Simply put, the fluid must form a typical flow velocity distribution (laminar flow velocity distribution and turbulent flow velocity distribution) in a circular pipe to be accurately calculated using a sophisticated mathematical model. However, this typical flow rate distribution must be formed by a fully developed tube flow, that is, a sufficiently long straight section can be formed. However, if the fluid flows through the spoiler (elbows, tees, valves, etc.), the flow velocity distribution will be distorted and vortex will be generated. In this case, the fluid will become insufficiently developed, and the flow rate will be any in the pipeline. A section is different and changes at any time. The flow rate distribution can only be restored to full development of the tube flow at the end of a very long straight section or with a rectifier installed. This is the root cause of many flowmeters requiring long enough straight sections.
For metal tube rotor flowmeters, the mechanical department standard stipulates that the front straight pipe section is at least 10 times DN. From the upstream flange end face of the flowmeter, the rear straight pipe section is at least 5 times DN, counting from the downstream flange end face of the flowmeter. DN refers to the flowmeter diameter.
For the glass rotor flowmeter, the mechanical department standard does not clearly specify the length of the straight pipe section before and after, but it is stipulated that if the flow regulating valve is to be installed, it must be installed 5-10 times DN downstream of the flowmeter, from the flange end face of the flowmeter. Start. DN refers to the flowmeter diameter.
The glass rotor flowmeter is an instantaneous flow measuring instrument that is simple to use, convenient to read, and widely used. In the scope of environmental protection equipment and instruments, the amount is up to 30,000 units/year. Therefore, it is extremely important to choose and use this instrument.
First, the variety of glass to flowmeter and glass rotor flowmeter can be divided into: ordinary type, reinforced vascular type, small flow and small shape, corrosion resistant, laboratory type, according to its application and adaptation range. Eight series of insulation type, alarm type and high pressure type. According to the national series of meter series, no matter which series, the maximum number of 12 diameters from 1 mm to 100 mm, the measurable flow range is: liquid (water) 0.1 ml / min ~ 40 m3 / h, gas (air) 1 ml / min ~ 1000 m3 / h. The glass rotor flowmeter used for environmental protection equipment generally has a diameter of no more than 10 mm, and the measured flow rate is a small flow range.
The selection of the glass rotor flowmeter can be considered from the following aspects.
1. The object of the measurement. That is, measuring the type of medium, pressure, and chemical properties. For liquid media, gas media, corrosion-resistant media should be selected for corrosive media.
2. The performance of the flowmeter itself. Generally speaking, after the above conditions are determined, if there is no big change in price, the needle valve is preferably placed on the upper part of the flow meter; if there is a large flow hole, it is a direct flow scale; the structure is simple; the external size is small, etc. . If it is a small flow range, the ball float type can be selected because it is stable during measurement, is not easy to accumulate dust, has high precision, and has good interchangeability.
3. According to the price. Generally speaking, the price with high precision is high. It is necessary to select the accuracy level of the meter according to the purpose of the measurement. For example, it is only necessary to control the throughput of the measuring medium. After the trial operation is adjusted, the throughput must be stabilized in the future, and the accuracy is secondary.
2. Scale correction of the glass rotor meteormeter The scale of the glass rotor flowmeter is obtained by the manufacturer under the conditions of the factory using water and dry air which are close to the ideal fluid. However, in the use of the flowmeter, there are two situations in which the scale value cannot be directly used: one is that the measurement medium is not water and air, and the other is that the medium is water and air, but its state (temperature, pressure) and scale The status is different. Thus, when the flowmeter is used, in order to obtain a correct measurement result, there is a problem that the scale value needs to be corrected. Therefore, solving the correction of the glass rotor flowmeter scale is the key to using this instrument.
Why is there a certain straight pipe section before and after the flowmeter when the speed flowmeter is installed? The velocity flowmeter is a flowmeter that directly measures the flow velocity of the closed pipe full pipe flow. Although the measurement method is the same, the measured flow rate is the same. Accuracy is not only related to the accuracy of the flowmeter itself, but also related to its installation position. The ideal installation position of the flowmeter is that the flow state of its upstream and downstream fluids is a single-phase steady flow, so that the upstream and downstream local resistance pipe fittings are in fluid flow state. The effect does not directly affect the measurement accuracy of the flowmeter.