Where does the water pressure in my house come from?

[ZenBeam]

I had looked at DWSD’s site last night, but digging deeper, I found a cite. From the Detroit Water and Sewerage Department’s
Master Plan page, Task C: Water Supply and Service Management Plan (PDF), :

@Water Supply and Service Management Plan

4.1 DWSD’s Existing Water System
DWSD presently provides treated water to nearly 4 million people in 126 suburban communities as well as the city of Detroit in southeastern Michigan. More than 230 billion gallons of water are pumped each year. All variations in DWSD’s customer water demands, including peak hour demands and fire flows, are satisfied through direct pumping. To maintain desired operating pressures, DWSD uses variable speed pumps and throttled pump discharge valves at pumping stations and at WTPs [Water Treatment Plants].

Currently, DWSD does not operate any elevated storage reservoirs to assist in meeting peak hour water demands. Instead, DWSD uses offline ground storage reservoirs located at most of the pumping stations to provide additional water beyond the amount that is pumped from WTPs. To fill the offline ground storage reservoirs, the current mode of operation requires water to be “bled” through throttled fill valves from the transmission system.

[Si_Amigo]

@ZenBeam

I had looked at DWSD’s site last night, but digging deeper, I found a cite. From the Detroit Water and Sewerage Department’s
Master Plan page, Task C: Water Supply and Service Management Plan (PDF), :

I am sure this fascinates everyone but I must reply. Needless to say the city of Detroit proper’s water supply is lacking many things but I wasn’t aware that towers were one of them. Their question and answer section implies that they do have some elevated tanks, but I suspect that most of these are being used as band aids and are not really towers. Still the Master Plan says they serve 4 million people and a lot of those people are served by water towers (I can see the Novi and Northville towers from home); but suspect that individual cities run these on their own. Nothing about Detroit surprises me anymore.

[suranyi]

@Xema

Here’s a (pdf) map of the water system that serves Oakland. Note that there are indeed several reservoirs in the hills north and east of the city (towers not needed there - the hills are plenty high).

In fact the water supply for the entire Bay Area is essentially the Sierra Nevada mountains, so the water is always under pressure naturally, just from the elevation difference from the mountains to the metro area. Reservoirs just hold the water at different levels on its journey.

[friedo]

@Hari_Seldon

The water in NYC comes from the Catskills and I believe that all its pressure comes from gravity flow.

The gravity-fed system in NYC provides enough pressure throughout almost all of the city to get up to about five stories. For any buildings taller than that, water tanks on the roof are used. There are a few small areas where pumping stations are used for ground-level pressurization.

NYC’s water system is truly a marvel of the modern world. It’s a shame it’s not more widely appreciated.

[Colophon]

Water towers seem to be a common sight in the American landscape, but I have hardly seen any in the UK. Does anyone know how UK mains are pressurised? Is it by pumps?

[aceplace57]

A lot of people don’t realize they have a water regulator. Around here it’s typically in the ground between the meter and the house.

In areas with basements the meter and regulator are probably there.

Water regulator protects your pipes from the citys higher pressure. Without it your pipes could burst.
http://www.atlantisplumbing.com/water-pressure-regulators.php

[robby]

@robby

…You can’t have water towers without pumps. You can have pumps without water towers, but would always need to keep the pumps running (which is not generally practical).

@Robot_Arm

I thought it was the other way around, kind of. If all your pressure comes from pumps, you need to have enough to meet the peak demand (when everybody is taking their morning shower, I would think) and then most of them sit idle the rest of the day. With a tower, the pump is sized for average demand; the level in the tank drops during peak demand and then it gradually refills.

I’ve also heard you don’t want the water sitting too long in the tank. After it’s been treated, you want the water used quickly before the treatment chemicals wear off and bacteria have a chance to start growing again.

What I was trying to say is that a water tower (in and of itself) does nothing for you. You have to first pump the water into the tower to get any benefit from it.

The other alternative (assuming you are not lucky enough to have reservoirs at a higher elevation than your service area) is to run pumps continuously. As has been noted, this is what Detroit apparently does, using variable frequency pumps. You still have to run the pumps continuously, though, so your electricity costs are going to be high.

Your other points regarding water towers are also valid.

[Musicat]

I’m sure a qualified hydrology engineer will come along to correct what I say or modify it, but the way I see the problem is that water is not easily compressible. Ordinary pumps can lift water or push on it, but it’s difficult to provide a constant, even, reliable pressure of a certain amount with a pump alone.

However, it’s pretty easy to provide the same constant, even, reliable pressure if you let gravity do it, or enclose the water in a tank which also contains a compressed gas like air – a substance which is highly compressible.

You pump water into a tank, even in spurts, or compress air into a tank at irregular intervals, and the resulting gravity or air pressure evens out the flow to a workable level.

[Xema]

@suranyi

In fact the water supply for the entire Bay Area is essentially the Sierra Nevada mountains, so the water is always under pressure naturally, just from the elevation difference from the mountains to the metro area.

That elevation difference helps to transport the water to nearby reservoirs. But the water pressure in Oakland is not related to the altitude difference between it and the Sierra Nevada mountains.

[robby]

@Musicat

I’m sure a qualified hydrology engineer will come along to correct what I say or modify it, but the way I see the problem is that water is not easily compressible. Ordinary pumps can lift water or push on it, but it’s difficult to provide a constant, even, reliable pressure of a certain amount with a pump alone.

However, it’s pretty easy to provide the same constant, even, reliable pressure if you let gravity do it, or enclose the water in a tank which also contains a compressed gas like air – a substance which is highly compressible.

You pump water into a tank, even in spurts, or compress air into a tank at irregular intervals, and the resulting gravity or air pressure evens out the flow to a workable level.

