What is flow (or residual) pressure defined as?

Flow pressure, or residual pressure, refers to the pressure measured within a piping system while water is in motion, typically during a flow test. This measurement is taken to determine how much pressure is available to overcome friction losses and to ensure that adequate pressure reaches fixtures and outlets during normal water use. When water is flowing, various factors such as pipe diameter, length, and any restrictions in the system affect the pressure due to the dynamic conditions present at that time.

In contrast, when water is static, the pressure measured (often referred to as static pressure) may be higher than the actual flow pressure because it does not account for the pressure drops caused by flow resistance. Understanding flow pressure is crucial for effective design and troubleshooting in plumbing systems, as it directly impacts the performance and reliability of water distribution to various fixtures and appliances.