How is the pressure known that does not change even when water is flowing?

The correct answer is static pressure. Static pressure refers to the pressure exerted by a fluid at rest or in a system where there is no flow. It remains constant regardless of whether water is flowing or not, making it crucial for understanding how systems behave under different conditions.

In fire protection systems, static pressure is often measured when the system is not flowing; it provides a baseline measurement that helps determine the possible performance of the system when water does flow. This is essential for ensuring that adequate pressure will be available during an emergency response.

Residual pressure, on the other hand, is the pressure remaining in the system when water is flowing, typically measured at a specific point during a discharge event. It accounts for pressure losses due to friction and other factors, which makes it different from static pressure.

Cyclic pressure refers to fluctuations in pressure that can occur in systems, and dynamic pressure is the pressure associated with the movement of a fluid, which is influenced by the flow rate and velocity. Both of these concepts are not applicable to the scenario of pressure that does not change during flow. Thus, static pressure is the relevant term for understanding pressure that remains consistent.