Analysis on the type selection and design of flow

Analysis on the type selection and design of flow regulating valve

flow regulating valve plays a very important role in the heating system of metering and charging. Therefore, how to correctly select and design the flow regulating valve is particularly critical! Starting with the structure and working principle of flow regulating valve, this paper puts forward some problems that should be paid attention to in the selection and design of regulating valve. In the selection and design of temperature control valve, while selecting the temperature control valve with the same diameter as the pipeline, it is also necessary to create an ideal differential pressure working condition for the selected temperature control valve; Electric control valve is a device suitable for flow regulation in computer monitoring system, which is generally used in unattended thermal power stations; For the manual balancing method, the periphery of the disc punched and cut in 4.6 should be free of burrs that affect the flatness. How to use the characteristic curve of the valve to analyze the regulating performance of the valve, and how to solve the problem that the valve is easy to cause steam water hammer under the condition of small opening; For the self operated flow control valve, pay attention to the requirement of minimum working difference when designing and selecting the type

key words: temperature control valve, electric control valve, balance valve, differential pressure control valve

the implementation of heat metering and charging in the heating system can save energy and improve the energy efficiency of the heating system. As far as the current situation is concerned, the energy efficiency of China's heating system is only about 30%. People often only pay attention to the heat loss of the boiler and the outside, and ignore the heat loss of the heat user. The heat loss of heat users is mainly caused by uneven cooling and heating. This part of heat loss is about 30 ~ 40%, which is quite considerable. From the perspective of energy conservation, the main purpose of metering and charging in the heating system is to tap the energy-saving potential of this part

metering and charging can save energy macroscopically through three ways: first, the flow regulating valve is installed to realize the flow balance, and then the uneven phenomenon of cold and hot is overcome; Secondly, through the function of temperature control valve, the free heat of solar energy, household appliances, lighting and other equipment is used; The third is to improve the energy-saving awareness of residents who use heat and reduce the unnecessary heat dissipation of opening windows. Two of the three energy-saving ways are realized by flow regulating valves. It can be seen that the flow regulating valve plays an important role in the heating system of metering and charging. Therefore, how to correctly select and design the flow control valve is very important

I. temperature control valve

1. Structure and working principle of radiator temperature control valve

the temperature control in the user's room is realized through the radiator temperature control valve. The radiator thermostatic control valve is composed of a thermostatic controller, a flow regulating valve and a pair of connectors. The core component of the thermostatic controller is the sensor unit, that is, the thermometer. The temperature bag can sense the change of ambient temperature and produce volume change, drive the displacement of the regulating valve spool, and then adjust the water volume of the radiator to change the heat dissipation of the radiator. The set temperature of the thermostatic valve can be adjusted manually, and the thermostatic valve will automatically control and adjust the water volume of the radiator according to the set requirements, so as to achieve the purpose of controlling the indoor temperature

the temperature control valve is usually installed in front of the radiator to achieve the room temperature required by residents by automatically adjusting the flow. Temperature control valve has two-way temperature control valve and three-way temperature control valve. The three-way temperature control valve is mainly used for the single pipe system with crossover pipe. Its diversion coefficient can change within the range of 0 ~ 100%, and there is large room for flow regulation, but the price is relatively expensive and the structure is complex. Some of the two-way temperature control valves are used to continuously improve the industrial batch production management system in the double pipe system, and some are used in the single pipe system. The resistance of two-way temperature control valve used in double pipe system is large; The resistance for single pipe system is small. The temperature sensing package of the temperature control valve and the valve body are generally assembled as a whole, and the temperature sensing package itself is the on-site indoor temperature sensor. If necessary, remote temperature sensors can be used; The remote temperature sensor is placed in the room requiring temperature control, and the valve body is placed in a certain part of the heating system

2. Type selection and design of temperature control valve

temperature control valve is the most important regulating equipment for flow regulation of heating system, and other regulating valves are auxiliary equipment, so temperature control valve is necessary. If a heating system is not equipped with a temperature control valve, it cannot be called a heat metering and charging system

in the design of temperature control valve, correct type selection is very important. The purpose of type selection of temperature control valve is to determine Kv value (flow coefficient) according to the design flow (under known heat load) and allowable resistance drop; Then the diameter (model) of the temperature control valve is determined by the Kv value. Therefore, the relationship between Kv value and diameter must be given in the design atlas or manufacturer's samples, otherwise it is not convenient for designers to use

in the selection and design of temperature control valve, it is not a simple choice of temperature control valve with the same diameter as the pipeline, that is, it is all done. Instead, it is necessary to create an ideal differential pressure working condition for the selected temperature control valve in the process of model selection. The normal working pressure difference of a temperature control valve is between 2 ~ 3mh2o, and the maximum is not more than 6 ~ 10mh2o. Therefore, the preset value range of the temperature control valve must be given to prevent noise from affecting the normal operation of the temperature control valve. When there are more than two caliber choices under the same Kv value, the temperature control valve with small caliber should be preferred to improve the regulation performance of the temperature control valve

II. Electric control valve

electric control valve is a device suitable for flow regulation in computer monitoring system. It is usually used in unattended heating stations. The electric control valve is composed of valve body, driving mechanism and transmitter. The temperature control valve is a device for self operated flow regulation through the temperature sensing bag, and there is no need for external power supply; The electric control valve generally requires a single-phase 220V power supply, which is usually used as the actuator of the computer monitoring system (regulating flow). Electric control valve or temperature control valve is the most important equipment for flow regulation in the heating system, and others are its auxiliary equipment

