What should never be done before starting a positive displacement pump?

The correct answer is to start with the discharge valve closed. This practice is crucial when operating a positive displacement pump because starting the pump against a closed discharge valve can lead to extremely high pressure build-up within the pump and the piping system. Positive displacement pumps work by capturing a fixed volume of liquid and forcing it into the discharge line, and if the discharge is closed, there is nowhere for the liquid to go. This can cause mechanical failure, damage to the pump, or even rupture of the discharge pipe due to excess pressure.

Ensuring that the discharge valve is open before starting the pump allows for proper flow and prevents any dangerous pressure build-up within the system. It's essential to always follow operational guidelines for starting positive displacement pumps to ensure safety and effective operation.