POSITIVE DISPLACEMENT FLOWMETERS (Fig. 6):
- This meter repeatedly entraps the fluid into a known quantity and than passes it out.
- The quantity of the fluid that has passed is based on the number of entrapments.
- The volume flow rate can be calculated from the revolution rate of the mechanical device.
FEATURES OF POSITIVE DISPLACEMENT (PD) FLOWMETERS:
- Design Pressure: 1500 PSIG (liquids)/ : 100 psig (gases)
- Design Temperature : 293 Deg. C (liquids)/ : -34 to 60 Deg. C (gases)
- Sizes: 6 mm to 400 mm
- Fluids : Clean Liquids/ gases
- Flow range: 0 – 20000 GPM (liquids)/ : 0 – 3000 cub.m/hr (gases)
- MOC: mostly in aluminum, stainless steel, plastics, hastelloy
- Accuracy is + 0.5% to + 1% of flowrate
- Range ability is 15 : 1
ADVANTAGES OF PD FLOWMETERS:
- Good accuracy and high range ability
- Can be used in viscous liquid flow
- Low to medium initial set up cost
- Require no power supply and available in wide variety of read out devices
DISADVANTAGES OF PD FLOWMETERS:
- Maintenance required at frequent intervals because of the `moving parts.
- High pressure drop due to obstruction
- Not suitable for low flow rate
- Not suitable for fluids with suspended solids
- Gas (bubbles) in liquid could significantly decrease the accuracy
THERMAL MASS FLOWMETER (Fig. 6):
- Operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer.
- Gas flow passes over two PT100 RTD transducers.
- The temperature transducer monitors the actual gas process temperature, whilst the self-heated transducer is maintained at a constant differential temperature by varying the current through it.
- The greater the mass flow passing over the heated transducer, the greater current required to keep a constant differential temperature.
- The measured heater current is therefore a measure of the gas mass flowrate.
FEATURES OF THERMAL MASS FLOWMETERS:
- Design Pressure: 1200 PSIG
- Design Temperature: 176 Deg. C
- Sizes: 15 mm to 1000 mm
- Fluids : Clean gases
- Flow range: 0 – 2500 SCFM
- MOC: mostly in stainless steel/ glass, teflon, monel
- Accuracy is +1% to + 2% of flowrate
- Range ability is 10 : 1 to 100:1
- Upstream length/ Downstream straight length is 5/ 3
ADVANTAGES OF THERMAL MASS FLOWMETER:
- No temperature or pressure compensation required
- Linear output (as temperature differential is proportional to mass flow)
- Can be used on corrosive process streams if proper materials are specified
- DC voltage or 4 to 20 mA dc outputs available
DISADVANTAGES OF THERMAL MASS FLOWMETER:
- Practical for gas flows only
- Subject to blockage by foreign particles or precipitated deposits due to small openings in flowmeter
- Power requirements excessive in larger pipe sizes
- Has to taken out of process line for servicing
- Accurate field calibration is difficult
CORIOLIS MASS FLOWMETER (Fig. 7):
- When a moving mass is subjected to an oscillation perpendicular to its direction of movement, Coriolis forces occur depending on the mass flow.
- When the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube.
- On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist.
- This twisting movement is sensed by a pick up and is directly related to mass flow rate
FEATURES OF CORIOLIS MASS FLOWMETERS
- Design Pressure: 345 bar
- Design Temperature: 200 to 426 Deg. C
- Sizes: 1.5 mm to 150 mm
- Fluids/ Applications : Liquids (clean/ dirty/viscous/ slurries) clean /liquified gases
- Flow range: 0 – 25000 lb/m
- MOC: mostly in stainless steel, hastelloy/titanium
- Accuracy is + 0.15% to + 0.5% of flowrate
- Range ability is 20 : 1
- Bidirectional flow measurement
ADVANTAGES OF CORIOLIS MASS FLOWMETERS:
- Capable of measuring difficult handling fluids
- Independent of density changes, flow profile and flow turbulence. Hence straight lengths are not required.
- No routine maintenance required since no moving parts
- High accuracy
DISADVANTAGES OF CORIOLIS MASS FLOWMETERS:
- Not available for large pipes (upto 150 mm only)
- High flow velocities required for detection resulting in high pressure drop
- Expensive compared to other flowmeters
- Difficulty in measuring low pressure gases.
APPLICATIONS OF FLOWMETERS:
CLEAN LIQUIDS/ GASES:
- Variable Area
- Magnetic (only liquids)
- Coriolis Mass Flowmeters
- Thermal mass flowmeter (only gases)
- PD meters
- Most suited: Magnetic/Coriolis Mass Flowmeters
- Limited applications: Venturi meters
- Most suited: Vortex meters
- Limited applications: Venturi meters/Thermal mass flowmeter/Variable area flowmeter
CORROSIVE LIQUIDS: Magneticflowmeters/Ultrasonic flowmeters
NON-NEWTONIAN LIQUIDS: Coriolis Mass Flowmeters
VISCUOUS LIQUIDS: Coriolis/Magnetic/Positive Displacement Meters
ABRASIVE SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters
FIBROUS SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters (limited applications)
- Most suited: Orifice DP meters/ Vortex flowmeters
- Limited applications: Venturi meters/Variable area meters
- Most suited: Orifice DP meters
- Limited applications: Venturi meters
CRYOGENIC: Venturi meters/Orifice plates
I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.