The sewage flow meter is based on the principle of pitot tube velocity measurement, and is a plug-in flow meter that estimates the flow rate by measuring the flow rate on the straight line in the pipeline. The utility model has the advantages of simple structure, low price, convenient assembly and disassembly, and small pressure loss. It is an energy-saving instrument from the aspects of low consumables and low operating costs. Under the current vigorous promotion of building a conservation-oriented economy, it is a recommended flow meter.
The flow rate Q is the fluid volume m3 (or mass kg) passing through a section A of the pipe in a unit time s, that is, Q = m3 / s = m2? m / s = A?V.
The flow rate can also be converted to the product of the pipe section A and the flow velocity V, but the flow rate in the industrial pipe is usually not constant, only the 桴 section is divided into a plurality of cell areas Ai, multiplied by the flow rate Vi through Ai, ie the flow rate. But this method is too cumbersome. Fortunately, no matter how complicated the flow velocity distribution in the pipeline flow rate, after a long straight pipe section (generally 30 times diameter), the flow velocity distribution inside the pipe will exhibit a fully developed turbulent flow symmetrically to the center of the circle under the viscous action of the fluid. In this case, it is only necessary to measure the flow velocity of N points in the diameter direction, and the flow value can be accurately estimated.
Using the principle of pitot tube velocity measurement, the flow rate of the fluid can be measured by the Bernoulli equation by measuring the total static pressure of the fluid. The sewage flowmeter is inserted into the pipeline along the diameter direction of the pipeline, and there are several pairs of total pressure pores flowing. Due to the different sinking speeds, the measured total pressure is not equal. After averaging in the high pressure chamber, the high pressure line is connected to the high voltage end of the transmitter; There are several pairs of back pressure holes on one side, and the measured back pressure (such as the back pressure values in the bit flow should be equal). After the low pressure chamber is averaged, it is connected to the low voltage end of the transmission through the low voltage line, ignoring some factors that have little effect, sewage flow. The calculated flow calculation formula can be expressed as
Where 1Q is the flow rate (m3/n); A is the coefficient depending on the unit of each parameter;
P is very good in the pipeline (m); ΔP, the difference between the high and low pressure after the average (Pa);
δ fluid density (kg/m3).
1. Simple structure and light weight, only a total of more than 10 parts.
2. The scope of adaptation is wide. It can be applied to a variety of fluids (gas, liquid, steam); caliber from 25 mm to 9 m, upper pressure limit up to 40 Mpa; upper temperature limit 1000 °C.
3. Energy saving is remarkable. The unrecoverable pressure loss is only a few tenths of the orifice plate, and the annual operating cost is 1/40 to 1/50 of the orifice plate.
4, easy to install. It is only necessary to form a round hole of about 40 mm on the pipe, and the seat can be welded. The continuous flow pattern can be decorated without interruption in the low pressure condition.
5. Long-term stability is good. No support parts, accuracy is not affected by dirt, corrosion, etc. Accuracy can guarantee the length of the straight pipe section can reach ±1%; when the length of the straight pipe section is not up to the requirement, the repeatability can also reach 0.2%, which is suitable for industrial control systems.
6, accuracy. When the length of the straight pipe can not be satisfied, due to the sampling nature, the accuracy is difficult to be higher than ± 2% to 3%, which is not suitable as a meter, especially for trade accounting.
7, anti-blocking. In the early sewage flowmeter, the back pressure is only a thin tube, which is easy to block, and requires high fluid cleanliness. The new sewage flowmeter adopts multiple points of back pressure and has a large cavity, which can improve the easy blockage.
The sewage flow meter was introduced to the market in the late 1960s. The earliest detection rod section was round at the beginning, and it was found that when the Reynolds number Re<105, the separation point on the circular section was 78°; and when Re>106, the separation The point will be changed to 130°. When Re is between 105 and 106, the separation point is uncertain, and the induced flow coefficient has a deviation of nearly ±10%. In the late 1970s, the diamond-shaped cross-section detection rod (foreign diamond Diamon) was determined at home and abroad to replace the circular section. Currently commonly used are the following:
1. Diamond-I type, launched by DSI Corporation of the United States in 1978. Since the sewage flow meter should generally be used in the bit flow, that is, the cross-sectional area of the pipe has a lateral flow, the back pressure should be equal, and only a little back pressure can be taken, and a thin tube with an inner diameter of about 3 mm is introduced to the transmitter, but Most of the on-site fluids are not clean enough, and often there are plugging faults. At present, there are still manufacturers in China, and foreign countries have long abandoned them.
2, Taba tube (Figure 2b), by the British Toba (TFL) flowmeter company, introduced in 1985, in the round detection rod, a large angle, forcing the fluid to separate at the hexagon, it and the diamond-I type early The used pressure-inducing lead-out tube has proved that not only does it have no excellent performance, but it also increases the disadvantages of easy clogging. This structure has been eliminated by domestic and foreign manufacturers. A domestic factory pushes a patented product, also known as the Toba tube, which actually welds a warhead on each total pressure detection hole, and the detection rod is still round. It has neither the advantages of the warhead type control face number nor the shortcomings of the uncertainty of the round section separation point. What is the ingenuity? Hard to understand!
