22/6/23 [email protected]1 6.2 Gross Volume Flow Rate Flow, Flow Volume, & Flow Flow, Flow Volume, & Flow Rate Rate There are different flow measurement sensors. Many flow sensors actually measure flow rate. o Mass flow rate o volumetric flow rate Some flow sensors measure flow velocity. Some sensors measure pressure differences Total flow (or flow volume) can be, therefore, derived from flow rate
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[email protected] 6.2 Gross Volume Flow Rate Flow, Flow Volume, & Flow Rate There are different flow measurement sensors. Many flow sensors.
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An obstruction in the flow, measure pressure differential before and after the obstruction
Low initial cost No moving parts Handle dirty media Easy to use Well understood technology Supported by AGA and API
Not highly accurate, particularly in gas flow Orifice plate and pitot tube can become clogged High maintenance to maintain accuracy Typically low turndown Pressure drop
LiquidsGases Steam
VortexInlineInsertion
Bluff body creates alternating vortices, vortex shedding frequency equal to fluid velocity
High accuracy No moving parts No maintenance Measures dirty fluids
Can be affected by pipe vibration Cannot measure low flows
Liquids GasesSteam
TurbineInlineInsertionDual turbine
Turbine rotates as fluid passes by, fluid velocity equal to blade rotational frequency
High accuracy Low flow rates Good for steam Wide turndown
Moving parts require higher maintenance Clean fluids only
LiquidsGasesSteam
MagneticMagElectromagnetic
Measures voltage generated by electrically conductive liquid as it moves through a magnetic field, induced voltage is equal to fluid velocity
High Accuracy Wide turndown Bi-directional No moving parts No pressure loss to system
Conductive fluids only Expensive to use on large pipes
Fluid velocity measured by time arrival difference of sound waves from upstream and downstream transducers
Low cost clamp-on installation Non-intrusive No maintenance Bi-directional Best for larger pipes
Typically not used on pipes < 2” Less accurate than inline or insertion meters Used primarily for liquids Susceptible to changes in fluid sonic properties
Most liquids (condensate)Gas (when spool-piece)
DopplerUltrasonic
Fluid velocity measured by sensing signals from reflective materials within the liquid and measuring the frequency shift due to the motion of these
reflective materials
Low-cost, clamp-on installation Non-intrusive Measures liquids containing particulates or bubbles Low maintenance Best for larger pipes
Can’t be used in clean liquids Less accurate than in-line or transit-time ultrasonic
Most liquids containing reflective materials
Thermal Mass
Measure heat loss of heated wire thermistor in fluid flow
Measure flow at low pressure Relative low cost Measure fluids not dense enough for mechanical technologies Easier to maintain than DP meter
Susceptible to sensor wear and failure Not very accurate Limited to fluids with known heat capacities
1. Compressibility Effects In compressible gas flows,compressibility effects change the value of the discharge coefficient. ★The compressible adiabatic expansion factor,Y
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.
