CTC 261 Hydraulic Devices 1 Objectives Calculate flow through an orifice Calculate flow over a weir Calculate flow under a gate Know how to compute discharge ratings for detention basin outlet structures
2 Orifices Hole in a wall/pipe through which water flows Square edge Beveled edge 3 Orifice
When water flows through an orifice the water contracts with a smaller area than the physical orifice opening (vena contracta) www.spiraxsarco.com www.diracdelta.co.uk 4 General Orifice Equation
Q=ca(2gh).5 Where: This should look familiar!! Q=discharge (cfs or cms) c=discharge coefficient (0.62 often used) a=cross-sectional orifice area (sq ft or sq meters)
h=total head (ft or m) g=gravitational constant (32.2 or 9.81) 5 Orifice Discharge Free Discharge Submerged Discharge Equation is the same. Head for the submerged discharge is the difference between upper and lower water surfaces 6
Orifice-Free Discharge Given: Dia=6, WSE=220.0 ft; Elev of orifice centerline=200.0 ft Q=ca(2gh).5 Q=0.62*0.196*(2*32.2*20).5 Q=4.4 cfs 7 Weir
Horizontal surface over which water is allowed to flow Used to regulate and measure flows http://www.flow3d.com/appl/weir.htm 8 Rectangular, Sharp-Crested Weir Q=cLH3/2
Q-flow (cfs) c-adjusted discharge coefficient (careful) c=3.27+0.4(H/P) where P is ht of weir above channel bottom L-effective crest length, ft L=L-0.1nH
L=actual measured crest length and n=# of contractions H-head above crest, ft 9 Rectangular, Broad-Crested Weir Q=cLH3/2
Q-flow (cfs) c-discharge coefficient (App A-5 English units) L-crest length, ft H-head above crest, ft Note: Dont adjust broad-crested weirs for contractions 10 V-Notch or Triangular Weir Q=c*tan(angle/2)*H5/2 c = 2.5 (but should calibrate)
11 Other Weir Types Cipoletti (trapezoidal) Ogee (dam spillway) youngiil.co.kr www.lmnoeng.com 12 13
Flow under a gate Sluice gate, head gate, diversion gate Depending on conditions, flow can be flat, have a hydraulic jump or be submerged Flow is modeled as an orifice Typical c=0.7 to 0.85 but should be determined experimentally 14 Siphon flow
Closed conduit that rises above the hydraulic grade line Has practical problems 15 Break 16 Detention Outlet Structures
Single Stage (culvert or orifice) Multi-Staged to handle different flows Combination of orifices &/or weirs 17 Single Stage Outlet Example (Ex14-3) An outlet consisting of a 12 pipe is proposed for a detention basin. The invert of the pipe is
320.0 feet and the top of berm is 325.0 ft. Compute the discharge rating for the outlet. Area=0.785 sq ft Assume c=0.62 Use orifice equation: Q=ca(2gh).5 18 Single Stage Outlet Example WSE (ft) h (to c/l of pipe) Q out (cfs) 320 0
0 321 0.5 2.8 322 1.5 4.8 323
2.5 6.2 324 3.5 7.3 325 4.5 8.3 19
Stage-Discharge Curve 9 8 7 Discharge (cfs) 6 5 4 3 2 1 0 321 322
323 324 325 326 327 328 WSE (ft) 20
Multi-Stage Outlet Example 14-4 (pg 349) 4 Orifice and 2 weirs L=1.5 and L=12.5 21 Multistage Outlet Equations: c*a*(2gh)^.5 orifice cLH^1.5 weir 22 Stage-Discharge Curve
90 80 70 Discharge (cfs) 60 50 40 30 20 10 0 559 560
561 562 563 564 565 566 WSE (ft) 23 Check Details
Check outflow pipe to make sure it can handle outflow Orifice would be submerged at some point, impacting h (Note----Q is insignificant compared to the weir flow) 24