Valves are usually penstocks, the leaf working in gun-metal guides. The smaller ones are worked by hand and the larger by power. They require balancing to reduce the effort of lifting and are better placed so that the water forces them against their seats. Hinged flap-valves are common at the ends of sewers, to open when the water presses against the inside.
Sewer Connections and Ventilation.—Fig. 6 shows the connection of two large circular sewers at about the same level, with access to either from the surface. Where there is great difference in level a "drop-pipe," fig. 7, or a ramp, may be used, tumbling bays or stairs being unsatisfactory on account of the agitation.
Unless there are objections, it is usual to ventilate the sewer through gratings in the cast-iron cover of the manholes, which latter are usually not more that 15o yards apart. Otherwise, con nections are made to shafts on the roadside, such shafts being about 3o feet high. Mechanical and other methods have been tried, but the effect is very local.
shaft is usually about 2 feet square and provided with step-irons. The floor may be concrete or brick or stone paved. The central channel should carry the maximum rate of dry-weather flow, so that the "benches" are normally free from water and deposits.
Sewers should never be at such a flat gradient as to require flushing to remove sediment, but if this is impossible, such flush ing may be accomplished by perambulating water-tanks fitted with a hose, or by underground tanks designed to automatically discharge at regular intervals by means of a syphon. In either case a very large quantity of water is required. Pumping should be avoided whenever possible on account of the expense, but if found inevitable, either special types of pumps, or ejectors working by compressed air may be used.
See G. Thomson, Modern Sanitary Engineering, pt. II. ; Baldwin Leatham, Sanitary Engineering; Moore and Silcock, Sanitary Engineer ing; G. S. Coleman, Hydraulics Applied to Sewer Design.
(G. S. Co.)