Orifice Meters for Water Flow Measurement
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Orifice Meters for Water Flow Measurement

   

Orifice Meters for Water Flow Measurement 1

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:

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. Larry Arrington, Dean.



Copyright Information

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