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Application of Differential Pressure Control Valve in Double - pipe Heating of Households
Abstract: In this paper, three application schemes of differential pressure control valve in double-pipe household heating system are analyzed and the principle of selection of each scheme is given. It is pointed out that double-pipe heating system of household metering is under design conditions Hydraulic calculation, the natural role of pressure head can not be considered, indoor and outdoor systems should be used in different programs. Keywords: pressure control valve household heat metering double pipe heating system I. Overview In household metering double pipe heating system, in order to make full use of household appliances, lights and the human body and other free calories, usually in each group of radiators Preset thermostatic valve installed, so the entire system is variable flow operation, the role of pressure in the temperature control valve with the flow rate changes. When the actual pressure difference larger thermostatic valve may produce noise, especially in the room when the heat load is small, the thermostatic valve will switch frequently, resulting in oscillation. In addition to causing unnecessary wear and tear, but also lead to the backwater temperature, and affect the system of other temperature control valve, so in a well-designed household metering double pipe heating system, on the one hand should be used in the system Thermostatic valve thermal power is always greater than or equal to 1, on the other hand the temperature control valve with the actual pressure should also be maintained within its allowable range [1]. Differential pressure control valve is also called self-operated differential pressure control valve. In variable flow system, it can keep the pressure difference of the controlled loop constant by inducing the pressure of two points in the heating pipe system to ensure the controlled loop In the regulation of the normal operation of the valve, then double household heating metering system design, the control valve should be how to arrange it? Usually have the following three programs: a. Differential pressure control valve is located only in the building heating inlet, control of the heating inlet pressure drop for the set value. b. For the next two-pipe system, the differential pressure control valve is located at the beginning of each set of common risers to control the differential pressure of the riser to a fixed value. c. Differential pressure control valve is located at the inlet of each household to control the indoor system pressure is constant. At present, in the actual design, the three options should be how to choose, a lot of controversy, only to ensure the smooth work of temperature control valve, the worst program 1, but the initial investment with the least; the best option 3, but its initial investment Highest; program 2 is between programs 1 and 3. Here are some analysis for these three programs, hoping to provide some useful suggestions for the design of the project staff, the choice of programs. In addition, it should be noted that the dual-pipe heating system discussed in this paper refers to a dual-pipe system in both indoor and outdoor environments. Second, the program analysis 1. Option 1: Differential pressure control valve is located only in the building's heating inlet As a two-pipe system, so to households as a unit, the heating system is a parallel relationship between households. ΔPS = ΔP1 + ΔP2-ΔP3 (1) Where: ΔP1 - building heating inlet differential pressure control valve control pressure difference; ΔP2 - calculated ΔP3 - Resistance loss from the pressure drop control point at the heating inlet differential pressure control valve to the calculated user inlet port for the return water line. (1) discussion of the parameters in the formula a. Building heating inlet differential pressure control valve control pressure difference ΔP1 during the operation of the system, ΔP1 is a fixed value, it depends on the design conditions, the most adverse heating system loop , From the heating inlet pressure drop control point to the end of the user port for import and export of resistance between the loss of resistance â–³ P "3, the end of the total loss of the user indoor system resistance â–³ P" s and the end of the user with Of the natural role of pressure head â–³ P "2. According to equation (1) are: â–³ P1 = â–³ P" 3 + â–³ P "s- â–³ P" 2 (2) b. The user with the natural role of pressure head ΔP2 ΔP2 The user is located in the floor and for the water supply and return pipe temperature [2]. During the operation of the system, ΔP2 is a constant variable. Therefore, under the design conditions, ΔP2 of the natural acting pressure head should be the minimum value when calculating the pressure difference ΔPS acting on the inlet of the household according to formula (1). Because if the value is larger, then according to formula (1) calculated by the role of household import pressure ΔPS larger, when the design of indoor systems based on ΔPS, the resistance loss of the pipeline and the temperature control valve may be larger, When the actual natural pressure head ΔP2 is less than the selected value, the role of household inlet pressure ΔPS will be lower than the design value, resulting in the actual pressure on the temperature control valve is less than the design value, at this time, even if the temperature control valve On, the heat provided by the radiator is still not enough to maintain the design room temperature, so in the design conditions, the natural role of pressure head ΔP2 should take the minimum. In this way, in actual operation, the natural role of pressure head ΔP2 is always greater than or equal to the minimum value, so to ensure that the