Less Friction, More Pressure

As I have related in a couple of recent blogs, I recently had my house re-piped. Old galvanized pipe was replaced with a combination of hard copper and “PEX” plastic tubing. PEX is a form of polyethylene (PE) with additional “cross-linking” (the “X” in PEX) to increase the strength of the plastic. PEX is particularly well-suited for repiping jobs because it’s flexible, and can thus more easily get “snaked” through existing wall spaces and around existing utilities (drains, electric, phone, etc.).
I’ve noticed an increase in household water pressure since the repiping, so I thought this would be a good time to talk about friction. No, not the kind between you and your supervisor, co-workers, or spouse: the hydraulic kind.
The pressure at a faucet or plumbing fixture is one of the great benefits of a public water system. Not only do we provide our customers with clean, healthful drinking water, but we also provide them with water pressure. This pressure means that they don’t have to use buckets to lift water into the toilet tank, washing machine, etc.; the pressure delivers the water to the intended location. But where does the pressure come from? And why do some experience lower pressure than others?
For me, pressure is easiest to visualize as the height of the water above the user. Mathematically, every 2.31 feet of height is equal to 1 psi of pressure. Typical household pressure is about 40 psi, which is pretty close to 100 feet of water height. In most cases, this height – which a physicist would call “potential energy” – is provided by pumping our water up to a storage tank at a level above our customers. Our piping systems contain this pressure – which is one reason that our pipes need to have significant strength.
Customers that are at relatively high elevations get less pressure than those at lower elevations, because there is less total height of water above their taps. So the pressure that a customer has at their house depends on their vertical location within the system.
But moving water through pipes requires energy expenditure – there’s no free ride. The energy lost in transit is called “friction.” And we see this energy loss as a drop in pressure: some of our potential energy was “spent” moving the water through the pipe.
The amount of friction lost in transit is a function of many things, including the length of the transit, the diameter of the pipe, the “smoothness” of the interior of the pipe (known as the “C” factor), plus the number of twists and turns and valves and other items that the water must flow through or around. Another extremely important factor is how fast the water is moving: as water velocity increases, friction increases even more. Looking at our distribution systems, customers generally have more pressure at night than during the middle of the day. This is because water flows – and therefore water velocities – are much higher during midday.
Back to my household pressure increase: I can point to two friction factors that were improved. First, the corroded old pipe resulted in a smaller effective diameter of the pipe – in essence, I now have larger pipes, at least on the inside. And second, plastics like PEX have extremely smooth surfaces. Both of these factors have made it easier for water to pass through my household plumbing, reducing friction, and increasing water pressure at the fixtures.
And now, I can’t take the pressure!