Reynolds number measures the ratio between inertial forces and viscous forces. It is normally used in fluid dynamics and heat transfer applications. For instrumentation it is used as an indicator whether the flow is turbulent of laminar
ReD = pVD/m where
ReD = Reynolds Number
P = density
V = velocity
D = diameter
M = viscosity
As a general rule, the higher the flow, the higher the Reynolds number or turbulence. The higher the viscosity, the lower the Reynolds number and flow laminar
There are 4 types of flowmeters
DP type β Orifice, Venturi, Flow Nozzle, and VA Flowmeter
PD (Positive Displacement) Type
Velocity Type
Mass Type
Things to consider when selecting a flow meter
Liquid or Gas?
Clean or Dirty
Accuracy
Turndown ratio
Erosive or corrosive
Viscosity
Flow Rate
Laminar or turbulent
Temperature
Single phase or multiphase
Venturi Type Flowmeter and Flow Nozzle type
Advantage
Low pressure drop. Venturi has the lowest pressure drop
Proven
Disadvantage
Expensive
Longer installation length
Low turndown ratio (3:1)
Like orifice plate, needs long straight length requirement
When to Avoid
High Turndown ratio applications
Needs to be used at highly viscous fluids
Highly corrosive or erosive
High viscosity or laminar flow. This is because the flow pattern of laminar flow is not evenly distributed. It is only distributed in the middle
Flow Nozzle (low pressure drop, but not as low as venturi)
Venturi Flowmeter
Pitot Tube
Averaging Pitot Tube (Annubar)
Positive Displacement Meter
Directly measures volume
Types
Oval Gear Meter
Nutating Disc
Rotating Impeller
Rotating Lobe
Advantages
Good accuracy
Good turn down ratio
Can be used in high viscosity fluids
Directly measure volume
Disadvantages
Has moving parts, requires high maintenance. Needs to lubricate
High pressure drop
Gas and bubble gas reduce accuracy, therefore would require air eliminator
Expensive for large diameters
Cannot withstand turbulent flow or high Reynolds number since turbulent flow
When not to use
Unless to be used for metering, avoid!. Why? Because it has high DP and requires high maintenance
Vortex Flowmeter
When liquid flow is obstructed, it creates vortices. The frequency of these vortices created is proportional to flow
Vortex meter size should not be more than 8β. This is because more than 8β it is not economic since itβs too large
Advantage
Little maintenance
Good accuracy
Disadvantage
Low to medium pressure drop
Requires low Reynolds number < 20,000. Therefore long straight pipe length is required
Types
When not to us
Corrosive applications
Turbine Flowmeter
Advantage
High turndown ratio(10:1)
High accuracy
Fast response
Disadvantage
Has moving parts, requires high maintenance
Not suitable for viscous fluid
Requires straight length requirement
Affected by gas blow by. If there is gas, it will damage the rotors
When not to use
For steam application
For liquid with dual phase (which has gas), do not use turbine meter as the presence of gas will cause the turbine to spin faster. This will effect the accurac
Cannot use when there is pulsating flow such as when monitoring reciprocating transmitter. Therefore requires straight length requirement
Ultrasonic Flowmeter
Advantage
No pressure loss
No moving parts
Can detect zero flow
Disadvantage
Expensive
Complex
Accuracy susceptible to noise
When not to use
Near a noisy location like situated next to a control valve
Vibrating pipe
Magnetic Flowmeter
Advantage
No pressure loss
Withstands high corrosion
High Rangeability (40:1)
Good accuracy (0.5 to 1%)
Disadvantage
Meter must always be full (should be vertical or gravitated installation)
Cannot measure gasses
Liquid needs to be conductive (1-5 uS/cm)
When not to use
Liquid cannot be full and have bubbles
Liquid is not conductive
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