Chapter 11. Control System Instrumentation

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Charity Bridges
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1 Chapter 11 Control System Instrumentation

2 Measuring Instrumentations Transducers and Transmitters The typical process measuring instrument consists of sensing elements and transmitters (driving elements). This combination of sensor and transmitter is called transducer. Sensor: is a phenomenon that detects process variable and produces a signal that can be measured like mv, current, pressure difference, etc. Transmitter: converts this phenomenon into a signal that can be transmitted such as current to air (I/P), volt to current (V/I), volt to pressure (V/P), etc.

3 Sensor Systems Sensor temperature sensors flow sensors level sensors pressure sensors composition analyzers Transmitter

4 The Control Relevant Aspects of Sensors The time constant/deadtime of the sensor The repeatability of the sensor

5 Sensor Terminology Span Zero Accuracy Repeatability Process measurement dynamics Calibration

6 Span and Zero Example Consider the maximum temperature that is to be measured is 350ºF and the minimum temperature is 100ºF. The zero is 100ºF and the span is 250ºF If the measured temperature is known at two different sensor output levels (i.e., ma s), the span and zero can be calculated directly.

7 Smart Sensors Sensors with onboard microprocessors that offer a number of diagnostic capabilities. Smart ph sensors determine when it is necessary to trigger a wash cycle due to buildup on the electrode surface. Smart flow meters use statistical techniques to check for plugging of the lines to the DP cell. Smart temperature sensors use redundant sensors to identify drift and estimate expected life before failure.

8 Transmitters A transmitter usually converts the sensor output to a signal level appropriate for input to a controller, such as current signal of range 4 to 20 ma or pneumatic signal of range 3-15 psig. Transmitters are generally designed to be direct acting. In addition, most commercial transmitters have an adjustable input range (or span). For example, a temperature transmitter might be adjusted so that the input range of a platinum resistance element (the sensor) is 50 to 150 C.

10 Transmitter/Controller May need additional transducers for G m if its output is in ma or psi. In the above case, G c is dimensionless (volts/volts).

11 Measurement / Transmission Lags Temperature sensor make as small as possible (location, materials for thermowell) Current or Pneumatic Transmission lines for Control Loop TM (s) T(s) 1 s +1 msc U A Pneumatic, usually, produces a pure time delay (no time delays for electronic lines); less common today compared to electronic transmissions; but it is useful for place of hot or inductive conditions. = s s s

12 FLOW MEASUREMENT What is flowrate?? Amount of material passing one point for certain time Flow Measuring Device: 1. Differential Pressure Meter - Orifice, Venturi, Nozzle 2. Electromagnetic Flowmeter 3. Vortex Flowmeter 4. Turbine Flowmeter 5. Ultrasonic Flowmeter

13 How To Choose the Right Flowmeter?? There are key questions you can ask yourself when trying to determine which flowmeter is the best choice for you. The purpose of the measurement and the physical characteristics of the fluid being measured are the two main considerations.

14 Some specifics to consider are: What is the fluid being measured (air, water, gas, etc?) If it's not water, what are the properties of the fluid you are measuring? What construction materials are acceptable? What are the track records of the various technologies you are considering? What are the minimum and maximum process pressures and temperatures?

Flow Nozzle Consist of an elliptical converging section and cylindrical throat section. Suitable for high-velocity, non-viscous, erosive flows.

15 Flow Nozzle Consist of an elliptical converging section and cylindrical throat section. Suitable for high-velocity, non-viscous, erosive flows. Flow Nozzles have a smooth elliptical inlet leading to a throat section with a sharp outlet. This restriction in the fluid flow causes a pressure drop. The flow can be calculated from the measured pressure drop This device has a greater overall pressure loss or operating cost in terms of head pressure than a Venturi but offers lower installation costs.

16 Orifice Venturi Nozzle

17 Paddle Type Orifice Plate

18 Sizing an Orifice for a Differential Pressure Flow Indicator b is the ratio of the orifice diameter to the pipe diameter. 0.2 < b < 0.7 Pressure drop at minimum flow should be greater than 0.5 psi. Pressure drop across the orifice should be less than 4% of the line pressure. Choose the maximum value of b that satisfies each of the above specifications.

Electromagnetic Flow meter This instrument operates based on Faraday s Law. It is ideal for liquids that conduct electricity. The magnetic field is developed by electric coils.

19 Electromagnetic Flow meter This instrument operates based on Faraday s Law. It is ideal for liquids that conduct electricity. The magnetic field is developed by electric coils. The conductive liquid forms an electric conductor as its move through the magnetic field established inside the flowmeter. This conductor in the magnetic filed will generate an electric voltage that is proportional to its average velocity.

