Working principle of flow control valves and pressure control valves

Working in the oil refining industry, I believe that flow control valves and pressure control valves operate on the same principle; the differences are as follows: 1. The media used are different; 2. The criteria for choosing between air-operated and spring-operated types can sometimes vary; 3. Their functions are different

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, so that the electric control valve or temperature control valve can 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; yet 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 network 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 high pressure differences, in actual operating conditions, as long as the valve’s coefficient is not 1, there is a large pressure difference across the valve when it is open to a small degree; whereas the pressure difference is smaller when it is open to a large degree. This results in the value of dG/dC increasing at small opening degrees and decreasing at large opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the valve’s coefficient, the greater this shift. For valves with linear characteristics, such shifts in performance lead to a reduction in the effective range of opening degrees available for 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 intense turbulent vortex areas behind the valve. The pressure in these vortex areas is very low; when this pressure drops below the saturation pressure corresponding to the water temperature, vaporization occurs, leading to steam hammer effects: 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 restricting 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. Furthermore, whenever pressure conditions are not a concern, it is advisable to install the alkaline balance valve on the return water pipe where the water temperature is lower. 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 supply. A self-acting balance valve regulates flow by maintaining a constant pressure difference before and after the orifice (with a fixed diameter); therefore, it can also be called a constant-flow valve. A constant flow valve operates on flow rate and is capable of locking 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 values ； 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 in 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 the requests of 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, allowing for adaptation to the heat demands of heavy users, thereby 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. Based on 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 across any range of adjustments, 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 primarily 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 loss of control in regulation does not occur. Therefore, this type of control valve can be used, which is beneficial for improving the control performance of the heating system.

Overview: A self-acting control valve is an energy-saving instrument that requires no external power source; it relies on the changes in pressure, temperature, and flow rate of the medium being controlled to carry out automatic regulation, and it possesses integrated functions of measurement, actuation, and control. Self-acting control valves are mainly divided into self-acting pressure control valves, self-acting differential pressure control valves, self-acting temperature control valves, and self-acting flow control valves. 1. Working principle of the self-acting pressure control valve (post-valve pressure control): The pressure P1 of the working medium before the valve is reduced through throttling by the valve core and seat, resulting in the post-valve pressure P2. P2 is introduced into the lower diaphragm chamber of the actuator through control lines and acts on the top plate; the force generated thereby balances the reaction force of the spring, determining the relative position of the valve spool and the valve seat and thus controlling the pressure behind the valve. As the pressure behind the valve, P2, increases, the force exerted by P2 on the top plate also increases. At this point, the force exerted by the top plate is greater than the reaction force of the spring, causing the valve core to move toward the valve seat, until the force from the top plate and the reaction force of the spring are in balance. At this point, the flow area between the valve core and the valve seat decreases, the flow resistance increases, thereby reducing P2 to the set value. Similarly, when the pressure P2 behind the valve decreases, the direction of action is opposite to that described above; this is the working principle of a self-acting (pressure behind the valve) pressure control valve. 2. Working principle of the self-acting pressure control valve (pressure control before the valve): The pressure P1 of the working medium before the valve is reduced through throttling by the valve core and seat, resulting in the pressure P2 after the valve. At the same time, P1 is delivered through control lines to the upper diaphragm chamber of the actuator, acting on the top plate; the force generated thereby balances the reaction force of the spring, determining the relative position of the valve core and the valve seat and thus controlling the pressure in front of the valve. As the pressure behind the valve, P1, increases, the force exerted by P1 on the top plate also increases. At this point, the force exerted by the top plate is greater than the reaction force of the spring, causing the valve core to move away from the valve seat until the force from the top plate balances the reaction force of the spring. At this point, the flow area between the valve core and the valve seat increases, the flow resistance decreases, thereby reducing P1 to the set value. Similarly, when the pressure behind the valve P1 decreases, the direction of action is opposite to that described above; this is the working principle of a self-acting (pre-valve) pressure control valve. 3. Working principle of the self-acting temperature control valve (heating type): The temperature control valve operates based on the principles of the incompressibility of liquids and thermal expansion and contraction. It is a self-acting temperature control valve for heating; when the temperature of the object being controlled is below the set temperature, the liquid inside the thermocouple contracts, which reduces the force acting on the actuator rod. Under the influence of the spring force, the valve element opens, increasing the flow rate of heating media such as steam and hot oil, thereby raising the temperature of the object being controlled. Once the temperature reaches the set value, the valve closes. After it closes, the temperature of the object drops, and the valve opens again, allowing the heating media to flow back into the heat exchanger and raise the temperature once more, thus maintaining a constant temperature for the object being controlled. The valve opening degree is related to the difference between the actual temperature of the controlled object and the set temperature. 4. Working principle of self-acting temperature control valves (cooling type): The working principle of self-acting temperature control valves used for cooling can be similar to that of those used for heating; the only difference is that the valve element opens and closes under the action of the actuator and spring force, in contrast to temperature rise valves. A cold medium flows through the valve body, and these valves are primarily used for temperature control in cooling systems. 5. Working principle of the self-acting flow control valve: When the medium to be controlled enters the valve, the pressure P1 before the valve is transmitted to the lower diaphragm chamber through the control line. The pressure Ps, which results from throttling by the throttle valve, is sent to the upper diaphragm chamber. The difference between P1 and Ps, namely △Ps = P1 – Ps, is known as the effective pressure. The difference between the thrust generated by P1 on the diaphragm and the thrust generated by Ps on the diaphragm, balanced against the spring force, determines the relative position of the valve spool and the valve seat, thereby determining the flow rate through the valve. As the flow rate passing through the valve increases, that is, as △Ps increases, P1 and Ps act on the lower and upper diaphragm chambers respectively, causing the valve core to move toward the valve seat. This changes the flow area between the valve core and the valve seat, resulting in an increase in Ps. The thrust exerted by the increased Ps on the diaphragm, combined with the spring force, balances the thrust exerted by P1 on the diaphragm at the new position, thereby achieving control over the flow rate. Conversely, the same applies. The flow rate of the medium under control is set by adjusting the relative position of the throttle valve and the valve seat