thermostat valve thermal power is always greater than or equal to 1, the room temperature can always reach the design value. However, since the influence of natural pressure head ΔP2 is more and more important, it is usually difficult to determine the minimum value for each user. For ease of design, the natural pressure acting on the inlet pressure ΔPS is calculated under design conditions The head ΔP2 may not be considered. c. Resistance loss from the differential pressure control point of the heating inlet differential pressure control valve to the calculated user supply inlet and return pipe ΔP3 In the variable flow system, the resistance loss of the return water pipe ΔP3 is a variable, It depends on the flow in the pipeline and on the length of the pipeline. In the design conditions, the maximum value, when the flow in the pipeline approaches zero, ΔP3 also approaches zero [1]. The same heating system when using the same program, its ΔP3 generally larger than the use of iso-programs [2], so according to equation (1) shows; ΔP3 caused by each user ΔPS fluctuations, the same program with a larger ratio system, This shows that the design should choose different systems. d. Effect of household inlet pressure differential ΔPS For the two-pipe system, when the heat load of the radiator is constant, when the differential pressure ΔPS of the household inlet is greater than the designed value, due to the regulation function of the thermostatic valve on the radiator, The flow of each pipe section of the system will remain unchanged [1], so the loss of resistance of each pipe section will not change. The added value of ΔPS acting as a household inlet will act on every thermostatic valve of indoor system equally. Thus, in the system design, as long as the guarantee operation, the role of household import pressure differential ΔPS is always greater than or equal to the total resistance loss of the indoor system under design conditions, we can ensure that in any case, the temperature control valve The actual pressure difference is always greater than or equal to the design value under the design conditions, so the thermostat valve thermal power is always greater than or equal to 1, the user can get the design at any time room temperature. So how should we design to make the role of household import pressure ΔPS is always greater than or equal to the total resistance loss of indoor system under design conditions? According to the previous analysis we can see: the design conditions, design, the role of natural pressure can not be considered, the pipeline resistance loss ΔP3 maximum. In the actual operation process, due to the existence of the natural role of pressure head, pipeline resistance loss ΔP3 is small, so according to equation (1) shows: During operation, household import pressure difference is always greater than or equal to the design conditions Therefore, under the design conditions, as long as the pressure difference between the inlet and outlet of the household is greater than or equal to the total resistance loss of the indoor system, the pressure difference of the inlet of the household inlet will always be greater than or equal to the design Total resistance loss of indoor system under operating conditions. This can be easily done in hydraulic calculations under design conditions. In addition, the fluctuation of ΔPS due to the effect of household inlet reflects the fluctuation of the differential pressure acting on each thermostatic valve in the indoor system. Therefore, as long as the maximum value of the pressure difference ΔPS between the inlet of the consumer and the consumer is ensured, Thermostatic valve does not exceed its maximum working pressure. According to the literature [3 ~ 4], we can see that under the design conditions, the resistance of the indoor system including the heat meter and the lock-up regulating valve should not generally exceed 30 kPa. Therefore, in the running process, as long as the maximum value of ΔPS is controlled not to exceed 30 kPa To ensure the normal operation of temperature control valve. (2) summary of the analysis of the program 1 Through the previous analysis shows: In order to ensure that the operation process, the actual role of the temperature control valve pressure does not exceed the maximum pressure of normal work, the user maximum pressure drop port does not exceed 30kPa, Therefore, according to equation (1), we have: ΔPS = ΔP1 + ΔP2 -ΔP3 kPa From the above formula we can see: When ΔP3 = 0, the role of household inlet pressure drop ΔPS maximum, so according to the above formula: ΔP1 ≤ 30 -ΔP2 kPa In the formula, ΔP2, which is the natural acting pressure head, is the maximum value of each user under design conditions [2], and its maximum value can be calculated as follows: ΔP2 = gH (Ïh-Ïg) kPa where H - for the next two-pipe system, the height of the building; under the double-pipe system for the next, the height of the building minus the height of the building's top layer, m. Ïh, Ïg-- design conditions, the temperature of the water supply and return corresponding to the density of water, kg / m3. Therefore, ΔP1≤30-gH (Ïh-Ïg) / 1000 kPa Therefore, when the differential pressure control valve is installed only at the heating inlet, the control pressure difference must be less than or equal to 30-gH (Ïh-Ïg) / 1000 kPa To ensure that the system is running, the role of pressure on the temperature control valve can be less than the normal working pressure of the maximum. In addition, since the calculation of hydraulic conditions under design conditions, regardless of the role of natural pressure head, so according to equation (2) are: â–³ P1 = â–³ P "3 + â–³ P" s Thus it is only when the design conditions Adverse loop resistance loss (â–³ P "3 + â–³ P" s) is less than 30-gH (Ïh-Ïg) / 1000kPa, can only be used when the program. 