PRT 140: Lesson 6 Flow Measurement

Objectives

  • Define major terms associated with flow and flow measurement
  • Identify common types of flow sensing and measuring devices
  • Discuss and demonstrate the difference between total volume, flow rate, volumetric flow, mass flow
  • Net and Gross Flow (temperature corrections)
  • Review P&ID symbols for flow instrumentation
  • Demonstrate relationship between dP and flow rate

Reading

Chapters 6 and 7

  • Analytical Variables and Instruments
  • Miscellaneous Measuring Instruments

Terms to Know

  • Reynolds Number, Laminar, Turbulent
  • Volumetric Flow
  • Mass Flow
  • Net and Gross Flow – not in textbook, and important
  • Flow Instrumentation per lecture and notes

Flow – volumetric and mass

  • Movement of fluid
  • Flow rate = volume/time, or mass/time
  • gpm – gallons per minute
  • SCFH – standard cubic feet per hour – vapor
  • BPD – barrels per day – oil production
  • Lbs/hr – pounds per hour
  • m/s – meters per second – a velocity value

Reynolds Number

4 factors (Q: What haven’t we given you?)

  1. Velocity of fluid
  2. ID of pipe
  3. Density of fluid
  4. Absolute viscosity of fluid

LAMINAR flow – < 2,000

Streamlined flow; velocity varies over diameter of pipe

Laminar flow can increase risk of corrosion and scaling – why?

A diagram of laminar flow
Laminar Flow
[140-6-1a]

TURBULENT flow – >4,000

Fully turbulent at > 10,000

Fluid flow is consistent, well mixed, usually the desired condition

A diagram of turbulant flow
Turbulent Flow

[140-6-1b]

Mass Flow – LIQUID

  • Mass/Time (M/T)
  • Density = mass/volume = M/V
  • Volumetric flow = volume/time = V/T
  • Volumetric flow x Density = (V/T) x (M/V) = M/T
  • Density varies with temperature, so you need to know the exact density at the flowing temperature to calculate mass
  • Balance and convert all units as needed.

Net vs. Gross Flow – LIQUID

(also applies to total volume)

Gross Flow = volumetric flow rate at actual conditions (‘observed’) – what most flow instruments measure

Net Flow = volumetric flow rate converted to flow rate at standard conditions – usually 60 deg F

WHY NET? To keep consistent – measure flow at -20 deg F, then product heats up to 40 deg F – different observed amount of product. You wouldn’t want to keep changing the ‘amounts’ used in records, procedures, designs, sales, etc.

Volume Correction Factors (VCF)

  • Need T and VCF to calculate
  • VCF – different for each liquid
  • Discuss: Why don’t we need P?
  • Find VCF for observed T
  • Net Flow = Gross Flow x VCF
  • Net Volume = Gross Volume x VCF
  • Many different types of VCF data – charts, formulas, programs, internal programming in the instrumentation control system
Temperature
Deg F VCF
25 1.07
30 1.06
35 1.05
40 1.04
45 1.03
50 1.02
55 1.01
60 1.00
65 0.99
70 0.98
75 0.97
80 0.96
85 0.95
90 0.94
95 0.93
100 0.92
105 0.91
110 0.90

MASS FLOW, NET FLOW – GASES

  • Need both T and P to calculate – why?
  • Our old friend – Ideal Gas Law
  • Remember: P and T in ABSOLUTE units
    P1V1  =  P2V2
    _____       _____
    T1             T2
  • Instead of Volume, think volumetric flow: Ft3/hr

Net flow, gas = look at volumetric flow

Condition 1 = observed T, P

Condition 2 = standard T, P

  • 14.696 Pounds per Square Inch (psia)
  • 60 Degrees Fahrenheit (oF) (520oR)

Use this formula to calculate SCFH from CFH data

Flow Instrumentation

  • Direct vs. Indirect Measurement
  • Direct measurement – positive displacement
    • Sound familiar?
    • Very similar to P-D pumps – chamber within meter physically moves a set volume
    • Counter tallies the number of times the chamber fills/empties

Flow Elements

  • Most common indirect measurements use dP
  • Orifice Plate, Venturi, Flow Nozzles, Annubar, Pitot

NOTE: These are just the sensing elements – still need some kind of transmitter to create data from the change in dP (differential pressure).

A photo of an orifice plate
Orifice Plate is used for measurement.
[140-6-2a]
A diagram of a flow nozzle
Flow Nozzle Sections
[140-6-2b]

Example Problems

B Orifice Plate A Measures flow using a tube with several openings and then averaging all flow measures.
D Flow Nozzle B Measures flow using a metal disc containing a drilled opening
E Venturi Tube C Measures flow using an L-shaped tube and another tube that compares the impinging pressure with static pressure.
C Pitot Tube D Measures flow using a tapered inlet device inserted into a flange connection/spool piece
A Annubar ® E Measures flow using a cone-shaped device with inlet and outlet components

dP vs. Flow

  • Flow rate is proportional to the square root of the differential pressure
  • Consider the full range of flow and dP that we measure
  • Look at % of full range:
  • The % of full flow range will vary as the square root of the % of full dP range, or…
  • The % of full dP range will vary as the square of the % of full flow range
  • %F = √%dP, or
  • %dP = (%F)2
  • dP = differential pressure through an element
  • F = flow rate
  • We’ll look at a change in the dP, and a change in the Flow

Example Problems

  1. 60 % dP
    • 60% = 0.60, square root = 0.775
    • = 77.5% flow
  2. 50% dP
    • 50% = 0.50, square root = 0.707
    • = 70.7% flow
  3. 45% dP
    • 45% = 0.45, square root = 0.671
    • = 67.1% flow
  4. 36% dP
    • 36% = 0.36, square root = 0.60
    • = 60% flow