Unlike butterfly valves and gate valves, which are used only for shutting off pipelines, piston flow control valves are valves capable of meeting various special regulation requirements. Its regulating function is achieved through the axial movement of a piston-like cylinder within the valve chamber, with its stroke being in line with the direction of the water flow in the pipe. The flow enters the housing along an axial arc; the flow channel within the piston valve is axially symmetric, so no turbulence occurs as the fluid passes through it. The change in the flow channel area is achieved through linear motion of a piston along the axial direction of the pipe. Regardless of the position of the piston, and no matter where the piston moves, the water flow cross-section within the valve chamber remains circular at all positions; it narrows toward the axis at the outlet, thereby achieving optimal cavitation protection and preventing damage to the valve body and pipes caused by cavitation that may result from throttling. The piston valve body is designed as a single unit, offering high flow capacity; the degree of opening is linearly related to the flow rate, which helps to effectively prevent cavitation and vibration. The inner shell is connected to the outer shell via streamlined guide ribs, which ensure that the stainless steel piston slides smoothly without any risk of tilting or malfunction. The end face upstream of the inner shell is spherical, allowing the water flow to follow a gradual transition; the piston is operated by a crank-slider mechanism installed within the shell. The piston valve features dual sealing, namely metal-to-metal and metal-to-rubber, to achieve bidirectional bubble-level sealing. Thus, the sealing system achieves a long service life and tight sealing. Due to the special structure of the piston valve, depending on the operating conditions, the water flow characteristics of the valve can be adjusted by changing the type of the outlet component at the downstream exit of the valve; this allows it to meet the requirements of different applications and achieve the best flow control effects. There are four types of outlet-regulating components: the first is type E, which features a bend-correcting structure and a sudden enlargement of the cross-section downstream of the end seat, thereby eliminating cavitation damage. It is suitable for applications with flow control, high pressure differences, and high backpressure, while the structure that straightens the flow path and enlarges the cross-section can reduce cavitation effects. The second S-type has a slotted sleeve as its closed guide component. It is suitable for applications requiring flow control, regulation of high pressure differences, and high back pressure; its adjustability matches the state of the fluid, enabling optimal performance. The third F-type has a short diffuser tube at the outlet section of the valve body, and is suitable for regulation as well as opening and closing operations, serving as a switch. The head loss during opening and closing is very low, and the resistance is minimal when it is fully open. The fourth type, LH or SZ, features a porous ring network designed to prevent cavitation, installed on the guide element that closes the piston inside the valve chamber. It is suitable for use in situations requiring regulation, cavitation prevention, high pressure differences, and low back pressure (at the inlet of reservoirs). Its flow control characteristics are optimized to meet the specific operating conditions, thereby eliminating the effect of cavitation. In water diversion applications, VAG piston valves can be used as relief valves and explosion-proof vent valves. During storage, the LH and E types of VAG piston valves can function as level or pressure control valves. In water transmission systems, the LH and S types of VAG piston valves can be used as flow control valves, while the E, LH, or SZ types can serve as drain valves or filling valves. In sewage and water treatment applications, VAG piston valves can act as protection valves for pump startup; they can also be equipped with special control outlet components and used as flow control valves in water treatment plants. The E type piston valve can also be used as a gas flow control valve in sewage treatment plants. For water supply purposes, VAG piston valves utilize different outlet components depending on the liquid conditions, and they are used as pressure/flow control valves, such as for balancing pressure in pipeline networks.

