Centrifugal air blowers have many uses.  The most common is to take filtered air and send it to a process, such as wwtp aeration tank diffusers.  Many industrial applications such as air knifes and water filter backwash also use filtered air through the blower.  However, vacuum system & exhauster applications are designed to suck up dirt, water, and all kinds of other impurities.  When their pre-filter systems fail, or become compromised, the "foreign material" really does a job on the impellers.

Impellers from a blower installed at an AC Spark Plug plant were worn so thin by the ceramic, you could literally put your finger through the impeller.  Landfill gas and water can cause so  much corrosion on the steel shaft & aluminum impellers that the impellers must be destroyed to get them off the shaft.  How bad does the impeller have to be to justify replacement? 

The key word here is justify.  If a blower suffered a severe failure that caused the impellers to rub on the casing, it is probably best to always replace all impellers.  But impellers cost $1000 each, or more. Can any of them be re-used?  Yes.  If the rubbing is minimal, or non-existent, and there is no evidence of cracking, or keyway/bore damage, dirt buildup, imbalance, and the impeller was not subjected to excessive heat and pressure during removal, than you can probably re-install the impeller. 

My rule of thumb is to never re-use an impeller that has had enough aluminum rubbed off that it could affect the balance.  Not because it would need to be re-balanced, but the severe rubbing could have caused microscopic hairline cracks inside the impeller that may not be visible on the surface.  An invisible crack may not weaken an impeller enough to fail right away, but it may "blow apart" within a year.  It is just too much of a gamble. 

In a pinch, with no replacement impellers available, I've used blue dye to confirm no hairline cracks exist.  And, of course, then re-balance the impeller.  Although the outer diameter is usually where any scraping appears, the front face of the impeller near the bore, can have more severe rubbing. Very close inspection of the impellers are a must.  

Identifying worn bearings before they fail, as well as simply replacing air filters regularly, are much more easily justified than having to replace a rotating assembly.   Invest in PM, it will save you money! 

Blower Performance & Design