Balance valve application analysis

First, the balance valve

The balancing valve correctly understands that it should be a hydraulic working condition balancing valve. From this concept, all valves used for balancing hydraulic conditions such as regulating valves, pressure reducing valves, self-operated flow control valves, and self-operated differential pressure control valves should be regarded as balancing valves for hydraulic working conditions - balancing valves. The product on the market called the balancing valve is only a manual regulating valve with a flow test function.

Static balancing valves are manual or manual balancing valves. Dynamic balancing valves are self-operated flow control valves and self-operated differential pressure control valves. Self-operated flow control valves have also been referred to as self-operated flow controllers and self-operated balancing valves. Self-operated differential pressure control valves are also known as Automotic Balamce Valves in Northern Europe.

Second, hydraulic conditions and hydraulic conditions balance

Generally speaking, the pipe network of heating and air conditioning is a closed-circuit circulating pipe network. The hydraulic working conditions refer to the pressure at each point of the system, the flow rate and pressure difference of each pipe section. By the formula â–³P=SG2

△P——pressure difference or resistance loss

S—— resistance coefficient of pipe section or system

G——Flow of pipe segment or system

It can be seen that the flow rate and pressure are related parameters, and the regulation of flow and pressure is mutual means and purpose. The decompression means is to reduce the flow of the upstream pipeline; reducing the flow rate must also reduce the pressure at the point before the pipeline or increase the pressure at the point after the pipeline. The change of flow will inevitably lead to the change of pressure; for the system with the same S value, the change of pressure difference will inevitably result from the change of flow. Therefore, there is no flow control valve that does not affect the pressure, and there is no pressure control valve that does not affect the flow.

The balance of hydraulic conditions refers to the rational distribution of fluids. In the heating and air conditioning pipe network, water is the heat carrier medium, and the reasonable distribution of water flow is the basis for the balance of thermal conditions. Taking the heating system as an example, the designer performs the calculation of the hydraulic working condition under the assumption that the branch flow is the design value. Due to the limitations of the pipe and the highest flow rate, it is almost impossible to achieve a hydraulic balance in design. This will inevitably lead to the near-end resistance coefficient not reaching the ideal design state, forming an imbalance phenomenon in which the near-end flow is too large and the far-end flow is insufficient.

Since the hydraulic working condition is designed as a design water pressure map, in actual operation, this water pressure map must be formed by the balance of the valve. The process of adjusting the hydraulic condition with a valve is a process of establishing a reasonable water pressure map. When the design is reasonable, the two water pressure maps will be well combined.

Since the running hydraulic condition is formed by the intersection of the working curve of the pump and the characteristic curve of the external network.

For the external network characteristic curve â–³P=SG2, since the S value of the parallel proximal branch will be smaller than the design value, the total S value is much smaller than the design value, and the circulating water pump operates under the small lift and large flow conditions, so that the water pump is on the large axis. Power, low efficiency point operation. In severe cases, the shaft power may be greater than the motor nameplate power, and the motor is over rated until the motor accident occurs.

The process of adjusting the net is to increase the proximal resistance with the balance valve, so that the S value of the proximal branch increases to the design value, and the total S value increases to the design value. The distribution of the near and far flow is even and reasonable, and the circulating water pump operates under the design conditions to achieve the purpose of saving heat, saving electricity and improving the heating quality.

The workers who run the post often misunderstand some imbalances in hydraulic conditions:

(1) The pump output is insufficient, and the actual lift of the pump is less than the nameplate lift, resulting in the end of the resignation.

In fact, due to the small resistance of the proximal branch line and large flow, the flow rate at the distal end is small, and the working point of the pump is offset at the working point of large flow, small lift and low efficiency.

(2) The resistance of the boiler or heat exchanger is large, and the resistance of all boilers or heat exchangers is far less than the actual resistance.

In fact, the increase in total circulating water will inevitably lead to increased resistance such as boiler heat exchangers. Water flow increases by 40% and resistance increases by 100%.

(3) The output of the boiler is insufficient. In fact, the temperature difference between the supply and return water cannot be greater after the flow rate is increased. Of course, the abnormal coal quality and wind system may also cause boiler output problems.

