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Week 3 Lecture 1: Pressure Measurement Lecture 2: Flow measurement Activity: Module 2, Module 3 Tutorial: Least Squares Algorithm

Hung Nguyen Department of Maritime Engineering

Lecture 1 Pressure Measurement

· Learning Outcomes:

­ State pressure and units for pressure, gauge, absolute and differential pressures ­ Describe pressure measuring methods: bellows type, Bourdon-tube, mercury barometer, manometers and rotameters ­ Describe pressure detection circuitry

Pressure and units

· Pressure is force per unit area (fluid, gas) · Units: Pascal (Pa), atmosphere, bar, psi Pascal: SI unit for pressure ­ the pressure or stress that arises when a force of one newton (N) is applied uniformly over an area of one square metre (m2). Pa = N/m2, kPa = 103 pascals, MPa = 106 pascals Atmosphere (atm): 1 atm = 101.325 Pa Bar: 1 bar = 105 Pa Psi (pounds-force per square inch absolute): 1 Pa = 0.000145 Psi, 1000 Pa = 0.145 Psi

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Pressure-unit conversion factors

· 1 torr = 1 mm mercury (mmHg) = 1.33322 mbar

Gauge, Absolute and Differential Pressures · All pressures must be measured relative to some reference.

· Absolute: p2 = 0 · Atmospheric (gauge): p2 = atmospheric pre · Differential: p2 = certain pressure

Pressure Measuring Methods

· General principle: Pressure can be converted to force by letting it act on a know area: diaphragm, Bourdon tube, bellows, flapper-nozzle, piezo-electric · High vacuum pressure measurement is not related to force measurement: column of liquid as in manometers · Differential pressure transmitter: converts pressure to other types of signal, e.g. electrical signals (4 mA ­ 20 mA)

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Mercury Barometer

· Evanelista Torricelli (1608-1647) invented the Torricelli Mercury Barometer in 1644. He used a long glass tube, closed at the upper end, open at the lower and filled with mercury.

http://scienceworld.wolfram.com/biography/Torricelli.html

Collection of Mercury Barometers

www.medfordclock.com/

Bellows Type Pressure Instrument

· Metallic bellows: 0.575psi, used with a heavy range spring up to 1000psi · Bellows: one-piece, collapsible, seamless metallic unit that has deep folders formed from very thin walled tubing · Diameter: 0.5 ­ 12 inches

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Bourdon Tube

· Principle and Range

Eugène Bourdon (1808-1884)

Pressure of Liquid (at depth)

· P: pressure at depth h, A: cross-sectional area of a cylinder, : density, and g: acceleration of gravity (9.81 m/s2)

p = gh p h= g

· h is called static head in mmHg or mm of water

Manometers

· U-tube Manometer · Well-typed Manometer · Inclined Manometer

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U-tube Manometer

· Simplest differential pressure instrument ­ liquid filled manometer

p1 - p 2 = gh

p1 - p 2 = ( - 1 ) gh

· : density of liquid in U-tube (Mercury) · >> 1

p1 - p 2 = gh

Well-typed Manometer

· Well-typed or Reservoir Manometer

p1 - p 2 = gh

Well-typed Manometer

· Well-typed manometer with zeroing adjustment

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Inclined Manometer

· = angle of inclination of manometer · h = vertical increase in head · d = movement of column along limb Pressure

d=

h sin

Summary of Manometers

· Use: for static pressure measurement, differential pressure measurement · U-tube & Well-type: 1 mm Hg or water to 1 m of Hg or water, approximately 1 mbar to 100 mbar or 1.5 bar · Inclined: 1 mm to 300 mm of water, approximate 1 mbar to 30 mbar · Advantages: not used in control systems

Pressure Detection Circuitry Transducer ­ D/P Transmitters

· Converts pressure to other forms of signal, electrical signal for indication and control · Output: volt (0-5V, 0-10V) or current (4mA ­ 20mA) · Commonly used types:

­ ­ ­ ­ ­ Bellows/Bourdon resistance (potentiometer) Inductive Linear variable differential transformer (lvdt) Capacitive Diaphragm & inductive or piezo-electric (crystal material)

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Resistance Transducer

· Combination of a bellows or a Bourdon tube with a variable resistor (potentiometer)

Inductive Pressure Transducers

· Inductive pressure transducer

Inductive

Lvdt

Capacitance Pressure Transducer

· Principle

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Diaphragm Inductive or Piezoelectric Pressure Transducers

