Double/ Multi stage pump - When in a casing two or more than two impellers are revolving then it is called double/ multi stage pump. Source: Internet
It may be desirable to use minor modification of the solder pots (e.g. titanium liners or impellers) used in wave-soldering, to reduce maintenance cost due to increased tin-scavenging of high-tin solder. Source: Internet
Pump efficiencies tend to decline over time due to wear (e.g. increasing clearances as impellers reduce in size). Source: Internet