[Abstract] When measuring the steam in the vortex flowmeter, the steam flow rate in the pipe must be limited to a range, and the vortex flowmeter can work normally. Some people use some means that seem to be unintentionally illegal, causing the downstream pressure to drop suddenly, causing the steam flow rate to exceed the upper limit of the flow rate measured by the vortex flowmeter. The vortex flowmeter cannot work normally, and the measurement is seriously small. Finally, the method of installing a current limiting device is proposed to control the steam flow rate to ensure the normal measurement of the vortex flowmeter.
At present, vortex flowmeters are widely used in steam metering. Vortex flowmeter is a kind of well-developed flowmeter. Its main advantages are: no moving parts, simple and firm structure, convenient installation, low maintenance cost, range ratio is generally above 10:1, and accuracy is relatively high. The measurement gas is generally between 1.0 and 1.5, and the measurement liquid is generally at the 1.0 level.
In the verification work for many years, we found that someone used a vulcanity of a steam vortex flowmeter. In the case of keeping the vortex flowmeter measuring pipe section intact, only making a fuss at the steam outlet can cause vortex flow. The metering is seriously small.
In Figure 1, we see that there is no modification in the metering section of the vortex flowmeter, but the large gas storage tanks behind it are very popular. The whole system automatically controls the front and rear valves according to the pressure change in the gas storage tank, thereby performing a circulating operation of inflating, deflation, re-inflation and deflation. The whole cycle is: at the beginning, the gas storage tank is empty, and the valve 1 and the valve 2 are closed. Then the valve 1 is quickly opened, and the upstream 0.8 MPa and the hot steam are vigorously filled into the empty can. Then, the gas tank is gradually inflated, and after the pressure is raised to a certain pressure, the valve 1 is closed, and the valve 2 is opened to allow the steam in the gas tank to be discharged for use. The main purpose of this operation is to allow most of the steam flowing through the vortex flowmeter to pass at a very high flow rate.
The critical pressure ratio is an important value in the analysis of the flow in the tube. When the ratio of the back pressure pb outside the outlet to the inlet pressure P1 before the critical interface is less than or equal to the critical pressure ratio, the steam flow rate reaches the critical value sonic speed in the critical section. .
When the overcurrent gas is superheated steam: k=1.3, γcr=0.546
Pcr is called: critical pressure.
So we get: By reducing the back pressure ratio, the flow rate through the steam can be raised to a very high level relative to the vortex flowmeter, even reaching the speed of sound, the supersonic steam can reach more than 500m / s.
The steam pressure from the heat network is generally higher than 0.8 MPa, and when the gas tank starts to be inflated, the pressure in the tank is almost normal pressure. According to the principle of critical flow of steam. When the front-to-back pressure ratio of the steam pipe and the vessel is lower than the critical pressure ratio γcr=0.546, the flow velocity of the steam in the pipe will reach the speed of sound. In this case, the pressure in the gas storage tank is calculated according to 0.4 MPa which is gradually increased to the middle of the inflation phase, and the back pressure ratio is 0.5. That is to say, in most of the steam passage phase, the back pressure ratio is less than 0.546, and the steam flow rate is maintained at the speed of sound, and the speed of sound is much higher than the upper limit of the flow rate measured by the vortex flowmeter. Through such an approach, the steam flow rate through the vortex flowmeter can be greatly increased, resulting in a serious vortex flowmeter measurement. Unscrupulous users are also arguing that all of my measuring instruments have passed the verification of the national legal metrological verification agency. Indeed, in this case, it is impossible to identify the problem by simply checking the flowmeter. We can judge that the high flow rate of the medium has a great influence on the metering performance of the vortex flowmeter, which makes it inaccurate. Why do you say that? Let us analyze the effect of high gas flow rate on the vortex flowmeter.
The vortex generator is placed in the fluid, and the fluid is alternately arranged on the two sides of the vortex generator in two rows of regularly staggered vortices. In a certain Reynolds number range, the frequency of the vortex is related to the geometry of the vortex generator. The resulting vortex frequency f is proportional to the flow rate, which can be detected by various sensors.
The vortex flowmeter is based on the Karman vortex principle and has the following relationship:
Where: b - the width of the spoiler, m; - the average flow velocity of the fluid flowing through the flowmeter, m / s; f - the frequency of the vortex, Hz; Sr - the Strohal number (dimensionless ).
