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Training Manual on Selection of Control Valves I. Uses of Control Valves Since the mid-20th century, automation of production processes has been an important means of ensuring efficiency and quality in large-scale industrial production. In automatic control systems for the production process, control valves are one of the key components. A control valve, also known as a regulating valve, is the main type of actuator. In the system, the control valve receives a control signal from instruments and uses mechanical force to adjust the flow rate and pressure of the medium being controlled; it is a type of terminal element. In process systems, control valves belong to the category of throttling components and function as resistance-modifying elements. At their core is a throttling aperture (throttling area) formed between a valve stem that can move and a stationary valve seat; by changing the position of this valve stem, the resistance characteristics of the control valve can be altered, which in turn affects the resistance characteristics of the process system. This allows for the regulation of flow rates, and it enables automatic control of certain process parameters such as flow rate, pressure, temperature, and liquid level, thus facilitating automation of the production process. Control valves are primarily used in the industrial automation systems of industries such as petroleum, chemicals, power plants, light industry, papermaking, pharmaceuticals, shipbuilding, and municipal services (including environmental protection). Given its crucial role in the safe and efficient operation of industrial systems, it is very important to select an appropriate control valve. II. Types and Structure of Control Valves A control valve consists mainly of three major components: the valve itself, the actuator, and the accessories for the control valve. 1. A valve consists of a valve body, internal components (such as the valve core, valve plate, valve seat, sleeve, etc.), a valve cover, and a packing box. It must possess the following basic elements: structural design, nominal diameter, nominal pressure, method of connection to pipelines, operating temperature range, materials used for the valve body and internal components, valve seat diameter, and rated flow coefficients (such as Cv, Kv, etc.), flow characteristics, and valve seat leakage rating. 2. Actuators: Actuators are usually of pneumatic or electric type; there are also hydraulic, pneumatic-hydraulic, or other special types. A: Generally speaking, pneumatic actuators are divided into diaphragm type and cylinder-piston type. The film type is mainly of the film-spring type; in special cases, there is also a film-type without springs ； Cylinder piston types are divided into horizontal types (usually connected to angle-action valves) and vertical types (connected to linear-action valves) based on the way they are connected to the valves ； Based on their mode of operation, they are divided into single-acting and double-acting types. B: Electric actuators are mainly divided into linear stroke electric actuators, angular stroke electric actuators, and multi-turn electric actuators. 3. Control valve accessories: The most commonly used control valve accessories are valve positioners and the air filter pressure regulator that must be used in conjunction with them. There are many other types of attachments; those we have used so far include: valve position signal generators (limit switches, valve position transmitters), handwheel mechanisms, electromagnetic directional control valves, pneumatic accelerators, quick exhaust valves, air-controlled directional control valves, hold-down valves, dampers (speed regulators), triple-break protection devices, and air source protection devices (which include electromagnetic directional control valves, air-controlled directional control valves, check valves, and air storage tanks). The type of accessories for control valves should be determined based on system requirements, and then selected accordingly. III. Introduction to Typical and Common Valve Types Valve Type, Characteristics, and Application Areas Straight-through single-seat valve (GLOBE): Low seat leakage (0.01% of the rated flow coefficient), low allowable pressure difference; suitable for general fluids in applications where low seat leakage or complete shut-off is required. The upward-guided single-seat valves that are widely used internationally today replace the upward-and-down guided single-seat valves in many applications. Compared to the latter, those with upward guidance offer smoother flow paths, lower pressure drop losses, higher flow rates, and a wider range of adjustable parameters. Straight-through two-seat valve (upward and downward guidance): The GLOBE valve has low unbalanced force and allows a higher pressure difference compared to single-seat valves; however, its flow path is complex and it experiences high seat leakage (0.1% of the rated flow coefficient). Suitable for applications with large pressure differences where no requirements are placed on leakage volume. Sleeve valve (double seat) – GLOBE type: it offers good stability, tolerates large pressure differences, and allows for easy replacement and maintenance of its internal components. Suitable for general fluids; it features low pressure drop, high flow rate, a wide adjustable range, good dynamic stability, low noise, and minimal cavitation corrosion. The cage valves in the CV3000 series feature guide vanes within the valve body cavity that improve the flow of the fluid around the sleeve, enabling smoother flow; as a result, they offer better dynamic stability, lower noise levels, and improved resistance to cavitation corrosion. The angle valve (ANGLE) is a derivative of the GLOBE valve, available in single-seat and cage types. The flow path is simple, facilitating self-cleaning and cleaning. Suitable for right-angle connections: suitable for media with high viscosity and particles. Due to its better flow conditions, the Venturi angle valve has replaced various other angle valves and is now used in a wide range of applications. The eccentrically rotary valve (cam-rotating valve) features a simple flow path and low leakage levels (0.01% of the rated flow coefficient). Compared to single-seat valves, it allows for a larger pressure difference, higher flow rates, better stability, and a wider range of adjustable