Flow Measurement and Control

Orifice Meter The orifice meter consists of an accurately machined and drilled plate concentrically mounted between two flanges. The position of the pressure taps is somewhat arbitrary.

Orifice Meter The orifice meter has several practical advantages when compared to venturi meters. Lower cost Smaller physical size Flexibility to change throat to pipe diameter ratio to measure a larger range of flow rates Disadvantage: Large power consumption in the form of irrecoverable pressure loss

Orifice Meter The development of the orifice meter equation is similar to that of the venturi meter and gives: where:  = ratio of orifice diameter to pipe diameter ≈ 0.5 usually S 0 = cross sectional area of orifice V = bulk velocity through the orifice C 0 = orifice coefficient ≈ 0.61 for Re > 30,000 –

There is a large pressure drop much of which is not recoverable. This can be a severe limitation when considering use of an orifice meter.

Pressure Loss in Orifice Meters

ASME Design Standards Fluid Meters: Their Theory and Applications, 6 th ed., American Society of Mechanical Engineers, New York, 1971 pp

Rotameters Rotameters fall into the category of flow measurement devices called variable area meters. These devices have nearly constant pressure and depend on changing cross sectional area to indicate flow rate. Rotameters are extremely simple, robust devices that can measure flow rates of both liquids and gasses. Fluid flows up through the tapered tube and suspends a ‘float’ in the column of fluid. The position of the float indicates the flow rate on a marked scale.

Rotameters Three types of forces must be accounted for when analyzing rotameter performance: Flow Gravity Buoyancy Flow Buoyancy Gravity For our analysis neglect drag effect

Rotameter Mass Balance Assume Gradual Taper Flow Between Float and Tube S 3 is annular flow area at plane 3

Rotameter Momentum Balance Note: p 3 = p 2 Must account for force due to float

Rotameter Mechanical Energy Balance 0 Assume: (Base velocity head on smallest flow area)

Rotameter Combining Momentum and Mechanical Energy Balance After Some Manipulation

Rotameter Assuming S f ≈ S a discharge coefficient can be defined C R must be determined experimentally. As Q increases the float rides higher, the assumption that Sf = S is poorer, and the previous expression is more nearly correct.

Other Flow Meters

Turbine Meter Measure by determining RPM of turbine (3) via sensor (6). Turbine meters are accurate but fragile.

Coriolis Meters When fluid is passed through a U-bend, it imposes a force on the tube wall perpendicular to the flow direction (Coriolis force). The deformation of the U-tube is proportional to the flow rate. Coriolis meters are expensive but highly accurate.

Pneumatic Control Valves

Orifice Meter Example A 2 in. Schedule 40 pipe carries 35 º API distillate at 50 ° F (SG=0.85). The flow rate is measured by an orifice meter which has a diameter of 1.5 in. The pressure drop across the orifice plate is measured by a water manometer connected to the flange taps. If the manometer reading is 20 in. of H 2 O, what is the flow rate of the oil in GPM ?

Now what ???