The
Anti-Vacuum Valve is a very special type of air valve. It's primary
function is to prevent the formation of vacuum in hydroelectric penstocks or large
diameter water mains, which might cause line collapse under such conditions of
flow as may result, for example, from too rapid a closure of an upstream head
gate or shut down valve, a downstream burst, turbine "runaway" or ordinary
emptying of a pipeline.
By virtue of its unique design,
the anti-vacuum valve reacts automatically, sensitively and positively, even after
long period of inactivity, to changes of pressure within a pipe, and whenever
necessary, permits air to flow in at a sufficiently high velocity, and at a low
enough induction pressure, to safeguard the line against collapse.
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Cowled
Inlet Type
An annular cowl shrouds the orifce, affording protection
to the orifice and the seating , and air flows through the ports provided around
the periphery of the body assembly. Application of this type should be confined
to situations where no damage is likely to occur to surrounding structures from
sudden intakes of air.
Duties
The
valve automatically allows induction of large volumes of air to prevent vacuum
formation and also provides an automatic means of ventilating a line when it
is being emptied of water and of exhausting air when it is being recharged.
Operation
The
valve element is in the form of a disk which is sensitively balanced by a counterpoising
mechanism. the disk guide pin is attached to a crosshead, to which is fitted at
either end a cranked lever that rocks about an intermediate pivot pin and carries
an adjustable counterweight on its outer arm. The parts are so arranged that by
adjusting the position of the counter weights, the valve can be balanced at any
desired point in its working travel. Thus, when swinging freely the valve may
be balanced at partially open position in which case, if it is closed by hand,
it is self-opening to the predermined point of equilibrium and vice versa. Also
attached to the crosshead is an oil dashpot which gives free opening, in a downward
direction, but offers resistance to closing, in an upward direction and avoids
all possibilites of oscillation of the suspended masses.
In action, therefore,
the valve cannot remain at either extremity of its travel unless it is acted upon
by some external force. During normal operation, the disk is held shut by the water
pressure in the pipe. Should the pressure on the underside of the disk fall below
that of the atmosphere, the valve will immediately open to admit air and break
the vacuum. With a very small vacuum, say 1 in of mercury or about 1/2 psi below
atmosphere, the valve opens fully and offers a wide passage for the free flow
of air. On the cessation of air inflow, the valve returns to a position of slightly
open, which is sufficient for the escape of air during refilling of the line.
When the rising water makes contact with the uderside of disk, closure is completed;
only a very small water pressure is required to close the valve, consequently,
the quantity of water over-flowing through the orifice during final closure is
negligible.
INFLOW OF RARIFIED AIR
Valve Orifice Size
(mm) |
Branch Size |
Depression Below Atmosphere (m) |
| 0.2 |
0.4 |
0.6 |
0.8 |
1 |
2 |
3 |
4 |
5 |
| Inflow of Rarified Air (cumecs) |
| 380 |
700 |
4.25 |
5.92 |
7.31 |
8.50 |
8.61 |
15.09 |
16.45 |
19.96 |
22.13 |
| 200 |
350 |
1.03 |
1.45 |
1.77 |
2.09 |
2.12 |
3.70 |
4.04 |
4.93 |
5.40 |
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