III. balance valve

balance valve is divided into dynamic balance valve and self operated balance valve. Whether manual balance valve or self operated balance valve, their function is to increase the resistance at the near end of the heating system and limit the actual operating flow not to exceed the design flow; In other words, its function is to overcome the redundant resource pressure head near the heating system, so that the electric control valve or temperature control valve can work under a permitted resource pressure head. Therefore, manual balance valve and self operated balance valve are auxiliary flow regulating devices of temperature control valve or electric control valve, but they are also very important. If the type selection is improper or the design is unreasonable, the electric control valve or temperature control valve cannot work well

1. Manual balance valve

1.1 working principle of manual balance valve

manual balance valve is manually adjusted at one time and cannot automatically change the resistance coefficient with the change of system working conditions, so it is called static balance valve. The object of the manual balance valve is resistance, which can play the role of manually adjustable orifice to balance the resistance of the pipe system, so as to achieve the resistance balance of each loop in the selection method of test tonnage of Jinan gold testing machine. It can solve the problem of steady-state imbalance of the system: when the operating condition is different from the design condition, the circulating water volume is more or less than the design condition. Because the balance valve balances the system resistance, it can distribute the new water volume according to the proportion calculated in the design, so that the flow volume of each branch will increase or decrease proportionally at the same time, Still meet the corresponding flow requirements under the current load

1.2 problems that should be paid attention to in the selection and design of manual balance valve (2)

(1) the valve characteristic curve determines the regulating performance of the valve, such as the flow curve of stop valve. If it is considered that the flow change between 95% and 100% is meaningless, then the opening from 0% to 5% realizes the whole process change of flow. Such a valve cannot be used as the balance regulation of hydraulic conditions. Because the theoretical characteristic curve of the valve is measured under the top pressure difference, and the actual working condition as long as the valve weight is not 1, the pressure difference between the front and rear of the valve is large at the small opening line. The large opening is the small pressure difference between the front and rear of the valve, resulting in the valve dg/dc value increasing at the small opening and decreasing at the large opening, so that the actual working curve of the valve shifts to the fast opening direction. The smaller the valve weight, the greater the shift, For valves with linear characteristics, the deviation of actual performance will reduce the opening space for effective adjustment of the valve, so the chord arc under the theoretical curve of the valve, such as equal percentage characteristics, is better. For valves with equal percentage characteristic curve, the actual working curve may be close to the linear characteristic when the valve weight is 0.3 ~ 0.5

(2) generally, the flow rate of the valve is too high at a small opening, and a vigorous turbulent vortex zone will be formed behind the valve. The vortex zone and the new pressure are very low. When the pressure here is lower than the saturation pressure corresponding to the water temperature, the flashing of water vapor will lead to steam water hammer phenomenon: serious noise, vibration of the valve and pipeline, damage to the valve, pipeline and pipe support. To prevent this kind of accident, first of all, in the design of the flow passage of the valves, it is considered that the valve plug and valve seat form a narrow throttling channel when the opening is small, which restricts the formation of vigorous turbulent vortex; Secondly, when selecting the valve, try to increase the valve weight to avoid the valve running under small opening. In addition, when pressure conditions are not involved, try to install the alkali balance valve on the return pipe with low water temperature

2. Self operated balance valve

2.1 working principle of self operated balance valve

self operated balance valve can automatically realize the flow balance of the system without external power supply. The self operated balance valve realizes the flow limit by keeping the pressure difference between the front and back of the orifice plate (fixed aperture) constant. Therefore, it can also be called a constant flow valve

the function object of the constant flow valve is the flow, which can lock the amount of water flowing through the valve, rather than the balance of resistance. It can solve the dynamic imbalance problem of the system: in order to maintain the high-efficiency operation of a single refrigerator, boiler, cooling tower, heat exchanger and other equipment, it is necessary to control the flow of these equipment to be fixed at the rated value; From the end of the system, in order to avoid the interaction of dynamic regulation, it is also necessary not to affect the pointer to return to zero and limit the flow at the end device or branch

problems that should be paid attention to in design

the disadvantage of self operated flow control valve is that the valve has the requirement of minimum working pressure difference. Generally, the minimum working pressure difference of 20KPa is required for products. If it is installed on the most unfavorable circuit, it is bound to require the circulating water pump to increase the working lift of 2 meters of water column. Therefore, the method of installing near end and disturbing far end should be adopted. Do not install this self operated flow control valve when the distance from the user to the heat source is greater than 80% of the heating radius

IV. differential pressure regulating valve

1. The principle of differential pressure regulating valve

the principle of differential pressure regulating valve is essentially the same as that of self operated balance valve. However, in the self operated balance valve, the orifice plate exists in the valve body as a component; There is no orifice in the differential pressure regulating valve, but the system behind the differential pressure regulating valve is regarded as an orifice. Therefore, the differential pressure value of the regulating valve actually refers to the pressure difference between the inlet and outlet of the subsequent system. It can be seen from the structure of the differential pressure regulating valve that the purpose of this regulating valve is to control the inlet and outlet pressure difference of the subsequent system to be fixed. The basic function is to automatically adjust the operating flow of heat users according to the demand of heat load of heat users. When the room temperature of a building is required to be reduced by some hot users, the opening of the temperature control valve in the corresponding room becomes smaller, resulting in the pressure difference of the differential pressure regulating valve becoming larger than the set value. At this time, the differential pressure regulating valve automatically reduces the valve core, increases the throttling effect, and reduces the system pressure difference until it returns to the set value