging gases or steam, full-opening safety valves should be used ； ② When discharging liquids, a slightly open safety valve is generally used ； ③ When releasing water vapor or air, a safety valve equipped with a wrench can be used ； ④ For safety valves used with gases having a set pressure greater than 3 MPa and a temperature exceeding 235°C, a safety valve with heat sinks should be selected to prevent the discharged medium from directly eroding the spring ； ⑤ Generally, for flammable, explosive, or toxic media, a closed system should be used, while for steam or inert gases, an open system can be employed ； ⑥ For discharging highly toxic, strongly corrosive, and extremely hazardous media, a bellows safety valve should be selected ； ⑦ In applications with high back pressure, back-pressure balanced or pilot-operated safety valves are recommended. 2 Drain valves and their selection methods: Drain valves are devices that automatically remove steam condensate, as well as non-condensable gases such as air, from heating equipment or steam pipelines, while preventing steam leakage. Since the steam trap functions to prevent steam from escaping while allowing water to drain, it enables uniform heating in steam heating equipment, making full use of the latent heat of steam to avoid water hammer in steam pipes. It is an energy-saving product that provides the necessary temperature and heat for various heating processes and equipment, ensuring their proper operation. 2.1 Classification of steam traps There are many types of steam traps, each with different properties. A steam trap must be able to “identify” steam and condensate in order to function as a vapor barrier and drain mechanism. “The identification of steam and condensate is based on three principles: density difference, temperature difference, and phase change. Three types of steam traps are manufactured based on three principles: mechanical, thermostatic, and thermodynamic. ① Mechanical steam traps, also known as float-type steam traps, utilize the density difference between condensed water and steam. Changes in the level of condensed water cause the float (which is spherical or barrel-shaped) to rise or fall, thereby opening or closing the valve disc and achieving the purpose of preventing steam from passing while allowing water to flow out. Mechanical steam traps have a low degree of subcooling; they are not affected by changes in operating pressure and temperature. Water is discharged as soon as it appears, preventing water from remaining inside the heating equipment, thus enabling the heating equipment to achieve optimal heat exchange efficiency. Mechanical steam traps include free-floating ball type, lever-ball type, float bucket type, and inverted bucket type ； ② Thermostatic steam traps utilize the temperature difference between steam and condensate to cause deformation or expansion of a temperature-sensitive element, which in turn drives the valve core to open and close the valve. Thermostatic steam traps exhibit a relatively large degree of subcooling; typically, this value ranges from 15 to 40 degrees. They can utilize part of the sensible heat contained in the condensate. As a result, there is always hot condensate present before the valve, with no steam leakage, thus contributing to energy savings. Thermostatic steam traps include bellows type, steam pressure or balanced pressure type, bimetallic type, and liquid expansion type ； ③ Thermodynamic steam traps operate on the principle of phase change; by utilizing the differences in flow velocity and volume as steam and condensed water pass through, they create a pressure difference between the upper and lower parts of the valve element, which in turn drives the valve element to open or close. Since the operating power of thermodynamic steam traps comes from steam, there is significant steam waste. Thermodynamic steam traps are divided into disc type, pulse type, labyrinth type, or microporous type. 2.2 Selection Guidelines for Drain Valves In heating processes such as steam transmission, steam-water separation, heating, drying, insulation, heat tracing, disinfection, distillation, concentration, cooking, heat exchange, heating, and air conditioning, drain valves are necessary to supply the temperature and heat required by various equipment. They also help to remove condensed water promptly and prevent steam leakage. When selecting a steam trap, it is required to have high sensitivity, to prevent steam from escaping while effectively draining water, to avoid any steam leaks, to improve steam utilization efficiency, to offer reliable performance, to withstand high back pressures, to have a long service life, and to be easy to maintain. Principles for selecting steam traps: ① If it is required that the equipment heat up quickly, that the heating temperature be tightly controlled, and that no condensed water accumulate within the heating equipment, then a mechanical trap capable of discharging saturated water should be chosen ； ② If the heating equipment has a large heating surface, there is no need to heat it up quickly, the temperature requirements are not strict, and it is possible to allow a certain amount of condensate water to accumulate, in which case a thermostatic steam trap should be used ； ③ Depending on the volume of drainage from the equipment used, it is essential to consider a safety factor when selecting a steam trap ； ④ The steam trap determines the drainage volume based on the pressure difference. The specific selection guidelines are as follows: ① The upper backpressure limit for mechanical steam traps is 80%, and they offer high performance; hence they are ideal steam traps for heating equipment in manufacturing processes. The valve sensitivity of small orifices is higher than that of large orifices. Except that the inverted bucket type steam trap can remove a small amount of hot and cold air, this type of trap cannot remove air. The free-floating ball type has a simple structure and high sensitivity; it can drain saturated water. However, it has poor resistance to water hammer and fouling. It is suitable for large-diameter pipes and high flow rates, but its manufacturing process is complex. ② The thermostatic steam trap can be installed above steam-using equipment and used solely as an air vent valve. The liquid expansion type is suitable for drain lines of heating pipelines and heating pipelines where a lower heat tracing temperature is required. Steam-pressure or balanced-pressure types have a simple structure and are sensitive in operation; they can drain water and air continuously, offering