Transcript Washington University ChE 433 Digital Process Control

Washington University
ChE 433 Digital Process Control Laboratory
Flow Meter
&
Control Valve
Lecture
Flow Meters, Transmitters
Flow - Most frequently measured process variable
Classifications
●
Head Producing
●
Positive Displacement
●
Velocity
●
Mass
●
Magnetic
Factors Affecting Flow
Flow rate related to velocity and area:
Q  VA
Reynolds Number; dimensionless number relating to flow
regime
Dv
3160* GPM * G
Re 


d
<2000 laminar; >4000 turbulent; between is transitional
Head Producing Flow Meters
Develop a permanent pressure loss across a flowing
obstruction.
Measures Fluid Volume Rate not Mass
Orifice Plate and Flanges
Orifice is a hole in a plate mounted between two special flanges
Head Producing Flow Meters
Differential Pressure, “D/P cell”
Mechanical type, nozzle baffle
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.
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.
Wedge Meter
wedge.pdf
Rosemount 485 Annubar® Primary
Head Producing
Head Producing Meters
Head, or Pressure, producing meters can also be
used as level and pressure transmitters
(Save this thought, More later)
Equations for Bore Size:
(See Notes)
Calculate Reynolds number by:
Dv 3160 * GPM * G 6.316 * w
Re 



d
d
Flow Coefficient, K for  0.35 to 0.65, usually target 0.6
K  0.5930  0.4  4  (0.0015   0.012 4 )
1000
Re
Calculate plate expansion factor and gas expansion factor, for
gasses.
Equations for Bore Size:
(See Notes)
Liquid Flow Equation:
GPM
K 
5.674 Fa d 2
2
G
hM
Gas or Vapour Equation, D bore diameter, inches; G in
lb./ft^3:
w
K
359 KDFaY GhM
Calculate as an iterative process, see notes for details
Washington University
Orifice Example
9/5/ 3
Pipe, orifice, and meter data
* Beta ratio
Actual pipe diameter
* Orifice diameter
Drain or vent diameter
.62750
2.06700 inches
1.29704 inches
.00000 inches
Type of orifice taps
Maximum differential (meter dry)
Upstream conditions
Fluid name
Fluid condition
Maximum flow
Ratio of normal to maximum flow
Normal flow
Flowing temperature
Flowing pressure
Viscosity
Density at flowing conditions
Density at 60 F and 1 atm
Pipe Reynolds no. at normal flow
Flange
250.000 in. H2O
Water
Liquid
100.000 gpm
.70000
70.000 gpm
100.0 deg F
50.000 psia
.69600 cp
62.15040 lb/ft3
62.40000 lb/ft3
153910.3
Reynolds Number Example
●
Q = 70 GPM
Dv
3160* GPM * G
Re 


d
●
G = 1.0 sp gr
●
 = 0.7 cp
●
d = 2.067”
●
R = (3160*70*1)/(2.067*0.7) = 153000
Positive Displacement, P.D. Flow Meters
Measures Fluid Displacement Volume
Fluid displaces a piston or vane, rotation
proportional to volume flow
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.
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
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
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.
Magnetic Flow Meter
Electrode Voltage, E, proportional to the velocity
of the flowing liquid, Faraday’s law.
E

B

V
B – Magnetic Field Intensity
Vector Cross Product of:
V – Fluid Velocity
Magnetic Flow Meter
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.
Flow Measurement
Flow and Pressure Transmitters.pdf
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.
Control Valves
Sliding Stem
Baumann 51000 Series Control Valve
(used in our lab)
Control Valves
Rotating Stem
Baumann 25000 Series Control Valve
Control Valves
Control Valve Details.pdf
Diaphragm Control Valve.pdf
Transducer.pdf
Valve Trim Selection
Use Equal Percent trim for Flow Control
Why?
Equal percentage trim compensates for the nonlinearity of the change in pressure drop through a
restriction
Equal % Trim in a flow loop results in “Linear”
installed characteristics
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
Liquid Pressure
Liquid Pressure can be tuned like a flow loop
Valve Trim Selection
What about level?