1. Washington University ChE 433 Digital Process Control Laboratory
Flow Meter
&
Control Valve
Lecture

2. Flow Meters, Transmitters
Flow - Most frequently measured process variable Classifications
Head Producing
Positive Displacement
Velocity
Mass
Magnetic

3. Factors Affecting Flow
Flow rate related to velocity and area:
Reynolds Number; dimensionless number relating to flow regime
<2000 laminar; >4000 turbulent; between is transitional

4. Head Producing Flow Meters
Develop a permanent pressure loss across a flowing obstruction.
Measures Fluid Volume Rate not Mass

5. Orifice Plate and Flanges Orifice is a hole in a plate mounted between two special flanges

6. Head Producing Flow Meters
Differential Pressure, “D/P cell”
Mechanical type, nozzle baffle

7. Head Producing Flow Meters
Used to create differential pressure D/P; Square root of D/P proportional to flow rate from Bernoulli’s equation. Must be in turbulent flow regime. Normal flow 70% of maximum
Can measure Liquids, Gasses or Vapours
Taps can be either flange, pipe 2 ½ by 8 Diameters, VC, elbow.
Use 2 ½ by 8 for large diameter pipes, orifice plates are expensive.
Need to provide long straight runs upstream and downstream, or straightening vanes.
~ 50% of the flow meter installations are DP
Can mount transmitter directly to orifice flanges with 3-valve manifold.
Use meter run for small diameter, 2” or lower, for better accuracy.

8. Head Producing Flow Meters
3 to 5 % accuracy; 3 to 1 turn down
Have permanent pressure loss, ~ 40” H2O typical. Extra Power = $$
There are several computer programs, some form vendors, to calculate the bore and differential. However, you will have to do this by spreadsheet or calculator.
Problems: Can be expensive to pipe, many fittings, potential for leaks, fugitive emission losses; sensing lines plug, freeze.
Need long straight run of pipe upstream/downstream, or straitening vanes.
Newer designs require less straight pipe run. There are several different designs, venturi tube, wedge, and Annubar of meter elements, but all develop a “pressure head” that increases with increasing flow.

9. Wedge Meter
wedge.pdf

10. Rosemount 485 Annubar® Primary
Head Producing
                                               

11. (Save this thought, More later)
Head Producing Meters
Head, or Pressure, producing meters can also be used as level and pressure transmitters
(Save this thought, More later)

12. Equations for Bore Size: (See Notes)
Calculate Reynolds number by:
Flow Coefficient, K for  0.35 to 0.65, usually target 0.6
Calculate plate expansion factor and gas expansion factor, for gasses.

13. Equations for Bore Size: (See Notes)
Liquid Flow Equation:
Gas or Vapour Equation, D bore diameter, inches; G in lb./ft^3:
Calculate as an iterative process, see notes for details

15. Washington University Orifice Example /5/ 3   Pipe, orifice, and meter data   * Beta ratio Actual pipe diameter inches * Orifice diameter inches Drain or vent diameter inches   Type of orifice taps Flange Maximum differential (meter dry) in. H2O   Upstream conditions Fluid name Water Fluid condition Liquid Maximum flow gpm Ratio of normal to maximum flow Normal flow gpm   Flowing temperature deg F Flowing pressure psia   Viscosity cp Density at flowing conditions lb/ft Density at 60 F and 1 atm lb/ft Pipe Reynolds no. at normal flow    

16. Reynolds Number Example
Q = 70 GPM
G = 1.0 sp gr
m = 0.7 cp
d = 2.067”
R = (3160*70*1)/(2.067*0.7) =

17. Positive Displacement, P.D. Flow Meters
Measures Fluid Displacement Volume
Fluid displaces a piston or vane, rotation proportional to volume flow

18. Velocity Measuring Flow Meters
Vortex Shedding Meter
Flowing liquid or gas hits a blunt object, the flow separates and oscillates around the object.
This pulsing frequency is proportional to the flow rate.
Can be sensed by pressure transducers, stress pulses
in the blunt object or by temperature change.
Von Karman Effect -
Need high Reynolds numbers, turbulent flow.

19. Velocity Measuring Flow Meters
Straight runs required the same concept as orifice plates, but shorter, 10 dia up 5 diameter down.
Popular; inexpensive, no extra piping required.
Caution for use with steam, Condensate can travel at a
high velocity and damage the bluff body.
Competitive costs below 4” line size, easy to install.
Require 3 to 12 feet/second liquid velocity.
15 to 1 turn-down

20. Coriolis Effect; Newton’s Second Law
Mass Flow Meter
Coriolis Effect; Newton’s Second Law
Force proportional to mass flow through tube.
When fluid is moving through the sensor's tubes, Coriolis forces are induced. These forces cause the flow tubes to twist in opposition to each other.
Problems with coating, amalgamation, tube corrosion, stress cracking.
Pulsing flow and vibration can cause errors.
Work closely with suppler, very specialized, engineered.
See Web sight for tutorial

21. - Mass flow rate - Volumetric flow rate – Density – Temperature
Mass Flow Meter
Multi-variable measurement, 3 in one meter:
- Mass flow rate - Volumetric flow rate – Density – Temperature
High accuracy (+/-0.1%) and repeatability which means improved product quality and reduced waste.
Easy installation; no special mounting
No flow conditioning
No straight pipe run requirements.

22. Magnetic Flow Meter
Electrode Voltage, E, proportional to the velocity of the flowing liquid, Faraday’s law.
Vector Cross Product of:
B – Magnetic Field Intensity
V – Fluid Velocity

23. Magnetic Flow Meter

24. The largest problem is the grounding.
Magnetic Flow Meter
Voltage is small 1 to 2 mv. Electrodes made of Platinum or Titanium. Tube material non-conductive, Teflon or ceramic.
Fluid must be “conductive” - many now work on very low conductive liquids
The meter must have a solution ground.
The largest problem is the grounding.
Our lab flow meters are magnetic flow meters.

25. Flow and Pressure Transmitters.pdf
Flow Measurement
Flow and Pressure Transmitters.pdf

26. Control Valves
Control Valve – A valve that uses a signal, electronic or pneumatic, to adjust its opening.
Failures, FO, FC - Fail Open or Closed
ATO - Air To Open ATC - Air To Close
Two Basic Designs:
Sliding Stem, globe gate, needle etc.
Rotating Stem, Ball, Plug, Butterfly, Disk, etc.

27. Control Valves Sliding Stem Baumann 51000 Series Control Valve
(used in our lab)

28. Baumann 25000 Series Control Valve
Control Valves
Rotating Stem
Baumann Series Control Valve

31. Control Valves Control Valve Details.pdf Diaphragm Control Valve.pdf
Transducer.pdf

32. Use Equal Percent trim for Flow Control
Valve Trim Selection
Use Equal Percent trim for Flow Control
Why?
Equal percentage trim compensates for the non-linearity of the change in pressure drop through a restriction

33. Equal % Trim in a flow loop results in “Linear” installed characteristics

34. Low Gain, 0.1 to 0.5, because of noise
Flow Control Settings
Low Gain, 0.1 to 0.5, because of noise
Reset 1.2 to 12 Seconds
No Rate!
Small Filter 1.5 to 3 sec Max

35. Liquid Pressure can be tuned like a flow loop

36. Valve Trim Selection
What about level?