【Daily Question 20090224】What are the criteria for selecting an actuator and a control valve?

1. Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

The selection of the actuator is primarily based on the \"maximum operating pressure difference\" specified by the system specialist; the force generated by the actuator must be greater than this pressure. Regarding the \"maximum operating pressure difference,\" the difference in pressure calculated before and after the valve when it is operating normally often turns out to be too low. The reasons for this may include the following: 1. When the valve is not operating properly, its opening degree may be very small, resulting in an **increase in the pressure before the valve.\" The maximum operating pressure difference refers to the pressure difference that may occur when the valve is about to close; in such a situation, the outlet of the fluid transfer equipment (such as pumps, fans, or air compressors) will reach the highest pressure at which the equipment can operate, and this value equals the rated outlet pressure of the fluid transfer equipment. 2. During abnormal operation, the pressure in the pressure vessel behind the valve may be lower than the pressure of the pressure vessel under normal operation. It is these two factors that cause the pressure difference before and after the valve to be much greater during abnormal operation than during normal operation, resulting in leakage in valves that do not leak under normal conditions when operating abnormally. Taking the feedwater system of industrial boilers as an example, the rated pressure of a boiler feedwater pump with a capacity of 35 t/h is usually 6.0 MPa, the bubble pressure is 3.8 MPa, and the pressure before the valve is 4.0 MPa under normal conditions. When the valve is operating properly, the pressure difference before and after the valve is approximately 0.2 MPa. If this value is used as the allowable pressure difference, almost any valve will meet the requirements. However, when the steam consumption suddenly decreases, the valve opening also decreases, and the pressure in front of the valve gradually increases until it reaches the rated pressure of the feed water pump. At this time, the pressure difference before and after the valve is 2.2 MPa; generally, it is difficult for a single-seat valve to prevent leakage under such a pressure difference. The pressure difference before and after the valve is used to calculate the flow capacity of the valve, namely the Cv value. The gas source pressure is mainly divided into two ranges: 0.5–0.8 MPa; I forgot the other range (but it’s definitely less than 0.5 MPa). This was determined at the initial design stage. Ordinary valves will suffice; if there are difficulties, the valve manufacturer can also find a solution (such as using an air storage tank or something similar)

Selecting control valves is too complicated; a simplified approach to selection is provided below. 1) Selection of valve type: For control valves, it is advisable to choose fully functional, lightweight ultra-light control valves as a replacement for other types of control valves. This approach helps to avoid the problems associated with selecting products that lack sufficient functionality, which could lead to improper selection (such as the clogging issues encountered with straight-stroke valves) ； The hassle of cutting off the relationship with the pressure difference, etc.). 2) Selection of the actuator: For the selection of the actuator: ① Import electronic actuators should be used to overcome a range of reliability issues ； ②The membrane-type actuator selects the compact series ； ③For piston-type actuators, rack-and-pinion types should be considered ； ④The pressure difference at which the valve closes is specified, and the size of the actuator is determined by the manufacturer. 3) Material selection is a complex issue. Since many corrosive agents do not affect polytetrafluoroethylene, it is advisable to use fully fluorinated corrosion-resistant valves, which simplifies the selection process for applications requiring corrosion resistance. However, corrosion-resistant alloys should be considered only when the temperature is above 180 °C, below –40 °C, or when PN ≥ 2.5 KPa. 4) Selection of spring range: For valves equipped with diaphragm actuators, positioners are used in most cases, allowing full utilization of a gas supply pressure of 250 KPa. A spring with moderate stiffness that can also provide a sufficient output force should be chosen, with a pressure range of 60–180 KPa ； Similarly, a pressure range of 150–300 KPa should be selected for the piston actuator. 5) Selection of flow characteristics: ① When the parameters are not certain, choose a logarithmic characteristic; ② When the response speed of the system being controlled is fast, such as in flow regulation or liquid pressure regulation, choose a logarithmic characteristic ； When the system’s response speed is slow, the level control system, the temperature control system, and the linear characteristic selection ； ③ When the S value is small, select logarithmic characteristics ； ④ Choose a logarithmic characteristic when the valve may operate at a low opening degree. 6) Flow direction selection: For straight-stroke control valves with a single seal, an open-flow type is usually chosen ； When cut-off and erosion prevention are required, the flow-blocking type should be selected (the stability of the flow-blocking type is poor, so corresponding stability measures should be considered). 7) Selection of packing: When a positioner is used, it is advisable to choose graphite packing that is wear-resistant, heat-resistant, has a long service life, and provides reliable sealing. 8) Selection of positioner and converter: Since a converter does not have the three functions of enhancing output force, increasing operating speed, or improving positional accuracy through positioning, a positioner should generally be chosen (the price of a positioner is similar to that of a converter). 9) Selection of solenoid valves: The reliability of solenoid valves should be given top priority; therefore, imported solenoid valves with high reliability should be chosen. Determine the relationship between the activation and deactivation of the solenoid valve and the main valve, and inform the manufacturer.

1. Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

Selecting the appropriate flow characteristics \ Choosing the suitable material for the control valve \ Selection of flow direction \ Selection of actuator

1. Select the appropriate structure and material for the control valve based on the process conditions； 2. Select an appropriate flow characteristic based on the characteristics of the process object ； 3. Select an appropriate valve size based on the process operating parameters ； 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

1. The control valve is determined based on uncertain process parameters, such as temperature, pressure difference before and after, maximum flow rate, minimum flow rate, etc； 2. The actuator is generally determined based on the pressure of the process medium, the differential pressure before and after the control valve, as well as the valve diameter ； 3. The material of the valve body is determined by the physical and chemical properties of the medium ；

Selection of control valves: 1. Criteria for selecting the structural type of control valves 1.1 Control function 1.2 Leakage class and shut-off pressure difference 1.3 Pressure and temperature resistance 1.4 Erosion, cavitation, corrosion 1.5 Fluid medium 1.6 Service life 1.7 Maintenance and spare parts 1.8 Performance-to-price ratio 1.9 Suggested order of selection: single (double) seat globe valve, cage-type single (double) seat valve, eccentric rotary valve, butterfly valve, angle valve, ball valve (V.O.), three-way valve, special control valves. 2 Determine the forward/reverse action mode of the control valve based on process requirements. 3 Select the material, packing, and accessories for the control valve according to process parameters. Selection criteria for actuators: 1.1 Reliability; 1.2 Cost-effectiveness; 1.3 Smooth operation with sufficient output force; 1.4 Simple structure; 1.5 Easy maintenance; 1.6 Light weight. 2 Suggested order of selection: Pneumatic diaphragm actuators (for linear motion), cylinder actuators (single-cylinder with spring return, double-cylinder – suitable for both linear and rotary motion), electric actuators, hydraulic actuators. 3 Selection of the spring range for the actuator: 3.1 A stiffer spring results in better stability of the control valve; 3.2 A softer spring provides greater output force. 3.3 It is recommended to choose a range of 60–180 kPa (usually 20–100 kPa or 40–200 kPa)

