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Instrumentation And Control Engineering

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Question Q 1 (a) Specify an instrument, in each case listing its operating features and benefits, which is suitable for measuring the following:  (i)           the flow-rate of fruit juice in the range 5-25 m3/hour (ii)          the flow-rate of sea water in the range 150-1000 m3/hour (iii)         the flow-rate of steam in a 50 mm nominal bore pipe (iv)         the flow-rate of oil in the range 8-20 m3/hour  NOTE, that differential pressure devices and variable area flow-meters are not allowed, and that in each case, the instrument is to be installed in a fully automated plant, and a 4-20mA output signal is required. (b) Describe briefly, the measuring principle used in the flow-meter specified above for the measurement of the flow-rate of fruit juice in the range 5-25 m3/hour, and for this instrument only, list the questions you would ask the end user prior to ordering the instrument.                                                                                                          (c) Specify two instruments suitable to detect the presence of flow during normal process operation, and list the benefits and disadvantages of each type (d) Specify a device describing briefly its principle of operation, that is typically installed downstream of a bursting disc as an indicator of disc rupture,               (e) A flip switch can be used to detect the presence of flow. What are the benefits and disadvantages of this type of switch? Suggest a better alternative. Q 2 (a) The filtrate flow from an upstream process must be transferred to a downstream unit operation a substantial distance away. To achieve this the filtrate is passed to a 1000 L buffer tank and pumped to the downstream operation. The filtrate flow-rate is not constant and must be maintained (the flow cannot be stopped) as any back pressure would damage the upstream filter unit. On completion of the operation the tank is cleaned using a hot water flush via a spray ball with the flush water directed to drain. Based on the specification that the process plant is fully automated and hygienic, sketch a P&ID of the tank and any associated equipment required to achieve this duty and explain the principle steps and basis of operation.       (b) On/off air driven valves are purchased, with the specification, normally closed or normally open (spring to close or spring to open), and with single or double feedback. Explain the above statement, and the principle of operation  (c) An inductive proximity switch is a device used in valve feedback, describe its construction, operating principle and beneficial features Q 3 (a) In an automatic process plant four communication signals are used to interface with the main control unit. With the use of an appropriate example in each case, outline the function of each of these four signals.                                                       (b) In continuous process control a ‘PID controller’ is installed which uses a combination of proportional, integral and derivative action. Why are these three modes of control used in conjunction?                                                                         (c) Give an example of a process situation where PID control is used and a further example where on/off control would suffice.                                                      (d) In an automatic process plant software and hard-wired interlocks are used. With the use of an appropriate example in each case explain the difference. Q 4 (a) Thermocouples, RTD’s (resistance temperature devices) and thermistors are used to measure temperatures on process plants, for each device: (i)           sketch the typical response characteristics,                        (ii)          the range of temperature measurement,                                   (iii)         the advantages and disadvantages,                                   (iv)         the principle of operation.                                                  (b) In certain instances change of state temperature sensors are used, for these:  (i)           discuss the advantages and disadvantages,                       (ii)          and give two examples of their use.    Q 5 (a) Discuss the factors that have to be considered on deciding whether a process plant should be designed to be fully automated, and specify the operating benefits.  (b) Explain why an FDS (Functional Design Specification) is an important document.  (c) In an automatic process plant four kinds of signal are used to interface with the main control unit. With an appropriate example in each case outline the function of each of these four signals.                                                                                               (d) If you were to purchase a rack type PLC (programmable logical controller), list the items required, and what determines their specification, and with an appropriate sketch show its typical construction.         Q 6 (a) It is imperative to use a Tag number to identify an individual plant item, explain why this is.                                                                                                                                (b) You have been appointed the lead engineer on the design of a process plant that is used to manufacture a cosmetic product. Part of that plant is the supply and installation of a pressurized vessel on a concrete mezzanine floor, which allows the mixing of two ingredients which are put into the vessel one after the other in a set ratio by mass from an upstream unit operation. The ingredients are pumped and have the consistency of pastes. The mixing must be performed at a set temperature of 90ï‚°C and maintained at that temperature and for this reason the mixing tank is jacketed with a sealed unit using water which is re-circulated through a plate and frame heat exchanger supplied with steam as the heating medium. After mixing for a set period of time the contents are transferred to an adjacent pressure reaction vessel for further processing. The vessel is finally cleaned using hot water on completion of the transfer of the mixed ingredients.    