The assignment is attached.
1 Task 1: (50 marks) Case Description: Hydraulic Turbine Trip - lubrication and cooling system of the combined bearing All the rotating mass of the turbine-generator group is supported axially in the prop bearing and radially in the guide bearing, both contained into the same oil container. All this set is called combined bearing. The combined bearing is sliding type, composed by two main and distinct surfaces, being the mobile part connected to the axle, and the fixed part constituent by the skids or sabots. The mobile part is composed by a polishing steel disc, commonly called mirror. There isn’t contact between the two surfaces because always it will have an oil film between them, whose function is to prevent the direct contact metal with metal and also to make cool, wasting the heat generated by attrition between the surfaces. The Figure (1) shows a diagram of the turbine- generator group to ease the localization of the combined bearing. The oil is removed from the container by a set of bombs; it is cooled through heat exchangers and led back to the container where the bearing active parts are immersed. The heat exchangers have as cooling fluid the water. There are two exchangers, one being normally in operation and the other as reserve. Due to the weight of the rotating mass and the hydraulic counter-attraction that come across the prop bearing, it is necessary that, during the machine start and stop, be injected oil between sabot and the prop bearing ring, to lubricate it. The prop bearing oil injection system forms a film of oil between the fixed and rotating parts, in the band of 0% to 50% of the nominal rotation. In its nominal rotation or even above 50% of it, the prop bearing is auto-lubricated. 2 The oil injection in prop bearing is realized by two high-pressure bombs, identified as “AG” and “AH”, and the oil cooling is also realized by two bombs, identified bombs as “AI” and “AJ”. Figure 1. Group Kaplan turbine-generator, detaching the combined bearing. 3 The FMEA analysis was developed according to standard procedures. The results of analysis of the functions, modes and effects of failure of the components of the lubrication and cooling system of combined bearing are shown in Appendix A. For the top event of lubrication and cooling system of the combined bearing Failure on Demand, develop a Fault Tree analysis using downloaded FTA software (Top Event FTA, freely downloadable). The failure probability of each basic event of the FT was calculated in accordance with the failure rate, as described in the Equation below, from the detailed information about each failure and about the time of duration of the failures. The failure rate of each basic event of the FT in study is shown in the Tab. (1). Table 1. Probability of basic events failure. Basic Event Failure rate (failure/hour) Basic Event Failure rate (failure/hour) Incrustation in the inox plaques of heat exchanger 01 3.975 x 10-4 Bobbin of the contactor of motorpump AI with dry resistance 3.960 x 10-5 Looseness in the inox plaque connections of heat exchanger 02 3.961 x 10-5 Bobbin of the contactor of motorpump AJ with dry resistance 3.960 x 10-5 Incrustation in the inox plaques heat exchanger 02 3.576 x 10-4 Bobbin of the contactor of motorpump AG with dry resistance 3.960 x 10-5 Deterioration of the filter element 01 3.960 x 10-5 Bad connection of the contactor bourns of motorpump AJ 3.960 x 10-5 The non-cited events in the Table (1) are not registered in the anomaly cards, that is, they represent failures that had not occurred in the last 3 years. For these events, it has been assumed a failure rate a corresponding to 1 failure in 15 years (7,615 x 10-6 failures/hour). 4 FTA Models 1. The following failure probability models are to be used for calculating the unavailability or PFD. Use the MTBF model for the repairable components, the FTA software referenced above uses the following formulae Where: q(t) Component unavailability λ Component failure rate µ 1/ Component repair time (MTTR) MTBF Mean time between failures MTTR Mean time to repair Questions: a) Develop an FTA model using the Top Event FTA or other similar software using the data provided in case study and Table 1. Present the fault tree (with at least four levels) for the event described above. (15 marks) b) Obtain the set of events that constitute the limit of resolution of the fault tree and identify and show the called basic causes and the occurrence probabilities, i.e. PFDs, of the basic causes, and determine the cut sets. (15 marks) c) What is the critical minimum cut set for the event? (10 marks) d) Suggest any drawbacks of using this analysis technique and make recommendations about improvement. (10 marks) 5 Task 2: (25 marks) Case Study: Chemical Recycling Plant Inclusion of recycle streams in chemical processes sometimes has to be done to improve plant economics, in spite of the fact that from the point of view of