where: = ratio of orifice diameter to pipe diameter ≈ 0.5 usuallyS0 = cross sectional area of orificeV = bulk velocity through the orificeC0 = orifice coefficient ≈ 0.61 for Re > 30,000
whereD2 = orifice, venturi or nozzle inside diameterD1 = upstream and downstream pipe diameterd = D2 / D1 diameter ratio
Discharge Coefficient - cd
Diameter Ratio d = D2 /
D1
Reynolds Number - Re
104 105 106 107
0.2 0.60 0.595 0.594 0.594
0.4 0.61 0.603 0.598 0.598
0.5 0.62 0.608 0.603 0.603
0.6 0.63 0.61 0.608 0.608
0.7 0.64 0.614 0.609 0.609
Performance of Orifice Flow Meter
Performance of Orifice Meter
The pressure recovery is limited for an orifice plate and the permanent pressure loss depends primarily on the area ratio. For an area ratio of 0.5, the head loss is about 70 - 75% of the orifice differential.The orifice meter is recommended for clean and dirty liquids and some slurry services.The rangeability is 4 to 1The pressure loss is mediumTypical accuracy is 2 to 4% of full scaleThe required upstream diameter is 10 to 30The viscosity effect is highThe relative cost is low
Performance of Venturi MeterHigh pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available.A discharge coefficient cd = 0.975 can be indicated as standard, but the value varies noticeably at low values of the Reynolds number.The pressure recovery is much better for the venturi meter than for the orifice plate.The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services.The rangeability is 4 to 1Pressure loss is lowTypical accuracy is 1% of full rangeRequired upstream pipe length 5 to 20 diametersViscosity effect is highRelative cost is medium
Performance of Nozzle Flow Meter
Discharge Coefficient - cd
Diameter Ratio d = D2 /
D1
Reynolds Number - Re
104 105 106 107
0.2 0.968 0.988 0.994 0.995
0.4 0.957 0.984 0.993 0.995
0.6 0.95 0.981 0.992 0.995
0.8 0.94 0.978 0.991 0.995The flow nozzle is recommended for both clean and dirty liquidsThe rangeability is 4 to 1The relative pressure loss is mediumTypical accuracy is 1-2% of full rangeRequired upstream pipe length is 10 to 30 diametersThe viscosity effect highThe relative is medium
# The unrecoverable overall pressure loss , , associated with a flow meter depends on ratio and flow rate. # The power, ,required to overcome any loss in a system is given by:
Orifice Meter ExampleA 5.25cm(2.067 in.) Schedule 40 pipe carries 35º API distillate at 50° F (SG=0.85). The flow rate is measured by an orifice meter which has a diameter of 3.81cm(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 50.8cm(20 in.) of H2O, what is the flow rate of the oil in m3/h(GPM) ?
Example A 10cm diameter square edge orifice plate is
used to meter the steady flow of 16 water ℃through a 20cm pipe. Flange taps are used and the pressure drop measured is 50cm Hg. Determine the pipe flow rate. The specific gravity of mercury is 13.6
expansion factors of the orifice a plate, venturi and flow nozzle, have been tabulated and are available in standard flow handbooks along with standardized construction, installation, and operation techniques.
When we select a velocity-based sensor to measure the flow rate (assume a laminar flow), where should we install the sensor to get an accurate reading on the flow rate?
RotametersRotameters 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.
RotametersThree types of forces must be accounted for when analyzing rotameter performance:
• Flow• Gravity• Buoyancy
Flow
Buoyancy
Gravity
For our analysis neglect drag effect
Variable Area Flowmeters
Fluid flow moves the float upward against gravity.
Float will find equilibrium when area around float generates enough drag equal to weight - buoyancy.
Some types have a guide rod to keep float stable.
Low Cost (pricing usually starts < $50)
Simple Reliable Design Can Measure Liquid or Gas
Flows Tolerates Dirty Liquids or Solids
in Liquid
Measuring Principles of Variable Area Flowmeters Flow Rate Analysis. The forces acting on the bob lead to equilibrium between: the weight of the bob fgVf acting downwards the buoyancy force 0gVf and the drag force F acting upwards.
