ACADs (08-006) Covered

Keywords

Flow rate, total flow, differential pressure, percent flow, cotameter, Bernouli.

Description

Supporting Material

1.1.2.1 5.4.1.7

Terminal Objective: Given the appropriate equipment and procedures the I&C Technician will calibrate and maintain flow instrumentation. Mastery will be demonstrated by successful completion of a Lab

Performance Exercises and written Exam.

Measuring Flow

Differentiate between Flow Rate and Total FlowDescribe the relationship between differential

pressure and flowGiven a percent range of DP sensed by an

orifice plate, calculate the percent flowDescribe the theory of operation of a given flow

measurement deviceDescribe the considerations for

removing/restoring a rotameter from/to service

Flow Rate vs. Total Flow

Flow Rate is the amount of fluid, either in mass or volume that passes a point in a given time. Examples: Pounds mass per hour, gallons per minuteWe measure flow rate using ultrasonics, venturi, orifice

plates, etc.

Total Flow is the flow rate multiplied by an amount of time. It is the accumulated or ‘totalized flow. Result will be in either mass or volume.Gas pumps and water meters measure total flow

Daniel Bernouli

• Discovered the square root relationship between DP and flow

• Bernoulli's Principle states that as the speed of the fluid increases the pressure in the fluid decreases

• Therefore, across a restriction in flow, as the fluid velocity increases, pressure decreases

• http://home.earthlink.net/~mmc1919/venturi.html

pressure ialDifferrent%% Flow

Pressur alDifferenti%Flow% 2

-or-

Flow Instruments

Flow

Example 1

• Pressure drop range: 0-100 in H2O• Flow: 0 to 100 GPM• Currently reading 25 in H2O• What is the flow?

Example 2

• Pressure drop range: 0-100 in H2O• Flow: 0 to 1000 GPM• Currently flow is 80 GPM• What is the differential pressure?

• What devices may be used for the square root function?

Discovered two centuries ago by Italian scientist GiovanniBattista Venturi (1746 - 1822).

Venturi Tube

Flow Nozzle

Elbow

Tap

Henri de Pitot (1695-1771)

Annubar

When flow exists,

there is a constant differential pressure across the float

Rotameter

Replacing a Rotameter

•Isolate the instrument

•Drain

•Verify correct replacement part

•Install

•Verify pressure tight

•Un-isolate slowly to prevent slamming the float

•In service leak check

Flow Instruments: DP Cell

Ultrasonic Flow Measurement Devices

Examples: Controlotron, UFM

Coriolis

• Offer direct mass flow, volume flow, density, and temperature measurement of liquids, gases, and slurries

• No moving parts• Mass flow rate causes the vibrating tubes to

twist• The twist of the tubes is proportional to mass

flowWatch the Coriolis Flow video-10 min

Electromagnetic Flow Measurement

Faraday’s Law

Flowmeter Pressure LossTypical

AccuracyRequired upstream pipe

diametersRelative

cost

Orifice Medium ±2 to ±4 10 to 30 Low

Venturi tube Low ±1 of full scale 5 to 20 Medium

Flow nozzle Medium ±1 to ±2 10 to 30 Medium

Pitot tube Very low ±3 to ±5 20 to 30 Low

Elbow meter Very low ±5 to ±10 30 Low

Positive Displacement High ±0.5 of rate None Medium

Turbine High ±0.25 of rate 5 to 10 High

Vortex Medium ±1 of rate 10 to 20 High

Electromagnetic None ±0.5 of rate 5 High

Ultrasonic (Doppler) None ±5 of full scale 5 to 30 High

Ultrasonic (Time of Travel) None ±1 to ±5 5 to 30 High

Mass (Coriolis) Low ±0.4 of rate None High

Other methods of measuring flow

Rotary Vane Positive Displacement-turbineNutating Disk or pistonHot wire method, temperature methodPelton WheelVortex sheddingCalorimetric methodOpen ChannelsWeirsTarget meter

Read OE18368 at end of ‘Flow’ section in handout.

Discuss: What happened?

Could it happen here? What can we do to mitigate it?

Discuss control of test leads.

On toMeasuring Temperature