% Total Flow vs. % Total dP

%F = √%dP

A chart showing the resulting curve of %F = √%dP
%F = √%dP
[image 140-6-3]

% Total dP vs. % Total Flow

%dP = (%F)2

A chart showing the resulting curve of %dP = (%F)2
%dP = (%F)2
[image 140-6-4]

% Span Calculations

  • Material covered more thoroughly in Week 10, but we’ve been using it all along.
  • Range = the Lower (LRV) and Upper (URV) in the range of the signal, instrument, or process value.
    • Example: 4-20 mA signal, the Range is 4 mA (LRV) to 20 mA (URV)
  • Span = the difference between the URV and the LRV
    • 4-20 mA signal, Span is 16 mA

SPAN, Operating Range

  • SPAN = URV – LRV
  • Operating Range is ‘LRV to URV’
  • Temperature transmitter calibrated for operating range 100 deg F to 400 degF
    • Span = 300 deg F
  • Temperature transmitter calibrated for operating range 1500 deg F to 1800 degF
    • Span = ?????

Scaling – Determining Values for % range

  • Scale represents 0-100% of measured process variable
  • 4 mA = 0%
  • 20 mA = 100%
%Span equals Value minus LRV multiplied by 100%, divided by Span.
%Span Equation
[image 140-6-5]

Span, %Span

Percent of ScaleInputOutput
0%500ºF4 mA
25%625ºF8 mA
50%750ºF12 mA
75%875ºF16 mA
100%1000ºF20 mA

Scaling: What is %span for operating data?

  • Measure operating range – low end is LRV, high end is URV, difference is Span
  • Calculate % span through the range:
    • Operating valuex = (% desired x Spanx) + LRVx
  • Example: What is the 35% point in a temperature scale that reads between 55 and 172 deg F?
    • LRV = 55 deg F
    • URV = 172 deg F
    • Span = 117 deg F (172 F – 55 F)
  • (.35 x 117 F) + 55 F = 96 F

Homework problem – %span

  • Flowmeter calibrated from 45 gpm – 230 gpm
  • Analog signal = 4 mA – 20 mA
  • Testing flow rates at listed % span?
  • What is LRV, URV, Span of Flow data?
  • What is LRV, URV, Span of mA data?

VALUEB = [(% SPANA) x SPANB] + LRVB

Example:

  • Value = [(0.14) x 185 gpm] + 45 gpm
  • Value = 25.9 gpm + 45 gpm
  • Flow Value = 70.9 gpm
  • Value = [(0.14) x 16 mA] + 4 mA
  • Value = 2.24 mA + 4 mA
  • Signal Value = 6.24 mA

Shop Demo – dP vs Flow

  • DAC Pump demo unit
  • Globe valve to create 3 psi dP in upper spool
  • Take readings of flow, dP at the following settings
  • Flow at 15 gpm, 12 gpm, 9 gpm, 6 gpm
  • What is 100% dP range?
  • What is 100% flow range?
  • How closely does it follow the calc plan?

Flow Instruments

Rotameter – Fluid flows through the device, lifting a free-floating indicator called a float. The position of the float is referenced to calibrated marks to indicate the flowrate.

Magmeters – Produces a magnetic field that penetrates the flow tube; liquid is the conductor flowing at right angles to the magnetic field. This creates an electrical potential, sensed by electrodes. (voltage)

Flow Instruments – Turbine Meter

  • Free-spinning turbine (fan) in flowing liquid-
  • The rpm of spinning fan is proportional to flow rate
  • Rpm generates a pulse
  • Calibrated with the ‘K-factor’ to determine actual flow rate – pulses/gallon

Flow Instruments – Mass Flow

  • Coriolis Meter – does not need external compensation for temperature, etc.
  • Fluid flows through a vibrating coil – sensors measure the twisting, oscillation, and can calculate velocity, flow, mass, etc. from all this data.
  • Video illustrates this principle
  • Very powerful tools – lots of data from one instrument.

dP transmitters

  • We’ve seen dP transmitters used to calculate dP, level, density – now we can calculate flow
  • Uses the Bernoulli principle, and the relationship between dP and flow rate (already discussed)

Vortex Meter

  • Common process meter – minimal pressure drop
  • The vortex element extends into the process fluid, disrupts flow – creates eddies (vortices) around the ‘bluff body’ of the element
  • Sensors pick up the pressure fluctuations caused by these eddies – the even signal is proportional to flow rate (calibrated)

Flow Meters – Drawing Symbols

Flow meter drawing symbols
Flow meter drawing symbols
[140-6-6]
Flow meter drawing symbols
Flow meter drawing symbols
[140-6-7]

PID from homework

What’s happening with these instruments?

An instrument diagram
What is going on in this diagram?
  • Why is TE connected to the FY?
  • Is this a gas stream or a liquid stream? How do you know?
  • What does the C mean?
  • What type of flow meter is it?
  • What type of TE is it?
  • What type of signals are used?
TagFunctional
Description
Notes
FE-001Flow ElementNote that there is a separate FE/FT drawn. Not always the case. If you draw only one, use the FT
FT-001Flow Transmitter
FY-001Flow ComputerFlow Computer is calculating the mass flow rate using flow and temperature data. The "Y" can indicate different instruments - have to look at the function.
FI-001Flow Indicator
TW-002Thermowell
TE-002Temperature ElementNote that this must be a combined TE/TT since it sends a signal.
TI-002Temperature Indicator