Steam is divided into saturated steam and superheated steam. In general, vortex flowmeter manufacturers will provide a table of saturated, superheated steam mass flow rates for user selection in order to determine the size of the caliber. For superheated steam, temperature and pressure compensation is required. For the true meaning of saturated steam, only temperature or pressure compensation is required. During the measurement, check the density table according to the temperature and pressure to obtain the required mass flow rate. In actual measurement, due to the change of operating conditions, both saturated steam and superheated steam may deviate from the original design state, which makes the relationship between the steam temperature pressure and its density generally changed, which affects the measurement accuracy.
Wet saturated steam
When the vortex flowmeter is installed in the pressure reducing valve, the wet saturated steam suddenly decompresses, and the fluid undergoes adiabatic expansion. The water droplets evaporate partially. At the same time, vaporization heat is absorbed from the liquid phase and the vapor phase, so that the vapor-liquid phase temperature is reduced. If the temperature does not decrease much or the humidity before the evaporation is high, the temperature will rapidly decrease to the saturation temperature corresponding to the new pressure and a new equilibrium will be established. At this time, the steam is still saturated steam if the pressure is reduced much or before the evaporation. When the humidity is low, the steam becomes superheated steam because the water droplets evaporate and the temperature is still lower than the saturation temperature corresponding to the new pressure.
　　 superheated steam
In the flow calculation device, the mass flow rate can be obtained based on the temperature and pressure of the superheated steam through the look-up table. However, when the superheated steam is transported over a long distance or due to improper heat insulation measures of the pipeline, the heat loss temperature is often decreased from entering the critical saturation superheated state, and even part of the steam is condensed into water droplets phase change occurs, then becomes wet saturated steam (supersaturated steam).
　　 The output of the vortex flowmeter is only proportional to the flow velocity of the fluid flowing through the measuring tube. When the saturated steam is measured, the influence of the water drop on the output of the vortex flowmeter is negligible. Therefore, it can be considered that the output of the vortex flowmeter is completely determined by It is caused by the saturation of the dry (saturated) portion of the steam, and the density of the dry portion can be accurately detected based on pressure compensation or temperature compensation. If the two parties agreed to settle the cost of the dry part of the steam and the condensate does not charge for steam metering, the effect of the phase change on the measurement is negligible and can be ignored. If the condensate is also charged as steam, the vortex flowmeter has a low measurement result.
After the above evaporation occurs, the former has no effect on the compensation. Only the dry part of the steam increases, and the dryness increases accordingly. The latter case is when the wet saturated steam becomes superheated steam. The effect on the flowmeter is as follows: three conditions:
(1) The steam has been considered to be in an overheated state during design, or it is difficult to determine in which state, or sometimes it is in an overheated state or sometimes in a saturated state. Therefore, if the temperature and pressure compensation is adopted, the above phase change has no effect on the measurement result.
(2) Considering saturated steam design and using pressure compensation, the phase change mentioned above will bring about a small error, that is, the compensation error caused by the density difference corresponding to the difference between the superheated steam temperature and the saturated steam temperature.
(3) Saturated steam is considered in design, but using temperature compensation, that is, using superheated steam temperature as the saturation temperature to check the density meter will generally cause greater errors.
There are three ways to solve the above problems:
(1) Before installing the total steam flow meter in the pressure reducing valve, since the above steam is not decompressed, there is no phase change problem. Therefore, before the pressure meter is installed in the pressure reducing valve, it is processed by the saturated steam compensation method to ensure the measurement. Accuracy.
(2) If the flowmeter can only be installed behind the pressure reducing valve, an additional pressure transmitter can be added for temperature and pressure compensation.
(3) If the stability of the pressure reducing valve is better, the upstream pressure value of the flow meter can be set as a constant value in the display instrument to perform temperature and pressure compensation.
Regulator compensation installation diagram:
Vortex flowmeter sample display:
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