The vortex flow meter is more commonly used in steam flowmeters now, with temperature and pressure compensation, and the measured result is mass flow. However, due to different environments, steam is divided into superheated steam and saturated steam. These are all possible changes in the density of steam, which will affect the measurement results.
1. Superheated steam
In the flow calculator, the mass flow rate can be calculated according to the temperature and pressure of the superheated steam, but when the superheated steam is transported for a long distance, or due to improper insulation measures of the pipeline, the temperature will often drop due to heat loss. The superheated state enters the critical saturation state, and even part of the steam condenses and undergoes a phase change and turns into water droplets. At this time, it becomes wet saturated steam.
The output of the vortex flow meter is only proportional to the flow rate of the fluid flowing through the measuring tube. When measuring wet saturated steam, the influence of water droplets on the output of the vortex flow meter is negligible, so it can be considered that the output of the vortex flow meter is entirely composed of It is caused by wetting the dry part (saturated part) of each steam, and the density of the dry part can be accurately detected by pressure compensation or temperature compensation. When steam is measured, if the two parties agree to settle the fee according to the dry part of the steam, and the condensed water is not charged, the impact on the relative measurement is negligible and can be ignored; if the condensed water is also charged according to the steam, the measurement result of the vortex meter is low.
2. Saturated steam
When the vortex gas flow meter is installed behind the pressure reducing valve, the saturated steam suddenly decompresses greatly, the fluid expands adiabatically, the water droplets partially evaporate, and at the same time absorbs the heat of vaporization from the liquid and vapor phases, reducing the temperature of the vapor and liquid phases. If the temperature is not lowered much or the humidity is higher before evaporation, the temperature will quickly drop 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 drops a lot or the humidity before evaporation If the temperature is lower, the temperature is still higher than the saturation temperature corresponding to the new pressure due to the evaporation of water droplets, and the steam becomes superheated steam.
After the above-mentioned evaporation occurs, the former one has no effect on the compensation, only the dry part of the steam increases, and the dryness increases accordingly. The latter situation means that the saturated steam becomes superheated steam. At this time, the influence on the flow meter is divided into three types: Cases:
(1) It has been considered in the design that the steam becomes superheated, or it is difficult to determine which state it is in, or sometimes it is superheated and sometimes saturated, so the temperature and pressure compensation is adopted, and the above phase change has no effect on the measurement results.
(2) Considering saturated steam in the design, and adopting pressure compensation, the above-mentioned phase change 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) The design is based on saturated steam, but temperature compensation is used, that is, the temperature of superheated steam is used as the saturation temperature to check the density table, which will generally cause large errors.
There are three ways to solve the above problem:
(1) Install the total steam flow meter before the pressure reducing valve. Since the above-mentioned steam has not been decompressed, there is no phase change problem, so install the flowmeter before the pressure reducing valve, and use the saturated steam compensation method to ensure the measurement Accuracy.
(2) If the flowmeter can only be installed behind the pressure reducing valve, a pressure transmitter can be added for temperature and pressure compensation.
(3) If the stability of the pressure reducing valve is good, the upstream pressure value of the flowmeter can be set as a constant value in the display instrument for temperature and pressure compensation.