CHAPTER 6
FLOW MEASUREMENT & MEASUREMENT OF LIQUID LEVEL
Flow measurement
A technique used in any process requiring the transport of a
material from one point to another
Types of flow
Laminar flow
Turbulent flow
Re < 2000
Re > 4000
Critical zone between
laminar & turbulent
2000<= Re <= 4000
Reynolds Number, the non-dimensional velocity
defined as the ratio of
•the inertia force (ρ u L)
the viscous or friction force (µ)
interpreted as the ratio of
dynamic pressure
(ρ u^2),
shearing stress (µ u / L)
Re = (ρ u^
2) / (µ u / L)
= ρ u L / µ
= u L / ν
Re = Reynolds Number (non-dimensional)
ρ = density (kg/m^3), lbm/ft^3
u = velocity based on the actual cross section area of the duct or pipe (m/s, ft/s)
µ = dynamic viscosity(Ns/m^2,lbm/s ft)
L = characteristic length (m, ft)
ν = kinematic viscosity
(m^2/s, ft^2/s)
Elements that needs to consider during flow measurement
Clean or dirty
Wet or dry
Hazardous/corrosive or safe
laminar or turbulent
Pressure (vacuum ~ high pressure)
Temperature
Flow rate
Flow measuring devices
Primary devices (quantity meter)
Measure the amount of fluid in terms of mass or volume that
flows past a given point in a definite period of time
Advantages
Simple, economic and extremely accurate
Device
.Nutating Disc Meter
The simple and reliable operating principle allows water to flow into the chamber, which causes the disc to nutate. Each disc nutation represents a fixed quantity of water, so the total flow is calculated by the total number of nutations.
Sliding-Vane-Type Meter
Application
domestic water
system
is a positive displacement flow meter that create a high accuracy measurement and low pressure loss by decreasing rotation resistance
Application
metering of gas and
refined petroleum products
Lobed impeller flow meter
Two rotating impellers, designed with a figure eight cross section, rotate in opposite directions due to the forces exerted by the gas flow being measured. The shape of the impellers prevents contact while the gap between them remains constant.
Advantages
Excellent measuring accuracy for gas measurements
No inlet and outlet sections required
No external power supply
Secondary devices or Rate Meters
its not measure the flow directly but instead measure another physical quantity
which related to flow.
Pitot Tube
Commonly used variable head velocity measuring device
Also known as the air-speed indicator used in aircrafts
Invented by Henri Pitot in 1732
In industrial applications, pitot tubes are used to measure air flow in pipes, ducts, and stacks, and liquid flow in pipes, weirs, and open channels.
