Topic 1 Pressure Measurement

Transcription

1 Week 6 Lecture: Topic 1: Pressure Topic : Flow Class Test 1 (1 hour) on Tuesday Tutorial 5: Transducers and temperature measurement Topic 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 Reading: Chapter 7 Pressure measurement Pressure/Unit Principles of pressure transducers Types of pressure transducers: Barometer, Manometers (indication) Pressure transducers (indication and control) D/P transmitters (indication and control) Applications of pressure transducers 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 (m ). Pa = N/m, kpa = 10 3 pascals, MPa = 10 6 pascals Atmosphere (atm): 1 atm = Pa Bar: 1 bar = 10 5 Pa Psi (pounds-force per square inch absolute): 1 Pa = Psi, 1000 Pa = Psi Pressure-unit conversion factors Gauge, Absolute and Differential Pressures All pressures must be measured relative to some reference. 1 torr = 1 mm mercury (mmhg) = mbar Absolute: p = 0 Atmospheric (gauge): p = atmospheric pre Differential: p = certain pressure 1

2 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, piezoelectric 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 0 ma) Mercury Barometer Evanelista Torricelli ( ) invented the Torricelli Mercury Barometer in He used a long glass tube, closed at the upper end, open at the lower and filled with mercury. Collection of Mercury Barometers Bellows Type Pressure Instrument Metallic bellows: psi, 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: inches Principle and Range Bourdon Tube Eugène Bourdon ( ) Pressure of Liquid (at depth) P: pressure at depth h, A: cross-sectional area of a cylinder, ρ: density, and g: acceleration of gravity (9.81 m/s ) p= ρgh p h = ρg h is called static head in mmhg or mm of water

3 Manometers U-tube Manometer Well-typed Manometer Inclined Manometer U-tube Manometer Simplest differential pressure instrument liquid filled manometer ( ρ ρ ) p p = gh 1 1 =ρ p1 p gh ρ: density of liquid in U-tube (Mercury) ρ >> ρ 1 p1 p = ρgh Well-typed Manometer Well-typed or Reservoir Manometer Well-typed Manometer Well-typed manometer with zeroing adjustment p1 p =ρgh Inclined Manometer θ = angle of inclination of manometer h = vertical increase in head d = movement of column along limb Pressure d = h sin θ θ 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 Disadvantages: not used in control systems 3

4 Example: Application of manometer 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 0mA) Commonly used types: Bellows/Bourdon resistance (potentiometer) Inductive Linear variable differential transformer (lvdt) Capacitive Diaphragm & inductive or piezoelectric (crystal materials) Pressure P1 D/P Transmitters 0-5V, -5-+5V 0-10V, V Resistance Transducer Combination of a bellows or a Bourdon tube with a variable resistor (potentiometer) Ref Pressure P Sensor Signal conditioner (converter, amplifier) 4-0mA 0-0mA Recorder or Indicator (digital) Inductive Pressure Transducers Inductive pressure transducer Capacitance Pressure Transducer Principle Inductive Lvdt More: 4

5 Diaphragm Inductive or Piezoelectric Pressure Transducers Differential Pressure Transmitters Principle of diaphragm and inductive pressure transducers Principle of diaphragm & Piezoelectric pressure transducers Functional Uses of P. Transducers Three basic functions: indication, alarm and control (pressure, flow, level) 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 Applications in Control Systems Flow control systems Pressure control systems Level control system Summary of Topic 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 Topic : Flow Measurement Learning Outcomes: State flow velocity, volumetric flow rate, mass flow rate Describe flow measuring methods Describe flow measuring methods Describe types of flowmeter: orifice, venturi, pitot, flow nozzle, rotameter, and differential pressure transmitter Reading: Chapter 8 5

6 Topic : Contents Flow (velocity, volumetric flowrate, mass flowrate) and units Principles of flowmeters Types of flowmeters Applications 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 defined as follows: Q = V x A where Q = liquid flow rate through the pipe (m 3 /s) V = average velocity of the flow (m/s) A = cross-sectional sectional area of the pipe (m ) Factors that affect liquid flow rate: 1. liquid's viscosity and density,. 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. multiply by from US GPM Flowrate Units Imp GPM US million gal/day to ft 3 /sec m 3 /hr liters/sec barrels/min barrels/day US GPM1) Imp GPM US million gal/day ft 3 /sec m 3 /hr liters/sec barrels/min barrels/day Bernoulli's equation Relationship between F.R. & Pressure V 1 V1 P 1 V 1 P restriction P1 V P = + + ρ ρ f V = K ρ Velocity, Volume & Mass Flows V = K ρ Velocity Volume flow Mass flow P Q = VA = KA ρ W = Qρ = KA ρ 6

7 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 Flow Measurement Methods D/P Sensors Orifice Venturi Tube Flow Nozzle Pitot Tube Other methods: Variable-area (rotameter) Positive displacement Elbow Tap Velocity methods Mass-related methods Ref: Differential Pressure Methods D/P Methods: Orifice Plate Differential-pressure flowmeters with variety of flow restricting elements (a through f). 0 = flow, 1 = differential-pressure transduction element, = pipe, 3 = orifice, 4 = nozzle, 5 = Venturi tube, 6 = Pitot tube, 7 = centrifugal elbow, 8 = centrifugal loop. 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 7

8 Upstream P1 Flowmeter (D/P Type) 0-5V, -5-+5V 0-10V, V Examples: D/P Typed Flowmeters Downstream P Sensor Orifice Venturi Elbow Signal conditioner (converter, amplifier) Inductance (lvdt) Capacitance 4-0mA 0-0mA Recorder or Indicator (digital) 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: 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. Adv & Disadv of some flow sensors Rangeability Sensor 1 Accuracy Dynamics Advantages Disadvantages (s) -low cost -high pressure loss orifice 3.5:1-4% of full span - -extensive industrial practice -plugging with slurries -lower pressure loss than -high cost venturi 3.5:1 1% of full span - orifice -line under 15 cm -slurries do not plug flow -good for slurry service -higher cost than orifice plate 3.5:1 % full span - nozzle -intermediate pressure loss -limited pipe sizes elbow 5-10% of full 3:1 - -low pressure loss -very poor accuracy meter span 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: annubar 3:1 turbine 0:1 vortex 10:1 shedding positive 10:1 or displace greater ment % of full span 0.5% of measurement 1% of measurement 0.5% of measurement 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 -poor performance with dirty or sticky fluids -high cost -strainer needed, especially for slurries -expensive -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 t sufficiently i 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: 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: 8

9 Flow control system Applications Current Status Control Lab Compressed air (pneumatic supply) Flow control Control system: valve DAQ and LabVIEW I/P Converter PLCs: updated Possible projects: control algorithms for PC-based controller flow control, hydraulic control Control laws: PID, self-tuning, optimal, neural network, etc. D/P Transmitter Orifice plate Summary of Topic Any Questions? Flow, units, measuring methods Flow measuring methods: Orifice, venturi, pitot, etc. Advantages and disadvantages Calibration and maintenance Applications of flowmeters Tutorial Tutorial 5: transducers and temperature 9