How is the gap between the fan inlet and the impeller determined?

According to the **standard of the People’s Republic of China: “Code for Construction and Acceptance of Compressor, Fan, and Pump Installation Projects” GB50275–98, when the equipment’s technical documents do not specify otherwise, the axial insertion depth should be 10/1000 of the outer diameter of the impeller； The radial clearance should be uniform, with its value ranging from 1.5/1000 to 3/1000 of the impeller’s outer diameter (the higher value applies when the outer diameter is smaller). High-temperature fans should also account for thermal expansion.

I would like to ask the expert: I’m also facing the issue of a gap between the centrifugal fan’s impeller and the air outlet, which results in low airflow from the fan. I checked the \"Code for Construction and Acceptance of Compressor, Fan, and Pump Installation Projects\" GB50275–2010, and it states that the axial insertion depth should be 8% to 12% of the outer diameter of the impeller, while the radial clearance on one side should be 1.5% to 4% of the outer diameter of the impeller. But the outer diameter of my impeller is 710 mm, and the calculated gap value is several dozen millimeters – that doesn’t seem right. What should it be, experts? My fan was manufactured in Denmark in 1985; there are no documents or drawings available. A few days ago, the bolts holding the impeller cover broke, and the inside of the fan was in a mess. So I created drawings of the impeller and the air inlets, but the airflow volume remains low.