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Saturated steam temperature and pressure compensation

Date: 2019-06-27
浏览次数: 103

Vortex Flowmeter-Saturated Steam Temperature and Pressure Compensation

(1) temperature compensation and pressure compensation are used to calculate the consistency of density saturated steam by look-up table method, which is essentially the same.  The reason for this is that the pressure and temperature of saturated steam have a single-value function relationship, and the density found from the steam temperature is consistent with the density found from the pressure corresponding to this temperature.  Therefore, both temperature compensation and pressure compensation are feasible in principle.

(2) The difference in investment is considered from the perspective of saving investment and reducing installation workload. Since the price of a platinum thermal resistor is only a few tenths to a fraction of that of a pressure transmitter, temperature compensation is more economical.

(3) The difference in compensation accuracy is related not only to the accuracy of temperature sensor and pressure transmitter, but also to the type of flowmeter, the working condition of specific measuring object and the range selection of pressure transmitter.  Generally speaking, temperature measurement has a great influence on compensation accuracy. The specific analysis is as follows.

① Influence of temperature measurement error on flow measurement results.  The relationship between temperature measurement error and flow measurement result has little influence on superheated steam, for example, if the temperature measurement error is 1℃, the uncertainty of flow measurement result caused by temperature compensation is about 0.096% r (differential pressure flow) to 0.19% r (vortex flowmeter).  What has great influence is that the temperature signal is used for compensation in saturated steam flow measurement. For example, the equilibrium temperature of saturated steam with pressure of 0.7Mpa is 170.5℃, and the corresponding density is 4.132kg/m3. If the temperature measurement error is -1℃, and the saturated steam density meter is checked based on this, the density is found to be 4.038kg/m3, causing the flow measurement error to be about -1.14℃ (differential pressure flowmeter) to-2.2% R (vortex flowmeter).

(2) Consideration of accuracy grade of temperature sensor.  The temperature measurement error is related to the accuracy grade of the temperature sensor and the measured temperature value. For example, for saturated steam with a pressure of 0.7Mpa, the error limit is 0.49℃ [4]. If the steam density meter is checked with this measurement result for compensation, the uncertainty of flow compensation is about 0.56% R (differential pressure flowmeter) to 1.11% R (vortex flowmeter).  However, if the B-grade platinum thermal resistor is used to measure temperature, the error limit will increase to 1.15℃, and the uncertainty of flow compensation will increase to 1.31% R (differential pressure flowmeter) to 2.61% R (vortex flowmeter).  Obviously, the error caused by the use of Class B platinum thermal resistor for such purposes is considerable, so it is generally not suitable for use.  Here, only temperature measuring elements with different accuracy levels are compared.  Of course, the error mentioned here is only the temperature measuring element. As for the uncertainty of the flow measurement system, the influence of flow secondary meter, flow sensor, flow transmitter, etc. must also be taken into account.

(3) pressure transmitter accuracy grade, pressure measurement error and its influence.  The pressure measurement error is related to the accuracy grade and measuring range of the pressure transmitter. For example, the pressure transmitter with 0.2 grade accuracy and 0 ~ 1MPa measuring range is selected to measure the saturated steam pressure of 0.7MPa, and the error limit is 2 kPa.  If the steam density meter is checked with this result to compensate, the uncertainty of flow compensation caused by this error limit is about 0.13% r (differential pressure flowmeter) to 0.25% r (vortex flowmeter).  Obviously, pressure compensation can obtain higher compensation accuracy than temperature compensation.

Vortex flowmeter of Guangzhou fangshi instrument co., ltd. adopts the integration of temperature and pressure compensation. saturated steam can be compensated by pressure or temperature, superheated steam can be compensated by temperature and pressure. customers do not need to install pressure transmitter, temperature sensor and secondary meter, which is not only convenient to install but also saves cost.

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