Error characteristics and usage precautions of liquid volumetric flowmeters

Volumetric flowmeters have been around for more than a hundred years. They were first used to measure water flow. The original flowmeter was a drainage pump or hydraulic press, which later developed into a flowmeter. By the late 1930s, it was widely used for oil product measurement. At present, it is still widely used in the measurement of petroleum products in China. It is very common and has a large amount of use.

1 Basic structure and classification of volumetric flowmeters

Volumetric flowmeters use mechanical measuring elements to continuously divide the fluid into unit volumes and accumulate and measure the total volume to achieve fluid measurement. It is somewhat similar to the way of measuring the volume of liquids with containers in daily life. The difference is that in industrial applications, the volume of the fluid needs to be continuously measured in the pipeline. Its working process is: the fluid continuously fills a certain "metering space" with a certain volume, and then continuously sends this part of the fluid to the outlet to flow out. In one measurement, the number of times these "metering spaces" are filled with fluids is continuously accumulated, multiplied by the volume of the "metering space", and the total flow through the flowmeter can be obtained.

There are many types of volumetric flowmeters, and their structural forms vary. According to the structural characteristics of their measuring elements, there are mainly oval gear flowmeters, Roots flowmeters, scraper flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disc flowmeters and many other types.

2 Error characteristics of volumetric flowmeters

For any flow measurement value, it must include two parts: one is the flow measurement value itself; the other is its error allowable range, otherwise it is incomplete. Therefore, for any flowmeter, its error characteristics must be understood. The so-called error characteristics are the relationship between the error value of the flowmeter and the flow measurement value. Discussing the error characteristics is to discuss and study the trend of the measurement error value changing with the flow measurement value.

Assume that the volume of the flow meter's measuring space is V0 (m3), and the number of rotor rotations in a certain period of time is N. Then the volume of fluid V (m3) flowing through the meter during this period of time is

V = NV0 (1)

Assume that the gear ratio constant of the meter is a. The value of a is determined by the gear ratio of the gear set that transmits the rotor rotation and the scale value of the meter pointer rotating one circle. If the meter indication value is I, its relationship with the number of rotor rotations N is

I = aN (2)

From equations (1) and (2), the relationship between the volume of fluid passing through the meter within a certain period of time and the meter indication value can be obtained:

The measurement error value E of the volumetric flow meter can be expressed by the ratio of the difference between the indicated value and the true value to the indicated value. The error value E can be expressed as

Substituting the relationship between the fluid volume V and the indicated value I into equation (3), we can obtain:

It can be seen from formula (5) that the error characteristic of the volumetric flowmeter is only related to the volume of the metering space V0 inside the flowmeter and the gear ratio constant a of the instrument. In other words, from the perspective of the measurement principle, the measurement error of the volumetric flowmeter is only related to the geometric structure of the flowmeter, and has nothing to do with the fluid properties and flow value. We call this error characteristic the ideal error characteristic of the volumetric flowmeter. Plotted as a curve, it is a horizontal line parallel to the horizontal axis, as shown by curve 1 in Figure 1.

Figure 1 Error characteristic curve of volumetric flowmeter

However, when we calibrate the volumetric flowmeter and draw the actual error characteristic curve, it is closer to the form of Curve 2. At low flow rates, the error value tilts sharply in the negative direction; as the flow rate increases, the error value gradually moves from the negative direction to the positive direction and stabilizes at a certain position, and the error curve is parallel to the horizontal axis. As the flow rate continues to increase, the error value will shift to the negative direction again. The actual error characteristic curve shows this trend of change because there is an inevitable leakage phenomenon in the volumetric flowmeter. The so-called leakage is that the fluid flows directly from the inlet to the outlet through the gap between the rotating part and the outer casing without being measured. The error characteristic relationship that takes the leakage phenomenon into account will be discussed below. [page]

Assume that the fluid leakage flow per unit time is represented by △g; the flow rate through the flowmeter is qV; the total fluid volume through the flowmeter is V; and the total leakage volume during this period of time is △V. In this way, △V can be expressed as:

Therefore, when there is leakage, when the rotor discharges N metering spaces of fluid, the actual volume of fluid passing through the flowmeter is

V = NV0 + △V (7)

Substituting equations (2) and (6) into equation (7), we can get:

Formula (8) can be rearranged into:

Substituting formula (9) into the error definition formula (4), we can obtain:

From the analysis of formula (10), it can be seen that since the volume of the metering space V0 and the gear ratio constant a are both constant values, the relationship between the error E and the flow rate is affected by the leakage flow rate per unit time △g.

