A valve for flushing water closets, urinals or slop sinks is shown in section in Fig. 149. It is known as the Vimometer and is operated in the following manner: When not in operation the parts are all in position shown in the illustration, and water which has flowed up through the hollow stem, a, fills the chamber, b, thus equalizing the pressure on both sides of the valve seat, c. When the handle of the valve is raised, the eccentric, d, opens the relief valve, e, which relieves the pressure in the chamber, b, and the pressure of water then unseats the valve, c, and flows to the closet bowl. When the handle is released it drops back into place thus allowing the relief valve to close. Water flowing through the hollow stem then equalizes the pressure in chamber, b, so the valve can seat by gravity. A fine mesh strainer covers the opening to the hollow stem, a, to keep out foreign matter which might clog the opening.

Flush valves cannot be successfully used unless there is sufficient volume and pressure to operate them. For high-pressure service they require a head of at least twenty feet; where this head is unavailable, special low-pressure valves should be used.

Vimometer valves can be regulated to discharge almost any desired quantity of water at each flush of a fixture. The usual amounts vary from 6 to 12 gallons, which are discharged in from 9 to 15 seconds' time. If the service pipe is not large enough to supply this quantity of water within the required time a flush valve cannot be successfully used.

Flush valves require a separate system of piping to supply them with water, their use is therefore confined almost entirely to buildings in which a large number of closets are installed. A separate tank to supply the water closets and urinals is usually provided, and in small installations where the number of fixtures are few, storage provision should be made for at least twice the quantity of water that can be discharged by all closets at one flush. When there are a great number of closets to be supplied, this rule need not be strictly adhered to, although it is better in all cases to install a tank with a capacity of at least two days' supply to guard against any interruption of pump or service.