Understanding Residual Pressure in Water Supply Systems
Introduction to Residual Pressure
When a building is supplied with water, the main entrance water valve sets the static pressure at, for example, 70 psi when closed. This static pressure is typically equivalent to the pressure at a city fire hydrant. However, when all faucets and valves in the building are fully open, the pressure at the end of the supply pipe is known as the residual pressure. If this residual pressure is close to the static pressure, it indicates that the water supply line is functioning optimally.
Residual pressure refers to the pressure at the end of a water supply line in a system with an active flow. This is a critical metric for ensuring sufficient water pressure remains to serve various needs, such as filling a fire hydrant or meeting the height requirements for a building.
Defining Residual Pressure
Residual pressure is the pressure that remains in a water supply system when all supply accessories (faucets, valves, tubs, etc.) are fully open. This pressure should be nearly equal to the static pressure at the entrance valve. If the residual pressure is close to the static pressure, it indicates a reliable water supply for the system. In contrast, if the residual pressure is significantly lower, it may indicate a need for pressure boosters or other improvements.
Minimum Pressure in Water Supply Systems
The minimum pressure in a water supply system is influenced by peak conditions, such as high water flow rates. This minimum pressure can vary based on the perceived usage of the term and the operator's experience. Static pressure is the height difference between the tap and the supply water tower. For example, a 70-foot (21.3 m) height difference would result in an approximate 30.3 psi static pressure, which would be slightly less when the water is flowing.
Basal and Static Pressure
The terms 'static' and 'basal' are used to describe the pressure in a water supply network. 'Static' refers to the system at rest, while 'basal' refers to the pressure at any given time. Water supply networks are never truly static, as they always have some flow. The pressure varies during the day, but is usually controlled by pumps, keeping it more stable.
Residual Pressure in Water Supply Testing
Residual pressure is a key metric in water supply testing. For example, during a hydrant flow test, a waterworks operator might measure a baseline pressure of 120 psi. When a hydrant is opened, the pressure drops to 110 psi. This 110 psi is the residual pressure, indicating the pressure available for other needs such as serving a building's height, sprinkler system, or plumbing system. It also helps determine if a fire pump or booster pump is necessary.
Proper testing is crucial to obtain an accurate residual pressure reading. Testing should be done at peak demand times, such as around 3:00 PM when the flow is highest, to ensure the lowest possible basal pressure. Testing during off-peak demand times, such as the pre-dawn hours, would yield a higher pressure reading, which does not reflect the most likely worst-case scenario.
In conclusion, understanding residual pressure is vital for ensuring reliable water supply systems. By knowing and maintaining residual pressure, water supply networks can better serve their communities and meet their needs, especially during high demand periods.