If it is assumed that the leakage flow rate △g of the volumetric flowmeter is a constant value, the change trend of its error curve can be discussed using formula (10).

When the flow rate is very small, in extreme cases, qV=△g, then the value in the brackets in formula (10) is 0, and the error value E tends to negative infinity. As the flow rate qV increases, the value in the brackets in formula (10) gradually increases, and the error value E also gradually increases in the positive direction. When the flow rate continues to increase and reaches △g, which is very small compared to qV, that is, qV/△g is very close to 0, formula (10) is transformed into formula (5), and the error curve tends to the ideal error curve.

3 Precautions in the use of volumetric flowmeters

Volumetric flowmeters are a type of flowmeter with high accuracy, but improper use will not produce ideal results. Only timely maintenance can make the flowmeter operate normally. Correct use and timely maintenance of flowmeters are the responsibilities that flowmeter operators should adhere to.

After the newly designed or reinstalled flow meter metering system has been installed and checked, it should be tested. The test run is carried out according to the following procedures: First, close the valves before and after the flow meter, and slowly open the bypass pipeline valve for the horizontally installed metering system (for the vertically installed system, open the main line valve) to allow the fluid to flow through the bypass pipeline to flush the remaining debris in the pipeline and balance the inlet and outlet pressure of the flow meter. If there is no bypass pipeline, a short pipe can be used to replace the flow meter to allow the fluid to pass. After the pipeline is flushed, remove the short pipe and replace it with the flow meter. Use it according to the regulations in the flow meter manual.

After the metering system is put into normal operation, the operator should pay attention to the following aspects:

(1) Exhaust gas: Usually, after the liquid sweep line, there is still a lot of air left in the pipeline. With the pressurized operation, the air flows through the volumetric flow meter at a higher flow rate. The active measuring element may run at an overspeed, damaging the shaft and bearings. Therefore, the flow rate should be increased slowly at the beginning to allow the air to escape gradually.

(2) Bypass pipe switching sequence: When the liquid flows from the bypass pipe to the instrument, the opening and closing should be slow, especially on high-temperature and high-pressure pipelines. When starting, the first step is to slowly open valve A, and the liquid will flow in the bypass pipe for a while; the second step is to slowly open valve B; the third step is to slowly open valve C; the fourth step is to slowly close valve A. When closing, follow the above reverse sequence.

(3) After starting, confirm that there is no excessive flow through the lowest pointer or word wheel and stopwatch. The optimal flow should be controlled at (70-80)% of the maximum flow to ensure the service life of the instrument.

(4) Check the filter: The filter net is most likely to be broken when the new line is started. After the trial run, check whether the net is intact in time. At the same time, when the filter net is clean and free of dirt, record the two parameters of pressure loss at the commonly used flow rate. In the future, there is no need to remove the net to check the blockage condition, that is, the degree of increase in pressure loss determines whether it needs to be cleaned.

(5) Measuring high-viscosity liquids: It is used for high-viscosity liquids, which are generally heated to make them flow. When the meter is out of service, the liquid inside it cools and becomes thicker. When it is reactivated, it must be heated first and the viscosity of the liquid reduced before the liquid is allowed to flow through the meter. Otherwise, it will bite the active measuring element and damage the meter.

(6) Avoid rapid flow changes: When using a volumetric flowmeter, care should be taken not to have rapid flow changes (such as using a quick-opening valve). Due to the inertia of the volumetric flowmeter, rapid flow changes will generate a large additional inertia force, causing damage to the rotor.

(7) When flushing the pipeline with steam, steam is prohibited from passing through the volumetric flowmeter.

4 Conclusion

The volumetric flowmeter is a relatively classic flowmeter and is still widely used in industrial dynamic measurement. Only by understanding the error characteristics of the volumetric flowmeter and doing a good job of use and daily maintenance can we ensure that the advantages of this type of meter are brought into play and better serve production. (end)