First off, you mean hydraulics, not hydrology.

Secondly, the type of pump that you are describing sounds like a positive-displacement pump. However, a centrifugal pump would be more commonly used in this application, and would be capable of providing steady pressure, even with an incompressible fluid like water. The downside, as I mentioned previously, is that you cannot ever shut all of the pumps down.

[Snnipe_70E]

@robby

First off, you mean hydraulics, not hydrology.

Secondly, the type of pump that you are describing sounds like a positive-displacement pump. However, a centrifugal pump would be more commonly used in this application, and would be capable of providing steady pressure, even with an incompressible fluid like water. The downside, as I mentioned previously, is that you cannot ever shut all of the pumps down.

In a large city water will be used at all times. Someone will be flushing a toilet, getting a drink of water, or some plant will be using water.
So not being able to shut off all the pumps is not a problem.

[guizot]

@Lance_Steele

Yeah that all makes sense. But I feel like in order to supply enough water to everyone in an urban area like where I live, there’d have to be water towers everywhere, yet I rarely see them.

This is what I’ve always wondered about. I occasionally see big tanks, but it just doesn’t seem like there are enough of them around for all the water that people in a city must be using. Especially for cities that are located in the middle a vast expanse of flat land, with no hillside reservoirs around. Isn’t the distance from the tank and the number of customers that can be served from a single tank limited if a certain pressure is to be provided?

[DHMO]

I have been a Water Treatment professional for 18 years—7 years in Florida and 11 years in Colorado.

For Florida, with almost no elevation, and the raw surface water sources usually being at the lowest point in the area, all pressure in the distribution network was accomplished by high-capacity pumps at the Treatment Plants. They would maintain 90-120 psi leaving the plant, and the pressure would taper off as you approached the periphery of the network. We had elevated storage tanks and re-pump stations at strategic locations in the community, which would empty during high demand periods (morning getting-ready-for-work and evenings), and would fill during low demand (1-5 AM, before the morning rush.)

In Colorado, most of the water comes from reservoirs in the foothills, several hundred feet higher than most of the area we serve, and gravity takes care of the bulk of the pressure. The same issues apply, however, and we have elevated tanks and re-pumps located in low-pressure areas throughout the distribution network.

The Chlorine residual leaving the plant is seasonally adjusted to the demand, so that at the limits of the outermost taps in our distribution, there will be sufficient disinfection to prevent bacterial growth.

Here in Denver, during the peak summer demand, the combined output of the three treatment facilities can exceed 500-600 million gallons per day, so yes, the pumps are running pretty much constantly. The elevated storage tanks are there mainly to provide pressure, but do not contain enough volume to be a primary source. We maintain several hundred million gallons of treated water in clearwells at the treatment plants, and around the community in reservoirs equipped with pumps and Chlorination equipment to satisfy the demand 24/7/365.

[robby]

@Snnipe_70E

In a large city water will be used at all times. Someone will be flushing a toilet, getting a drink of water, or some plant will be using water.
So not being able to shut off all the pumps is not a problem.

Except that you then have to take that into account with respect to maintenance (both preventive and corrective). Also you may need more backup pumps.

Finally, your electricity costs will be higher due to constantly running the pumps. Some utilities are able to save money by pumping water up into towers at night when demand is low and electricity costs are low, then using the stored water the next morning when demand peaks.

It’s not that constantly running pumps is a “problem,” per se; but you do have to consider the various tradeoffs involved.

[Snnipe_70E]

@robby

Except that you then have to take that into account with respect to maintenance (both preventive and corrective). Also you may need more backup pumps.

Finally, your electricity costs will be higher due to constantly running the pumps. Some utilities are able to save money by pumping water up into towers at night when demand is low and electricity costs are low, then using the stored water the next morning when demand peaks.

It’s not that constantly running pumps is a “problem,” per se; but you do have to consider the various tradeoffs involved.

For a large city it would take thousands of tanks.

[TriPolar]

@Snnipe_70E

For a large city it would take thousands of tanks.

Large cities usually have thousands of tanks. They’re on top of the buildings, which need to use tanks to maintain water pressure above the first few floors.

[R.P.McMurphy]

@friedo

The gravity-fed system in NYC provides enough pressure throughout almost all of the city to get up to about five stories. For any buildings taller than that, water tanks on the roof are used. There are a few small areas where pumping stations are used for ground-level pressurization.

NYC’s water system is truly a marvel of the modern world. It’s a shame it’s not more widely appreciated.

That’s mostly correct, 5 stories and all. There are a lot of tanks on top of higher buildings that are in use and maintained to provide pressure. Proper maintenance of those tanks is an ongoing business. However, in many more modern buildings electrical pumps are used without having to have the holding tanks.

And yes, the gravity fed system for water delivery is a marvel. The Romans understood that when they built the aqueducts.

[Dag_Otto]

I’d also guess that the tanks on top of buildings are not the water utility’s equipment. The point of demarcation is probably the water meter at or near ground level. Everything past the meter belongs to the building.

[Meeko]

@AndrewL

Water towers. Great big tanks of water up on stilts. Pumps run all day to push water up into them, and when you open the faucet water flows downhill through the pipes and out of the tap.

:eek:

I actually wondered about two weeks ago, what purpose water towers served. If water needs to be stored, that wouldn’t explain why they are so high up.

Hence, I was going to ask why Water towers as such, existed.

Guess this one is answered before I ask it.

[ZenBeam]

@Dag_Otto

I’d also guess that the tanks on top of buildings are not the water utility’s equipment. The point of demarcation is probably the water meter at or near ground level. Everything past the meter belongs to the building.

I’d further guess that whether tall buildings have individual water tanks or just use pumps would be independent of whether the municipal supplier has water towers or not.