What are the characteristics of compressed air flow meter?

 

Features of compressed air flow meter

 

① Large vibration. Not every air flow meter is installed in a place with large vibration. However, air flow meters installed in compression plants and blower rooms must consider vibration. This vibration mainly comes from compressors and blowers. The vibration of the machine can be transmitted to far away places through air pipes or air ducts. Among them, the reciprocating compressor has the largest vibration. The vibration generated by large reciprocating compressors during operation often causes the plant and the surrounding ground to vibrate together, posing a threat to the accurate and reliable operation of related air flow meters. It causes the fulcrum of the lever-type differential pressure transmitter to move, causing the instrument to drift in indication. Vibration causes the vortex flow sensor to generate interference signals corresponding to the vibration frequency, causing the flow indication to be significantly higher.

 

② Gas carries water. Compressed air is taken from the atmosphere, which always contains a certain amount of water vapor. The water vapor content is expressed by the water vapor partial pressure ps. The saturated partial pressure of water vapor in the atmosphere is a function of the atmospheric temperature (see Table 3.4). On rainy and foggy days, the water vapor partial pressure in the outdoor atmosphere reaches saturation, that is, the relative humidity reaches 100%. At this time, compressing the atmosphere is like compressing a sponge that has absorbed enough water. As the volume decreases, a corresponding amount of water will precipitate. This is the simple principle of why compressed air carries water. On sunny days, the relative humidity of the atmosphere is low, but as it is compressed and the volume is reduced to a fraction of the original, the water vapor partial pressure will increase accordingly, and it is also possible to enter a saturated state and precipitate water droplets.

 

The larger-caliber orifice flowmeter used to measure the flow of compressed air often has water accumulated in front of the orifice, which will affect the measurement accuracy. There is often a section of water in the pressure pipeline, which causes the differential pressure measured by the differential pressure transmitter to be inconsistent with the differential pressure generated by the throttling device. These are common reasons for 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 on the surface of the annular chamber and the inner wall of the pipeline, making its surface rough. When the iron oxide produced by corrosion becomes dry under certain conditions, it is easy to fall off the inner wall of the pipe and be carried to the front of the orifice by the airflow, which will also affect the flow indication. Therefore, when stopping for maintenance, these powdery and blocky garbage should be removed.

 

③ Pulsating flow. Most of the outlet fluids of compressors and blowers contain certain pulsations. For example, reciprocating compressors show half-wave pulsations, as shown in Figure 3.13. It can be observed on site that the outlet pressures of compressors and blowers have obvious swings. Among them, the outlet pulsation frequency of positive (fixed) displacement blowers is relatively high, generally tens of hertz, while the outlet pulsation frequency of reciprocating compressors is relatively low, generally a few hertz. Flow pulsation causes the indications of various flow meters such as differential pressure flowmeters and vortex flowmeters to be high, 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 pulsation on the indication of the flow meter. One is to set a buffer tank at the outlet of the compressor to filter out the pulsation, and install the flow meter behind the buffer. In fact, the systems of reciprocating compressors are designed in this way; the second is to install the flow meter away from the pulsation source, so that the gas volume of the process pipeline and its pipe resistance can be used to form a low-pass filter to attenuate the pulsation.