Self-differential pressure control valve is used in HVAC engineering

Abstract: The working principle of the self-operated self-pressure differential control valve is introduced, and the application of the valve in cold and heat source protection and central heating projects is discussed.
Keywords: Self-operated, pressure difference control valve, cold and heat source protection, central heating
Introduction
The function of the commonly referred to self-operated differential pressure control valve is to control the differential pressure of a certain branch or user in the network so that it is basically constant, while the differential pressure consumed by itself changes. It is through adjustment that the differential pressure is basically constant. Its own opening is used to adjust the pressure difference consumed by itself to achieve a constant pressure difference of the controlled object. This kind of pressure difference control valve has been widely used in heating and air-conditioning projects, especially in household metering and heating projects, so it is familiar and understood by everyone. This article introduces a self-operated differential pressure control valve with different functions. Its function is to control its own differential pressure, so it can be called a self-operated differential pressure control valve. At the same time, its application in HVAC engineering is discussed.
1. Structure and working principle
Here, we take the ZY47-16C self-differential pressure control valve as an example to introduce the working principle of the self-differential pressure control valve. Figure 1 is a schematic diagram of the structure and working principle of the valve. The spring, pressure-sensitive diaphragm and valve stem are bonded together, and the outlet pressure P2 is introduced into the sealing cavity above the pressure-sensitive diaphragm through the pressure guide tube, and the inlet pressure P1 is at the bottom of the pressure-sensitive diaphragm. The pre-compression amount of the spring is determined according to the set value ΔPs of P1-P2 (hereinafter referred to as the set pressure difference), even if the elastic force of the spring is equal to the force of the pressure-sensitive film on the serpentine springs under the set pressure difference condition. And select the spring according to the principle that the stroke of the valve plug is much smaller than the pre-compression amount of the spring. In this way, in the equilibrium state of any opening of the valve, the inlet and outlet pressure difference ΔP of the valve is approximately equal to the set pressure difference ΔPs. Strictly speaking, if the opening is different, the ΔP of the equilibrium state is not equal. Obviously, as the opening increases, the ΔP of the equilibrium state increases. However, through the selection of the spring, it is possible to control the deviation of the equilibrium state ΔP relative to ΔPs within a certain range (such as 10%) within the full stroke of the valve plug.
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Figure 1 Structural diagram of ZY47-16C self-pressure difference control valve
The work of the self-operated self-pressure difference control valve in the system can be divided into two situations for explanation: ① The current status is closed. If the pressure difference ΔP between the front and rear of the valve is less than the set pressure difference ΔPs, it will continue to close, and it is a shut-off valve. If ΔP is greater than ΔPs, the pressure-sensitive membrane overcomes the elastic force of the flat spring, drives the valve plug to rise, and the valve opens; when a balanced state is reached, the inlet and outlet pressure difference ΔP approximately falls back to the set pressure difference ΔPs. ②The current status is open. If the system operates stably, the inlet and outlet pressure difference ΔP is approximately the set pressure difference. If ΔP increases due to changes in system operating conditions, the valve will be opened larger and the flow rate will increase; when the equilibrium state is reached, ΔP will approximately fall back to ΔPs. When the valve is at its maximum opening, ΔP is greater than ΔPs, and the valve no longer has the ability to regulate the pressure difference. If the inlet and outlet pressure difference ΔP is less than ΔPs due to changes in system operating conditions, the valve will be closed and the flow rate will decrease. When the equilibrium state is reached, ΔP will approximately rise to ΔPs. When the valve is closed and ΔP is less than ΔPs, it no longer has the ability to regulate the pressure difference and becomes a shut-off valve. In short, when the self-operated self-pressure difference control valve is in the closed state, ΔP must be greater than ΔPs before it can be opened; when it is in the open state, the opening can be automatically adjusted to keep the pressure difference between the front and rear of the valve basically constant.
2. Application of self-differential pressure control valve in HVAC engineering
2.1 Application in protecting hot and cold sources
In recent years, oil and gas units have been widely used in heating projects. Due to the implementation of metering and charging for heating, users’ awareness of independently adjusting the flow rate has been greatly enhanced. In addition, the consumption of domestic hot water changes greatly within a day, resulting in a large range of changes in the flow rate of the heating system. If the flow rate is too small, it may cause local boiling of the fuel and gas unit, which may cause damage to the unit. For the chiller in the air conditioning system, if the chilled water flow is too small, it may also cause partial freezing of the evaporator pipe, thereby damaging the unit. For the above two situations, as shown in Figure 2, a self-operated self-pressure difference control valve can be installed on the bypass pipeline. Due to user adjustment and other reasons, the system flow rate decreases, and the pressure difference ΔP before and after the pressure difference control valve will increase accordingly. When ΔP is greater than the set pressure difference ΔPs, the pressure difference control valve opens to increase the flow of water through the cold and heat sources. flow to ensure the safe operation of the unit. When the pressure difference control valve is in the open state, the pressure difference before and after the valve can always be kept basically constant. The flow rate through the valve changes inversely with the flow rate of the user system. That is, if the flow rate of the user system decreases, the flow rate through the differential pressure control valve will increase; conversely, if the flow rate of the user system increases, the flow rate through the differential pressure control valve will decrease. This ensures that the flow rate through the cold and hot sources does not change too much, which not only protects the cold and hot sources, but also improves the stability of the unit’s operation.