Control valves are used to regulate the flow rate, pressure, and level of a medium. Based on the signals from the control point, the opening degree of the valve is automatically adjusted, thereby enabling the regulation of the flow rate, pressure, and level of the medium. Control valves include electric control valves, pneumatic control valves, and hydraulic control valves, among others. A control valve consists of an electric actuator or a pneumatic actuator, along with the control valve itself. They are adjusted and generally divided into straight-through single-seat and straight-through double-seat types; the latter features high flow capacity, low imbalance, and stable operation, making it particularly suitable for applications with large flow rates, high pressure drops, and low leakage. The flow capacity Cv is one of the main parameters for selecting control valves. The flow capacity of a control valve is defined as the volume of fluid that passes through the valve per hour, when the valve is fully open, with a pressure difference of 0.1 MPa across it and a fluid density of 1 g/cm3. This value is known as the flow capacity, or flow coefficient, and is denoted by Cv; its unit is t/h. The flow characteristic of a control valve is the relationship between the relative flow rate of the medium passing through the valve and its opening degree, under the condition that the pressure difference across the valve remains constant. The flow characteristics of control valves include linear, equal percentage, and parabolic characteristics. The meanings of the three injection characteristics are as follows: (1) Equal percentage characteristic (logarithmic). For the equal percentage characteristic, the relative stroke and relative flow rate do not follow a linear relationship; at each point along the stroke, the change in flow rate resulting from a unit change in stroke is proportional to the flow rate at that point, and the percentage change in flow rate remains constant. Therefore, its advantage is that the flow rate change is small when the flow rate is low, while it is large when the flow rate is high; in other words, it maintains the same regulation accuracy at different opening degrees. (2) Linear characteristic (linearity): The relative stroke and relative flow rate under a linear characteristic have a linear relationship. The change in flow rate caused by a change in unit stroke remains constant. When the flow rate is high, the change in the relative value of the flow rate is small; when the flow rate is low, the change in the relative value of the flow rate is large. 3) Parabolic characteristic: the flow rate varies proportionally to the square of the stroke, exhibiting an intermediate behavior that is roughly between linear and equal percentage characteristics. From the analysis of these three characteristics, it can be seen that in terms of regulation performance, the equal percentage characteristic is the best, as it offers stable regulation and excellent performance. Moreover, the parabolic characteristic offers better regulation performance than the linear one, allowing any of these flow characteristics to be chosen depending on the requirements of the application. Shanghai Faner Valves – Control valve series: imported control valves, control valves imported from Germany, those imported from the UK, and those imported from the United States. Pneumatic control valves: pneumatic diaphragm single-seat control valves ZMZP, ZMAP, ZMBP; pneumatic diaphragm double-seat control valves ZMAN, ZMBN. Pneumatic compact diaphragm control valves ZJHP; pneumatic compact diaphragm sleeve control valves ZJHM; pneumatic three-way split control valves ZAZQ, ZMAQ; pneumatic three-way merge control valves ZAZX, ZMAX; pneumatic lightweight control valves of types ZXMAP, ZXMAN, ZXMAM; pneumatic high-pressure angle control valves ZMAS, ZMBS; pneumatic control and shut-off O-ring ball valves of type ZSHO; pneumatic piston-type control and shut-off butterfly valves ZSCW; pneumatic V-type wafer ball valves ZSHV; pneumatic V-type flanged ball valves ZSHV; pneumatic piston-type quick shut-off valves ZSPQ; pneumatic diaphragm-piston type shut-off valves ZMQ, ZSQ; pneumatic diaphragm valves ZMGT; pneumatic fluorine-lined control valves ZMPF; CV3000 series control valves HLS, HTS, HCB, HPS, HPC, HAA, HAC, HCP, HPN. Electric control valves: electric single-seat control valves ZAZP, electric double-seat control valves ZAZN, electric sleeve control valves ZAZM; intelligent control ball valves ZRQO; electronic electric control valves ZDLP, ZDLM; electronic electric double-seat control valves ZDLN; electronic electric three-way merge control valves ZDLQ, ZDLX; electric three-way split control valves ZAZQ; electric three-way merge control valves ZAZX; electric V-type control valves ZKJV; electronic V-type control valves ZDJV; electric compact single-seat control valves ZAJP, electric compact sleeve control valves ZAJM; compact electronic control valves ZDHP, ZDHM; electric control butterfly valves ZAJM. Self-acting control valves: self-acting pressure control valves ZZYP, ZZV, ZZVP; self-acting temperature control valves ZZWP, ZZWEP. Valve body material: cast steel, stainless steel, chrome-molybdenum steel. Operating pressure: 1.6–32 MPa; operating temperature: -40°C–600°C. Valve port diameter DN15-DN200. Connection method: flange. Manufacturing standard: National standard.

Flow control valves, pressure control valves, and other control valves are essentially the same; they are simply named according to the process variable they regulate. They are all throttling elements, and it is through the degree of opening of these throttling elements that the flow resistance characteristics of the piping system are adjusted, thereby changing the flow rate and influencing other process parameters. It can be simply understood as a flow-limiting orifice plate with a variable flow area. Additionally, the issues of critical flow and non-critical flow should be considered.