[Information] The functions and characteristics of commonly used compressed air flow meters
1. The role of compressed air flowmeter Compressed air is an important secondary energy source for enterprises and institutions, most of which is converted from electrical or thermal energy by compressors. When the air pressure value is low, it is generated by the blower, and the working fluid converted from a large amount of energy is managed to receive the effect of saving energy and improving the level of equipment management. It is the main purpose of compressed air flow measurement. In the production process of chemical industry, there is an important process oxidation reaction, which uses air as a raw material and another certain raw material to carry out a chemical reaction under specified conditions. If the air mass flow rate is too large or too small, it will be safe for production and products. Quality and the consumption of valuable raw materials have a critical impact. In this case, the accuracy of air flow measurement is particularly high, and most of them are equipped with automatic adjustment. 2. Characteristics of compressed air flow measurement 1. Large vibration. Not every air flow meter is installed in a place with high vibration. But the air flow meter installed in the compression plant and blower room must consider vibration. This vibration mainly comes from the compressor and the blower. The vibration of the machine can be transmitted to a far place through the air pipe or the air pipe. Among them, the largest vibration is the reciprocating compressor. The vibration generated during the operation of the large-scale reciprocating compressor often drives the plant and the surrounding ground to vibrate together, which threatens the accurate and reliable operation of the relevant air flow meter. It causes the pivot point of the lever-type differential pressure transmitter to move, which causes the indication value to drift. Vibration causes the vortex flow sensor to generate interference signals corresponding to the vibration frequency, causing the flow indication to be significantly higher. 2. The gas carries water. Compressed air is taken from the atmosphere, and the atmosphere always contains a certain amount of water vapor. The water vapor content is indicated by the water vapor partial pressure ps. The saturated partial pressure of water vapor in the atmosphere is a function of atmospheric temperature (see Table 3.4). In rainy and foggy days, the partial pressure of water vapor in the outdoor atmosphere reaches saturation, that is, the relative humidity reaches 100%. At this time, compressing the atmosphere is like a sponge that presses on enough water. As the volume shrinks, there will be a corresponding amount Water precipitates. This is the simple principle that compressed air carries water. In fine weather, the relative humidity of the atmosphere is low, but as it is compressed, the volume shrinks to a fraction of the original, the partial pressure of water vapor will increase accordingly, and it may also enter a saturated state and precipitate water droplets. Large-diameter orifice flowmeter used to measure the flow of compressed air. There is often water in front of the orifice, which will affect the measurement accuracy. There is often a section of water in the impulse piping, which causes the differential pressure measured by the differential pressure transmitter to be inconsistent with the differential pressure generated by the throttle device. These are common causes of errors caused by water in the air. In addition, due to the high content of nitrogen oxides in the urban atmosphere, the water droplets contained in the compressed air are acidic, causing corrosion of the surface of the ring chamber and corrosion of the inner wall of the pipeline, making the surface rough. When the iron oxide produced by corrosion becomes dry under certain conditions, it is easy to fall off from the inner wall of the pipe and be brought to the orifice plate by the airflow, which will also affect the flow indication. Therefore, during parking inspection, these powdery and massive garbage should be removed. 3. Pulsating flow. Most compressor and blower outlet fluids contain certain pulsations. For example, reciprocating compressors exhibit half-wave pulsations, as shown in Figure 3.13. At the scene, it can be observed that the outlet pressure of the compressor and the blower swings significantly. Among them, the positive (fixed) displacement blower outlet pulsation frequency is higher, generally tens of hertz, while the reciprocating compressor outlet pulsation frequency is lower, generally a few hertz. Flow arteries cause the differential pressure flowmeter, vortex flowmeter and other flowmeters to display high values, causing the float in the float flowmeter to jump up and down. There are two common methods to eliminate and weaken the influence of flow artery motion on the flowmeter indication. One is to set up a buffer tank at the compressor outlet to filter the pulsation, and install the flowmeter behind the buffer. In fact, the system of the reciprocating compressor is designed in this way; the second is to install the flowmeter away from the pulsation source, so that the gas capacity of the process pipe and its pipe resistance can be used to form a low-pass filter to attenuate the pulsation.