20 Example of a Magnetic Flow Meter

Vortex Flow meter Also know as vortex shedding flowmeters or oscillatory flowmeters. It measures the vibrations of the downstream vortexes caused by the barrier placed in a moving stream.

21 Vortex Flow meter Also know as vortex shedding flowmeters or oscillatory flowmeters. It measures the vibrations of the downstream vortexes caused by the barrier placed in a moving stream. The velocity of vortex flowing in the stream is directly related to the stream velocity. A device that counts the vortices passing per second will also measure the flowrate. Advantage: Low cost installation-do not required impulse tubing and valve manifold. Disadvantages: Vortexes are inhibited in viscous fluid at low flow rate. At high fluid velocity the obstructions may introduce excessive pressure drop-limited to higher flow rate.

22 Turbine Flowmeter Turbine meters have a spinning rotor with propeller-like blades that is mounted on bearings in a housing on the central longitunidal axis of the pipeline. Magnets are embedded in the rotor housing and a pickup coil, isolated from the fluid is placed outside the rotor blades. The rotor spins as water or other fluid passes over it. The rotating magnets induce a voltage pulse in the coil each time they pass the coils. Pulse frequency is proportional to the velocity Disadvantages: sensitive to viscosity changes, require maintenance at their bearing.

24 Ultrasonic Flow meter Ultrasonic flowmeters can be categorized into two types based on the installation method: clamped-on and inline. The clamped-on type is located outside of the pipe and there are no wetted parts. It can easily be installed on existing piping systems without worrying about corrosion problems. Clamped-on designs also increase the portablility of the flowmeter. The inline type, on the other hand, requires fitting flanges or wafers for installation. However, it usually offers better accuracy and its calibration procedures are more straightforward. Ultrasonic flowmeters measure the traveling times (transit time models) or the frequency shifts (Doppler models) of ultrasonic waves in a pre-configured acoustic field that the flow is passing through to determine the flow velocity.

25 Ultrasonic Flow meter Time Transit Model - A pair of transducers is placed on the pipe wall, one on the upstream and the other on the downstream. The time for acoustic waves to travel from the upstream transducer to the downstream transducer is shorter than the time it requires for the same waves to travel from the downstream to the upstream. The larger the difference, the higher the flow velocity.

26 Ultrasonic Flow meter Doppler Model - rely on the Doppler effect to relate the frequency shifts of acoustic waves to the flow velocity. It usually requires some particles in the flow to reflect the signals.

27 Pressure-Measuring Devices Most liquid and all gaseous materials in the process industries are contained within closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled. Types of pressure sensors are: U-Tube Manometer Bourdon Tube Sensor Bellows-Type Sensor Strain Gauge Sensor

28 U-Manometer

29 Bourdon, Bellow and Diaphragm Sensor Bourdon: A bourbon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. Bellow: A bellows is a closed vessel with sides that can expand and contract Diaphragm: A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation In all, the displacement can be related to pressure. Displacement is convert to electrical signal ~ required secondary element.. Used elastic material

30 Bourdon Tube Sensor

31 Bourdon Tube Sensor

32 Bellows-Type Sensor

33 Diaphragms Sensor

34 Strain Gauge Sensor The electrical resistance of a metal wire depends on the strain applied to the wire. Deflection of the diaphragm due to the applied pressure causes strain in the wire, and the electrical resistance can be measured and related to pressure. Fluid Force Diaphragm Displacement (detected by strain gauge sensor) and the resistance change (detected by Wheatstone bridge).

35 Strain Gauge Sensor

36 Strain Gauge Sensor

37 Temperature Sensing Systems Temperature control is important for separation and reaction processes, and temperature must be maintained within limits to ensure safe and reliable operation of process equipment. Temperature can be measured by many methods; several of the more common are described in this subsection. You should understand the strengths and limitations of each sensor, so that you can select the best sensor for each application. In nearly all cases, the temperature sensor is protected from the process materials to prevent interference with proper sensing and to eliminate damage to the sensor. Thus, some physically strong, chemically resistant barrier exists between the process and sensor; often, this barrier is termed a sheath or thermowell, especially for thermocouple sensors.

38 Temperature Sensing Systems There are several methods used to measure temperature. The followings are just few of these methods, which may be employed. The capillary tube (fluid thermometer) A capillary tube is a very thin tube. This type of thermometer has a bulb filled with mercury. When the temperature rises, the mercury in the bulb expands. This expansion pushes the mercury higher in the capillary tube.

