Nib asked
How do skyscrapers get water to the top?
No pump pulls water up a tower and no single pump pushes it all the way: water climbs in stages, with a pump and a tank every stretch, because each foot of height adds weight and pressure.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
A skyscraper has taps a hundred floors up, yet the water main under the street can usually push water up only a few floors on its own. Something in the building has to do the lifting.
The city's own pressure only reaches a few floors.
check yourselfIf the city main only reaches a few floors, what has to lift the water higher?
2no suction
A pump at the top of a pipe does not pull water up. It only lowers the pressure there, and the air pressing on the supply below pushes the water in. One atmosphere can hold up a column of water about 10.3 metres tall, so suction can never lift water higher than that, however strong the pump.
Suction tops out near 10.3 metres, so tall buildings push water up from below.
a common mix-upA pump does not suck water up a skyscraper. Air pressure can only hold up about 10.3 metres of water.
check yourselfWhy can a suction pump never lift water more than about 10 metres?
3the weight
Water has weight, so a column of water presses on whatever is under it. Each foot of height adds about 0.433 pounds per square inch at the bottom, which is the same as saying 2.31 feet of water for every pound per square inch. Double the height and you double the pressure.
Pressure at the bottom grows by about 0.433 psi for every foot of water above it.
check yourselfHow much pressure would a 100 foot column of water make at its base?
4too much
The Burj Khalifa is 829.8 metres, or 2,722 feet, tall. An unbroken column of water that high would press about 1,180 psi on the bottom. The International Plumbing Code asks for a pressure-reducing valve whenever the pressure inside a building would pass 80 psi, and 80 psi is only about 185 feet of water. One pump pushing straight to the top is therefore not an option.
One unbroken column to the top would mean about 1,180 psi at the bottom, far above the 80 psi limit.
check yourselfAbout how many feet of water make 80 psi?
5in stages
A tall building does not use one giant pump. It is split into stretches. Pumps at the bottom lift water to a tank partway up, more pumps there lift it to the next tank, and so on. Each stretch only has to carry the weight of its own water.
Water climbs a tower in stages, a pump and a tank every stretch.
6seven stops
The Burj Khalifa, the world's tallest building, has seven two-storey mechanical floors spread up its height, where its pumps and water tanks sit. Its water supply averages just under 250,000 gallons a day. In a tank-fed design, the water in a tank simply flows down by gravity to the floors beneath it.
The Burj Khalifa lifts water through seven mechanical floors, and gravity does the last stretch down from each tank.
check yourselfWhy does each stretch of the tower need its own pump and tank?
A skyscraper's water never makes one giant leap: it is passed up from tank to tank, each pump lifting only a small slice of the height.