The simplest & most common flow meter is a fixed Orifice in a pipeline. This Orifice creates a pressure differential (i.e. pressure drop) which is proportional to flow. But the flow rate versus differential pressure is a square root function, thus at the high end of the scale the graduations are far apart and very easy to read. At the low end of the scale, the graduations are so close together that reading becomes impossible. The net result is a meter with only a 3 OR 4 to 1 usable range.

  
  The limited rangeability of this type of meter can be overcome by adding a bypass leg around the Orifice, which has a pressure drop equal to the drop across the Orifice and bypassed flow is directly proportional to the mainline flow. This results in a meter, which has a linear function and not a square root. Since the flow meter has a 10 to 1 range, the Orifice meter will now have a 10 to 1 linear flow range, which is as easy to read at the bottom as the top of the scale. This principle can be used virtually any pipeline size, handling almost any flow rate and metering either liquid OR gas. It is one of the most versatile meters available. Its only limitation is that the solid particles OR slurries can clog the Orifice and prevent proper operation.  
  BY-PASS METERING COMPONENTS  
  The Bypass flow meter consists of a mainline Orifice plate, a set of Orifice Flanges Or a carrier ring, a flow meter, range Orifice, Bypass piping with Isolation Valves.  
 
  • Mainline Orifice Plate: - The purpose of the mainline orifice plate is to create the pressure differential. As the flow rate varies across this mainline orifice, the pressure differential across this plate also varies and it is this variation, which makes the meter function.
  • Orifice Flanges / Carrier Ring: - Orifice flanges / carrier rings serve two functions. First, they hold the mainline orifice plate in the proper position within the pipeline. Secondly they channel a portion of the flow out of the high-pressure side of the orifice and return it again to the low-pressure side. This function may be accomplished with other than flange taps. The Bypass piping may be connected directly to the pipeline in the form of pipe taps, radius taps OR vena contracta taps.
  • Variable Area Flow Meter: - As discussed above, a flow meter (Glass Tube Rotameter) is placed in the Bypass piping to measure the bypass flow.
  • Range Orifice: - The range orifice is a small orifice placed in the bypass piping either before OR after the flow meter. This orifice is sized so that the total pressure drop through the bypass-piping equals the pressure differential across the mainline orifice.
  • Bypass Piping: - The bypass piping connects the upstream side of the mainline taps to the inlet side of the flow meter and the downstream side of the mainline taps to the outlet of the flow meter. Since the pressure drop through the bypass piping has a direct bearing on the accuracy of the flow meter, it is necessary to keep the bypass piping as short as possible with as few fittings as possible.
 
  DESIGN FEATURES  
 
  • Dowel Pin, side plate construction
  • Detachable flow scales
  • Externally adjustable packing glands
  • Connections can be rotated 360 degree at 90 degree
  • Laminated safety glass shielding
  • Scale length of 180-200 mm
 
  PERFORMANCE  
 

Accuracy

 +/- 2% F.S.D.
Repeatability
 0.5 %
Rangeability
 5:1 for 1250 mm WC D.P., 7:1 for 2500  & 3750 mm WC D.P.,
10:1 for 5000 & 10000 mm WC D.P.
Scale length 
 180-200 mm
Pressure Ratings
 Maximum Operating Pressure is 25 Kg/Cm2.
Temperature Ratings
 Maximum Operating Temperature
rating is 121 o  C for Gas services and 93 o  C for Liquid services.
Connections
 Flanged.
Enclosure  IP 55 OR IP 65 on request
 
  MATERIALS OF CONSTRUCTION  
 

Mainline Orifice Plate

 316SS, 304SS, P.P.
Orifice Flange / Carrier Ring
 M.S., 316SS, P.P.
Range Orifice
 316SS
Bypass Piping with Isolation Valves
 M.S., 316SS
Glass Tube Rotameter:-
 
Tube
 Borosilicate Glass
Float
 SS 316, PTFE, others on request
Packing 
 Neoprene, PTFE, Silicon, Viton
End Fitting
 M.S., C.S., SS304, SS316, CF-8M, CF8,
C.I. PTFE Lined, SS PTFE Lined,
PVC, others on request
Frame & Cover M.S. OR SS
   
 
   
 
CAPACITY TABLE – GLASS TUBE BY PASS ROTAMETER FOR LIQUIDS
Mainline Size Sch. 40 Flange Tapings (M3/Hr)
Differential in mm W.C.
1250 2500 3750 5000 10000
1.5 7 10 12 14 20
2 11 16 20 23 33
2.5 16 23 28 33 47
3 25 36 44 51 72
4 44 62 76 88 125
5 69 98 119 138 196
6 100 141 173 200 282
8 173 245 300 346 490
10 273 386 473 546 772
12 387 548 671 775 1097
14 468 663 812 937 1326
16 612 866 1061 1225 1732
18 775 1096 1342 1550 2192
20 963 1362 1668 1926 2724
24 1393 1970 2413 2786 3940
 
   
   
 
CAPACITY TABLE – GLASS TUBE BY PASS ROTAMETER FOR GASES
Mainline Size Sch. 40 Flange Tapings (SCFM)
Differential in mm W.C.
1250 2500 3750 5000 10000
1.5 130 190 235 270 385
2 220 315 385 445 635
2.5 315 450 555 640 905
3 490 695 855 990 1400
4 1020 1450 1775 2050 2900
5 1605 2270 2780 3215 4545
6 2310 3270 4005 4630 6545
8 4000 5655 6930 8000 11320
10 6295 8905 10910 12600 17820
12 8930 12500 15000 17500 24600
14 10785 15255 18685 21580 30520
16 14095 19930 24415 28190 39870
18 17840 25230 30905 35685 50470
20 22170 31355 38400 44345 62715
24 32065 45350 55545 64135 90705