In the June 2012 issue of Reed, David Widelock ’69 replied to my “Burning Question,” saying I should have kept doubting the fireman’s statement that with higher pressure the rate of water flow decreased. David cites his experience as a landscape architect and the Golf Engineering Associates Technical Help Series, which says, “Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.”
David and golf engineers are certainly correct that higher pressure at the source will increase rate of flow through a pipe. But what happens downstream can be surprising. A thought experiment will illustrate. Imagine you are watering your lawn with a garden hose. The valve to which the hose is connected is fully open, but you close the nozzle so water doesn’t flow at all. Now imagine you poke a small hole into the hose anywhere along its length. What happens? Water shoots out the hole and into the air. The higher the pressure in the hose, the higher the water will shoot up, but the rate of flow through the hose is close to zero. Now imagine you open the nozzle fully (or take it off completely). The rate of flow through the hose shoots up, but what happens at the hole you poked into the hose? The water stops shooting up; pressure has dropped to just about zero. If the local water supply increases power to its pumps, then the rate of flow will increase, but pressure in the hose will remain near zero because there’s nothing to impede the flow.
If you don’t want to ruin a perfectly good garden hose, you can try the same experiment by pretending to blow a stopped-up trumpet. Try to force air past your closed lips: high pressure, but zero rate of flow. Now open your lips and allow the air to flow. What happens to the pressure?