The water transmission system
As part of its responsibility, Toronto Water conveys large volumes of water between water treatment plants, pumping stations, storage reservoirs and to points of supply to the local District distribution systems (of former municipalities) that make up the City of Toronto and the southern part of York Region. The average daily amount pumped (and repumped) into the transmission system is about 2900 megalitres.
The transmission system consists of about 500 kilometres of trunk water mains ranging in diameter from 400 to 2500 millimetre (mm), the greater number being in the 900 to 1250 mm range. The older parts of the system are lead or rubber joint cast iron pipe, 400 to 1050 mm in size.
Since 1949, all pipe 750 mm diameter and larger has been laid in steel, lined with cement mortar and encased in concrete with all joints welded. Mains laid in tunnels are placed inside a grouted liner plate and the space between the liner plate and the steel main is filled with concrete.
Except under special circumstances, in crossing sewers or under railway tracks, water mains are laid about 1.5 metres deep, just below the frost line, and hence follow ground contours. Mains are usually located in public streets both for ease of installation and quick access for maintenance.
Water mains have several appurtenances, located in chambers, which require preventive maintenance. A chamber is an underground room or vault accessible from street level by means of covered surface maintenance hole openings. In the past, chambers were usually constructed of brick masonry or reinforced concrete. Currently, we are using circular precast concrete sections in most cases.
The following are the usual water main appurtenances:
- Line valves: are generally gate or butterfly valves one or two sizes smaller in diameter than the main piping. These are used to isolate sections of the main as required for operation or maintenance.
- Branch valves: Branches are the connections through which water is supplied to the local distribution systems. The branch valves are part of the mains and are usually gate valves ranging in size from 400 to 600 mm. They are used to isolate supply to the local systems, when required, for repair or maintenance to either the local or trunk main.
- Maintenance holes: are covered openings placed in the pipe line to permit access approximately every 150 metres for inspection, cleaning and maintenance.
- Air valves: The 100 mm double acting air valves used in our system are located at all high points in a main to allow continuous release of the small quantities of air trapped in the water. They also permit the entry of large volumes of air, when necessary, to prevent collapse of the pipe under pump failure conditions, where a vacuum is established inside the main. Air valves are often mounted on the access maintenance hole cover in a chamber.
- Drain valves: are 300 mm gate valves located at the low points in the mains so that portions can be drained for cleaning, repair or maintenance. They are always incorporated on a standard access maintenance hole cover to allow entry at the drain point.
- Check valves: are used when flow in only one direction is required, and are normally full pipeline size.
- Pressure reducing valves: When a low point in a district is isolated from the normal supply mains to that elevation, a pressure reducing valve is installed to allow the higher district supply to be directed into the low area without causing breaks or other high pressure problems.
- Meters: are devices installed in the main to measure the flow within the transmission system and at supply points to York Region for billing purposes. Flows are measured by a variety of meter types: venturi, magnetic, mechanical turbine or displacement.
- Meter registers: For venturi and magnetic flow meters, a separate meter register chamber is required. These special chambers are heated and well drained to protect the registering equipment. They are usually installed off the water main line and outside the pavement, for easier access.