Can depend on the size of the main, Fire Service Standpipe is 2.5" diameter, opening that fully off the end of a 3 or 4" main isn't going to help matters...
(A Mate of mine was working on a Road surfacing crew a few years back, was told to go and fill the bowser up with water. He found a convenient hydrant, connected up and started filling. Before too long a Lady came out her house and said her tap had started running brown water. He told her they were aware and that's why he was drawing off water for testing, before quickly packing up and making haste from the scene...)
The Yellow 'H' signs tell the Fire Service the diameter of the main and distance from the sign, if they need a lot of water, they'll look for a hydrant being fed from a larger size main.
The brigade don’t suck from a hydrant they rely on the hydrant pressure if the engine did try to suck the hose would collapse with air pressure if you put a hard hose on and sucked you would collapse the main once again with air pressure ask any firefighter if an engine is on a hydrant you keep the suction on zero to just relay the water. Bob
Makes sense not to use suction but if there were a couple of tenders drawing from the main then on max throughput it would probably be enough to drop the available pressure on a district supply, certainly locally .... distance from the pump station/res/tower notwithstanding.
Perfect example is a customer of mine lives on the 3rd floor of a tenement and when the swimming pool across the road is tapping the mains, there's over a 1 bar dynamic drop on their supply. It does get quite pitiful actually, plays havoc with their combi. There is work ongoing to upgrade the mains though I believe.
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