Flow MeasurementFlow Measurement

Instrument Training Material

AimAim

Understand the basic principle of flow measurement.

Knowing different type of flow measuring device available in YYCG.

Flow!Flow!

What?– Movement of a liquid or a gas from

one point to another. Requirement for flow to occur?

– Upstream pressure MUST be higher then downstream.

Flow Profile TypesFlow Profile Types

Flow MeasurementFlow Measurement Through a closed piping is one of the most

important aspects of process control. Flow is generally measured by measuring

velocity VV through a known area AA.

AAVV

Flow MeasurementFlow Measurement Volumetric flow:

Q vQ v = A · V = A · VWhere:

A = the cross sectional area of the flow carrier (pipe)V = the fluid’s velocity

Units: – m3/day, – m3/hr, – liters/min, – gal/min, – ft3/sec,

Flow MeasurementFlow Measurement Mass flow,

Q m = Q m = Q v · Q v · Where:Qv is the Volumetric Flow Rate.is the density of the liquid.

Units:– kg/hr,– ton/day,– lb/hr,– lb/sec.

Flow MeasurementFlow Measurement Flow rate.

– easure of flow at specific point of measurement. Reynolds Number

– Basic equations of flow assume the velocity of flow is uniform across a given cross section.

– Re = vD/– Where = the flow velocity– D the inside diameter of the pipe– is the fluid density– is the fluid velocity

Differential Pressure Flow meterDifferential Pressure Flow meter

Contains two elements– Primary element

Some type of restriction that is place inside a pipe to cause a differential pressure

– Secondary element Normally a differential pressure transmitter

DP Flow MetersDP Flow Meters

When liquid flow through the restriction– Velocity increases

– Pressure decreases Pressure upstream is higher than the

pressure downstream

DP Flow MetersDP Flow Meters Flow rate through the restriction is

proportional to the square root of the DP across the restrictionV = k Sq (DP)V = k Sq (DP)

The DP TX measures the different in pressure or delta P and converts that value to an output that can be used by signal conditioning equipment to indicate flow rate

Dp

Head Meters or DP Flow MetersHead Meters or DP Flow MetersBased on measuring the differential pressure created by the fluid when flowing through a restriction installed in the process line.Velocity is directly proportional to the square root of the differential pressure across the restriction,restriction, ; where constant

V = k dPV = k dP

where ß = d / D ß = d / D;; d d is constriction diameter in m; D D is pipe diameter in m; dP dP is deferential pressure in N/m2;p -p - the density of the liquid, kg/m3; C - C - discharge coefficient(actual flow/theoretical flow).

Q v = A · V =Q v = A · V =p d p d 22

4422pp

111-ß1-ß 44 dPdPCC ••••

22pp

111-ß1-ß 44k = k =

Head Meters or DP Flow MetersHead Meters or DP Flow Meters Generally cheaper, simple,

reliable and offer more flexibility than other flow measurement methods.

Consists of two components: Primary device - placed in the pipe to restrict the flow and develop a differential pressure.

Secondary device - measures the differential pressure and provides a readout or signal for transmission to a control system.

Types of Primary ElementsTypes of Primary Elements Orifice plate

– Widely used in industrial applications

– It is simple and least expensive

– Produces a greater overall pressure loss

– Effectively utilized for clean fluid where line pressure losses or pumping costs are not critical

Orifice plateOrifice plateOperating principle: Plate inserted in process line. Differential pressure developed across the orifice

plate is measured to determine the flow rate. Maintain steady flow through the orifice plate, the

velocity must increase as it passes through the orifice.

Lowest pressure occurs where the velocity is the highest.

Greater the flow larger the dp across the orifice plate.

Types of Primary ElementsTypes of Primary Elements Type of orifice Plate:

Types of Primary ElementsTypes of Primary Elements Venturi Tubes.

– exhibit a very low pressure loss.– the largest and most costly.– operate by gradually narrowing the diameter of the pipe.– Their applications are generally restricted to those

requiring a low pressure drop and high accuracy reading.

Types of Primary ElementsTypes of Primary Elements

Flow nozzlesFlow nozzles may be thought of as a variation on the Venturi tube. The nozzle opening is an elliptical restriction in the flow but with no outlet area for pressure recovery. The flow nozzle is a high velocity Flowmeter used where turbulence is high such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls between that of the Venturi tube and the orifice plate.