settings. Butterfly valves have a compact structure and are generally suitable for applications with low pressure differences, high flow rates, and low requirements regarding leakage. They are especially appropriate for use with thick pasty substances and media containing particles. For applications with high requirements regarding leakage levels (Levels IV and V), soft seat or hard seal designs can be used. Ball valves of the V-BALL and O-BALL types feature a compact design, a high flow coefficient, and excellent sealing performance; they are suitable for use with fluids such as pulp, sewage, and those containing fibers or particles. Type O ball valves are used for shut-off applications and are required to be leak-free; they are not suitable for regulating purposes. V-ball valves are appropriate for regulating applications. In recent years, there has been a tendency to use hard-seal designs, which increases their service life but raises the cost of the equipment. The three-way ball valve is a derivative of the straight-through O-ring ball valve, developed by combining the characteristics of a three-way control valve. There are L-type and T-type ones; the L-type is used for direction reversal, while the T-type is used for merging and splitting streams, as well as for direction reversal. Three-way control valve: Three-way control valves are commonly used in heat exchangers, and they are divided into merge valves and split valves. The internal structure of the Vu series tee valves is superior to that of the traditional tee valves used in boilers, allowing them to operate reliably at high temperatures. Multi-pole pressure relief valves mostly employ a labyrinth-type multi-pole pressure reduction structure, which provides protection against nuclear attacks and is resistant to erosion. Suitable for applications with high temperature and high pressure differences in water. Our company currently uses two types of internal components: one is the commonly known sugar-coated fruit style, and the other is a structure with multiple interchangeable throttling elements. IV. Introduction to Other Valve Types 1. Some Derivatives of GLOBE Valves A. Sleeve Single-Seat Valve. Our company has HLC and HSC models; the valve seat sealing is of the single-seat type, with a sleeve-guided design. The unbalanced force is low, thus allowing a large pressure difference; the leakage level when the valve seat is closed is high, at ANSI Class IV. Since the HLS small-bore single-seat valve is not suitable for handling working pressure differences greater than 3.0 MPa when regulating liquid media, the HLC can be used as a substitute ； Furthermore, thanks to the robust sleeve protection (sleeve material 17-4PH, hardness HRC45), it can be used in vaporization and flashing applications. However, the flow capacity of the sleeve valve is one order of magnitude lower than that of a single-seat valve; the maximum Cv for HLC (DN25) is 4. Furthermore, a certain margin should be left for the allowable pressure difference. B, TCB. It is a special cage-type control valve; like all two-seat valves, it also has two sealing surfaces. However, the lower sealing surface is sealed by a traditional narrow beveled surface, while the upper sealing surface, thanks to the high-performance sealing ring installed on the sleeve, features a sliding radial seal. Currently, this model can only be used in temperature ranges below -17 to 300 degrees. And the diameter is DN100 or less. C, HCBE. It is also a pressure-balanced control valve; its internal component is a pilot-operated spool guided by a sleeve, it features a single-seat sealing design, and its leakage level is ANSI Class IV. Based on feedback from the usage sites, HCBE sometimes experiences oscillation phenomena, and it is almost impossible to fix. In my opinion, this is due to the fact that no detailed simulation tests were conducted during the development of this product, resulting in the failure to obtain meaningful parameters and an inability to anticipate potential failures under certain operating conditions. It is recommended to choose the diaphragm seal type to avoid operating under high pressure differences (above 0.8~1.0 MPa). D When the medium is a toxic substance, especially a toxic gas, or when the medium is expensive, considering that slight leaks may occur at the dynamic seal of the packing box as the valve stem moves, a bellows is used as a dual-sealing mechanism to completely eliminate the possibility of leakage from the gaps around the moving valve stem. When selecting a bellows valve, attention should be paid to the allowable pressure difference, which is generally lower than that of valves of the same type. Domestic bellows, due to their thicker sheets, have a lower allowable pressure difference compared to imported bellows. E. Corrosion-resistant valve. For use in highly corrosive media, employing corrosion-resistant metals as materials sometimes fails to meet the corrosion resistance requirements; or although such metals do meet these requirements, their high cost makes them uneconomical to use. As a result, we turn to cheaper non-metallic corrosion-resistant materials. Therefore, in addition to corrosion-resistant metallic materials, within the limits permitted by process pressure and temperature, plastics such as polytetrafluoroethylene or other non-metallic corrosion-resistant materials (such as ceramics) are used for valve bodies and valve components, which are referred to as plastic valves or ceramic valves ； Alternatively, a corrosion-resistant plastic or other non-metallic corrosion-resistant material can be coated on the surface of the valve’s metal material, in the area that comes into contact with the fluid; such valves are known as plastic-lined valves, ceramic-lined valves, or rubber-lined valves. The nominal pressure of these valves is generally PN1.0MPa, and their operating temperature ranges from -30 to 150 degrees Celsius. F. Insulated jacket valve. When the crystallization temperature of the medium being handled is lower than the ambient temperature, or when the temperature of the liquid drops causing an increase in viscosity, or when the fluid solidifies, a thermal insulation jacket can be installed on the valve body and the upper valve cover to maintain the medium at a certain high temperature. The insulation fluid is generally steam (our company’s requirements for insulation jackets specify the pressure of the insulation steam)