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Mechanical Industry Between The Bearing And Valve Principle
Aug 12, 2018

The function of the self-actuated pressure difference control valve is to control the pressure difference of a branch or a user in the network and make it basically constant, while the pressure difference consumed by itself is changed. This pressure difference control valve has been widely used in heating and air conditioning engineering, especially in household metering heating engineering. This paper introduces a kind of self-operated pressure difference control valve with different functions. At the same time, its application in hvac engineering is discussed.

1. Structure and working principle

Taking zy47-16c self-pressure difference control valve as an example, the working principle of self-pressure difference control valve is introduced. Figure 1 shows the structure and working principle of the valve. The spring, the pressure sensitive membrane and the stem are cemented together, and the outlet pressure P2 is imported into the sealed cavity above the pressure sensitive membrane through the pressure guide, and the lower part of the pressure sensitive membrane is the inlet pressure P1. According to the set value of P1 and P2 Δ Ps (hereinafter referred to as the set pressure difference) to determine the spring precompression, even if the spring tension and setting the differential of spring force is equal under the condition of pressure sensitive film. And according to the stroke of the plug far less than the spring precompression of the principle to choose the spring. This makes any opening balance valve, the valve inlet and outlet pressure differential Δ P and setting the differential Δ Ps approximately equal. Strictly speaking, the opening is different, equilibrium Δ P is not equal. Obviously, with the increase of the opening, equilibrium Δ P is enlarged. But through the selection of spring, can completely within the whole journey of the plug, the equilibrium state of Δ P relative to Δ Ps deviation control within a certain range (for example, 10%).

The operation of the self-actuated pressure difference control valve in the system can be explained in two cases: the current status of the feed is closed. If the valve pressure difference before and after Δ P is less than the set pressure differential Δ Ps, continued to shut down, then is a shutoff valve. If Δ P is greater than the Δ Ps, the pressure sensitive film to overcome the spring tension, drive the valve plug, open valve; Differential pressure at the inlet and outlet of the equilibrium state, approximate Δ P back to set the differential Δ Ps. The current state of invocation is on. If the system stable running, inlet and outlet pressure differential Δ P approximation for setting pressure difference. If the change of the system working condition, make Δ P increases, the valve open big, traffic increase, Balance, Δ P approximate back to Δ Ps. Valve for the biggest opening, Δ P > Δ Ps case, no longer have the ability to control pressure difference of the valve. If due to the change of system operating mode, the inlet and outlet pressure differential Δ P < Δ Ps, the small valve, flow decreases, and reaches an equilibrium state, Δ P and approximate rise to Δ Ps. Until the valve is closed, appear Δ P < Δ Ps, no longer have the ability to control pressure difference, and become a shutoff valve. Self-reliance type itself, in short, differential pressure control valve in the closed position, Δ P must be greater than Δ Ps can open; In the open state, the opening can be automatically adjusted to keep the pressure difference before and after the valve basically constant.

2. Application of self-pressure difference control valve in hvac engineering

2.1 application in protecting cold and heat sources

In recent years, in the heating engineering, the fuel oil and gas units have more applications. As a result of metering and charging for heating, users' awareness of self-regulation of the flow is greatly enhanced. In addition, the consumption of domestic hot water changes greatly in a day, which makes the flow of heating system have a large range of changes. If the flow rate is too small, it may cause partial boiling of the fuel and gas units, and thus damage the units. For the chiller unit in the air conditioning system, if the refrigerated water flow is too small, it may cause partial freezing of the evaporative discharge pipe, which will damage the chiller. For the above two cases, as shown in FIG. 2, self-operated differential pressure control valves can be installed on the bypass pipe road. Due to reasons such as the user to adjust the system flow decreases, and the differential pressure before and after the pressure difference control valve Δ P will increase, when Δ Ps Δ P is greater than the set pressure difference, pressure differential control valve opens, increased by the flow of cooling and heat sources, to ensure the safe operation of the unit. When the pressure difference control valve is open, the pressure difference before and after the valve can always be kept basically constant. The flow through the valve changes opposite to that of the user system. That is, the flow rate of the user system decreases, and the flow rate through the pressure difference control valve increases. On the contrary, if the flow rate of the user system increases, the flow rate decreases through the pressure difference control valve. In this way, it can guarantee that the flow through the cold and hot source does not change too much, which not only protects the cold and hot source, but also improves the operation stability of the unit.

