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What kind of compensation should be used for saturated steam?

(1) Check the table method to obtain the consistency of density. The saturated steam adopts temperature compensation and pressure compensation, which are essentially the same. The reason is that the saturated steam has a single-valued function between the pressure and the temperature, and the density detected from the steam temperature is consistent with the density detected by the pressure corresponding to the temperature. Therefore, the use of temperature compensation and pressure compensation is feasible in principle.
(2) The difference in investment is considered from the perspective of saving investment and reducing installation workload. Because the price of a platinum thermal resistor is only one-tenth to a fraction of that of a pressure transmitter, temperature compensation is used. economic.
(3) Difference of compensation accuracy How much compensation accuracy can be obtained by temperature compensation and pressure compensation, which is not only related to the accuracy of temperature sensor and pressure transmitter, but also the type of electromagnetic flowmeter and the working condition of specific measurement object. The range selection of the pressure transmitter is related. In general, the temperature measurement has a great influence on the accuracy of compensation. The specific analysis is as follows.
1 The influence of temperature measurement error on flow measurement results. The relationship between temperature measurement error and flow measurement results has little effect on superheated steam, such as superheated steam with a temperature of 250 °C. If the temperature measurement error is 1 °C, the uncertainty of the flow measurement result is caused by temperature compensation. 0. 096% R (differential pressure flow) to 0.19% R (vortex flowmeter). The most influential is the temperature signal used for the compensation of the saturated steam flow measurement, for example, the saturated steam with a pressure of 0.7 MPa, the equilibrium temperature is 170.5 ° C, the corresponding density is 4.132 kglm3, if the temperature measurement error is 1 ° C, and according to The saturated vapor density meter was found to have a density of 4.038 kg/m3, causing a flow measurement error of about 1.14% R (differential pressure flowmeter) to the vortex flowmeter.
2 temperature sensor accuracy level considerations. The temperature measurement error is related to the accuracy level of the temperature sensor and the measured temperature value. For example, the saturated Yanqi with a pressure of 0. 7MPa is temperature-measured with a grade A platinum thermal resistance, and the error limit is ±0.49 °C [4], if used The result of the measurement is to check the steam density table for compensation, and the flow compensation uncertainty is about 0. 56% R (differential pressure flowmeter) to soil 1.11% R (vortex flowmeter). However, if the temperature limit is increased by the B-class platinum thermal resistance, the error limit is increased to 1.15 ° C, then the flow compensation uncertainty is increased to 1.31% R (differential pressure flowmeter) to 2.61% R (vortex). Street flow meter). Obviously, the error that can be caused by Class B platinum RTDs for such applications is considerable, so it is generally not suitable. Only comparative comparisons of temperature measuring elements of different accuracy levels are made here. Of course, the error mentioned here is only the link of the temperature measuring component. As for the uncertainty of the flow measuring system, it must be included in the influence of the flow secondary meter, the flow sensor, the flow transmitter, and the like.
3 pressure transmitter accuracy level, pressure measurement error and its impact. The pressure measurement error is related to the accuracy level and range of the pressure transmitter. For example, a pressure transmitter with a 0.2-level accuracy and a measurement range of 0 to 1 MPa is used to measure a saturated steam pressure of 0.7 MPa with an error limit of ±2 kPa. If the steam density meter is used to check the steam density table, the flow compensation uncertainty caused by this error limit is about ±0.13% R (differential pressure flowmeter) to ±0.25% R (vortex flowmeter) . Obviously, the compensation accuracy of pressure compensation can be higher than the temperature compensation.
(4) Difficulties in implementation The above two methods of compensation are feasible. They are only discussed in principle, and other problems will arise in the implementation.
1 Installation is difficult. As described in Section 3.1.5 above, the differential pressure flowmeter used to measure the mass flow of saturated steam, if temperature compensation is selected, the temperature measurement casing is too close to the throttle to interfere with the flow state or simply It is not possible to install to the desired location and modify the plan.
2 Enters the overheated state due to the phase change. For dry saturated steam, when flowing through the vortex flowmeter at a higher flow rate, the adiabatic expansion due to pressure loss tends to cause the steam to enter a superheated state. At this time, it is still regarded as saturated steam, and the saturated steam density is checked according to the steam temperature. In the table, the values obtained are significantly higher.
For the above various reasons, when measuring the saturated steam mass flow rate, only the temperature is measured, and the density meter is checked accordingly, thereby calculating the mass flow rate, which is not applied in practice.

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