39 Thermocouple Consist of two dissimilar metal and connected ~ voltage generated Hot junction ~ measure temperature Cold junction ~ reference (known temperature) E 1 = voltage generated by T 1 (hot junction) E 2 = voltage generated by T 2 (cold junction) E t = E 1 E 2 Hot junctio n E1 Cold junctio n E2

40 When the junctions of two dissimilar metals are at different temperatures, an electromotive force (emf) is developed The emf is calculated using the following equation: emf (volts) = (T h T c ) o C x b b = constant (V/K), T (K) Different combinations of metals result in different voltage- temperature characteristics. In industry, thermocouples are usually classified by a one-letter type designation that describes their response.

41 Standard thermocouples Type Materials Normal Range J Iron-constantan -190 o C to 760 o C T Copper-constantan -200 o C to 371 o C K Chromel-alumel -190 o C to 1260 o C E Chromel-constantan -100 o C to 1260 o C S 90% platinum + 10% rhodium- platinum 0 o C to 1482 o C R 87% platinum + 13% rhodium- platinum 0 o C to 1482 o C

42 Thermocouple are often insulated electrically with ceramic material (high temperature) and sheathed in stainless steel Used thermowell for effectively seal off the process fluid or gas. Advisable to use thermowell to prevent heat loss and personnel injury Temperature sensor without thermowell Temperature sensor with thermowell

43 RTD (Resistance Temperature Detector ) The electrical resistance of many metals changes with temperature, metals for which resistance increases with temperature are used in RTDs Temperature is determined from the change in the electrical resistance of the metal wire. Linear relationship using equation R T = R o (1+aT) R T = the resistance at temperature, T R 0 = the resistance at base temperature of 0 C T = the temperature of the sensor (to be determined from R T ) a = the temperature coefficient of the metal.

44 a = R 100 / R R 100 = Resistance at 100 o C (steam point) R 0 = Resistance at 0 o C (ice point) Limitation: o C

45 RTD sensitivity, a can be noted from typical value of metal used, Platinum = / o C Nickel =0.005 / o C The effective range of RTDs principally depend on the type of wire used Platinum RTD = -100 to 650 o C Nickel RTD = -180 to 300 o C

46 Thermistor: This sensor is similar to an RTD, but applies metals for which the resistance decreases with increasing temperature. The relationship is often very nonlinear, but thermistors can provide very accurate temperature measurements for small spans and low temperatures. Thermisters are made from oxides of metals such as copper, nickel, cobalt and lithium which are blended to produce the required resistance-temperature characteristics. Most Thermistors have negative non-linear temperature coefficients. Thermistors are produced in many shapes and sizes such as beads, discs and probes and may be coated in a glass or steel sheath for added protection and strength.

47 Filled systems: A fluid expands with increasing temperature and exerts a varying pressure on the containing vessel. When the vessel is similar to a bourbon tube, the varying pressure causes a deformation that changes the position detected to determine the temperature. The tube in a filled-system temperature indicator can be filled with a liquid and vapour. When the temperature rises, more liquid is changed to vapour. The increase vapour pressure straightens the spiral bourdon tube pressure instrument. The figure below show how the movement of the bourdon tube, caused by a pressure change, is read as a temperature change

48 Table: Summary of temperature sensors Sensor Type Limits of Application ( C) Accuracy 1,2 Dynamics: t (s) Advantages Disadvantages Thermocouple type E: chromel-constantan type J: iron-constantan type K: chromel-nickel type T: copper-constantan -100 to 1000 ±1.5 or 0.5% for 0 to 900 C 0 to 750 ±2.2 or 0.75% 0 to 1250 ±2.2 or 0.75% -160 to 400 ±1.0 or 1.5% for -160 to 0 C see note 3 -good reproducibility -wide range -minimum span of 40 C -temperature vs. emf not exactly linear -drift over time -low emf corrupted by noise RTD -200 to T see note 3 Thermister -40 to 150 ± 0.10 C see note 3 Bimetallic - ± 2% - Filled system -200 to 800 ± 1% 1 to 10 -good accuracy -small span possible -linearity -good accuracy -little drift -low cost -physically rugged -simple and low cost -no hazards -self-heating -less physically rugged -self-heating error -highly nonlinear -only small span -less physically rugged -drift -local display -not high temperatures -sensitive to external pressure

49 LEVEL MEASUREMENT Level = a measurement of the height of the free surface of the liquid from a fixed datum or reference. Level accuracy:- Smoothen the process operation. Comply the custom and taxing regulation. Level can represent the amount of asset and related to money matter Tank contains liquid and sometime the solid. Sometimes the liquid solidifies, very corrosive, and vaporizes create difficulties.

50 Differential Pressure Level Measurement The difference in pressures between to points in a vessel depends on the fluids between these two points. If the difference in densities between the fluids is significant, which is certainly true for a vapor and liquid and can be true for two different liquids, the difference in pressure can be used to determine the interface level between the fluids. Usually, a seal liquid is used in the two connecting pipes (legs) to prevent plugging at the sensing points.

