Sharing of approaches for diagnosing on-site air flow issues

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Yesterday, we received a report from the client------Asahi Glass Display Glass (Shenzhen) Co., Ltd., stating that the air flow meter was giving inaccurate readings. Upon going to the site for a detailed inquiry, the staff there said that since this flow meter was installed in 2011, its measurement data has hardly been checked. Recently, the company is conducting energy management assessments, which has led to an increased focus on monitoring air flow. Currently, three air compressors are in operation in the air compressor station; the rated flow rate for each compressor is 44 Nm3/min (as indicated on the nameplate). However, the instantaneous value read by the flow meter is often between 1500 and 1600 Nm3/h, which is equivalent to 25 to 26.7 Nm3/min – there is a significant difference. (It should be noted that, according to the staff on site, this flow meter measures the flow rate in the main pipeline.) On-site inspection: A thermal gas mass flow meter was installed on a main pipeline with a DN200 diameter. Due to its high installation location, the signal is fed into the PLC. The instantaneous flow rate displayed on the PLC screen was monitored for 3 minutes; it remained very stable, varying basically within the range of 1500–1600 Nm3/h. To identify the problem, the company’s on-site maintenance staff climbed up using a ladder to reach the flow meter installation site. Check the display of the on-site flow meter. It also varies within the range of 1500–1600 Nm3/h, which rules out signal transmission issues. The maintenance staff reported that there was a valve at the installation location of the flow meter; I climbed up to take a look myself and found a butterfly valve about 0.3 meters behind the flow meter. The handle felt relatively loose; when the butterfly valve was opened to its maximum position, there was no change shown on the flow meter, indicating that the valve had no effect on the flow meter. Based on various indicators and a thorough understanding of thermal gas mass flow meters, it is believed that there should not be any major problems with the flow meter; instead, the focus should be on process-related factors. Due to the messy layout of the pipelines on site and their complex routing, I asked the staff there about the location of the gas storage tanks. There were 4 such tanks on site; by following the exhaust pipes leading from these tanks, it was found that the thermal mass flow meter was installed only on Tank No. 1, while no flow meters were installed on the exhaust pipes of the other three tanks. As shown in the figure: The only flow meter on site was installed during the first phase of the system construction at the start of plant operation. As the production scale of the plant increased, the original air compression system became insufficient to meet the demands, so a second-phase air compression system was added. Due to these series of changes, the employees on site did not understand the actual function of the flow meter; they thought it measured the total volume of air in the entire air compression system. In reality, however, this flow meter only measures 1/4 of the total air volume in the entire air compression station (this is a theoretical value, as the actual air volume is related to the actual consumption of air at the downstream end). The total air output from the three compressors is 44 X 3 = 132 Nm3/min, which equals 132 X 60 = 7920 Nm3/h. The theoretical value for one compressor is slightly over 1,000 cubic meters, which confirms my assumption. Later, the head of the electrical department agreed with my view, saying that the volume of gas output by that flow meter should be correct based on the actual load at the site. Thoughts: When encountering problems on-site, never be misled by the surface symptoms reported by the customer, and do not arbitrarily suspect the flow meter. Consider the issue comprehensively and work through it step by step until the root cause is identified.