Water vapor, as its name suggests, is a gas phase, but it turns into a liquid phase under certain conditions. A water droplet in saturated water vapor is originally a liquid phase, but it is vaporized into a gas phase under certain conditions. This is the phase change that is often encountered in steam mass flow measurements. Ruian Wanjiang Auto Parts Co.,Ltd , https://www.wanjiangautoparts.com
In the derivation mass flow meter, the key link is to obtain the steam density. After the steam phase change occurs, the relationship between the steam temperature pressure and its density is usually changed. Therefore, it must be taken seriously.
(1) Gas-phase liquid-phase transition After superheated steam is transported over long distances, it often goes from a superheated state to a saturated state due to heat loss and temperature drop, and even part of steam condensation changes into water droplets. How much influence these droplets have on flow measurement results is illustrated below.
There is a commonly used superheated steam with a pressure of 1.0 MPa. Its flow is qm. Assuming that 10% of the qm is condensed into water droplets after long-distance transport, qm1 is assumed to be the qm1, and the part that remains in the gaseous state is qms. By definition, the wet steam is known. The degree of dryness is X= (3.22)
Because of adopting temperature and pressure compensation, according to the critical saturation state look-up table, the steam (dry) density at this time is Ïs=5.6808kg/m3, and the density of the water drop is known as ÏL=882.47 kg/m3 at this time. Obviously, the volumetric flow rate of the water droplets and the dry part of the steam is qvl=qml/Ï1 (3.23)
Qvs=qms/Ï2 (3.24)
Where qv1 - the volume flow of water droplets, m3 / s;
Qvs - Volumetric flow rate of the dry portion of steam, m3/s.
By definition, the ratio Rv of the steam partial volumetric flow to the total volume flow qv of wet steam is Rv = (3.25)
Because (3.26)
So Rv= (3.27)
In this example, Rv=99.93%, it can be seen that the volume ratio of the water droplets in the wet steam is negligible.
1 The influence of humidity on the measurement results when using vortex flowmeters. The output of the vortex flowmeter is only proportional to the flow rate of the fluid flowing through the measuring tube. When measuring the wet saturated vapor, the effect of the water drop on the output of the vortex flowmeter is negligible. Therefore, the output of the vortex flowmeter is entirely determined by Caused by the dry part of wet steam. The density of the dry part, whether it is pressure compensation or temperature compensation, can be more accurately detected.
The results of steam metering often serve as a basis for economic settlement between supply and demand sides. If both parties agree to settle the cost by the dry part of the steam, the condensed water will not be charged. In this case, the influence of the phase change on the measurement is negligible and can be ignored. If the condensate is also charged in the same way as steam, the vortex meter has a low measurement value of 1-X.
2 The influence of humidity on the measurement results when the orifice flowmeter is selected. Known from the standard of GB/T2624-1993 [1], the orifice plate flow meter has the formula (3.1) when measuring the mass flow of steam.
When the heat of superheated steam is lost and it leaves the superheated state, only two situations may occur. One is to enter the critical saturation state, and the other is to enter the supersaturation state. If you enter the critical saturation state, the flowmeter will theoretically not increase the error because, in equation (3.1), р1 detected from the vapor pressure is consistent with the actual density. If you are oversaturated, the situation is complicated.
It is generally believed that the fluid exhibits a homogeneous flow when the steam quality is high (X≥95%). Temperature and pressure compensation can be performed in the usual way; however, some errors occur. In equation (3.1), р1 is the density of the actual fluid wet saturated steam, which is greater than the critical saturation state, and the lower the dryness, the greater the density. However, according to the pressure, the density of the critical saturation state is found, which is smaller than the actual density, so the mass flowmeter results in negative errors. When humidity is not measured, X is an unknown number, so it is unknown how much the measurement result is low.
When the steam quality is low (X≤95%), the stratified flow occurs in the pipeline, resulting in greater errors.
(2) Wet saturated steam turns into superheated steam Wet saturated steam turns into superheated steam, which generally occurs when the wet saturated steam suddenly decompresses, and the fluid undergoes adiabatic expansion.