Propose a design to achieve the requirement specified above. Consider in your design the following criteria:   The medium must never be in contact with air. The water jacket is sealed and must not be subjected to a pressure increase in the temperature control process. The method used to transfer the mixed product to the second vessel if pigging or pumping is not possible, as the vessel is in close proximity to the first. The mixed product must not cool down during the transfer between vessels The plant is fully automatic.   In your answer you should include a P&ID and briefly explain the function of the design and the reason behind any equipment used. (c) On commissioning of the unit operation designed in part (b) the end user has asked if the plant can be modified such that after the mixing period the contents can be cooled prior to transfer. Suggest a possible solution and explain the method of operation, including a simple sketch of the relevant section of the P&ID.              Q 7 a)Identify the options available to the process engineer when measuring the flow-rate of liquid in pipelines that are to be installed as part of a new fully automated process plant. Your answer should include four types of flow-meter each with a different operating principle, and in each case you should list the operational and installation advantages and disadvantages. Differential pressure devices should not be included.   b) Identify a choice of options available to the process engineer when ‘measuring’ the level of a liquid in a vessel, which is installed as part of a fully automated process plant, give the advantages and disadvantages of each method (c) Identify a choice of options available to the process engineer when ‘measuring’ temperature, in the case of each instrument, briefly describe their operating principle         (d) If you were required to supply an instrument to a Pharmaceutical Company in the context of liquid flow measurement what questions would you ask the end user.                                                                                                                                                                                   (e) If you were required to supply a flow switch to a Pharmaceutical Company specify two options that would meet the duty, and in each case, explain the device’s operating principle.   (f) If you were required to supply a device to be installed downstream of a bursting disc, in order to indicate rupture of the disc, suggest a device that would meet the duty, and explain the operating principle of the device.  (g) Automatic air driven valves are supplied normally open, normally closed or air/air driven with single or double feedback. Explain these terms.           (h) Automatic control valves can be specified to control flow or pressure upstream of the valve, explain why. What problem can occur if there is an excessive pressure drop across a control valve?     (i) If you were required to supply a flow control valve to a Pharmaceutical Company, what questions would you ask the end user to fully specify the valve? (j) Explain the principle of operation of a control valve and discuss the options available to the valve manufacturer. In your answer include the valve opening characteristics, direct and reverse acting, function of a positioner, use of a cam, and fitting of mechanical stops. Q8 (a) List the factors that have to be considered prior to specifying a pump for a given   duty.                                                                                                                  (b) Centrifugal pumps can be split into various designs. Identify the options available. (c)With the use of appropriate graphs, for centrifugal pumps only explain the relationship between pressure and volumetric flow, and power consumed and volumetric flow.                                                                                                        (d) Positive displacement pumps require in certain instances the inclusion of additional equipment. If a positive displacement pump was fitted in a fully automated plant, what equipment is essential and why, and what equipment is required in certain cases and why?                                                                                                                                        (e) An instrument is to be installed in a storage vessel to act as a low level switch, in order to protect a discharge pump from dry running. Describe two devices that would meet this duty.                                                                                                           (f) If a device cannot be fitted to the storage vessel, suggest a suitable instrument, and the point of installation, which would prevent the dry running of the discharge pump. Q 9 (a) Discuss the importance of certification in the manufacture and supply of instrumentation. Your answer should include the following certificates: IP, 3A, EHEDG, ATEX, FM, CSA, calibration, material and conformity.                              (b) Instruments are in certain cases certified as suitable for installation in pipe-work systems that undergo pigging. What is pigging, what is involved and how is the pipe-work design adapted for pigging?                                                (c) The following contact materials PEEK, PFA, PTFE, and EPDM are used in hygienic process plants. In each case, listing the beneficial properties, explain why?   (d) Instruments can be purchased to specifically measure the following parameters: ORP; Redox; Brix; and conductivity. Define each parameter.    