The pier face downstream of the gate slots may be protected against
the effect of flow separation and possible cavitation (see Section 4.6)
either by recessing it against the upstream pier face (by about 0.1 w, where
w is the width of the slots) and rounding off the downstream corner by a
radious 0.1 w or by a bevel of a slope about 1:12 (Fig. 6.1); for further
details see, e.g. Ball (1959).
6.2.3 Radial gates
Radial (Tainter) gates are usually constructed as portals with cross-bars
and arms (straight, radial, or inclined), but could also be cantilevered over
the arms. Their support hinges are usually downstream but (for low heads)
could also be upstream, resulting in shorter piers.
In equation (6.1), 0.11k0.15 and n0.07 for PB150 kN m. The
usual range of heads and spans for radial gates is 2H(m)20 and
3 B(m)55, with (BH)max550 m^2. Radial gates may be designed for
more than 20 MN per bearing.
The advantages of radial over vertical lift gates are smaller hoist,
higher stiffness, lower (but longer) piers, absence of gate slots, easier
automation and better winter performance.
The gate is usually hoisted by cables fixed to each end to prevent it
from twisting and jamming (Fig. 6.2). If the cables are connected to the
bottom of the gate its top can be raised above the level of the hoist itself, if
the layout of the machinery allows it.
6.2.4 Drum and sector gates
Drum and sector gates are circular sectors in cross-section. Drum gates are
hinged upstream (Fig. 6.3) and sector gates downstream; in the latter case
the hinge is usually below the spillway crest by about 0.1Hto 0.2H. Gates
on dam crests are usually of the upstream hinge type, with the hinge about
0.25Habove the downstream gate sill and a radius of curvature of rH.
The heads can be as high as 10 m and the spans 65 m. Drum gates float on
the lower face of the drum, whereas sector gates are usually enclosed only
on the upstream and downstream surfaces.
Both types of gates are difficult to install, and require careful mainte-
nance and heating in winter conditions; their main advantages are ease of
automation and absence of lifting gear, fast movement, accuracy of regula-
tion, ease of passing of ice and debris, and low piers.
270 GATES AND VALVES