Answer: 1. Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

1. Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

1. Process conditions 2. Safety issues 3. Energy conservation

Metered billing contributes to macro-level energy savings through three main approaches: First, flow control valves are installed to achieve flow balance, thereby overcoming the issue of uneven temperature distribution； Secondly, through the action of the temperature control valve, the free heat from solar energy, household appliances, lighting, and other devices is utilized ； Third, it has raised the energy-saving awareness among residents who use heat, reducing unnecessary heat loss from opening windows and similar actions. Of these three energy-saving approaches, two are achieved through flow control valves. It is evident how important flow control valves are in heating systems that use metered billing. Therefore, it is very important to know how to properly select and design flow control valves. I. Temperature control valves 1. Structure and working principle of radiator temperature control valves The temperature in a user’s indoor space is controlled through the radiator thermostatic control valve. The radiator thermostatic control valve consists of a thermostatic controller, a flow control valve, and a pair of connectors; the core component of the thermostatic controller is the sensor unit, namely the temperature bulb. The thermal bulb can change its volume in response to changes in the surrounding temperature, which in turn causes the valve element to move, thereby adjusting the amount of water flowing through the radiator and thus altering its cooling capacity. The set temperature of the thermostatic valve can be adjusted manually; it automatically controls and regulates the water flow to the radiator according to the set parameters, thereby achieving the goal of controlling the indoor temperature. Thermostatic valves are generally installed in front of the radiator, and by automatically adjusting the flow rate, they help achieve the room temperature desired by the residents. Thermostatic valves are divided into two-way thermostatic valves and three-way thermostatic valves. Three-way temperature control valves are primarily used in single-tube systems with bypass pipes; their flow division ratio can vary within the range of 0 to 100%, offering a large degree of flow regulation. However, they are relatively expensive and have a more complex structure. Two-way temperature control valves are used in some dual-pipe systems and in some single-pipe systems. The two-way temperature control valve used in dual-tube systems has high resistance ； The resistance for single-tube systems is low. The temperature sensing element of the thermostatic valve and the valve body are generally assembled as a single unit, with the sensing element itself serving as the sensor for the indoor temperature at the site. If necessary, a remote temperature sensor can be used ； The remote temperature sensor is placed in the room where temperature control is required, while the valve body is located at a certain point in the heating system. 2. Selection and design of thermostatic valves: Thermostatic valves are the primary devices used for regulating flow in heating systems; other control valves serve only as auxiliary devices. Therefore, thermostatic valves are essential. A heating system cannot be considered a heat metering billing system if it does not have a temperature control valve. In the design of temperature control valves, proper selection is very important. The purpose of selecting a temperature control valve is to determine the KV value (flow coefficient) based on the design flow rate (under a known heat load) and the allowable pressure drop ； Then, the diameter (model) of the temperature control valve is determined by the KV value. Therefore, design catalogs or manufacturer samples must provide the relationship between KV values and diameter; otherwise, it is not convenient for designers to use them. In the selection and design of temperature control valves, it is by no means sufficient to simply choose a valve with the same diameter as the pipeline. Rather, it is necessary to create ideal pressure difference operating conditions for the selected temperature control valve during the selection process. The normal operating pressure difference for a thermostatic valve is between 2 and 3 mH2O, with a maximum not exceeding 6 to 10 mH2O. To this end, it is necessary to specify the range of preset values for the temperature control valve in order to prevent noise generation and ensure its proper operation. When there are two or more valve sizes available for the same KV value, it is preferable to choose the valve with the smaller diameter, in order to improve the control performance of the temperature control valve. II. Electric control valve: The electric control valve is a device used for flow regulation in computer-based monitoring systems. It is generally used in unattended heat stations. An electric control valve consists of a valve body, a driving mechanism, and a transmitter. A temperature control valve is a device that performs self-acting flow regulation through a temperature sensor, and it does not require an external power supply ； Electric control valves generally require a single-phase 220V power supply, and are typically used as actuators in computer-based monitoring systems to regulate flow. Electric control valves or temperature control valves are the main devices for flow regulation in heating systems, with the rest being auxiliary devices. III. Balance valves: Balance valves are divided into manual balance valves and self-acting balance valves. Whether it is a manual balance valve or a self-acting balance valve, their function is to increase the resistance at the upstream end of the heating system, thereby preventing the actual flow rate from exceeding the designed flow rate ； In other words, its function is to overcome the excess head pressure at the upstream end of the heating system, allowing the electric control valve or temperature control valve to operate under an allowable head pressure. Therefore, both manual balance valves and self-acting balance valves are auxiliary flow control devices for thermostatic valves or electric control valves; they are extremely important. If the selection is improper or the design is unsound, neither the electric control valve nor the thermostatic valve can function properly. 1. Manual balance valve 1.1. Working principle of the manual balance valve: The manual balance valve is adjusted manually once, and it is unable to change its resistance coefficient automatically in response to changes in the system conditions; hence it is called a static balance valve. The manual balance valve acts on resistance; it functions as a manually adjustable orifice to balance the resistance in the piping system, thereby achieving resistance equilibrium across various circuits. It is capable of resolving issues related to the system’s steady-state imbalance: when operating conditions differ from those designed, the flow rate of water may be higher or lower than the specified value. Since balance valves are used to counteract the system’s resistance, they can distribute the new flow rate in accordance with the calculated proportions, ensuring that the flow rate in each branch increases or decreases proportionally, thus still meeting the flow requirements under the current load. 1.2 Issues to consider in the selection and design of manual balance valves (2) (1) The valve’s characteristic curve determines its regulating capabilities; for example, in the case of globe valves, if it is considered that flow rate changes within the range of 95%–100% are insignificant, then a flow rate variation from 