Principle of diaphragm and inductive pressure transducers

Principle of diaphragm & Piezo-electric pressure transducers

Differential Pressure Transmitters

http://www.foxboro.com http://www.enercorp.com http://www.pressure-transducer.com

Functional Uses of P. Transducers

· Three basic functions: indication, alarm and control · Environmental concerns:

­ Atmospheric pressure: Pressure instrument is sensitive to variations in the atmospheric pressure surrounding the detector ­ cause the indicated pressure to change ­ Ambient temperature: resistance of components, then reduce accuracy and reliability ­ Humidity: high moisture affects electric/electronic equipment: short circuits, grounds, corrosion

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Summary of Lecture 1

· Pressure measurement: units for pressure, absolute, gauge and differential pressures · Pressure measuring methods: mercury barometer, Bourdon tube, bellows type, manometers (U-tube, Well-type and Inclined) · Pressure detection circuitry (transducers) · D/P transmitters

Any Questions?

Lecture 2: Flow Measurement

· Learning Outcomes:

­ State flow velocity, volumetric flow rate, mass flow rate ­ Describe flow measuring methods ­ Describe types of flowmeter: orifice, venturi, pitot, flow nozzle, rotameter, and differential pressure transmitter

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Flow Measurement

· Various physical properties are considered: density, pressure, flow rate (velocity, volume flow rate and mass flow rate) and viscosity · Flow measurement involves liquids (water, oil), gasses (compressed air) and pneumatic and hydraulic systems · Piping and pumping systems · Offshore gas and oil industry

Flow Rate

Volumetric flow rate is defines as follows: Q=VxA where Q = liquid flow through the pipe (m3/s) V = average velocity of the flow (m/s) A = cross-sectional area of the pipe (m2)

Factors that affect liquid flow rate: 1. liquid's viscosity and density, 2. the friction of the liquid in contact with the pipe. 3. Reynold number: a dimensionless unit defined as the ratio of the liquid's inertial forces to its drag forces.

Flow Rate Units

multiply by US GPM Imp GPM US million gal/day 0.00144 to ft3/sec m3/hr liters/sec barrels/min barrels/day from US GPM1) 1 0.8327 0.00223 0.2271 0.0631 0.0238 34.286

Imp GPM

1.201

1

0.00173

0.002676

0.2727

0.0758

0.02859

41.176

US million gal/day

694.4

578.25

1

1.547

157.7

43.8

16.53

23810

ft3/sec

448.83

373.7

0.646

1

101.9

28.32

10.686

15388

m3/hr

4.403

3.67

0.00634

0.00982

1

0.2778

0.1048

151

liters/sec

15.85

13.20

0.0228

0.0353

3.60

1

0.3773

543.3

barrels/min

42

34.97

0.0605

0.0937

9.538

2.65

1

1440

barrels/day

0.0292

0.0243

0.000042

0.000065

0.00662

0.00184

0.00069

1

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Bernoulli's equation

· Relationship between F.R. & Pressure V1 P1 1 2 2 V1 P1 V2 P2 + = + + 2 2 3 V2 P2

restriction

2

f V2 = K

P

Velocity, Volume & Mass Flows

· Velocity

V=K

P P

· Volume flow

Q = VA = KA

· Mass flow

W = Q = KA P

Flow Measurement Methods

Numerous types of flowmeters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity, and mass meters. Differential pressure devices (also known as head meters) include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbowtap meters, target meters, and variablearea meters

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Flow Measurement Methods D/P Sensors

Orifice

Flow Nozzle Pitot Tube

Other methods: Variable-area (rotameter) Positive displacement Velocity methods Mass-related methods

Venturi Tube

Elbow Tap

Ref: http://www.omega.com/techref/flowcontrol.html

Differential Pressure Methods

Differential-pressure flowmeters with variety of flow restricting elements (a through f). 0 = flow, 1 = differential-pressure transduction element, 2 = pipe, 3 = orifice, 4 = nozzle, 5 = Venturi tube, 6 = Pitot tube, 7 = centrifugal elbow, 8 = centrifugal loop.

D/P Methods: Orifice Plate

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Venturi-tube Method

Flow Nozzle

Upstream pressure sensor

Downstream pressure sensor

Nozzle shrinks down the cross-section area of the pipe and create pressure differential

Examples: D/P Typed Flowmeters

The Model 340S SteaMeter is a differential pressure type flowmeter capable of measuring saturated steam flow.