Third, the network pressure map analysis and the installation position of the balance valve

The process of adjusting the net is a process that uses a balancing valve to achieve reasonable flow rates for each branch. The near-end capital indenter is larger than the user's need to use the indenter, which inevitably leads to excessive flow. Must use the valve to consume rich heads rich head = capital indenter - need indenter), if the user water supply pipe is installed with balance valve to adjust the network, then P3 is approximately equal to P4, P2 pressure line is shown in Figure 3, nearly parallel P4 . If the user installs the balance valve to adjust the network, P2 is approximately equal to P1, and the P3 pressure line is nearly parallel to P1.

The actual water supply pressure in the room is P2, and the return water pressure is P3. If the pressure is too low, the operation will be emptied. If the pressure is too high, the pressure of the component with low pressure rating (such as the radiator) will be destroyed.

Therefore, for the pipe network with large terrain difference, the installation position of the balance valve should be considered according to the above factors. That is to say, in the low-lying terrain, the balance valve of the building should be installed in the water supply to ensure that the indoor pressure is not maintained; in the higher position of the terrain, the balancing valve should be installed in the return water to ensure that the user does not empty.

For large direct-connected pipe networks, such as power plant condensing heating pipe network, the heating radius is very large, and the pressure difference between the external network and the returning water is very large. Therefore, special consideration should be given to the installation position of the balancing valve.

The external network of the condensate supply network of a power plant in Yantai is supplied with a water pressure difference of 52 m water column. Considering the pressure resistance of the radiator, the end return pressure is set to 0.35 MPa (35 m water column), and the front return pressure is only 0.1 MPa ( 10 meters water column), and the front water supply pressure is as high as 0.62MPa (62 meters water column). If the balance valve is installed on the return pipe, the controlled user's return water pressure P3 may be close to 0.6MPa, which will cause pressure damage of the radiator; The balance valve is installed on the water supply pipe, and the water pressure P2 of the near-end user is only a dozen meters of water column, which inevitably causes the operation to be emptied. Therefore, from the design, the balance valve should be installed for the return water to form the water pressure map of Figure 4.

The specific method is to install a self-operated flow control valve into the water supply pipe of the household entrance, and install a manual balancing valve on the branch return pipe of the building group whose topographic height difference does not exceed 10 meters. Here, the self-operated flow control valve is responsible for controlling the distribution flow; the manual balancing valve adjusts the pressure so that the pre-valve pressure reaches a full water operating condition of 0.25 MPa. The self-operated flow control valve only controls the pressure according to the flow rate “eyes”. If the return pipe is installed, the pressure is not to be manually adjusted, and a pressure damage accident has occurred. When the self-operated valve is installed and the water supply is not manually adjusted, the operation may be emptied and the heating effect may be affected, and an accident may not occur.

Fourth, the concept of user active variable flow and heat source active variable flow

For the heating system, under the traditional heating system, it is an evenly distributed heating mode. This heating mode generally adopts a constant flow quality regulation heating mode. There are also a few large-scale pipe networks that adopt the quality and adjustment method for the purpose of saving operating power. However, under the premise of average generation heat, the change of fluidity is only determined by the outdoor temperature change. Therefore, the control method only considers the single temperature of the outdoor temperature to control the rotation speed of the heat source circulation pump to realize variable flow operation. This variable flow rate can be defined as the heat source active variable flow mode.

In the operation mode of heat metering, the change of heating load and circulating water flow depends on the user's demand. The change of the total circulating flow of the system is determined by the user's change. This variable flow mechanism can be defined as the user's active variable flow mode.

Some industry insiders have proposed that the indoor system of metering and charging adopts a horizontal spanning tubular system and attempts to operate along the fixed flow mode. Here, it is estimated that whether the horizontal span can realize the flow operation, the fixed flow operation mode wastes the running electrical energy. To be abolished.

The metering and charging flow control scheme is the best feasible solution with the following scheme: taking 3-5 end supply and return water pressure difference signals as the control signal of the thermal circulation flow, and circulating the water pump when all the differential pressure signals are greater than the set value Reduce the speed, when any pressure difference is less than the set value, the circulating water pump increases the speed.

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