The traditional way to protect the cold and heat source is to install the electric pressure differential control valve on the bypass pipe road. When the system flow rate decreases, making the front and rear pressure difference of the electric valve larger than the set pressure difference, the electric signal drives the electric valve to open, making the cooling and heat source unit maintain the minimum flow. However, the electric pressure difference control valve is not as reliable as the self-operated pressure difference control valve because of its dependence on power supply and transmission line. In addition, the price is much higher than the latter. Therefore, the traditional electric control valve can be replaced by the self-driven differential pressure control valve in protecting the cold and heat source. By the way, it is inappropriate to install solenoid valve on the bypass pipe road as shown in FIG. 2, since the solenoid valve is only in the closed and full open state, so every action of the solenoid valve will have a significant impact on the flow of the user system.

2.2 application in central heating system

In central heating engineering, there are low buildings (shorter buildings or low-lying buildings) and high buildings (tall buildings or high-lying buildings) for heating users. If the pressure condition of heat network meets the requirement that the radiator of low buildings is not damaged, the high buildings will be emptied. If the pressure working condition of the heat network meets the requirement that the high building does not empty, the pressure on the radiator of the low building will exceed its pressure bearing capacity. With its own pressure difference control valve can often resolve this contradiction.

Figure 3 is an example of a low-lying heat source with a wide elevation difference. According to the topographic features, the pressurized pump is set in the proper position of the water supply pipeline, and the self-operated differential pressure control valve is set in the proper position of the backwater pipeline. During system operation, the pressure difference before and after the pressure difference control valve can be kept basically constant. In this way, the dynamic water pressure line of the network is divided into two parts. The dynamic water pressure line of the front is relatively low, which can meet the requirement that the radiator of low building is not damaged. The hydrodynamic pressure line at the rear is relatively high, which can meet the requirement of high building not to empty. When the system stops running, the hydraulic head of the whole network has a consistent trend, while the pressure difference control valve tries to keep the original pressure difference basically unchanged by reducing the opening, until the pressure difference control valve closes. At this point, the pressure difference control valve, together with the check valve on the supply line, separates the back of the network from the front. The hydrostatic pressure line at the front of the network is ensured by a water replenishing and pressure fixing device set on a heat source. The static water pressure line at the back of the network is ensured by the fixed pressure supplementary water pump which is equipped with the pressure difference control valve.

1 heat source 2 circulation pump 3 system supply pump 4 own differential pressure control valve 5 pressurized pump 6 check valve 7 network rear supply pump 8 water supply pressure regulating valve 9 hot users

On the contrary, if the terrain is very different, and the heat source is high, as shown in FIG. 4, the pressure difference control valve is installed at the appropriate position in the water supply pipeline and the pressure pump is installed at the appropriate position in the backwater pipeline according to the features of the terrain. During the operation of the system, the pressure difference before and after the pressure difference control valve can be kept basically constant, so the hydrodynamic pressure line at the back of the network is relatively low, which can meet the requirement of the low-building radiator not being damaged. The hydrodynamic pressure line at the front of the network is relatively high, which can meet the requirement that no emptying occurs in high buildings. When the system stops running, the pressure difference control valve closes automatically, along with the check valve on the return pipe, isolating the back of the network from the front. The static water pressure line at the front of the network is ensured by the water supply and pressure fixing device set in the heat source, while the static water pressure line at the back of the network is ensured by the water regulating valve on the water supply pipe road before and after connection.

1 heat source 2 circulation pump 3 system supply pump 4 own differential pressure control valve 5 pressurized pump 6 check valve 7 rear water supply pressure regulating valve 8 hot users

3, conclusion

When the self-actuated pressure difference control valve is closed, if the pressure difference before and after the valve is less than the set pressure difference, the valve will continue to close. If the pressure difference before and after the valve is greater than the set pressure difference, the valve opens. In the open state, the opening can be automatically adjusted, so that the pressure difference before and after the valve is basically constant.

Self-operated differential pressure control valve can be used to protect the cold and hot source. Compared with the traditional electric control protection, it has the advantages of reliable control and low price.

The self-actuated pressure difference control valve can be used to solve the contradiction of different demands on pressure condition caused by the difference between high building and low building height in central heating engineering.