Sluice Gate Flow Measurements
Sluice gates are used for controlling and measuring flow rates in open channels and rivers, mainly in connection to hydro power plants
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Sluice gate flow metering is often used to measure the flow rate in open channels. The sluice gate is also often used to regulate or modulate the flow.
The sluice gate flow rate measurement is based on the Bernoulli Equation principles and can be expressed as:
1/2 ρ v12 + g h1 = 1/2 ρ v22 + g h2 (1)
where
h = elevation height
ρ = density
v = flow velocity
The pressure components are in general irrelevant since the pressures upstream and downstream are the same (p1 - p2 = 0).
Assuming that the velocity profiles are uniform in the upstream and downstream section the Continuity Equation gives:
q = v1 A1 = v2 A2 (2)
where
q = flow rate
A = flow area
(2) can be modified to:
q = v1 h1 b= v2 h2 b (3)
where
b = width of the sluice
h1 = upstream height
h2 = downstream height
Combining (1) and (3), gives the "ideal" equation:
q = h2 b [ 2 g (h1 - h2) / (1 - (h2 / h1)) ]1/2 (4)
Assuming h1 >> h2 (4) can be modified to:
q = h2 b [ 2 g h1 ]1/2 (5)
This is approximately true when the depth ratio h1 / h2 is large, the kinetic energy upstream is negligible (v1 is small) and the fluid velocity after it has fallen the distance (h2 - h1) ≈ h1 - is:
v2 = [ 2 g h1 ]1/2 (6)
The ideal equation (3) can be modified with a discharge or contraction coefficient:
q = cd h2 b [ 2 g h1 ]1/2 (7)
where
cd = discharge or contraction coefficient
The discharge coefficient cd is a function of the opening height and the height of "vena contracta"
cd = h1 / ho (8)
where
ho = height sluice opening
Typically the discharge coefficient is approximately 0,61 for depth ratio ho / h1 < 0,2.
Sluice Gate Specifications
The most commonly used specification for sluice gates used in water and wastewater treatment plants is ANSI/AWWA C560-00. This specification should be used as guidance for selection of the gates, operating equipment and associated hardware.
Example - Flow Rate through a Sluice Gate
Water flows under a sluice gate with an opening height of 0,4 m. The width of the sluice is 3 m and the height from the water surface to the bottom of the sluice is 10 m.
Since h1 >> h2 and the depth ratio 0,4/10 < 0,2 - the contraction coefficient can be set to 0,61 - and equation (7) can be used for flow calculation:
q = 0,61(0,4 m)(3 m)[ 2(9.81 m/s2)(10 m) ]1/2
= 10,25 m3/s
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