During the summer, I often see kids playing by a fire hydrant. They were the ones who opened it and they enjoy themselves and whatnot. I have a couple questions on this;
Is it illegal to open a fire hydrant? What’s the penalty?
Does each hydrant have a limited supply? Or is it hooked to a central supply that unleashes it to wherever its needed?
It is highly illegal to open a fire hydrant unless you’re a fire fighter and there is a fire nearby. The penalty depends on local law; in a lot of places there is a simple lack of enforcement.
They are hooked up to the municipal water system. In some places, there may be a separate, high-pressure system dedicated to feeding the hydrants.
Hydrants are fed by common water mains beneath the roadway. (It is the same water that is delivered to the houses, shops, and factories along the street.)
It is generally illegal to open a hydrant. When the hydrant is opened, it simply lets all the water stream out at the pressure of the water line. One hydrant would probably do little more than waste some water, but allowing the practice encourages people all up and down the line to open the hydrant nearest them and many hydrants opened along the line will reduce pressure so that homes do not have water, factories do not have water, and in the case of an emergency, there would not be enough pressure for the fire department to control a fire.
A (very) few cities have purchased equipment to be threaded onto one of the valves, providing a controlled flow fountain for the neighborhood, thus reducing the amount of water lost to tampering while providing soe heat relief for the neighborhood kids.
If a hydrant is opened, do people call and say that a hydrant was opened or does something let them know? Usually everyone around the kids doesn’t mind but somehow the cops are always there within 20 minutes.
In addition to fire suppression, hydrants may also be used by municipalities for cleaning, lawn watering or other purposes. In any case, unauthorized use is prohibited both because this may lessen the pressure in the main, but also because the hydrant is not metered - the city pays for it. Most hydrants require a non-standard pipe wrench or pentagonal hydrant wrench to open. Not impossible to circumvent, but a deterrent for the casual would-be hydrant user.
It generally takes a phone call. There may be a few hydrants that have electronic monitoring, but most hydrants are already expensive enough without adding a power source (or an electric line) to something that some drunk driver or snowplow is going to knock over. People who need water or who want fire protection are going to be eager to keep unauthorized use to a minimum.
Municipal water authorities monitor pressure at various points on the grid, and have manual or automatic valving/booster pumps to increase pressure during peak demand.
Sprinkler systems which are connected to a central monitoring station may also react if supply side pressure drops greatly.
Most large cities have changed hydrant caps and stems from the pentagonal male to a flush triangular fitting to discourage unauthorized hydrant opening during hot weather. IIRC, Philadelphia Parks & Recreation will lend sprinkler devices to neighborhood persons so kids can cool off and play in the water without dropping mains pressure to unacceptable levels for fire suppression activities.
It seems to me that I have read that at least in New York City, the fire dept. used to open hydrants during hot summer days in Manhattan. Of course, that was way back when the city only had 6000000 people and the pressure on the water supply wasn’t as bad as now.
Among the many reasons that it is illegal to open hydrants is that you have to know how to furn them off. You have a large volume of water in motion when the hydrant is opened and it has a lot of inertia. Closing the valve too quickly can blow the hydrant right off the pipe.
You’re correct in theory, Sir. In practice, it takes a good half dozen turns + to shut down a hydrant and open the french drain below the frost line to drain the hydrant barrel, so hydrant shutdown won’t cause problems with ‘water hammer’.
We do address that point in pumper operations training because a high volume master stream (1500GPM) being shut down rapidly by an operator can damage the buried mains, and/or the pumper(s) in the supply.
With pumpers regularly exceeding $500K nowadays, intake and discharge relief devices are standard to protect the investment from damage resulting from a foolish pump panel moment. 
I read an interesting book on Philadelphia police life several years ago that was written by an ex Philly cop (1960s era).
He devoted a page or two to the whole issue of unauthorized hydrant use. He mentioned how all squad cars were equipped with a hydrant wrench specifically for the purpose of shutting illegally-opened hydrants, and that this was one of the more tedious parts of his job during the summer months.
He did sympathize with the children, pointing out that this was really the only relief many of them had from the suffocating heat; he even made mention of the special fountain device that tomndebb spoke of, so it may be a rare fixture these days, but that isn’t because it is a new fangled rare fixture.
On closer reading I see that danceswithcats already mentioned Philadelphia’s sprinkler heads. Well, my anonymous Philly cop book seems to agree.
It was always illegal.
However, the city supplies special caps to be attached to the hydrants that allowed them to be opened, but with a restricted flow. This article (dated today) indicates that they still supply the caps free of charge. Once you have the cap, you can use the hydrant for a sprinkler, but if you don’t have one, it’ll cost you a $500 fine.
Correction: the release was dated in 2003, but I’m sure the program still exists.
You can still get sprinkler caps in NYC. One of my jobs entails photographing real estate in the five boroughs, and during the summer months I partake of many a ‘Bronx carwash’; when the spray extends far enough into the street you can rinse off one side of your car by cruising slowly through.
It’s not perfect, of course. If you want to do the other side you’ve got to find a street with another open hydrant on the proper side. Turning around at the first one just pisses off the kids.
Now that we’ve had thorough discourse on urban hydrants, and the reasons why their use by kids alleviating summer heat is discouraged, I want to add one item on quasi-rural fire hydrants because of a peculiar term used for some of them, which may come up in discussion.
A dry hydrant is just what it sounds like – a hydrant which, when opened, will not emit water under pressure, but no flow whatsoever.
The reason for them is that they are the roadside terminus of a buried pipe which leads back to the bottom of a nearby lake, pond, or year-round stream. A pumper connected to the dry hydrant will draw water up through that pipe, enabling it to be used for firefighting, but when not in use, the hydrant auto-dries itself by gravity.
They are usually hooked up to the same mains as the potable water lines (though as mentioned, a dedicated fire main can be used) and as such the volume of water available is limited only by the city’s supply. In many areas, businesses (usually factories, warehouses, etc.) may be required to install a tank in order to store fire-fighting water on site so that the city’s water pressure isn’t adversely affected.
Now, the rate at which a hydrant can supply water is limited by factors such as static pressure and pipe size. Fire departments will test hydrants regularly as part of a maintenance program. In addition flow tests will be done by engineers when designing sprinkler systems.
The city I grew up in (Kaukauna, WI) would periodically open the hydrant across the street (and others I assume). It had something to do with flushing the pipes (rust???). I know they always warned the neighborhood beforehand, dunno if it was because the reduced water pressure or rust issues or what.
Brian
All water systems will flush their hydrants periodically. This is not so much for rust per se (both ductile iron mains and plastic mains should be generally free of rust), as for general sediment. A fire hydrant is basically on a dead end pipe extending off the main. There is a “T” connection extending off the main and running a few feet away from the main, sometimes in a straight line, sometimes with a right angle “elbow” to where the hydrant is to be placed. To the end of that pipe the hydrant barrel is mounted with an(other) 90° bend going up to the surface. The water flowing through the main never cycles through the smaller pipe leading to the hydrant, so any particulate matter that drifts into the smaller pipe simply settles to the bottom After a while, this sediment can build up and clog the hydrant access pipe. Opening the hydrant every couple of years allows any sediment to be blasted out onto the lawns or street, clearing the pipes.
There can be “rust” (or, at least, mineral deposits) in the pipes, depending on the source of the water. Where I grew up in Rochester, MI, the village was fed from a well that was very high in iron deposits. The streets in my subdivision had permanent orange stains on the gutters near the hydrants from the flushing. On the other hand, Detroit gets its water (mostly) from Lake Huron and Cleveland gets its from Lake Erie and the sediment from those pipes is not usually “rusty” at all.