The traditional way to protect hot and cold sources is to install an electric differential pressure control valve on the bypass line. When the system flow decreases and the pressure difference between the front and rear of the electric valve is greater than the set pressure difference, the electric signal drives the electric valve to open to maintain the necessary minimum flow of the cold and heat source unit. However, due to its dependence on power supply and lines for transmitting electrical signals, electric differential pressure control valves are not as reliable as self-operated differential pressure control valves. In addition, the price is also much higher than the latter. Therefore, in terms of protecting cold and heat sources, it is completely possible to use a self-operated self-pressure differential control valve to replace the traditional laser cutting copper electrode electric row valve. By the way, it is inappropriate to install a solenoid valve on the bypass pipeline shown in Figure 2, because the solenoid valve only has two states: closed and fully open, so every time it moves, it will affect the flow of the user system. greater impact.
2.2 Application in central heating system
In central heating projects, this situation often occurs: heating users include low buildings (shorter buildings or buildings with lower terrain) and high buildings (high-rise buildings or buildings with higher terrain). If the pressure condition of the heating network If the requirement that the radiators of low buildings are not crushed is met, emptying will occur in tall buildings; if the pressure conditions of the heating network meet the requirements of high buildings without emptying, the pressure on the radiators of low buildings will exceed its pressure-bearing capacity. This contradiction can often be solved with the help of its own pressure difference control valve.
Figure 3 is an example where the terrain height difference is very different and the heat source is located at a low place. Following the terrain characteristics, a pressurized water pump is installed at an appropriate location in the water supply pipeline, and a self-operated self-pressure difference control valve is installed at an appropriate location in the return water pipeline. During the operation of the system, the pressure difference before and after the differential pressure control valve can remain basically constant. In this way, the dynamic water pressure line of the network is divided into two parts. The dynamic water pressure line in the front part is relatively low, which can meet the requirements of the radiator in low buildings not to be crushed; the dynamic water pressure line in the rear part is relatively low. High, which can meet the requirements of high-rise buildings without emptying. When the system stops running, the water heads of the pressure measuring pipes in the entire network tend to reach a consistent trend, while the pressure difference control valve tries to maintain the original pressure difference basically unchanged by reducing the opening until the pressure difference control valve closes. At this time, the pressure difference control valve and the check valve on the water supply pipeline work together to isolate the rear part of the network from the front part. The hydrostatic pressure line at the front of the network is guaranteed by a water supply constant pressure device set at the heat source. The hydrostatic pressure line at the rear of the network is guaranteed by a constant-pressure water replenishing pump equipped with a differential pressure control valve.

Figure 3 Self-differential pressure control valve is used in central heating projects (1)
1 heat source 2 circulating water pump 3 system supply water pump 4 own pressure difference control valve 5 pressurized water pump 6 check valve 7 network rear supply water pump 8 supply water pressure regulating valve 9 heat user
On the contrary, if the terrain is very different and the heat source is at a high place, as shown in Figure 4, follow the terrain characteristics and install its own pressure difference control valve at the appropriate location of the water supply pipeline, and install a pressurized water pump at the appropriate location of the return water pipeline. . When the system is running, the pressure difference before and after the pressure difference control valve can be kept basically constant, so that the dynamic water pressure line at the rear of the network is relatively low, which can meet the requirements of the radiator in low buildings not to be crushed; in front of the network The dynamic water pressure line at the bottom is relatively high, which can prevent emptying of tall buildings. When the system stops running, the pressure difference control valve automatically closes and, together with the check valve on the return line, isolates the rear part of the network from the front part. The hydrostatic pressure line at the front of the network is guaranteed by the water replenishment constant pressure device installed on the heat source, and the hydrostatic pressure line at the rear of the network is guaranteed by the water replenishment regulating valve on the water replenishment pipeline connecting the front and rear.

Figure 4 Self-differential pressure control valve is used in central heating projects (2)
1 heat source 2 circulating water pump 3 system supply water pump 4 own pressure difference control valve 5 pressurized water pump 6 check valve 7 rear water supply pressure regulating valve 8 heat user
3. Conclusion
When the self-operated self-operated pressure difference control valve is in a closed state, if the pressure difference between the front and back of the valve is less than the set pressure difference, it will continue to close; if the pressure difference between the front and back of the valve is greater than the set pressure difference, the valve will open. When it is in the open state, the opening can be automatically adjusted to keep the pressure difference between the front and back of the valve basically constant.
The self-operated differential pressure control valve can be used to protect hot and cold sources. Compared with traditional electric control protection, it has the advantages of reliable control and low price.
The self-operated self-operated differential pressure control valve can be used to solve the contradiction in the different pressure working conditions requirements caused by the huge difference in height between tall buildings and low buildings in central heating projects.