The Kv value of the flow capacity of the regulating valve is an important parameter of the regulating valve. It reflects the ability of the fluid to pass through the regulating valve, that is,Control valveCapacity.According to the calculation of the Kv value of the flow capacity of the regulating valve, the caliber of the regulating valve can be determined.In order to correctly select the size of the regulating valve, the rated flow coefficient Kv value of the regulating valve must be accurately calculated. Control valveThe definition of the rated flow coefficient is: under specified conditions, that is, the pressure difference between the two ends of the valve is 105Pa, the density of the fluid is 1g/cm3, When the rated stroke flows through the regulating valve to m3/h or t/h flow rate.
1. Calculation of Kv value of general liquid
a. Non-blocking flow
In the formula: FL—pressure recovery coefficient, refer to Table 1. FF—Liquid critical pressure ratio coefficient, F=0.96-0.28
Pv—the saturated vapor pressure (absolute pressure) of the liquid at the inlet temperature of the valve, refer to Table 4 to Table 10. Pc—the critical pressure of material thermodynamics, see Table 2 and Table 3. QL—Liquid flow m3/h. ρ—Liquid density g/cm3 P1—pressure in front of the valve (absolute pressure) KPa. P2—pressure after the valve (absolute pressure) KPa. b. Blocked flow
In the formula: the meaning and unit of each letter are the same as before.
2. Low Reynolds number correction(Calculation of Kv value of high viscosity liquid)
When the viscosity of the liquid is too high, the Reynolds number decreases, which changes the flow state of the fluid. When Re<2300, the fluid is in a low-speed laminar flow, so the Kv value calculated according to the original formula has a large error and must be corrected.At this time, the calculation formula is: In the formula: φ—viscosity correction coefficient, obtained by checking the diagram of Re.For valves with only one flow path such as single seat valve, sleeve valve, angle valve, etc.: Re=70000
For double-seat valves, butterfly valves and other valves with two parallel flow paths: Re=49600
In the formula: K'v—the flow capacity calculated without considering the viscosity correction. γ—Kinematic viscosity of fluid mm2/s.
3. Calculation of the Kv value of the gas:
a. General gas when P2>0.5P1 When P2≤0.5P1
Where: Qg—gas flow rate m under standard conditions3/h, Pm—
(P1, P2 are absolute pressures) KPa, △P=P1-P2 G—gas specific gravity (air G=1), t—gas temperature °C b. High pressure gas (PN>10MPa) When P2>0.5P1,
When P2≤0.5P1,
In the formula: Z—gas compression coefficient, can be found in GB2624-81 "Design, Installation and Use of Flow Measurement Throttle Device".
4. Calculation of the Kv value of steam
a. Saturated steam when P2>0.5P1, When P2≤0.5P1
In the formula: Gs—steam flow rate Kg/h P1 and P2 meanings and units are the same as the previous K—steam correction coefficient The K value of some steam is as follows: water vapor K=19.4 methane, ethylene vapor K=37 ammonia vapor K=25 propane, propylene Steam K=41.5 Freon 11K=68.5 Butane, isobutane steam K=43.5 b. Superheated steam when P2>0.5P1
When P2≤0.5P1
In the formula: △t—water vapor superheat degree ℃ Gs, P1, P2 meaning and unit are the same as before.
5. Calculation of Kv value of two-phase flow:
When the medium is a gas-liquid two-phase flow, the Kv value of liquid and gas is generally calculated separately, and then added to obtain the total Kv value of the regulating valve. This method is based on the viewpoint that the two fluids are independent of each other and do not affect each other, but in fact As the liquid and gas phase components change, the fluid state tends to be different, so the error in calculating the Kv value is relatively large. In actual use, wet steam and water containing water vapor are commonly used.The following introduces a method for calculating the Kv value of a mixed liquid of steam and condensate. The calculation steps are as follows:
(1) According to the content of steam and condensate in the mixed medium, find the vaporized liquid.
(2) Check the correction coefficient Ks from the graph according to the vaporization rate and the inlet pressure P1.
(3) Divide the known flow rate by Ks to obtain the corresponding steam flow rate as the calculated flow rate.
(4) Use the formula for calculating the Kv value of steam to find the Kv value.
6. Caliber selection steps:
According to the process, determine the caliber of the regulating valve, the specific steps are:
(1) First calculate the maximum flow Q according to production capacity and equipment loadMaxAnd minimum flow Qmin
(2) Select the S value according to the selected flow characteristics and system characteristics, and then determine the minimum pressure difference △P according to the pressure distribution and pipeline lossminAnd maximum pressure difference △PMax
(3) According to the calculation formula of circulation capacity, find the Kvs at the maximum flow rate.
(4) According to the standard of Kvs in the selected product type, select a Kv value greater than Kvs and close to it.
(5) According to the selected Kv value and flow characteristics, verify the opening degree of the regulating valve, and the opening degree is required to be between 10% and 90%.
(6) Calculate R and check the adjustable ratio.
(7) After the name item is qualified, it will be determined according to the Kv valueControl valveCaliber.