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Mechanics of Fluids

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570 Unsteady flow<br />

Fig. 12.7 Gate valve.<br />

a knowledge <strong>of</strong> the precise way in which the continuous valve movement<br />

occurs is unnecessary: eqn 12.9 gives h2 − h1 for a reduction <strong>of</strong> velocity<br />

from u1 to u2, regardless <strong>of</strong> the manner in which u changes with time.)<br />

Let us suppose that the steady flow conditions before the valve closure<br />

begins are represented by a head h0 at the valve and a velocity u0 in the pipe.<br />

If at the end <strong>of</strong> the first chosen interval the head and velocity are respectively<br />

h1 and u1 then these must be related by eqn 12.9:<br />

h1 − h0 = c<br />

g (u0 − u1) (12.10)<br />

Another relation between h1 and u1 is required if either is to be calculated.<br />

For the simple case in which the valve discharges to atmosphere, the valve<br />

may be regarded as similar to an orifice, and so<br />

Au = Q = C dAv<br />

�<br />

(2gh) (12.11)<br />

where A represents the cross-sectional area <strong>of</strong> the pipe in which the velocity<br />

is u, Av the area <strong>of</strong> the valve opening and C d the corresponding coefficient<br />

<strong>of</strong> discharge. Putting C d(Av/A) � (2g) = B in eqn 12.11 we have<br />

u = B √ h (12.12)<br />

The factor B is usually known as the valve opening factor or area coefficient.<br />

It should be noted that Cd is not necessarily constant, and the variation <strong>of</strong> B<br />

with the valve setting usually has to be determined<br />

�<br />

by experiment for each<br />

design <strong>of</strong> valve. From eqn 12.12 u1 = B1 h1 and simultaneous solution <strong>of</strong><br />

this equation with eqn 12.10 gives corresponding values <strong>of</strong> h1 and u1.<br />

Such a closure in which B varies linearly with time t, that is,<br />

B = B0(1 − t/T), is frequently termed straight-line closure. It does not imply<br />

uniform motion <strong>of</strong> the valve. With a gate valve (Fig. 12.7), for example, in<br />

which the gate is moved uniformly, the rate <strong>of</strong> reduction <strong>of</strong> area is initially<br />

small, but very much greater towards the end <strong>of</strong> the closure. It is therefore<br />

common to arrange for such valves on long pipe-lines to close slowly during<br />

the final stages <strong>of</strong> the movement.<br />

A wave returning from the reservoir is negative in sign and it thus <strong>of</strong>fsets –<br />

partially at least – rises in pressure caused by further reductions <strong>of</strong> velocity.<br />

A numerical example will show how allowance is made for the reflected<br />

waves.<br />

Example 12.2 A pipe carries water at a (mean) velocity <strong>of</strong> 2 m · s −1 ,<br />

and discharges to atmosphere through a valve. The head at the valve<br />

under steady flow conditions is 100 m (pipe friction being neglected).<br />

Length <strong>of</strong> pipe = 2400 m; wave celerity c = 1200 m · s −1 . The valve

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