The Strohar number is a dimensionless parameter that is related to the shape of the vortex generator and the Reynolds number. Figure 3 shows the relationship between the Strouhal number of the triangular vortex generator and the Reynolds number of the pipe.
Figure 3 The relationship between the Stoll Hall number and the Reynolds number
As can be seen from Fig. 3, in the range of Re D = 2 × 104 ~ 7 × 106, the Strouhal number can be regarded as a constant. The vortex flowmeters we use are all designed in such a range that the Strouhal number is considered constant. Therefore, when we use the vortex flowmeter, we must avoid the Reynolds number of the measured medium outside the range of 2×104～7×106. Beyond this range, the Strouhal number is no longer constant, and the frequency measured by the vortex flowmeter And the flow rate is no longer a simple proportional relationship. That is to say, exceeding the range of Reynolds number 2×104～7×106 violates the design principle of the vortex flowmeter. At this time, the vortex flowmeter cannot be measured normally. Because the Reynolds number is related to the medium flow rate, so for the specific flow rate of the medium, for steam, the flow rate measurement range of the vortex flowmeter is controlled between 5m/s and 60m/s, and the upper limit of the range of the good vortex flowmeter is the most. Continue up 20%. Therefore, when selecting the diameter and flow measurement range of the vortex flowmeter, it is necessary to ensure that this flow rate limit is met. Most vortex flowmeters have no way to measure high velocity media. Therefore, in the appeal case, the ordinary vortex flowmeter is used to measure the steam under the sonic speed, and the result is completely untrustworthy and cannot be used.
The vortex flowmeter is a digital meter that detects the vortex frequency through a sensor. The electrical performance of the flowmeter must be operated under the conditions appropriate for it. Let us look at the vortex flowmeter to detect the vortex frequency at high flow rates. An experiment with a high flow rate vortex flowmeter is cited. In the online real-time spectrum analysis, the experiment found that high-speed flow rates higher than 80m/s often occur on pipelines with diameters of DN80 and below, and nearly half of them have high flow rates exceeding 100m/s. Moreover, the flow rate is as high as 180m/s. In the case of a general vortex flowmeter, when the flow velocity of the medium is too high, a sharp leak phenomenon occurs, which causes an error that is difficult to estimate.
The result of the leakage wave seen in Figure 4 is that the detected pulses are no longer continuous and a gap has occurred. Therefore, in this case, the trend of measurement results is generally small. At high flow rates, fluid movement behind the vortex generator is more complicated. The vortex sensor detection signal requires a certain degree of clarity, and if the flow rate is too high, the flow field becomes more complicated. At this point, the sensor will be seriously disturbed, and the sharpness of the target signal will drop sharply, making the vortex flow sensor undetectable or undetectable.
We can see that the leakage wave of the vortex flowmeter at high flow rate is very obvious. It is the use of this loophole in the vortex street, so that the vortex flowmeter leaks a lot of waves at the ultra-high flow rate, resulting in the final flow being much smaller. Actual traffic.
In addition to the devices in the above case, there is also a hot water station that puts steam directly into the pool to heat the water. These methods are used to try to make the pressure at the steam outlet suddenly drop, and the back pressure ratio is suddenly reduced. To greatly increase the flow rate of steam. Even if the speed of sound is not reached, it is much higher than the upper limit of the measurement of the vortex flowmeter, resulting in abnormal operation of the vortex flowmeter. Therefore, in order to ensure the normal measurement of the vortex flowmeter, we must pay attention to the measurement range of the vortex flowmeter, and the flow velocity inside the pipe must be limited to the measurement range of the vortex flowmeter.
For those who have a sudden pressure drop at the back end to increase the steam flow rate, it is possible to find a way to limit the steam flow rate to a reasonable range. For example, a current limiting device is used to install a critical flow venturi nozzle on the line behind the vortex flowmeter.
When the steam passes through the critical flow venturi nozzle, when the pressure ratio on the nozzle is lower than or equal to the critical pressure ratio of the nozzle, the throat of the nozzle forms a critical state, and the mass flow of steam flowing through the nozzle reaches a maximum. At this time, the mass flow of steam is not affected by changes in downstream conditions. According to this principle, we can safely limit the maximum flow through the vortex flowmeter by installing the critical flow venturi nozzle before the pipe section where pressure dip can occur. The reason for choosing a venturi nozzle is to reduce the pressure loss.
Know more about Problems in the selection and measurement of measuring equipment for steam flow meters