Measuring Flow 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

Measuring Flow What are we really measuring? Volume? Mass? Instantaneous flow or how much flow has passed by in an second? Minute? Hour? Flow rate is the instantaneous flow but is expressed in the time domain, as in gallons per second or pounds-mass per hour, assuming flow remains constant for that time period. Total flow is how much flow HAS PASSED in the last second, minute, hour or other time period. It is an integration of flow over time. The gas pump is a total flow device. Who cares how many gallons per second pass as long as you get the right amount of gas. Difference between volumetric and mass flow rate: Gallons are a unit of volume Pounds are a unit of mass 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 Read the objectives

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 Daniel Bernouli, 1700-1782, Dutch scientist

-or- If we put the output of a differential pressure transmitter directly to an indicator, then vary flow it would look like this For 0% flow we have zero dp For 10% we only get 1% dp For 50% flow we’re up to 25%dp For 75% flow we have 56.25%dp and for full flow we get full dp Talk about square root extractors Or simply marking the face plate Show the high inherent amplification of square root extractors on their low end and talk about noise in the process Differential pressure flow measurement stinks on the low end

Flow Instruments The downside: measuring flow with differential pressure requires and obstruction in the pipe, typically an orifice plate. The problem with obstructions in your pipe: They rob energy by creating a permanent pressure drop. Your process never gets back up to pressure after the orifice. At 10¢/kWh and continuous operation, a 1.65 psi pressure drop in a 1000 gpm flow will cost $1000 per year in electrical costs Also discuss turbulence caused by the obstruction

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?

Different orifice plates for different uses. What do you want to pass? Is your process dirty? Sludge? Which direction should an orifice plate face? Draw a cross section.

Manometer / U-tube flow indicator

Discovered two centuries ago by Italian scientist Giovanni Giovannin Battista Venturi, 1746 to 1822 Discovered two centuries ago by Italian scientist Giovanni Battista Venturi (1746 - 1822).

Venturi tube reduces total head loss but still does not bring it to zero Costs lots more than an orifice plate Venturi Tube

Flow tube or Flow Nozzle

Elbow Tap Used in the circ water system

Henri de Pitot (1695-1771) Used in the condensate system Tell story of the bent pitot tubes in the condensate lines Henri de Pitot (1695-1771)

Annubar Good on air systems, HVAC Generates low differential pressures, inches of water or fractions of an inch Annubar

there is a constant differential pressure across the float Rotameter Rotameters and the various floats 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 Notice the faceplate and how it is marked. The marking does it’s own square root extraction. Note the distance between 0 and 10, then the spacing between 20 and 30 Flow Instruments: DP Cell

Ultrasonic Flow Measurement Devices Examples: Controlotron, UFM Two methods of ultrasonic flow measurement: Doppler - good on dirty fluids and slurries, not terribly accurate Time of travel - best on clean fluids with not obstructions for the sound, fairly accurate Ultrasonic Flow Measurement Devices Examples: Controlotron, UFM

Watch the Coriolis Flow video-10 min 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 Watch the Coriolis Flow video-10 min

Electromagnetic Flow Measurement Faraday’s Law

Turbine Flow Meter Used in Radwaste control on slurry mixtures of resin Tell story of John Cole and I getting crapped up Signal generated by the rotating turbine is proportional to flow rate Pulses no square root conversion Magnetic pickup No ‘hole’ in the process piping

Tradeoffs

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