51 Differential Pressure Level Measurement Upper Tap Vapor Diaphragm Liquid DPT Lower Tap

52 Buoyancy Sensor Buoyancy = displacement By Archimedes principle, a body immersed in a liquid is buoyed by a force equal to the weight of the liquid displaced by the body. Thus, a body that is more dense than the liquid can be placed in the vessel, and the amount of liquid displaced by the body, measured by the weight of the body when in the liquid, can be used to determine the level.

53 Buoyancy Sensor

54 Buoyancy Sensor

55 Float Sensor The float of material that is lighter than the fluid follows the movement of the liquid level. The position of the float, perhaps attached to a rod, can be determined to measure the level.

56 Capacitance Sensor A capacitance probe can be immersed in the liquid of the tank, and the capacitance between the probe and the vessel wall depends on the level. By measuring the capacitance of the liquid, the level of the tank can be determined

57 Capacitance Sensor

58 ANALYTICAL MEASUREMENT The term analyzer refers to any sensor that measures a physical property of the process material. This property could relate to purity (e.g., mole % of various components), a basic physical property (e.g., density or viscosity), or an indication of product quality demanded by the customers in the final use of the material (e.g., gasoline octane or fuel heating value). Used to measure the physical properties Standalone in laboratory or installed near to equipment.

59 ph Meter Discuss and answer all the questions What is the purpose of ph meter?? How it s measured?? Explain standard hydrogen electrode. List and describe types of electrodes.

60 Conductivity Meter Discuss and answer all the questions What is the purpose of conductivity meter?? Explain polarization effect. How to measure conductivity?

61 Pneumatic Control Valve Called Actuator System Control Valve Valve body Valve actuator I/P converter Instrument air system

62 Cross-section of a Globe Valve

63 Pneumatic Control Valve Typical Globe Control Valve Air to Close Valve, Fail Open

64 Pneumatic Control Valve Air to Open Valve, Fail Close

65 Types of Globe Valves Quick Opening- used for safety by-pass applications where quick opening is desired Equal Percentage- used for about 90% of control valve applications since it results in the most linear installed characteristics Linear- used when a relatively constant pressure drop is maintained across the valve

66 f(x) Inherent Valve Characteristics QO Linear =% Stem Position (% Open)

67 Control Valve Design Procedure Choose a control valve so that the average flow rate results when the valve is 2/3 open. After the valve has been sized, check to ensure that the maximum and minimum flow rates will be accurately metered.

68 Additional Information Required to Size a Control Valve C V versus % open for different valve sizes. Available pressure drop across the valve versus flow rate for each valve. Note that the effect of flow on the upstream and downstream pressure must be known.

69 Valve Sizing Example Size a control valve for max 150 GPM of water and min of 50 GPM. Therefore, choose the valve size so that valve is approximately 67% open at 100 GPM.

70 Determine C V at 100 GPM Use the valve flow equation (Equation 2.1) to calculate C v For DP, use pressure drop versus stem position (e.g., Table 2.2) C v ( x) K F m DP /

71 C v versus % Valve Travel for Different Sized Valves Body % Valve Opening Size in C v

72 Check Max and Min Flows Ensure that the flow rate will be accurately controlled at the maximum and minimum flow rates. At minimum flow rate valve should be at least 10-15% open. At maximum flow rate the valve should be at most 85-90% open.

73 Valve Deadband It is the maximum change in instrument air pressure to a valve that does not cause a change in the flow rate through the valve. Deadband determines the degree of precision that a control valve or flow controller can provide. Deadband is primarily affected by the friction between the valve stem and the packing.

74 Valve Actuator Selection Choose an air-to-open for applications for which it is desired to have the valve fail closed. Choose an air-to-close for applications for which it is desired to have the valve fail open.

75 Optional Equipment Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position. Greatly reduces the deadband of the valve. Positioners are almost always used on valves serviced by a DCS. Booster relay- provides high capacity air flow to the actuator of a valve. Can significantly increase the speed of large valves.

76 Control Relevant Aspects of Actuator Systems The key factors are the deadband of the actuator and the dynamic response as indicated by the time constant of the valve. Control valve by itself- deadband 10-25% and a time constant of 3-15 seconds. Control valve with a valve positioner or in a flow control loop- deadband % and a time constant of seconds.

77 See ex 9.1 for A to O and A to C

EMaSM. Principles Of Sensors & transducers

EMaSM. Principles Of Sensors & transducers EMaSM Principles Of Sensors & transducers Introduction: At the heart of measurement of common physical parameters such as force and pressure are sensors and transducers. These devices respond to the parameters