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Description of flow measurement error caused by fluid temperature change

The flow temperature measurement causes the flow measurement error. A 220t/h boiler in a factory in Qingdao, the steam flow rate and the influent flow rate are measured by the orifice flowmeter. The boiler has been running at full capacity for a long time, but there is a mystery. That is, the steam flow rate is always about 2% higher than the sum of the boiler feed water flow rate and the desuperheating water flow rate. It is reasonable to say that the balance difference calculated according to the measurement results of the differential pressure flow meter can reach ≤ 2%, but the operator is still not satisfied. Therefore, the instrument personnel recalculated the orifice calculation book, re-inspected each instrument, and found no problems in checking the instrument installation. Finally, the instrument manufacturer is required to explain. The relevant personnel made a more comprehensive investigation of the situation of the two measurement systems. Finally, when asked whether the actual operating conditions of the fluid deviate from the design conditions in the orifice calculation book, the operating personnel explained that the deaerating water temperature is 50 ° C lower than the design condition, and this deviation is in the influent flow meter. Neither the corresponding correction nor the temperature compensation was introduced. Due to this deviation of temperature, the actual density of the fluid is increased by 4%, and the inflow meter is 2% lower.

Example 3 The liquid temperature rises, the volume expands, and the volume flow rate increases accordingly.
Two DN100 electromagnetic flowmeters in a chemical plant in Jiangsu Province measure the two dilute acids of the two pipes respectively, merge into the main pipe, and measure the total flow by the DN200mm electromagnetic flowmeter. The use unit reports to the instrument manufacturer that the total meter flow is 120% to 130% of the sum of the sub-meter flows, and it is considered that all three instruments are inaccurate. According to the field, the pipeline pressure is 0.6MPa absolute pressure, the liquid temperature of the two tubes is 30 °C, the mixed liquid is heat exchanged by the reactor before entering the total meter, and the temperature is raised to 180 °C. The temperature expansion coefficient of the diluted acid is similar to that of water. The increase in volume from 30 ° C to 180 ° C is about 12%. It can be judged that the difference between the total table and the sum of the sub-tables is mainly due to the change in liquid temperature. In addition, 0. 6MPa absolute pressure 158.5 ° C water has begun to boil, flow through the flow of the meter, a part of the steam in the liquid, will also increase the total meter volume flow reading, it can be considered that the total table reading is found 20% ~ 30% of the reasons [10].

4 Verify the accuracy of the steam flow meter with the amount of condensed water. Many Yanqi users use the heat in the steam. After the steam is metered by the flow meter, it is sent to the heat equipment. The steam releases the heat and becomes equal-quality condensed water, which is then discharged from the steam trap. The condensed water is collected for a period of time, the mass is measured, and then the accuracy of the steam meter is verified by comparing with the total amount of the steam meter in the same period of time. This method is a simple and easy method that is often used in the installation of electromagnetic flowmeters, but the following two points should be noted.
a. Condensed water always has a small amount of steam when it is discharged from the steam trap. It should be considered when comparing the total amount. It is better to discharge the steam trap into the bottom of the vessel containing the appropriate amount of cold water so that all the residual steam becomes condensate. Then measure.
b. If saturated steam flows through the flow meter, the effect of the entrained water droplets on the equilibrium calculation must be considered. Nowadays, the method of measuring the steam flow using a vortex flowmeter is very common, and the vortex flowmeter basically does not respond to the water droplets in the steam, but the water droplets are contained in the drain of the steam trap, so if the humidity of the steam is 5%, then, the total amount of condensed water is 5% higher than the total amount calculated by the steam flow meter. The difficulty is how much the humidity of the steam is difficult to measure. Only know that if the steam trap is installed on the pipeline before the flow meter is introduced, the stratified flowing water can be discharged, and the water droplet content in the steam is about 0 to 5% (mass ratio).

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