Q 10 (a) A Tag number is imperative in the identification of an individual plant item, explain why this identification is necessary.                                                                            (b) Multi-national companies operate under global standards, using preferred suppliers, outline the benefits and disadvantages of this policy           (c) Accreditation is important, what certificates are typically required to ensure a process plant is accredited.                                                                               (d) Instruments can be supplied as intrinsically safe. What certificate would be supplied with an intrinsically safe instrument, and what methods could the manufacturer use to design a device to be intrinsically safe.         Q11 (a)Many companies insist on named manufactures, this is not only for instruments but also for the supply of all equipment, to maintain global standards. What are the reasons behind this policy, outline the possible advantages and disadvantages in your answer, considering not only the company itself, but also any other organisation who are designing or supplying process plant to the company. (b) Describe the operating mechanism of a Bourdon Tube pressure gauge. How is this instrument adapted for use in a modern automated hygienic plant? Suggest an alternative device, outlining its measuring principle and it benefits.  (c)You are the lead engineer in the design of a process plant that produces a cosmetic product, a suitable method to ensure that at the end of a production run the surface hold up in the transfer pipe-work is minimised is pigging. What is pigging and how is the design adapted to account for this.  d) Differential pressure transmitters can be used to determine the level in a pressurized vessel, or the pressure drop across a filter installed downstream of a pump. Discuss any problems if this type of instrument is used.  e)With the use of appropriate graphs, for centrifugal pumps only explain the relationship between pressure and volumetric flow, for pumps operating in series and for pumps operating in parallel. (Your answer should include the plant system curve, the individual pump curves and the combined pump curve)  f) If centrifugal pumps are installed in series, for the downstream pump what additional information must be supplied to the pump manufacturer.             Q12 (a)Compare and contrast, with the aid of drawings, where appropriate, the design characteristics of orifice plates and venturi tubes.                                                 (b) It is desired to measure the flow-rate of a process fluid (density 1000 kg/m3) in a 100 mm internal diameter pipe. The normal operating mass flow-rate is approximately 200,000 kg/hr and the maximum anticipated flow-rate is 250,000 kg/hr. It is proposed to use an orifice plate to measure the flow-rate, but for process reasons the pressure drop across the meter must not exceed 0.15 bar. (c) Calculate the diameter of the orifice bore to achieve this. (d) What is a reasonable figure for the minimum flow-rate that can be measured with this orifice plate installed?                   (e) Based on your knowledge of the operating characteristics of orifice plates and venturi tubes in the measurement of flow-rate, suggest an alternative use (apart from the measurement of flow-rate) for each item.                                                    (f) Describe the measuring principle used in the operation of electromagnetic flow- meters and vortex-shedding flow- meters. What applications are these types of instruments suitable for, and why?            g)The measurement of flow-rate in an open channel can be determined using a rectangular notch (weir), and applying the ‘Francis’ formula:  h) If Q represents the volumetric flow-rate (m3/s), what do the letters B, n and D represent?                                                                              i) Water flows in an open channel, across a rectangular weir, which occupies the full width of the channel. The width of the weir is 0.5 m and the height of water flowing over the weir is 100 mm. Calculate the volumetric flow-rate m3/s. j) What would be the height of the water flowing over the weir if the water flow-rate was increased to 0.04 m3/s?        k) Specify an instrument, which is to be installed in a fully automated plant, which is suitable for measuring the following, noting that differential pressure devices are not allowed, and in each case list the instruments features and benefits:  (i)           Fruit juice at a flow-rate of 20m3/hour (ii)          Carbon dioxide at a temperature of 50ËšC, and a flow-rate of 40m3/hour (iii)         Steam at 2 barg in a 25 mm ID tube. (iv)         Ultra pure water at a flow-rate of 180m3/hour (v)          Suspensions at a flow-rate of 36m3/hour l) Describe the measuring principle used in the instrument specified to measure the flow-rate of ultra pure water above. m) Flow switches can be used to detect the flow of liquid, describe three different devices that can be used for this purpose. In each case outline their advantages and disadvantages, their principle of operation, their typical use and point of installation. Do not include devices used downstream of bursting discs. n) You are the lead engineer on an expansion of an existing process plant, and in this capacity you are organising a HAZOP. The plant is fully automated. The members of the HAZOP team must have an understanding of how the plant operates. What two documents must be distributed to the HAZOP team prior to the day of the HAZOP exercise? Explain the importance of each document. o) If it was not possible to fit a device to the storage vessel suggest an alternative instrument, and its point of installation, which would prevent the dry running of the pump. p)Identify a choice of options available to the process engineer when ‘measuring’ temperature, in the case of each instrument, briefly describe their operating principle. q) Four temperature measuring devices are commonly used, thermocouples, RTD’s, thermistors and radiation pyrometers. In each case give an indication of a typical temperature response, state the general measuring range of the instruments, and list their advantages and disadvantages.   