0 Preface
Flow meter will detect the measured fluid parameters at the same time, it will cause the measured fluid energy loss. In the field of flow detection of gases such as steam and natural gas, orifice flowmeters are widely used. The flow meter also becomes an important factor in energy loss while achieving effective metering. Scientifically selecting flow meters to effectively solve the contradiction between accurate metering and energy consumption of meters is an important task in energy measurement. After theoretical analysis and practical summary, we propose to use the elbow flowmeter instead of the common orifice plate differential pressure flow measurement device in the gas flow measurement to achieve both accurate measurement of gas flow and orifice plate throttling device. The pressure loss, to achieve energy-saving effect.
1 Measuring principle and energy saving principle of elbow flowmeter
1.1 Measuring principle of elbow flowmeter
When the fluid flows through the bend pipe, due to the diversion effect of the curved pipe wall, the flow velocity inside the fluid will gradually increase when flowing into the bend pipe, while the outside flow rate gradually decreases, which forms the respective over- flow section. Approximate the trapezoidal velocity distribution, and this trapezoidal velocity distribution reaches the limit state at the bend section of 45°. The flow velocity distribution of each mass point in the 45° section of the elbow is shown in Figure 1.
Due to the complexity of the fluid flowing through the elbow flowmeter, it is not possible to deduce a simple mathematical expression from the usual theoretical methods. Instead, we can only use dimensional analysis to establish a model that covers all possible influencing factors. On complex mathematical expressions. According to dimensional analysis principle: The relationship between the average flow velocity v of the fluid flowing through the elbow flowmeter and the pressure difference Δp between the inside and outside of the elbow can be expressed by the Euler number Eu:
Where: Re is the Reynolds number; Fr is the Froude number; Ma is the Mach number; R/D is the ratio of the bending diameter; L1, L2 are the length of the straight tube before and after; (λ1, λ2) represents the position of the pressure hole taken outside; Λ3, λ4) indicates the position of the pressure hole inside; Δ is the roughness of the pipe wall; β1 and β2 are the angles between the straight pipe section and the bend pipe.
According to Euler's definition of Eu, the above formula can be further rewritten as:
Formula (3) establishes the working principle of the fluid flow through the elbow flowmeter. According to the definition of Euler's number Eu, the working principle of the elbow flowmeter can be expressed as: the fluid kinetic energy (Ïν2) flowing through the elbow flowmeter and The pressure difference (Δp) inside and outside the elbow has a proportional relationship. The proportional coefficient (flow coefficient) α is Reynolds number, Froude number, Mach number, bending ratio, length of front and rear straight pipe sections, position of pressure hole (inside and outside), connection angle between straight pipe section and elbow pipe, elbow pipe The roughness of the inner surface, etc. as a function of the influencing factors.
Equation (4) gives the functional expression of the theoretical flow coefficient α. The determination of this coefficient can be determined by solving the Navier-Stokes differential equation that contains the relevant influencing factors.
1.2 energy saving principle of elbow flowmeter
For thermal power generation enterprises, whether the elbow flowmeter replaces the orifice plate throttling device has the energy-saving effect and can be calculated indirectly through the turbine power of the generator set.
The throttling of the orifice plate will reduce the steam pressure, which means that the functional force will be reduced, resulting in irreparable energy loss. The pressure loss generated by orifice plates is usually calculated according to equation (5):
In the formula: β is the diameter ratio of the throttle; Δp is the differential pressure (kPa) of the orifice plate.
Turbine Power Pi Calculation of Generator Sets:
In the formula: D is the steam turbine steam flow; h0 is the inlet steam steam enthalpy; hh is the heating steam enthalpy; ηi is the steam turbine relative internal efficiency (about 82%); ηm is the turbine mechanical efficiency (about 98%) Ηg is generator efficiency (about 98%);
The initial parameters (inlet steam parameters) p1, T1, h1 and extraction parameters p2, T2, h2 directly affect the power of the turbine.
For monitoring and metering needs, power plants usually install orifice plates at the outlets of boilers and steam turbine inlets to cause the initial pressure p1 to decrease; in addition, steam supply manifolds outside the steam turbines are installed with throttling orifice plates, resulting in steam outlet pressure for the total heat output. The increase in p2, the change of these two parameters will cause the turbine to reduce the effective power generation.
The elbow flowmeter is installed in the turning of the pipeline to replace the existing elbow without adding new resistance. Therefore, the steam quality will not be degraded during use. If an orifice tube flow meter is used instead of the orifice plate throttling device, the initial pressure p1 of the steam turbine and the total outlet pressure p2 of the heating will be increased under the condition that the boiler outlet pressure is constant, thereby increasing the power generation efficiency of the steam turbine and reducing the flow of the throttling element. Loss of energy, to achieve energy-saving purposes.
2 Real-time calibration of the flow coefficient of the elbow flowmeter
To evaluate the actual effect of the bent tube flowmeter instead of the orifice plate, it is first necessary to ensure that the accuracy of the measurement after the bend tube flowmeter instead of the orifice plate does not decrease. We use a standard instrument for normal pressure critical flow sonic nozzles with a measurement uncertainty of 0.25% (k=2), using air as the test medium, and two DN200 elbow flowmeter measurement devices with serial numbers JZ123 and JZ124 ( The flow meter coefficient is calibrated in real-time by a measurement system consisting of an elbow flow sensor, a flow totalizer, and a temperature and differential pressure transmitter. The verification results of JZ123 are shown in Table 1 (usual flow measurement range of the elbow flowmeter is 1800 to 4500 m3/h).
The actual flow rate of the elbow flowmeter of the other elbow flow metering device numbered JZ124 is 0.59. The results of the indication error and repetitive items are very close to the data in Table 1.
