Why should a positive displacement pump never be started against a closed discharge valve?

A positive displacement pump operates by trapping a fixed volume of fluid and then forcing it through the discharge line with each cycle. If the discharge valve is closed while the pump is running, the fluid has nowhere to go, which leads to an increase in pressure within the pump and discharge line. This excessive pressure can exceed the pump's design limits, potentially resulting in catastrophic failure, such as rupturing the pump casing, rupturing piping, or causing seals to fail. These types of failures can be dangerous and costly, resulting in not only equipment damage but also safety hazards for personnel.

In contrast, the other options present scenarios that do not correctly address the primary concern with starting a positive displacement pump against a closed discharge valve. Reducing energy consumption is not applicable in this context, as the pump will likely consume more energy while trying to overcome the blockage. While regulatory compliance may indeed have provisions related to safe operational practices, the fundamental engineering principle driving the need to avoid running against a closed valve is the risk of excessive pressure and the potential for damage it brings. Improving flow rate is irrelevant when the discharge is blocked, as it directly contradicts the function of the pump in this scenario.