Control valves are a crucial component of industrial automation control systems, often referred to as the "limbs" of production process automation and the final element of the control system. Correctly selecting control valves is key to ensuring stable and normal system operation. Control Valve Selection Scope Control valve selection typically includes the following aspects: Control valve structure and material selection: Control valves include single-seat valves, double-seat valves, bellows-sealed valves, valve body separation valves, sleeve valves, low-noise valves, angle valves, three-way valves, diaphragm valves, butterfly valves, ball valves, eccentric rotary valves, etc. Materials include cast iron, carbon steel, various stainless steels, copper, aluminum, titanium, bronze, Hastelloy, Monel alloy, etc. Users primarily select materials based on factors such as the temperature, pressure, corrosiveness of the controlled medium, and whether cavitation or erosion occurs. This includes the selection of materials for the valve body, valve cover, valve core, valve stem, valve seat, gaskets, and sealing packing. Control Valve Flow Characteristic Selection: The flow characteristic of a control valve refers to the relationship between relative stroke and relative flow rate. Common characteristics include linear flow characteristics, equal percentage flow characteristics, quick-opening flow characteristics, parabolic flow characteristics, etc., and the selection must be based on the characteristics of the process. Control Valve Diameter Calculation: Calculate the flow coefficient C of the control valve based on the process conditions, and then select the diameter according to the flow coefficient C value table for the corresponding structural type of control valve. Control Valve Actuator Selection: Control valve actuators are available in pneumatic, hydraulic, and electric types. Pneumatic actuators are suitable for fire and explosion-proof applications, with a low failure rate, but require an independent instrument air source; hydraulic actuators are suitable for applications with particularly high thrust or torque, offering smooth operation, but are bulky, expensive, and used in small quantities; electric actuators have readily available drive sources, and explosion-proof models are suitable for fire and explosion-proof applications. Their reliability has improved significantly in recent years, and they have become the mainstream actuator product. Control valve unbalanced force verification: Unbalanced force verification is an indispensable step to ensure the normal operation of control valves. Its essence is to verify whether the output force of the actuator is greater than the sum of the unbalanced force of the medium, the frictional force during valve operation, and the weight of the valve core (of course, the output force should have a certain margin during verification). To simplify calculations, valve manufacturers calculate the allowable differential pressure for commonly used valves based on operating conditions and list it in a table attached to the selection guide. Thus, unbalanced force verification is transformed into allowable differential pressure verification. There are two verification methods: one is to check whether the allowable differential pressure value meets the requirements based on the output force of the selected actuator; the other is to select an actuator with a suitable output force based on the allowable differential pressure value. When the verification fails, it may be necessary to reselect the valve from the valve structure selection stage. Some control valve selection calculation materials often focus on diameter calculations, but completely omit the unbalanced force verification. Therefore, some designers often only perform the first four steps and neglect the last one. In most cases where the fluid pressure is not high, omitting this verification will not affect the normal operation of the control valve. However, for production processes like boiler feedwater that may involve medium to high pressure, failure to perform calibration can result in feedwater valves not closing completely, thus affecting valve operation. More seriously, due to a lack of understanding of the concept of allowable differential pressure, when this problem occurs, people often cannot determine the cause and blame the control valve manufacturer, assuming it's a product quality issue, leading to prolonged unresolved problems. This article will first introduce the maximum allowable differential pressure value of control valves, then analyze related errors through several field operation examples, and summarize the issues that should be considered when selecting control valves in the future. Maximum Allowable Differential Pressure Value of Control Valves Taking data from the "Control Valve and Auxiliary Device Selection Sample" of Sichuan Instrument Factory 11 as an example, Table 1 lists the maximum allowable differential pressure values ​​of commonly used feedwater control valves for industrial boilers. Table 1 Maximum Allowable Differential Pressure Values ​​(MPa) of Several Control Valves Notes: 1. Ⅰ - Indicates AA, BA, CB sleeve and valve plug combination Ⅱ - Indicates DC, CC, DD, CD sleeve and valve plug combination 2. KHTS and KHSC in the table are products imported from Yamatake Corporation of Japan. Analyzing Table 1, the following conclusions can be drawn: The maximum permissible differential pressure value of a valve is related to its structural form, diameter, actuator power (electric or pneumatic), and actuator thrust. Single-seat control valves have low maximum permissible differential pressure values ​​and may not always meet the requirements of industrial boiler feedwater regulation systems. Double-seat control valves, however, offer a significantly better solution and typically meet the requirements. The situation with electric sleeve control valves differs from expectations. The maximum permissible differential pressure values ​​for the two types of sleeve-and-plug combinations differ greatly. Type II sleeve-and-plug electric sleeve control valves have very low maximum permissible differential pressure values, even lower than ordinary single-seat control valves. Type I sleeve-and-plug electric sleeve control valves have very high maximum permissible differential pressure values, even higher than their corresponding double-seat control valves. Care must be taken when designing and selecting these valves. Therefore, for industrial boiler feedwater regulation systems, double-seat control valves or Type I sleeve-and-plug combinations should be selected whenever possible. Due to diaphragm limitations, pneumatic control valves have lower maximum permissible differential pressure values ​​than electric control valves of the same diameter. When using the