From the measured data in Table 1, we can conclude that the precision measurement equipment of the elbow flow metering device is stable and reliable, and the flow coefficient of the elbow flow meter is determined by the real flow calibration. The elbow flow meter can operate at a wide flow rate. Accuracy of 1.5% in the measurement range, can completely replace the traditional orifice plate differential pressure flowmeter in the measurement characteristics.
3 Example analysis of energy efficiency
The following examples of technological changes in the steam measurement system of the Nanjing Xinsu Thermal Power Plant will be used to analyze the energy-saving effect of an orifice flowmeter instead of an orifice flowmeter. The plant has a total of 2 furnaces and 1 machine, usually 1 furnace and 1 machine. The outlet of the boiler and the steam inlet of the steam turbine are all installed with orifice flowmeters. When No. 1 furnace is in operation, the boiler outlet steam flows through orifice plates, valves, etc., to the inlet of the turbine, the pressure is reduced by 0.2 MPa; when furnace No. 2 is in operation, the pressure reduction reaches 0.3 MPa, causing the inlet pressure of the turbine to be always lower than The design value is about 0.1MPa. They urgently need to solve the problem of excessive pressure loss in operation of No. 2 furnace. Due to the expansion of the diameter, the investment is large. Decided to phase out the main components of pressure loss - 2 orifice flowmeters. The specific relevant parameters before the technical transformation of No. 2 furnace are shown in Tables 2 and 3.
3.1 Resistance loss
The above parameters are the actual data in the flowmeter calculation book. According to equation (5), the pressure loss generated by the two orifice plates of No.2 furnace and steam turbine at the common flow rate of 75t/h is 55.9kPa and 54.4kPa, respectively, and the total pressure loss is as high as 110.3kPa.
3.2 Energy Saving Calculation
The orifice plate of the steam flowing through the outlet of the boiler and the inlet of the steam turbine is an adiabatic throttling process and the steam enthalpy does not change. Turbine power changes can be calculated using a Morrillian map of entropy, as shown in Figure 2 (h is 焓, s is entropy). Knowing the pre-throttle states p1, t1 and the throttling pressure p′1, the throttling state parameters can be determined on the hs map according to the characteristics that the throttling values ​​before and after the throttling are equal. As shown in FIG. 2, the parameters of point 1 are p1, t1, and h1. In FIG. 2, a point 1 is crossed by the horizontal line of the definite line and p′ 1 to obtain the parameter after throttling. The work of the steam turbine is a reversible adiabatic expansion process (ie, isentropic process). When the steam is reversibly adiabatically expanded from point 1 to the extraction pressure p2 before throttling, the available enthalpy drop is h1-h2, and after throttling, When the water vapor is also reversibly adiabatically expanded to pressure p2, the available enthalpy drop is h'1-h'2, apparently h1-h2>h'1-h'2, and steam work is reduced after throttling.
The steam pressure caused by orifice elements, such as orifice plates, is reduced. The resulting energy loss can be calculated as follows:
1) The turbine inlet pressure will increase 0.1103 MPa without throttling, the initial parameters p0 = (3.38 + 0.1103) = 3.4903MPa, T0 = 437 °C, according to engineering thermodynamics, can be calculated h0 = 3305.185kJ/kg, s0 = 6.95348 ;
2) After adding the throttle, the initial parameter p'0 = 3.38 MPa, and h'0 = h0 from Figure 2, find s0 = 6.96756;
3) before the throttle heating extraction pressure ph = 0.7MPa, sh = s0, then hh = 2893.8458kJ / kg;
4) After the throttling extraction parameters p'h = ph = 0.7 MPa, s'h = s' 0, from Figure 1 to find h'h = 2900.8642 kJ / kg.
The turbine power drop value Px calculated from equation (6) is:
The energy loss caused by the throttling of the first two orifice plates of a steam turbine causes the steam turbine to generate 115.15 kW less per hour. The use of elbow flowmeters instead of orifice plates has a significant energy saving effect. The three-month energy-saving money can be used by the elbow flowmeter to recover all the investment in the technical transformation of the entire flowmeter equipment. If you consider the benefits of energy conservation, the abrasion resistance of the elbow pipe (stability of the metering characteristics), and the advantages of no leakage, the advantage is even more obvious.
3.3 Comparison of data before and after reconstruction
Before and after the implementation of the technical reform, the relevant data of the 2# furnace are shown in Tables 4 and 5 respectively.
From the actual comparison data in Tables 4 and 5, it can be seen that when the 70t/h flow rate of the main steam pipe of the 2# furnace before the technical transformation passes through two sets of orifice flowmeters, the total pressure drop is 0.3MPa on average, including the pressure loss of the orifice plate and the pipeline. Loss of resistance along the way, loss of local resistance. After the transformation, the total pressure drop was 0.2 MPa, and the pressure loss was reduced by 0.1 MPa. By comparing the main steam flow of No. 2 furnace with the inflow flow rate and turbine inlet flow rate, it is further verified that the accuracy of the elbow flowmeter in the range of 1 : 5 can reach 1.5%, and the metering performance is better than the original orifice flow rate. The meter fully meets the accuracy requirements for steam flow measurement.
4 Conclusion
From the theoretical, experimental and field practical applications, the performance of the elbow flowmeter is comprehensively described. The real-flow calibration experiment can be seen that the elbow flowmeter has high accuracy and stable performance, and fully meets the industrial application conditions; the comparison data before and after field application fully demonstrates the energy-saving effect of no pressure loss, under the current situation of increasingly tense energy. It is of great significance and is worthy of promotion and application in thermoelectric and other industries.
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