2. Scenario 2: Set pressure differential control valve on each set of common riser This scheme is only suitable for the next back two-pipe system. Referring to the previous analysis of the formula (1) parameters, the program under the design conditions for hydraulic calculation, the natural role of pressure head can not be considered, so the pressure control valve control pressure difference ΔP1 is equal to the common riser The most adverse loop in the design conditions of the loss of resistance (â–³ P "3 + â–³ P" s), where â–³ P "3 for the standpipe pressure control point to the household entrance between the supply and return pipe resistance Loss, in addition, to ensure that all users on the shared riser in the course of the operation of the household import pressure difference ΔPS does not exceed 30kPa, ΔP1 should also be less than or equal to 30-gHÏh-Ïg) / 1000kPa, when ΔP1 is greater than the value Should not be used in Option 2. 3. Option 3: In each household to introduce pressure control valve For large heating systems, when the program can not be used when 1 and 2, the program should be used.The pressure control valve control pressure The difference ΔP1 is equal to the total resistance loss of the most unfavorable loop of the indoor system under design conditions, including the resistance of the household heat meter and the lock-up control valve, ΔP1 should be less than or equal to 30kPa [3 ~ 4] Stand on the pipe just set stop valve or gate valve, from the closing effect Program, due to the regulation of the differential pressure control valve in the system operation, the natural role of pressure head and system flow changes will not affect the work of indoor temperature control valve.However, in order to run in the process To ensure the normal operation of the differential pressure control valve, the differential pressure should always be greater than or equal to the design pressure differential.The differential pressure control valve design pressure equal to the design conditions of its own resistance and its control pressure and the difference between Natural working pressure head can be taken as safety margin when hydraulic calculation of outdoor shared riser pipe and water supply and main pipe under design conditions, because it will make hydraulic calculation more complicated if we consider natural acting pressure head on the one hand , On the other hand the natural role of indentation inappropriate value, will lead to run, the differential pressure control valve is less than the pressure differential design, it may lead to differential pressure control valve even if fully open, the flow through Can not meet the user requirements.In addition should be noted in the design: the heating system used in the pressure control valve generally have the maximum working pressure limit, when the role in the valve on the actual When the pressure difference exceeds its maximum working pressure difference, the valve will be crushed, so when using the schemes 2 and 3, if the outdoor pipeline pressure difference in the heating inlet exceeds the heating system Differential pressure control valve maximum working pressure difference, it is necessary to introduce other types of heating inlet pressure control valve to control the entire heating system pressure difference at this time, the differential pressure control valve control pressure equal to the heating system Total resistance loss of the most adverse loop under design conditions 4. Indoor and outdoor system forms For indoor systems, based on the same reasoning described above for the resistance loss ΔP3 of the return piping, in order to reduce the operating temperature The range of fluctuation of the pressure difference of the control valve should be chosen as the iso-system.For the outdoor systems of the schemes 2 and 3, it is also recommended to use the iso-system.As the same heating system, the total resistance loss of the system is generally Than using the same program smaller [2]. In this way it is possible to reduce the capital expenditure required for the inlet of the heating system. Third, the conclusions (1) household metering double pipe heating system under design conditions for hydraulic calculation, the natural role of the pressure head can not be considered, indoor and outdoor systems should be used in different procedures. (2) When Option 1 is used, the control pressure difference ΔP1 of the differential pressure control valve shall be equal to the total resistance loss (ΔP "3 + ΔP" s) of the most unfavorable loop of the heating system under design conditions, and ΔP1 should Less than or equal to 30-gHÏh-Ïg) / 1000 kPa. (3) When Option 2 is used, the control pressure difference ΔP1 of the differential pressure control valve shall be equal to the total resistance loss (ΔP "3 + ΔP" s) of the most adverse loop on the riser at design conditions, and ΔP1 It should also be less than or equal to 30-gHÏh-Ïg) / 1000 kPa. (4) Scenario 3 is suitable for large-scale heating systems. The control pressure difference ΔP1 of the differential pressure control valve should be equal to the total resistance loss of the most unfavorable loop of the indoor system under the design conditions, and includes the household heat meter and the lock adjustment Valve resistance, ΔP1 should be less than or equal to 30kPa. REFERENCES 1 Gottfried Lerenat, Aubel, Ed., Heating Control Technology, Beijing: China Building Materials Industry Press, 1998 2 He Ping, Sun Gang, Edited, Heat Engineering (New Version), Beijing : China Building Materials Industry Press, 1993 3 DBJ01-605-2000 Technical Specifications for Design of New Central Heating Residential Household Heat Metering (Beijing Standard) 4 DB29-26-2001 Design Regulation for District Heating and Dwelling Heating (Tianjin Standard)