How flow resistance affects pressure, and why bore, demand and supply conditions matter when comparing readings.
Static pressure and running (dynamic) pressure
Static pressure is the reading with the outlets shut and nothing flowing. In a gravity supply it comes from the head of water; a mains-fed or pressurised system has its own source of pressure. Running pressure is measured with water flowing at a stated rate. Pipe walls, fittings and valves resist the flow, so pressure is lost along the way and less is left downstream. A pump starting or a changing supply can upset the comparison, so record where you took the reading, the flow, and which outlets were running.
Why friction loss grows so quickly
Friction loss rises sharply as water moves faster. Forcing the same flow through a smaller bore raises the velocity, because flow rate equals velocity multiplied by the cross-sectional area — shrink the area and the water must move faster to get the same volume through. That higher velocity brings a much bigger friction loss: for a fixed length of pipe, resistance to flow rises steeply as the bore shrinks, so even a small reduction in pipe size makes a large difference to what survives as pressure at the outlet.
What this means on site
- A good static pressure reading does not guarantee a strong flow: once water moves, friction losses can use up much of what looked available.
- Opening several outlets at once increases the combined flow through any shared pipework, which raises velocity and friction loss there and weakens what each outlet receives.
- A long run, a small bore, or extra bends and fittings all add friction loss, on top of whatever static head the system has.