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Publication #AE22

Orifice Meters for Water Flow Measurement1

Allen G. Smajstrla and Dalton S. Harrison2

Orifice Meters for Water Flow Management

Good water management requires that flow rates be accurately measured. A simple but economical and accurate device for the measurement of flow rates from pipes discharging to the atmosphere is the orifice meter. The orifice meter is sometimes called a pipe orifice, an end-cap orifice, or more descriptively, a circular orifice weir.

Orifice meters are usually circular orifices placed at the end of a horizontal discharge pipe as illustrated in Figure 1 . Flow rates are calculated from the orifice characteristics and a measure of the pressure behind the orifice. The head (or pressure) on the orifice is measured with a water manometer to obtain a high degree of accuracy (Typically within 2% of true values).

Figure 1 . 

Because of the simplicity of design and the few components of the orifice meter, it can readily be constructed by individuals with only average mechanical skills. An orifice meter can be constructed from a pipe end-cap which has been properly drilled, if the discharge pipe is threaded to allow the end cap to be added for flow measurements or removed after its use.

The cost of the orifice meter is minimal when self-constructed. The orifice meter is very accurate because of the sensitivity of the water manometer, and, because of the absence of moving parts or the need for calibration, it is often used as a means of calibrating other flow-measuring devices.

Details of Construction

Details of construction are illustrated in Figure 2 . The orifice consists of a perfectly round hole in the center of a circular steel plate. The orifice must be cut with clean, square edges. The plate must be 1/16-inch thick around the circumference of the hole. The plate is fastened against the outer end of a level discharge pipe so that the orifice is centered on the pipe. The end of the pipe must be cut square so that the plate will be vertical. A properly machined pipe end-cap may be used as the orifice plate.

Figure 2 . 

The bore of the pipe should be smooth and free of any obstruction that might cause excessive turbulence. This includes ensuring that there are no elbows, valves or other fittings closer than 6 feet upstream from the orifice. This approach must also be straight and level.

To measure pressure head on the orifice with sufficient accuracy, a glass tube manometer should be used. Exactly 24 inches from the orifice plate, the pipe wall is tapped mid-way between the top and bottom with a 1/8-1/4 inch diameter hole. Burrs inside the pipe caused by the drilling or tapping should be filed to ensure a smooth surface. The glass tube is connected with a short piece of flexible tubing to a pipe nipple threaded into the tapped hole. The glass tube is mounted vertically with a scale set to measure the height of water in the tube above the centertine of the pipe. When water is pumped through the orifice so that it is flowing full, the height of the water in the tube is the pressure head on the orifice.

Orifice Equations

For any given size of orifice discharge pipe, the rate of flow varies in a known manner with the pressure head as measured with the glass tube manometer. Standard tables, such as Table 1 , give the flow in gallons per minute (gpm) for various combinations of orifice diameter and pipe diameter

Discharge through the orifice is computed from equation 1 .

equation 1 . 

whereQ = orifice discharge (gpm),

C = coefficient which varies with the ratio of the orifice diameter to the pipe diameter, as well as with all of the other factors affecting flows in orifices of this construction (from Figure 3 ),

Figure 3 . 

A = cross sectional area of the orifice (in 2 ), and

h = head on the orifice measured above its centerline (in).

The value of its coefficient, C, varies with the ratio of the orifice diameter to pipe diameter because of changes in streamlines is flow converges through orifices of various sizes. The value of C is also a function of friction losses through the orifice. Therefore, the values of C given in Figure 3 are only applicable when the orifice meter is constructed as previously described.

Example Calculations

The use of Equation 1 is illustrated by an example. The following data apply:

  • Orifice constructed as previously described

  • Orifice diameter = 4 in

  • Discharge pipe inside diameter = 8 in

  • Pressure head, h = 30 in

Calculations are performed in 4 steps:

1.Ratio of orifice diameter to pipe diameter = 4 in / 8 in = 0.5

2.From Figure 3, C = 0. 587

3.Cross sectional area of orifice = pi d 2 /4 = (3.14)(4 in) 2 /4 = 12.56 in 2

4.Q = 8.02 CA h

Q = 8.02 (0.587) (12.56 in2) 30in

Q = 324 gpm

Comparing this result with that shown in Table 1 (325 gpm) shows the utility of Equation 1 . The discrepancies between values in Table 1 and those calculated result from inaccuracies in interpolating C values from Figure 3 .