good performance with low leakage. However, their resistance to water hammer and fouling is poor. They have a wide range of applications and can be used as air release valves in steam systems. Bellows types are widely used for draining water in heating systems, and can also serve as air release valves in steam systems. The bimetallic strip type has low operating sensitivity; it can drain water continuously, exhibits good drainage performance, and features a relatively large and adjustable degree of supercooling. It is suitable for use in applications ranging from low to high pressures, with a maximum service temperature of up to 550°C. Resistant to dirt and water impact. ③ Thermodynamic steam traps have high sensitivity to condensate; they feature small operating elements, low inertia, and fast switching speeds. Disc-type steam traps are suitable for high-temperature and high-pressure steam pipelines, dryers, heaters, and superheated steam equipment. Pulsating steam traps have a low back pressure and are suitable for siphon drainage in rotary drying drums, enabling the removal of a certain amount of cold and hot air. Labyrinth or microporous steam traps have a simple structure and can continuously drain water and air. The microporous type is suitable for small displacements, while the labyrinth type is suitable for very large displacements. 2.3 Installation requirements for drain valves Thermal-driven drain valves should be installed in horizontal pipes ； Float-type steam traps should be installed horizontally; those located outdoors must have appropriate anti-freezing measures in place ； Bimetallic sheet steam traps can be installed horizontally or vertically ； Pulsating steam traps are generally installed on horizontal pipes with the valve cover facing upward ； Inverted bucket type steam traps should be installed horizontally and not tilted ； When installing a steam trap, the flow direction arrow indicated on its body should be in line with the direction of the condensate flow in the pipeline; otherwise, the steam trap will lose its effectiveness. 3 Pressure Reducing Valves and Their Selection Methods A pressure reducing valve is a valve that reduces the inlet pressure to a desired outlet pressure by throttling through a control element, and it is able to maintain a relatively constant outlet pressure despite changes in inlet pressure and flow rate, by utilizing the energy of the medium itself. Its function is to rely on sensitive components such as diaphragms and springs to change the position of the valve disc, thereby reducing the pressure of the medium and achieving pressure reduction. Under normal circumstances, the outlet pressure of a pressure relief valve should be less than 0.5 times the inlet pressure. Fluctuations in the inlet pressure of the pressure reducing valve should be kept within 80% to 105% of the set inlet pressure value; exceeding this range may affect the valve’s performance during the pressure reduction phase. For the convenience of operation, adjustment, and maintenance, pressure relief valves should generally be installed on horizontal pipes. 3.1 Classification of pressure reducing valves Pressure reducing valves can be classified into bellows-type (direct-acting) pressure reducing valves, piston-type pressure reducing valves, and diaphragm-type pressure reducing valves. ① Bellows-type (direct-acting) pressure reducing valve. It is a relatively simple pressure relief valve, a direct-acting pressure relief valve, equipped with a flat diaphragm or bellows. Standalone structure; no need to install external sensing wires downstream. It is the smallest in size and most cost-effective among the three types of pressure relief valves, designed specifically for medium to low flow rates. The accuracy of directly-acting pressure relief valves is typically (+10% to -10%) of the downstream set point ； ② Piston-type pressure reducing valve. This type of pressure relief valve combines two types of valves – a pilot valve and a main valve – into one. The design of the pilot valve is similar to that of a directly-acting pressure reducing valve. The exhaust pressure from the pilot valve acts on the piston, causing it to open the main valve. When the main valve is too large to be opened directly, this design utilizes water pressure to open it. Therefore, this type of pressure relief valve offers higher capacity and accuracy (+5% to -5%) at the same pipe size compared to directly-acting pressure relief valves. Similar to direct-acting pressure regulators, the pressure regulator senses pressure internally, without the need for external sensing wires ； ③ Membrane type pressure relief valve. In this type of pressure reducing valve, double diaphragms replace the piston found in internally guided pressure reducing valves. This increased diaphragm area enables the opening of a larger main valve, and at the same pipe size, it has a higher capacity than internal pilot piston pressure reducing valves. Additionally, the diaphragm is more sensitive to pressure changes, with a precision of (+1%~-1%). The higher accuracy is due to the positioning of the downstream sensing wire (outside the valve), where there is less turbulence of gas or liquid. This pressure relief valve is flexible and can use different types of pilot valves, such as pressure valves, temperature valves, and air loading valves. 3.2 Selection guidelines for pressure reducing valves Pressure reducing valves have a wide range of applications; they can be used in equipment and pipelines for steam, compressed air, industrial gases, water, oil, and other liquid media. The specific selection guidelines are as follows: ① Bellows-type direct-acting pressure reducing valves are suitable for low-pressure steam, air, and similar media with medium to small pipe diameters ； ② Thin-film direct-acting pressure relief valves are suitable for medium and low pressures, as well as for steam or water and other fluids with medium and small diameters ； ③ Pilot piston type pressure reducing valve, suitable for steam, air, and water media at various pressures, sizes, and temperatures; when made of stainless acid-resistant steel, it is suitable for various corrosive media as well ； ④ In applications where the operating temperature of the medium is high, piston-type pressure reducing valves are generally used ； ⑤ When a high sensitivity is required, a spring-diaphragm type pressure reducing valve can be used ； ⑥ In cases where the medium is air or water (liquid), a directly-acting diaphragm pressure regulator or a pilot-diaphragm pressure regulator is generally selected.