At which point in operation does a pump seal primarily prevent air from entering?

The primary function of a pump seal in preventing air from entering occurs when the pump is operating under negative pressure. In this scenario, the pressure inside the pump is lower than the atmospheric pressure outside. If there is a failure in the sealing mechanism during this condition, it could allow air to be drawn into the pump, leading to cavitation, loss of prime, and reduced efficiency. The effectiveness of the seal is critical here, as any breach can compromise the entire operation of the pump system by allowing air to mix with the liquid being pumped, which can cause various operational issues.

In contrast, during the priming process, while ensuring that the pump is filled with liquid, the focus is more on achieving and maintaining a full pump housing than specifically preventing air ingress. When the discharge valve is open, the system is designed to allow flow under normal pressure conditions, and the primary concern is more with maintaining appropriate flow rates rather than sealing against air entry. Lastly, when the pump speed is low, the pressure conditions could be favorable, but negative pressure situations are more critical as they directly relate to potential air entry through sealing failures. Thus, understanding that air ingress is a significant concern during negative pressure conditions highlights the importance of effective sealing in pump operation.