Types of Primary ElementsTypes of Primary ElementsPitot TubesPitot Tubes consists of two hollow tubes that

sense the pressure at different places within the pipe. These tubes can be mounted separately in the pipe or installed together in one casing as a single device. One tube measures the stagnation or impact pressure (velocity head plus potential head) at a point in the flow. The other tube measures only the static pressure (potential head), usually at the wall of the pipe.

Advantages of DP flowmetersAdvantages of DP flowmeters Universally suitable for applications

involving liquids, gas and steam. Excellent method, even under extreme

process conditions, high temp and high pressure.

Dp transmitters can be replaced during operation with process shutdown.

Robust primary elements, no entirely mechanical and no moving parts.

Disadvantages of Dp flowmetersDisadvantages of Dp flowmeters Suitable for low-viscosity fluids Metering gas flow require steady

process conditions, pressure, temperature must be constant

Measuring system required additional valves and impulse lines

Variable area flowmetersVariable area flowmeters Simple and Cost Effective for measuring Liquid or

Gases. Consist of upright, tapered tube in which a float

made of glass or metal hovers on the fluid flow entering the bottom of the tube.

The force exerted on the float by the fluid depends on its density, viscosity and rate of flow.

Variable area flowmetersVariable area flowmeters Higher the flow rate and, consequently, the force of

the flow, the higher the float will rise in the tapered, graduated tube.

The annular gap between the float and the wall of the tube widens as the float moves up, until the force acting on the float are balanced and the float hovers at a steady height.

Variable area flowmetersVariable area flowmeters

Advantages:– Usable for liquid, gas or steam application

– Low cost method of flow metering due to the simple design

– No power supply needed

– Devices with sight glasses allow for an easy and dependable on-site process and flow monitoring

– Low pressure losses

Variable area flowmetersVariable area flowmeters

Disadvantages.– Measuring accuracy depends on the process

conditions and the fluid properties.

– Fluid specific calibration necessary.

– Limited turndown (Max. 10:1).

– Affected be entrained matter.

– Can be used only for low-viscosity fluids.

– No totalizing function.

– Meter has to be installed in a vertical pipe.

Vortex MetersVortex Meters The operating principle.

– based on the phenomenon of vortex shedding known as the von Karman effect.

– As fluid passes a bluff body, it separates and generates small eddies or vortices that are shed alternately along and behind each side of the bluff body.

– These vortices cause areas of fluctuating pressure that are detected by a sensor.

– The output depends on the calibration constant, or K-factor.

Vortex EquationVortex Equation

Vortex FrequencyFlow Rate =

K-factor

Vortex MeterVortex Meter In this example, the passage of a vortex causes a

slight bow of a wing placed downstream of the bluff body.

The bend is measured by a piezoelectric crystal sensor in contact with the top of the wing.

E +H Promag

Magnetic Flow meterMagnetic Flow meter Based on Faraday’s Law of Magnetic Induction,

which states that a voltage is induced into a conductor moving through a magnetic field.

With electromagnetic measuring principle, the flowing fluid is the moving conductor.

Magnetic Flow meterMagnetic Flow meter The induced voltage is

proportionally related to the flow velocity.

Using the pipe cross-sectional area, the flow volume is calculated.

The DC magnetic field is generated by a switched direct current of alternating polarity.

Turbine Flow MeterTurbine Flow Meter Uses a multi-bladed rotor that

is supported by bearings within a pipe section perpendicular to the flow.

Fluid drives the rotor at a velocity that is proportional to the fluid velocity and, consequently, to the overall volume flow rate.

A magnetic coil outside the meter produces an alternating voltage as each blade cuts the coil’s magnetic lines of flux.

Turbine Flow MeterTurbine Flow Meter Typical schematic of pick-up

assembly and generated waveform are shown.

When the blade at point A passes the cone point B , the magnetic field produced by the permanent magnet is deflected by its presence, generating a voltage in the coil (positive pulse in the waveform).

Turbine Flow MeterTurbine Flow Meter A negative pulse is

produced when the blade travel from B to C.

One pulse represents a unit volume, hence, turbine output can be rated in pulses per gallon or any other units desired.

Thank YouThank You

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