B. VENTURI METERS
a. A venturi tube also measures flow rates by constricting fluids and measuring a differential pressure drop.
b. In the upstream cone of the Venturimeter, velocity is increased, pressure is decreased.
c. Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument
Features of Venturimeters
a. Design Pressure: No limitation. Limited by DP transmitter/ pipe press ratings.
b. Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
c. Sizes: 25 mm to 3000 mm
d. Fluids/ Applications: Clean Liquids/ clean gases
e. Limited applications: Dirty /corrosive/viscous Liquids & Dirty gases
f. Flow range: limited only by pipe size and beta ratio.
g. MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze / haste alloy)
h. Accuracy: It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of DP transmitter varies from ±0.1% to ±0. 3% of full scale error.
i. Rangeability is 3:1 to 5:1.
j. Upstream length/ Downstream straight length is 20 / 5
Advantages of Venturimeters
a. Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costs
b. No process interruption for exchange of DP transmitter.
c. Can be used for temperature extremes-Cryogenics or High Temperatures
Disadvantages of Venturimeters
a. Highly expensive
b. Larger and heavier to handle.
C. ANNUBAR FLOWMETER
a. The probe is installed in the media line as a pressure sensor.
b. With the flow, the probe records both the static and the dynamic pressure via the probe openings.
c. In the minus chamber of the annubar, lying on the opposite side, only the static pressure has any effect
d. The differential pressure corresponds to the dynamic pressure in the pipeline & the flow can is calculated directly.