PRT 140: Lesson 6 Flow Measurement

LAMINAR flow – < 2,000

Streamlined flow; velocity varies over diameter of pipe

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

TURBULENT flow – >4,000

Fully turbulent at > 10,000

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

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

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).

Example Problems

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

Span, %Span

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?

VALUE_B = [(% SPAN_A) x SPAN_B] + LRV_B

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

PID from homework

What’s happening with these instruments?

• 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?