Flow Measurement

Muhajir Ab. RahimSchool of Mechatronic Engineering

Universiti Malaysia Perlis

Introduction

• In the process or manufacturing industries, the movement of raw materials, product and waste are very important.

• Optimum performance of some equipment and operations require specific flow rates.

• Generally, there are two types of flow; solid-flow and fluid-flow.

Solid-Flow Measurement

• A common solid-flow measurement occurs when material in the form of particles, such as crushed material or powder, is carried by a conveyor belt system.

• The flow is usually described by a specification of mass or weight per unit time that is being transported by the conveyor system. (kg/min or lb/min)

• Flow rate can be calculated by;Q = WR L

Q = flow (kg/min or lb/min)

W = weight of material on section of length L

R = conveyor speed (m/min or ft/min)

L = length of weighing platform (m or ft)

Exercise

• A coal conveyor system moves at 100 m/min. A weighing platform is 5m in length, and a particular weighing shows that 75kg coal are on the platform.

a) Find the coal delivery.

b) What kind of flow sensor is used?

a) Q= (WR)/ L = [75kg][100m/min]/[5m] = 1500kg/min

b) Load cell (strain gauge) is used to measure the weight of the coal

Fluid-Flow Measurement

• The determination of fluid physical properties like pressure, density, viscosity and velocity are very important in liquid flow measurement.

• The viscosity of a fluid refers to its physical resistance to flow. The most widely used viscosity unit is centipoise.

• The velocity of a flowing fluid is its speed in the direction of flow. The behavior of a fluid is determined by its velocity.

• Reynolds Number is used to describe the nature of flow.

R = vDρ μ

v = velocity (ft/s)

D = inside diameter of pipe (ft)

ρ = fluid density (lb/ft3)

μ = viscosity (centipoise)

Flow Characteristics

1. Laminar flow of a liquid occurs when its average velocity is comparatively low and the fluid particles tend to move smoothly in layers, with the fastest moving layers toward the center and the slowest moving layers on the outer edges of the stream. (parabolic shape)

2. Turbulent flow occurs when the flow velocity is high and the particles no longer flow smoothly in layers and turbulent or rolling effect occurs. (the layers disappear and the velocity across the stream is more uniform)

R < 2000

R > 4000*R = Reynolds Number

Flow Measurement

• Flow measurements are normally indirect measurements using differential pressures to measure flow rate.

• One of the most common methods for measuring the flow of liquids in pipes is by introducing a restriction in the pipe, and measuring the pressure drop that result across the restriction.

• When such a restriction is placed in the pipe, the velocity of the fluid through the restriction increases, and the pressure in the restriction decreases.

• As the flow increases, the pressure drops.

Flow Measurement

Q = K(∆p)1/2

V = Q/A F = ρQ

Q = volume flow rate (gal/min or m3/h)

K = a constant for the pipe and liquid type

∆p = drop in pressure across the restriction

V = flow velocity (m/min or ft/min)

Q = volume flow rate

A = cross-sectional area of flow carrier/ pipe

F = mass or weight flow rate (kg/h or lb/h)

ρ = mass density or weight density

Q = volume flow rate

The most common instruments used for measuring fluid flow are;

1) Restriction Flow Sensors

2) Pitot Tube

3) Obstruction Flow Sensor

4) Electromagnetic Flow Meter

Flow Measuring Instrument

Restrictions Flow Sensors

Venturi

Nozzle

Orifice Plate

Venturi Tube- the most expensive and the most accurate, difficult to install

Flow Nozzle- simple and cheap, but less accurate

Orifice plate- simple and inexpensive, the easiest to install but less accurate

Pitot Tube

• It is used to measure flow rate at a particular point in a flowing fluid (liquid or gas)

• The Pitot tube consists of a tube with a small opening at the measuring end. This small hole faces the flowing fluid.

• When the fluid contacts the Pitot tube, the fluid velocity is zero and the pressure is at the maximum.

• This small hole or impact opening, provide the higher pressure for pressure measurement. While the ordinary pressure tap provides the lower pressure reading.

Obstruction Flow Sensor

Rotameter

Moving vane flow meter

Turbine flow meter

Obstruction Flow Sensor• In Rotameter, the obstruction is a float that rises in a vertical tapered

column. The lifting force and thus the distance to which the float rises in the column is proportional to the flow rate. The lifting force is produced by the differential pressure that exists across the float, because it is a restriction in the flow. This type of sensor is used for both liquids and gases.

• A moving vane flow meter has a vane target immersed in the flow region, which is rotated out of the flow as the flow velocity increases. The angle of the vane is a measure of the flow rate. If the rotating vane shaft is attached to an angle-measuring sensor, the flow rate can be measured for use in a process-control application.

• A turbine type of flow meter is composed of a freely spinning turbine blade assembly in the flow path. The rate of rotation of the turbine is proportional to the flow rate. If the turbine is attached to a tachometer, a convenient electrical signal can be produced.

• In all of these methods of flow measurement, it is necessary to present a substantial obstruction into the flow path to measure the flow. For this reason, these devices are used only when an obstruction does not cause any unwanted reaction on the flow system

Electromagnetic Flow Meter

• This technique is only valid for electrical conducting fluids, by measuring the emf induced across the fluid stream when it passes through magnetic field.

How to Select?

• Before you start measuring a flow rate, you need to know, what type of fluid that you are dealing with. What is the fluid properties like viscosity, conductivity, etc.

• The higher accuracy of instrument, the expensive it will be, and more difficult to be installed.