The SITRANS DS III transmitter is available for measuring pressure, absolute pressure, differential pressure, flow or level.

Ref: http://www.sea.siemens.com

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Examples: Orifice Plate Flowmeter

Daniel® Senior® Orifice Flow Meter provides a convenient way to change orifice plates under pressure during line flow. The "Senior" meter saves users time and dollars by eliminating costly by-passes, valves and other fittings. Daniel® Simplex® Orifice Plate Holders have universal orifice plates and sealing units, and users trust them for fast, economical removal and insertion of orifice plates.

Ref: http://www.emersonprocess.com/

Adv & Disadv of some flow sensors

Sensor orifice Rangeabilit y1 3.5:1 Accuracy2 2-4% of full span Dynamics (s) Advantages -low cost -extensive industrial practice -lower pressure loss than orifice -slurries do not plug -good for slurry service -intermediate pressure loss -low pressure loss -low pressure loss -large pipe diameters -wide rangeability -good accuracy -wide rangeability -insensitive to variations in density, temperature, pressure, and viscosity -high reangeability -good accuracy Disadvantages -high pressure loss -plugging with slurries -high cost -line under 15 cm -higher cost than orifice plate -limited pipe sizes -very poor accuracy -poor performance with dirty or sticky fluids -high cost -strainer needed, especially for slurries

venturi

3.5:1

1% of full span

-

flow nozzle elbow meter annubar

3.5:1

2% full span 5-10% of full span 0.5-1.5% of full span 0.25% of measurement

-

3:1

-

3:1

-

turbine

20:1

-

vortex shedding positive displace ment

10:1

1% of measurement

-

-expensive

10:1 or greater

0.5% of measurement

-

-high pressure drop -damaged by flow surge or solids

Calibration of Flowmeters

All flowmeters require an initial calibration. Most of the time, the instrument is calibrated by the manufacturer for the specified service conditions. However, if qualified personnel are available in the plant, the user can perform his own calibrations. The need to recalibrate depends to a great extent on how well the meter fits the application. Some liquids passing through flowmeters tend to be abrasive, erosive, or corrosive. In time, portions of the device will deteriorate sufficiently to affect performance. Some designs are more susceptible to damage than others. For example, wear of individual turbine blades will cause performance changes. If the application is critical, flowmeter accuracy should be checked at frequent intervals. In other cases, recalibration may not be necessary for years because the application is noncritical, or nothing will change the meter's performance. Some flowmeters require special equipment for calibration. Most manufacturers will provide such service in their plant or in the user's facility, where they will bring the equipment for on-site calibration. Ref: http://www.omega.com/techref/flowcontrol.html

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Maintenance

· · A number of factors influence maintenance requirements and the life expectancy of flowmeters. The major factor, of course, is matching the right instrument to the particular application. Poorly selected devices invariably will cause problems at an early date. Flowmeters with no moving parts usually will require less attention than units with moving parts. But all flowmeters eventually require some kind of maintenance. Primary elements in differential pressure flowmeters require extensive piping, valves, and fittings when they are connected to their secondary elements, so maintenance may be a recurring effort in such installations. Impulse lines can plug or corrode and have to be cleaned or replaced. And, improper location of the secondary element can result in measurement errors. Relocating the element can be expensive. Flowmeters with moving parts require periodic internal inspection, especially if the liquid being metered is dirty or viscous. Installing filters ahead of such units will help minimize fouling and wear. Obstructionless instruments, such as ultrasonic or electromagnetic meters, may develop problems with their secondary element's electronic components. Pressure sensors associated with secondary elements should be periodically removed and inspected. Applications where coatings may occur are also potential problems for obstructionless instruments such as magnetic or ultrasonic units. If the coating is insulating, the operation of magnetic flowmeters will ultimately be impaired if the electrodes are insulated from the liquid. This condition will be prevented by periodic cleaning. With ultrasonic flowmeters, refraction angles may change and the sonic energy absorbed by the coating will cause the meter to become inoperative.

·

·

Ref: http://www.omega.com/techref/flowcontrol.html

Summary of Lecture 2

· Flow, units, measuring methods · Flow measuring methods: Orifice, venturi, pitot, etc. · Advantages and disadvantages · Calibration and maintenance

Any Questions?

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Activity

· Module 2 · Module 3

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