Every podunk town in America seems to have a water tower, but do they actually serve any purpose other than to advertise the town’s slogan or high school sports team? I can’t imagine that even the biggest of these towers can supply the whole town, and even so, what’s the purpose of having elevated towers rather than storing water below ground? There’s probably a simple explanation behind all this, but until someone points it out the suspense is killing me.
The water pressure in the town is supplied by gravity, and it’s nice and constant.
Put water up in tower…water wants to run downhill…A heavy weight of water = pressure without having to constantly run a pump to create that pressure (the pump only runs to ‘top up’ the tower as required).
A water pump that’s powerful enough to supply 100% of the demand at peak times would be large and expensive. And when it’s not at peak, it would be inefficient. A way around this problem is to employ a “capacitor,” which is the water tower. The pump will pump water into the tower during times of light demand (e.g. the middle of the night). During times of peak demand, the water tower will provide the necessary pressure.
There are also lower tanks around, not always on the same property, for capacity that can be readily pumped back into the tower faster than the original source. And they may be hard to spot, being 20-30 feet high and behind buildings or screening trees.
Water towers use mother nature’s own unrefined pressure to drive water to end users, instead of pumps. Of course there has to be a main pump to get it from the treatment plant up to the tower, but all in all a much simpler design for those communities that can take advantage of it.
The towers can supply quite a large number of simultaneous end users, but certainly not the whole town at once. Hopefully the system was engineered to accomodate the average anticipated demand, plus a few hundred extra GPM for padding.
Like CurtC sez…
I’m surprised nobody’s posted this yet:
There are some towns so bereft of any significance that all they have to boast about is their water tower. Take a ride down the Garden State Parkway, and watch for the sign welcoming you to “Union, New Jersey, home of the worlds’s tallest watersphere”. Look up, and, sure enough, there is a very tall, spherical water tower.
Sad.
So let me get this straight: even though the pumps have to expend energy to get the water up into the towers, the towers are still more efficient because they can pump the water up there during periods of light demand, whereas otherwise during periods of heavy demand the water pump would have much greater difficulty dealing with the demand than the natural gravity of a tower? This seems to be what is being said but I just want to make sure I’ve got it straight.
The water tower is typically filled during the middle of the night. Water usage drops drastically from about midnight to 4am. The pump can slowly fill the elevated tank (that’s what engineers and the like call water towers) during this time with minimal pumping effort. A pump is basically just a motor with an impellor attached to the long drive shaft, which sticks down into a pit full of water and provides the pumping force. Like any motor, water pumps have an efficiency curve. The ratio of power in vs. power out is much greater at lower speeds than at high speeds.
So using an elevated tank does two things for you. One is that it gives you cache of water to draw from during peak hours when the water demand for the city/municipality may be greater than how much water per hour the supplier can provide to the city. So the elevated tank doesn’t have to supply enough water for the entire town, it just has to make up the difference between the incoming water supply and the outgoing water demand. The second function is that they provide additional water pressure. It’s not efficient to run your pumps at full speed during the day and then practically turn them off at night. Instead, you can “offload” some of the pressure to the elevated tank and run the pumps at a slower, more efficient speed. The trade off is that you also have to run them during the night. But the tank helps you average out the pump’s work over a longer period of time, which is more efficient, and probably saves wear and tear on the motor.
Additional factoids:
- Water pressure is often measured in “feet of head”. Meaning that water pressure due to gravity is varies only with the height of a column of water, regardless of the quantity of water. A 2,000,000 gallon tank 100’ tall provides just as much pressure as a 50,000 gallon tank 100’ tall. The conversion rate is roughly 1 p.s.i. = 0.43 feet of head. So a 100’ tall tower will provide roughly 43p.s.i. at the base. To fill the tank, a pump sitting at it’s base will have to pump at 43p.s.i.
- Pump stations and elevated tanks are usually located right next to each other at the highest point in a city, not to show off the tower, but because you get “free pressure” from gravity. If all of the demand is downhill from the pump station, it doesn’t have to work as hard to get meet the pressure requirements.
- You’ll also usually find ground storage reservoirs (a tank of water sitting on the ground) at the pump station next to the elevated tank. The GSRs are mostly used to fill the elevated tank, if there is one. They provide extra capacity, but no appreciable pressure. The flip side is that it doesn’t take much pressure to fill them up, either.
Er, that should have said the the ratio of power in to power out is much greater at higher speeds than at lower speeds. Slower is more efficient (generally speaking). And I misspelled impeller :smack:
I’m surprised nobody’s posted this yet: Why are water towers are elevated, while petroleum tanks are on the ground? 