Premature failure of a vacuum pump impeller rotor recovered from a pitch impregnation plant

A vacuum pump that was integral to a pitch impregnation system had rapidly deteriorated, leading to compromise in quality of products being manufactured. It was found that a particular vacuum impeller rotor group had worn, with unacceptable vacuum levels being the result. A replacement rotor was installed whilst cause of premature failure was investigated. However, in a matter of just months of service, the replacement impeller group also deteriorated. Finding the underlying mechanism of failure and developing a cost effective solution to the problem became paramount in terms of both product quality and rapidly increasing production cost. It was determined that the root cause of failure was stress corrosion cracking (SCC). The leaded red brass vacuum pump vane rotors had become sensitised as a result of ammonia liberated directly from the coal tar pitch impregnation medium. It was determined that changing the material of construction to a stainless steel alternative would not provide a cost or process effective solution. Spray coating the brass vane rotors with an epoxy coating presented a quick simple and cost effective solution to the problem.