r) Based on your knowledge of instruments specify two methods to prevent the drying running of a centrifugal pump, if the pump is being used to empty a vessel in an automated process plant. In each case sketch a simple P&ID showing the position of any instrument used and explain your reasoning.                                                                                                                                                     Answers: For measuring flow rate of fruit juice in the range 5-25m^3/hour magnetic meter instrument can be used. a) i) Operating features of magnetic flow meter Magnetic flow meter transmitter has several kinds of operating features. It provides unparallel process of magnetic capabilities with the help of its coupled class performance. Its operating features are summarized below; An electrochemical emf is formed during the action which results the electrolytic reaction between the process of ion conducting and the metal electrode. When the conductors situated in the magnetic field inductive coupling are formed which can be also referred to as the quadrature of the a.c voltage. The operating principles run on the faraday’s law of induction. Benefits Various advanced diagnostics provides information of the unparallel that can enhance the performance of the meter. Isolated local operator interface make dist easy to design and makes it safe. It is also available for designing of the remote mount. ii) For measuring flow rate of sea water in the range 150-1000 m3/hour helical gear instrument can be used. Operating features and benefits of helical gear The helical gears may be used in several kinds of applications as it can be mounted in two types of shafts such as either 900 shafts of non-intersecting or parallel. The operating characteristics are smoother than any other instrument. The concentricity of the close connection between the diameter of the outside and the diameter of the patch which are smooth and easy to operate. The strength of the tooth is better due to the wrap of the helical around it. iii)   Nutating disk flow meter can be used for flow-rate of steam in a 50 mm nominal bore pipe. Operating features and benefits of Nutating disk Power source is not required for this type of measurement. The cost of the operating subsystem is low. It can be used for measuring the follow meter of the fuel. iv) For measuring the flow-rate of oil in the range 8-20 m3/hour magnetic meter instrument can be used. The operating features and the benefits of the magnetic meter instrument have been discussed previously Operating principle of magnetic flow meter The operating principle of magnetic flow meter is based on the principle of laws of Faraday’s electromagnetic induction. An electric current is applied to the package coil of the flow meter. The magnetic field (B) is generated in the coil due to this electromagnetic induction. When the flow of the liquid is applied to the instrument a very small voltage is induced in the coil. The induced voltage is proportional to the velocity of the flow of the liquid. Two electrodes of the coils are connected to the input of the circuit. The microprocessor then calculates the flow of the volumetric of the liquid and control of the input. The operating principle can be written by, u= K*B*v*d, where u is the induced voltage in the coil, B is the field of the magnetic strength between the two electrodes, d is the distance between the two different electrodes of the coil, v is the liquid velocity and k is the calibration factor. How can it measure the flow of a liquid? What are the capacity factor and size? How can it different from flow meter? What is the power used by the measurement instrument? c) Two types of instruments are proportional counter and the GM detector which can be suitable for detection the presence of flow during normal process operation. The advantages of GM detector This kind of detector is reliable and very useful for several kinds of indoor applications. Very easy to use and available in various kinds of shapes and sizes.   The advantages of proportional counter It can be used in various kinds of applications. There is no dead time for the various activity sources. d) Burst disc indicator device can be installed in the downstream of a bursting disc as an indicator of the disc rupture. Operating principle of Burst disc indicator device     Once the burst disc indicator has ruptured, it can be beneficial for shutting down of the plant. Burst is special kind of indicator circuit which is installed to the downstream of the burst disc during the breaking of the disc rupture. The input signal is frequently received by the control room of the plant. e) Advantages of the flip switch Installation process is easy. Long lasting. Versatility. Disadvantages of the flip switch Resetting is required. Loss of the temporal information. The better alternative switch is the magnetic switch as it is easy to use as well as to operate. 2 a)   Operating principle of the PID tank controller Plant This part of the controller system is to be controlled. Feedback The feedback part devices are used to measure the several kinds of variables which can be used to control. Error signal Error signal is measured during the operation of the PID controller. Disturbances Disturbances can be defined as such kinds of systems which can cuase the disturbance of the normal flow of the program. For example, the electrical intereference. Controller The controller is most important part of this system which is used to set the point, for processiong of the error and provides an accurate output of the signal.  