same actuator, since its thrust is a fixed value, the unbalanced force increases with increasing valve diameter, resulting in a decrease in the maximum permissible differential pressure. In electrically controlled valves, the maximum permissible differential pressure depends not only on the valve's structural form (single-seat, double-seat, sleeve type I, sleeve type II) but also on the actuator model and thrust. For example, the KHTS type electric single-seat control valve offers three actuator options: the domestically produced DKZ, the Sino-Japanese joint venture 361 LSA, and the German Hartmann-Brown RS. Each manufacturer also offers various actuator thrust options. When the maximum permissible differential pressure in the production process is slightly lower than a certain permissible value in Table 1, besides selecting a valve with a different structural form, another option is to try using an actuator with a larger thrust. For control valves of the same structure and diameter, using actuators with the same thrust is recommended, but the maximum permissible differential pressure will differ depending on the manufacturer. Refer to the relevant manufacturer's catalog for verification. Examples of Selection Errors Example 1: A factory's 20 t/h industrial boiler feedwater control valve initially used a KHTS type electric single-seat valve with a 361LSA-20 actuator. During operation, a small leak occurred. The product manual indicated a maximum permissible differential pressure of 1.49 MPa, lower than the boiler's normal operating maximum differential pressure of 2.0 MPa. After switching to a 361LSA-50 electric actuator, the maximum differential pressure reached 3.38 MPa (see Table 1), meeting the boiler's normal operating requirements. This example illustrates that by using a higher-thrust electric actuator, it's possible to meet the maximum permissible differential pressure requirements of the control valve. Example 2: A factory's 35 t/h industrial boiler's remote manual control design for superheated steam temperature used ZAP-6.4, Dg50 electric single-seat control valves. During operation, the valve failed to close completely. Reviewing the original design drawings revealed that no calculations (including flow capacity calculations) were performed on the control valve. The designer explained that calculations were unnecessary for a manual remote control valve. Upon checking the product manual, it was discovered that although the thrust of the electric single-seat control valve reached 6.4 kN, the maximum permissible differential pressure was only 1.4 MPa, lower than the maximum differential pressure during normal boiler operation (feed pump pressure 6.0 MPa, boiler drum pressure 3.9 MPa, ΔP≈2.1 MPa), resulting in significant valve leakage. This problem was resolved by using the original feedwater ball valve with an electric rotary actuator. This example illustrates that even for manually controlled valves, the maximum permissible differential pressure should be calculated during design and selection. Third example: During a visit to a carbon plant's calcining kiln waste heat boiler, it was found that the boiler drum water level regulation circuit was not in automatic mode. When inquiring about the cause, the on-site technicians told me that the electric control valve had a problem with not closing completely. Upon inspection, it was a ZAP 6.4, Dg50 electric single-seat control valve with an electric actuator thrust of 2.5 kN. After analyzing the issue with the on-site technicians, we discovered that the problem likely stemmed from the electric single-seat control valve's maximum permissible differential pressure not meeting the operational requirements (the feedwater pump outlet pressure at this site is 4.0 MPa, and the steam drum operating pressure is 2.5 MPa). We then consulted the product catalog for the control valve. The electric single-seat control valve for model Dg50 offers actuator options of 0.4 and 2.5 kN thrust, with maximum permissible differential pressures of 0.15 MPa and 0.94 MPa respectively. Although the designer selected the higher-thrust 2.5 kN electric actuator, its maximum permissible differential pressure of 0.94 MPa was still lower than the maximum differential pressure during normal operation (feedwater pump outlet pressure 4.0 MPa - steam drum operating pressure 2.5 MPa = 1.5 MPa). The factory has now replaced the electric single-seat control valve with an electric double-seat control valve. The maximum permissible differential pressure of the double-seat control valve has increased to 11.9 MPa, significantly higher than the maximum differential pressure that might occur in the feedwater system. This example illustrates that when selecting control valves for automatic control loops, the maximum permissible differential pressure value must be verified to ensure normal valve operation. Fourth example: The automatic control valve design for the feedwater system of a 35 t/h boiler in a coal mine gangue power plant used ZKZP 6.4, Dg65 electric single-seat valves. During the initial heating period after boiler commissioning, it was discovered that the feedwater control valve could not close completely, resulting in significant leakage. The situation was slightly better during normal operation. Initially, a product quality issue was suspected. However, after carefully reviewing the instruction manual, it was discovered that the maximum permissible differential pressure value for this model of control valve was only 1.83 MPa. Although this was not significantly different from the differential pressure across the control valve during normal operation (approximately 2.1 MPa), the slight leakage was tolerable. However, during the boiler heating phase, the differential pressure across the control valve became extremely large (potentially reaching 6.0 MPa due to the steam drum pressure being 0), causing the aforementioned problem. A temporary solution was to close the manual gate valves before and after the control valve during the boiler heating phase and use a process-specific manual bypass valve to control the feedwater flow. However, in the long run, the valve still needs to be replaced. This example illustrates that when calculating the maximum permissible differential pressure, it is necessary to consider not only the maximum permissible differential pressure under normal operating conditions but also the maximum permissible differential pressure under abnormal operating conditions. In conclusion, although calculating the maximum permissible differential pressure when selecting control valves for water supply regulation systems may seem like a small matter, neglecting it could lead to significant losses. Therefore, sufficient attention should be paid to this issue when selecting control valves under medium to high pressure differential conditions.