0% to 5% already covers the entire range of possible flow rates. Such valves cannot be used for balancing flow rates in hydraulic systems. Since the theoretical characteristic curve of a valve is determined under maximum pressure differences, in actual operating conditions, as long as the valve’s authority factor is not 1, there is a large pressure difference across the valve at low opening degrees, while this pressure difference is smaller at high opening degrees. As a result, the value of dG/dC increases at low opening degrees and decreases at high opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the valve’s authority factor, the greater this shift. For valves with linear characteristics, such shifts in performance lead to a reduction in the effective range of opening degrees for precise regulation; therefore, it is better to use a characteristic curve that follows a lower chord arc, such as an equal percentage characteristic. For valves with equal percentage characteristic curves, the actual operating curve may approach a linear characteristic when the valve authority is between 0.3 and 0.5. (2) Usually, when a valve is opened to a small degree, the flow velocity through the valve is too high, resulting in vigorous turbulent vortical zones behind the valve. The pressure in these vortical zones is very low; when this pressure falls below the saturated pressure corresponding to the water temperature, vaporization occurs, leading to steam hammer: severe noise, vibration of the valve and pipes, and damage to the valve, pipes, and pipe supports. To prevent such accidents, it is necessary first to consider, in the design of the valve flow channels, the formation of a narrow throttling passage between the valve plug and the valve seat at low opening degrees, thereby restraining the formation of intense turbulent vortices ； Secondly, when selecting valves, try to increase their valve authority as much as possible to avoid operating them at low opening degrees. Additionally, whenever possible and without involving pressure conditions, the alkaline balance valve should be installed on the return water pipeline with lower water temperature. 2. Self-acting balance valve 2.1 Principle of operation of the self-acting balance valve: A self-acting balance valve can automatically achieve flow balance in a system without the need for an external power source. A self-acting balance valve limits flow by maintaining a constant pressure difference before and after the orifice (with a fixed aperture); therefore, it can also be called a constant-flow valve. A constant flow valve operates on flow rate and is capable of locking in the amount of water flowing through it, rather than focusing on balancing resistance. He is able to address the issue of dynamic imbalance in systems: to ensure the efficient operation of individual devices such as refrigerators, boilers, cooling towers, and heat exchangers, it is necessary to maintain their flow rates at rated levels ； From the perspective of the system’s end points, in order to avoid mutual interference from dynamic adjustments, it is also necessary to limit the flow rate at the end devices or branches. It should be noted in the design that the drawback of self-acting flow control valves is the requirement for a minimum operating pressure difference; typical products require a minimum operating pressure difference of 20 KPa. If such valves are installed on the most unfavorable circuit, it will necessitate an additional head of 2 meters of water column for the circulation pump, so it is advisable to install them near the source rather than at the far end. Such self-acting flow control valves should not be installed when the user is more than 80% of the heating radius away from the heat source. IV. Differential Pressure Control Valve 1. Principle of the differential pressure control valve The principle of the differential pressure control valve is essentially the same as that of a self-acting balance valve. In self-acting balance valves, however, the orifice plate exists as a component within the valve body ； In a differential pressure control valve, there is no orifice plate; instead, the system downstream of the control valve is regarded as an orifice plate. Therefore, the differential pressure value of the control valve actually refers to the pressure difference between the inlet and outlet of the system downstream of it. From the structure of the differential pressure control valve, it can be seen that the purpose of this type of control valve is to maintain a constant pressure difference between the inlet and outlet of the system it is connected to. The basic function is to automatically adjust the operating flow rate of heat users according to their heat load requirements. When a building requires lower room temperatures due to some heat users, the opening degree of the temperature control valves in those rooms decreases. This leads to an increase in the pressure difference across the differential pressure control valve, which exceeds the set value. At this point, the differential pressure control valve automatically reduces the opening of its valve element, thereby increasing the throttling effect and reducing the system’s pressure difference until it returns to the set value. The ultimate result is a reduction in flow rate, enabling adaptation to the heat demands of heavy users and thus minimizing the frequent operation of the temperature control valve. When hot users request an increase in room temperature, the function of the pressure difference control valve is exactly the opposite (3). 2. Issues to consider during design: Some believe that pressure differential control valves should be installed on each household’s system or riser pipes. According to simulation calculations, if balance valves (including manual and self-acting types) or differential pressure control valves are installed at the thermal inlet of the building (with proper design), then within any adjustment range, the pressure difference before and after the indoor temperature control valve will not exceed 6–10 mH2O; thus, the temperature control valve can operate under reasonable conditions. Therefore, it is unnecessary and uneconomical to install too many differential pressure control valves. V. Selection of flow control valves when the circulating water pump operates at variable flow rates. This mainly refers to the selection of manual balance valves, self-acting balance valves, and differential pressure control valves. When the circulating water pump operates at variable flow rates, the manual balance valve is out of balance in a proportional manner, which is most favorable for the operation of the temperature control valve ； However, its drawback is that it requires too much manual operation, making it difficult to achieve ideal adjustment. The circulating water pump operates with variable flow, and the ideal set pressure difference at the inlet of each heat user should vary according to outdoor temperatures. In this regard, both self-acting balance valves and differential pressure control valves are not ideal, but regulation loss of control does not occur. Therefore, this type of control valve can be used, which is beneficial for improving the control performance of the heating system.

1. Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.

Select the appropriate structure and material for the control valve based on the process conditions. 2. Select an appropriate flow characteristic based on the characteristics of the process object. 3. Select an appropriate valve size based on the process operating parameters. 4. Select an actuator with sufficient thrust based on the force applied to the valve stem. 5. Select appropriate auxiliary devices according to the requirements of the process.