Ensuring Accuracies

The orifice meter provides the capability of measuring flow rates with accuracies within 2% of the actual rates if it is properly constructed and operated. Besides constructing the parts accurately and setting up the device correctly in the field, additional precautions must be taken to obtain accurate results. First, the diameter of the orifice should be less than 0.8 and greater than 0.4 of the inside diameter of the discharge pipe. The value of C changes rapidly as the ratio of diameters increases. Therefore, to ensure greatest accuracy, the ratio should be less than 0.7.

Second, the manometer tube must be completely free of obstruction and air bubbles when measurements of pressure head are made. Air bubbles can be eliminated by lowering the manometer tube to allow water to flow through it.

Finally, the orifice must flow completely full for Equation 1 or the data in Table 1 to apply. One method of ensuring this is to select an orifice diameter such that the manometer water level is above the top of the discharge pipe.

Summary

Orifice meters are easily constructed and economical flow-measuring devices for irrigation pump discharges. Accuracies within 2% of actual flow rates are obtainable if construction and operational practices outlined in this paper are followed. Because of the absence of moving parts or the need for calibration, the orifice meter is often used as a means of calibrating other flow measuring devices.

References

  1. Briggs, G. F. and A. G. Fielder, eds. 1966. Ground Water and Wells. Johnson Division, Universal Oil Products. 440 pp.

  2. Israelsen, 0. W. and V. E. Hansen. 1962. Irrigation Principles and Practices. John Wiley and Sons. 447 pp.

  3. SCS National Engineering Staff. 1964. Measurement of Irrigation Water. Section 15, Chapter 9. SCS National Engineering Handbook. SCS. USDA. 72 pp.

Tables

Table 1. 
Table 1. Discharge from circular pipe orifices.
Head (inches) 3-in. orifice 4-in. orifice 5-in. orifice 6-in. orifice 7-in. orifice 8-in. orifice
4-in. Pipe 6-in. Pipe 6-in. Pipe 8-in. Pipe 6-in. Pipe 8-in. Pipe 8-in. Pipe 10-in. Pipe 10-in. Pipe 10-in. Pipe
G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m.
6 108 82 160 150 305 240 408 345
8 122 94 185 170 350 280 458 395 600 935
10 133 104 205 190 393 316 508 445 666 1040
12 146 114 225 208 430 346 556 490 728 1120
14 157 123 243 224 465 376 599 530 785 1194
16 167 132 257 238 495 402 636 568 838 1266
18 178 140 271 252 524 426 672 604 887 1336
20 187 148 285 266 548 449 708 636 933 1404
22 197 156 299 279 572 470 744 664 979 1471
24 205 164 310 291 596 488 776 692 1022 1529
26 214 171 323 303 620 504 805 720 1064 1585
28 222 177 335 314 644 520 831 747 1104 1641
30 230 183 346 325 668 536 857 773 1143 1697
32 239 189 357 335 692 552 882 799 1181 1753
34 246 195 369 345 715 568 907 824 1218 1809
36 254 200 380 354 737 584 931 847 1251 1865
38 260 205 390 363 759 600 955 867 1281
40 266 210 401 371 781 616 979 887 1311
42 272 214 411 380 800 631 1001 906 1341
44 278 219 420 388 820 645 1023 925 1371
46 284 224 429 396 837 659 1045 944 1401
48 290 229 440 405 855 672 1067 963 1431
50 296 234 448 413 872 686 1089 982 1461
52 302 238 457 421 888 700 1110 100 1491
54 307 243 465 429 904 714 1130 1018 1520
56 313 248 472 437 919 727 1150 1036 1548
58 317 252 480 445 934 739 1170 1052 1574
60 323 257 489 453 948 751 1190 1068 1598
62 328 262 496 461 961 763 1209 1084
64 333 266 504 469 974 775 1227 1099
66 338 271 513 475 988 787 1245 1113
68 343 275 520 483 1002 799 1263 1127
70 349 280 525 491 1016 811 1280 1140

Footnotes

1.

This document is AE22, one of a series of the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date July, 1987. Reviewed July, 2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.

2.

Allen G. Smajstrla, assistant professor; and Dalton S. Harrison, professor, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.