Measuring Flow.ppt

ACADs (08-006) Covered 1.1.2.1

5.4.1.7

Keywords

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

Supporting Material

Measuring Flow

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.

 Differentiate between Flow Rate and Total Flow  Describe the relationship between differential pressure and flow  Given a percent range of DP sensed by an orifice plate, calculate the percent flow  Describe the theory of operation of a given flow measurement device  Describe 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 minute We 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/~mmc191 9/venturi.html

%Flow  %Differrential pressure -or-

%Flow  %Differential Pressur 2

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 Giovanni Battista Venturi (1746 - 1822).

Venturi Tube

Flow Nozzle

Elbow Tap

Henri de Pitot (1695-1771)

Annubar

Rotameter

When flow exists, there is a constant differential pressure across the float

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 flow Watch the Coriolis Flow video-10 min

Electromagnetic Flow Measurement

Faraday’s Law

Pressure Loss

Typical Accuracy

Required upstream pipe diameters

Relative cost

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

None

Medium

Flowmeter Orifice

Positive Displacement

High

±0.5 of rate

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-turbine  Nutating Disk or piston  Hot wire method, temperature method  Pelton Wheel  Vortex shedding  Calorimetric method  Open Channels  Weirs  Target 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 to Measuring Temperature