b) Principle of operation of on/off air valves This kind of valve can be referred as the modulating pilot valve. The main valve a affecting the downstream valve b in different ways which are summarized below; When the valve of the pilot ‘a’ is closed then no water can be passed through the downstream to the main valve that is closed by pressure of the upstream and after that the valve is introduced the chamber a. When the valve b is open then it can allow passing the drainage pressure to the chamber of the downstream. At this point the pressure of the water is controlled by the control chamber which is nearly equal to the downstream of the chamber. The throttling valve b is flowing through a fixed volume of the water to the control chamber. c) Operating principle of inductive proximity switch Detection of the inductive proximity sensors The inductive proximity sensor can detect the loss of the magnetic due to its eddy current which is generated at the time of external magnetic field. The magnetic AC field is produced on the coil of the detection which can change the impedance due to the loss of eddy current. Several kinds of pulse response sensors are used to generate the eddy current which can detect the change of the time. Proximity sensor is used to detect the change of the capacitance between the object and the sensors. Advantages of proximity switch Switching rate is high. It is capable for working in any kind of environment condition. Very accurate.  3 a) The four types of communication signals are actuator, controller, and sensors and the feedback which are used in automatic process plant. The actuator responds to the sensors with the help of various command of the responder. For example, the gas flow in the heating system is used to monitor the pump. The controller signal is sued to control the overall process of the system and can embed with the computers. Sensors are used for measuring the physical state of the level like liquidity level. Feedback is used to control the error signal. b) Proportional mode is used in PID controller for getting continuous variables output which are used to provide the error signal between the set point and the variable process. Integral mode is used in PID controller to reset the output bias automatically which is used for processing of the adjustment of the process error. Derivative mode is used in PID controller to change the output as per the rate of the change of the error in the output.   c) The example of a process situation where PID control is used is the electrical resistance furnace. The PID loop control is used to control of the temperature resistance and it is used to stabilize the system. The example of the on/off control is thermostat which is used for measuring the temperature d) Differences between the Hardwired and software The hard wired interlocks provide intrinsic type protection of several kinds of equipments. The example of hard wired interlocks is Fuse. The hard wired interlocks provide inventory of the class for using of the sub detector. In software the protection is normal level. The example of the software is switched on /off of a system. It provides the several kinds of requirements as per the requirements of the actions. 4 a) Response characteristics of thermistor   Response characteristics of thermocouple Response characteristics of RTD   Range of temperature for measurement of the thermocouple -2670 C to 23160 C Range of temperature for measurement of the thermistors -100°C to 500°C Range of temperature for measurement of the RTD -240°C to 649°C The advantage of the thermistors Accuracy is high. High sensitivity. The advantage of the thermocouple Linearity is better. Temperature range is high. The advantage of the RTD Accuracy is better. Linearity is high. The disadvantage of the thermistors Temperature range is low. Not highly linear. The disadvantage of the thermocouple The operating cost is high. Accuracy is not so good.  The disadvantage of the RTD The range of the temperature for operating system is medium. Sensitivity is not so good. Operating principle of thermocouple Two different types of wire are joined at the end of the junction point which can be called as the thermo elements. Two types of thermo elements are denoted as the positive and negative element. One end of the junction is immersed in the air which is to be measured and other end is called the tail. Operating principle of Thermistors The resistance of the Thermistor can be determined by, R = Ro ek   and K = β(1/T – 1/To) where R is the resistance of the thermistor at different temperatures of T and e can be denoted as the Naperian logarithms and β is the constant range. Operating principle of RTD RTD operating system can be determined by, Rt= R0[1+α(t-t0)] where Rt is the temperature of the resistance, R0 is the temperature of the resistance at t0 and α is the temperature coefficient. b) Advantages Cost Disadvantages Self heating Non linearity Examples of the temperature sensor The example of temperature sensors are Thermocouple, full thermometer system, bimetallic thermometer and RTD.4 5 a) The factors are the actuator, controller, feedback and the sensors b) Importance of FDS Development can know the exact develop. Quality can be assured in accordance with the testing process. Tracking numbers are requirements. The client can be assured about the testing. c) The four types of communication signals are actuator, controller, and sensors and the feedback which are used in automatic process plant. The actuator responds to the sensors with the help of various command of the responder. For example, the gas flow in the heating system is used to monitor the pump. The controller signal is sued to control the overall process of the system and can embed with the computers. Sensors are used for measuring the physical state of the level like liquidity level. Feedback is used to control th
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