• Where Vf is the volume and f is the density of the bob, γffg 0 is the density of the fluid, γ00g and • g is the gravitational acceleration:
Rangeability: 10 to 1 Pressure Loss: Medium Accuracy: 1 to 10% Straight Run Required: None Viscosity Effect: Medium Relative Cost: Low Sizes: <= 4” Connections: Threaded or Flanged Type of Output: Linear
Vortex Flowmeter Liquid, Gas, and Steam 1-12” (25 to 300mm) Temperature up to 750°F(400°C) EZ-Logic menu-driven user interface In-process removable sensor (below
750psig) Fully welded design with no leak path Optional remote mount electronic Accuracy
Insertion Vortex Meter Liquid, Gas, and Steam Model 60/60S Hot Tap, retractable Model 700 Insertion low temp, low pressure Model 910/960 Hot tap, retractable
960-high temp up to 500°F (260°C), high pressure
Optional Temperature and/or Pressure Transmitter
Line sizes 3-80” (76 to 2032mm) No moving parts EZ-Logic menu driven user interface Accuracy
Liquid ±1.0% of rate Gas and Steam ±1.5% of flow rate test
Liquid, Gas, and Steam Liquid flow velocity down to 1 ft/sec Model 60/60S Hot Tap, retractable Model 700 Insertion low temp, low pressure Model 910/960 Hot tap, retractable
960-high temp up to 750°F (400°C), high pressure
Optional Pressure and/or Temperature Transmitter
Line sizes 3-80” (76 to 2032mm) EZ-Logic menu driven user interface Nominal Accuracy
No Moving Parts Flow Range 1 to 15 ft/s (0.3 to 4.5 m/sec) Accuracy ±1.0% of Full Scale 1/2 to 20” Line Size Microprocessor-based electronics with
optional local display Maximum Fluid temperature 160°F (70°C) Model 2300 for acids, solvents, De-ionized,
and ultra pure water (1/2 to 8”) Model 2200 Fixed Insertion for (2 to 20”) Model 1200 for water, water/glycol (1-3”) Model 3100 retractable insertion (3-20”) Models 1200 and 2200 have Aluminum
Enclosure option for wet environments or heavy industrial installations
Ultrasonic flowmeters can be divided into Doppler meters and time-of-travel meters.
Doppler meters measure frequency shift caused by liquid flow. 2 transducers are mounted in a case. The frequency shift is proportional to the liquid velocity.
Time-of-travel meters have 2 transducers mounted on each side of the pipe. A time difference proportional to the flow can be detected.
Ultrasonic Flowmeters There are various types of ultrasonic flowmeters in use for
discharge measurement: (1) Transit time: This is today’s state-of-the-art technology
and most widely used type. This type of ultrasonic flowmeter makes use of the
difference in the time for a sonic pulse to travel a fixed distance.
First against the flow and then in the direction of flow. Transmit time flowmeters are sensitive to suspended solids
or air bubbles in the fluid. (2) Doppler: This type is more popular and less expensive,
but is not considered as accurate as the transit time flowmeter.
It makes use of the Doppler frequency shift caused by sound reflected or scattered from suspensions in the flow path and is therefore more complementary than competitive to transit time flowmeters.
Principle of transit time flowmeters.
Transit Time Flowmeter
Principle of Operation The acoustic method of discharge measurement is based on
the fact that the propagation velocity of an acoustic wave and the flow velocity are summed vectorially.
This type of flowmeter measures the difference in transit times between two ultrasonic pulses transmitted upstream t21 and downstream t12 across the flow.
If there are no transverse flow components in the conduit, these two transmit times of acoustic pulses are given by:
Since the transducers are generally used both as transmitters and receivers, the difference in travel time can be determined with the same pair of transducers. Thus, the mean axial velocity along the path is given by:
Example
The following example shows the demands on the time measurement technique:
Assume a closed conduit with diameter D = 150 mm, angle = 60°, flow velocity = 1 m/s, and water temperature =20°C.
This results in transmit times of about 116 s and a time difference
t =t12 – t21 on the order of 78 ns. To achieve an accuracy of 1% of the corresponding full-scale
range, t has to be measured with a resolution of at least 100 ps (1X10–10s).
Standard time measurement techniques are not able to meet such requirements so that special techniques must be applied.
Digital timers with the state-of-the –art Micro computers will make it possible to measure these time difference.
24 VDC Power (draws less than 100 ma) 115 VAC/ 230VAC or 12 VDC Optional Outputs 4 – 20 ma of Flow Rate Outputs 12 VDC Pulses of Totalized Flow (Solid State,
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.