3, diamond type - type II
The total pressure and back pressure detection holes are all 2 to 4 pairs, and after being averaged in the high and low pressure chambers, they are respectively led to the high and low pressure ends of the differential pressure transmitter. The Probar product (Fig. 2C-1) introduced by the US DSI in the late last century (1984) is composed of three profiles (one diamond, two triangles) combined into a detection rod. In Germany, the intra-automation company introduced an integrated structure in the 1990s called Itabar. It has two internal pressure chambers for high and low pressure chambers. It has good strength, is not easy to leak, and is resistant to high strength. After hot steel, the pressure can reach 40Mpa and the temperature can reach 100°C. Probar and Itabar's test rods have a composite structure that allows the temperature transmitter to be inserted into the detector bar to form an integrated smart mass flow meter.
4, warhead type
Launched in 1992 by Veris, USA, it is called Verbar.
Verbar has roughened the surface of the front end of the warhead (X/Ks≈200), which is believed to ensure the formation of a turbulent boundary layer after treatment, which improves measurement accuracy. The transition from laminar flow to turbulent flow affects accuracy, but this effect is negligible compared to other factors, while the position of the warhead and static pressure points makes the output differential pressure relative to other types of sewage flow. The low bias is a lot, which affects its selection at low density and low flow rate.
5, T type
The T-shaped structure is being convected in 2001. It was introduced by DSI in the United States. There are two rows of dense total pressure detection holes (about 2 mm in diameter) or pressure-taking grooves. The back-flow back measurement uses two rows of back pressure holes. It is believed that such a design can achieve a "more" speed profile, which is beneficial to improve accuracy. In fact, the total pressure hole is either dense enough to become a notch, and only the flow velocity in a certain diameter direction of the pipe can be measured. This design is meaningless when the straight pipe is not long enough and the flow rate in the diameter direction is not sufficient to reflect the entire cross section. The replacement of the total pressure hole by the notch has occurred decades ago, and there is no practical value without the implementation. Secondly, the smaller total pressure hole (or groove) is easy to block. In fact, it is not the manufacturer's high-pressure zone formed in front of the T-shaped detection rod, and the dust is not easy to enter. If so, is there a need for a wiper on the windshield of the car?
Sewage flowmeters have many advantages, such as energy saving, simple structure, and easy installation... but they are blessed by the blessings of the blessings and the sorrows of the blessings. These advantages also inevitably bring some shortcomings, such as accuracy. Not high enough, easy to block... As mentioned above, the sewage flow meter is a plug-in type instrument with sampling characteristics. When the straight pipe section cannot meet the requirements, no matter what type of test rod is used, it is difficult to achieve the accuracy of ±1% promoted by the manufacturer. Sex can be better than 0.5%.
More than 20 years ago, W?Rahmeyer and others have verified it. They installed the sewage flowmeter 2 to /2D after the resistance (valve, elbow φ...), that is, systematically tested under conditions of insufficient development of turbulence. Tests have shown that when the straight pipe section is less than 4~5D, the deviation of the flow coefficient can reach more than ±8%, and the repeatability can be better than 1%. It is indicated that for the sampling instrument of the sewage flow meter, the influence of the length of the straight pipe on its accuracy is crucial. The impact on repeatability is negligible!
In practical applications, since the sewage flowmeter is particularly suitable for large pipelines, it is generally difficult to ensure a sufficiently long straight pipe section, that is, it cannot have high accuracy, and the sewage flowmeter has no foothold in the process industry. Instruments generally have the following three uses:
1 For the measurement of trade and economic accounting, the accuracy should be placed first.
2 For the detection of the signal source of the industrial control system, the repeatability is the main one.
3 is used to monitor whether the process is working properly, and reliability is a priority.
For example, for a boiler combustion system, air flow must be measured to regulate the amount of fuel to ensure an optimal air to fuel ratio. At this time, if the air is measured by a sewage flow meter, as long as its output can reflect the change of the air, the two have a one-to-one correspondence with a single-valued function, and the randomness is good, that is, the repeatability is good. As for the absolute amount of air, people don't need to know. Secondly, the sewage flowmeter is also suitable for monitoring whether the process flow is normal. For example, China's West-East Gas Pipeline and wood-diameter trunk flowmeter do not involve metering and charging. In 96 trunk flowmeters, 50 sewage flowmeters are used. , accounting for 52%.
In the large-caliber flow detection and monitoring system, the sewage flow meter is often the preferred instrument with its remarkable energy-saving effect and excellent cost performance.
Because of the many influencing factors, the flow meter has a lot of corresponding varieties. At present, there is no flow meter that can replace other instruments and dominate the world. For each type of instrument, it can only use its strengths and avoid weaknesses, and play in its own field. effect. Industry experts are worried that the sewage flow meter is not reliable as the basis for metering and trade settlement because of insufficient accuracy. It is recommended that:
ⅰ. Select one type of sewage flowmeter test piece at home and abroad as a standard form
ⅱ. Establish a database of flow coefficients;
ⅲ. Conduct experimental research on site installation conditions.
These recommendations seem tempting, but implementation is unrealistic. Who is organizing an international standardization plan? China's more than 20 years of reform has pushed all research institutes to the market. No one has ever asked basic research. To implement such a plan, who will pay for it?
In recent years, some flow meters have developed rapidly. According to the “Flow Research” report, in recent years, sewage flow meters and ultrasonic flow meters have developed rapidly, with annual sales growth rates of 10.4% ey 6.9%. Both instruments can achieve an accuracy of ±0.5%, which is fully qualified for metering and trade settlement. The research of sewage flowmeters, manufacturers should overcome the mentality of "have the drawbacks, learn the money", must apply this plug-in instrument to the field of measurement and trade settlement, can continuously summarize the problems in the application, can be tested It is a great contribution to fully utilize the sewage flow meter in the monitoring field.