process control it can seriously affect system performance making it more sluggish because the overall time constant is increased. The plant has two main operating units: a reactor and a stripper (see Figure 2). Fresh feed, consisting of reactant A and some of product P, is fed to the reactor. The reactor is a continuous stirred tank. The irreversible reaction that occurs is of first order: A to P. The reactor output is fed to the stripper. Most of unreacted A is separated from product P there. The plant´s product, with a small mol fraction of A (XAB) is obtained at the stripper´s bottom. The stripper´s output at the top is recycled to the reactor. The plant is designed around a fixed feed composition of XA00 = 0.9. The inventory of the column base is controlled through the manipulation of the bottom product flowrate. The cooling water flowrate to the condenser is used to control the pressure. Perfect control is assumed for this loop. A Proportional-Integral controller manipulates the flowrate of steam to the reboiler of the stripper in order to regulate the purity of bottom's product. Develop safety requirements specification (SRS) (use the provided template as an example), for controllers at the output stream B, XAB, reactor affluent, condenser and the recycle stream. The SIL for each SIF is required to be SIL2 with at least an RRF of 200. 6 Figure 2. Chemical plant with a recycle stream. Task 3: (25 marks) The first step in achieving compliance is to prepare and to implement a “Functional Safety Management Plan”. Functional safety management simply applies quality management to systems that are designed to control risk. Functional safety management simply applies quality management to systems that are designed to control risk. Functional safety refers to “Safety Instrumented Systems” that implement “Safety Instrumented Functions” (SIFs) as part of a company’s overall risk management strategy. a) List at least five quality management principles in the context of product realization and safety management. (5 marks) b) List five principles of risk management. (5 marks) 7 c) Provide a list of requirements for a functional safety management plan (FSMP) and describe each requirement briefly. (15 marks) Appendix A LUBRICATION AND COOLING SYSTEM OF COMBINED BEARING FUNCTION To dissipate the heat generated in the combined bearing and lubricate its components. COMP. COMPONENT FUNCTION FUNCTIONAL FAILURE FAILURE MODE FAILURE CAUSE FAILURE EFFECT 1. Filter Filter the oil Do not filter the oil 1.1 Deterioration Disruption of the filter mesh - Risk of contaminating the oil load with residues Disruption of the O-rings Obstruct the oil flow 1.2 Clogging. High differential pressure Excess of impurities in the filter element - Consignment in the system lubrication andcooling 2. Circulation motorpumps AI e AJ Pump the oil Operate below of the 1,2 bar pressure 2.1 Low oil pressure on the motorpumps exit Oil leakage on the mechanical stamp - Turns off the priority bomb and turns on the reserve bombin the low pressure. In case that it also fails, it provokes TRIP in the generating unit. - Disturbance in the normal functioning system (failure inthe lubrication and cooling) Coupling damage Corrosion due to contaminated oil or a bad quality one Cavitations of the gears due to presence of air in the oil Electric defect (damaged poles) Abnormal noise 2.2 Noise Wasted rolls - Risk that the motorpumpbreaks Rolls badly lubricated Overheating 2.3 Thermal relay performing Loss of internal parts, provoking attrite with the motorpump axel - Turns off the priority bomb and turns on the reserve bombin the low pressure. In case that it also fails, it provokes TRIP in the generating unit. - Disturbance in the normal functioning system (failure inthe lubrication and cooling) Excessive lubrication Bad lubrication Operate below of the pressure of 35 bar 3.1 Low oil pressure on the exitof the motorpumps Oil leakage on the mechanical stamp - Turns off the priority bomb and turns on the reserve bombin the low pressure. In case that it also fails, it provokes TRIP in the generating unit. - Disturbance in the Coupling damage Corrosion due to contaminated oil or a bad quality one Cavitations of the gears due to presence of air in the oil 8 3. Injection motorpump sAG e AH Pump the oil Electric defect (damaged poles) normal functioning system (failure inthe lubrication and cooling) Abnormal noise 3.2 Noise Wasted rolls - Risk that the motorpumpbreaks Rolls badly lubricated Overheating 3.3 Thermal relay performing Loss of internal parts, provoking attrite with the motorpump axel - Turns off the priority bomb and turns on the reserve bombin the low pressure. In case that it also fails, it provokes TRIP in the generating unit. - Disturbance in the normal functioning system (failure inthe lubrication and cooling) Excessive lubrication Bad lubrication 4. Valves Isolate system components and supervisionand control accessories Do not isolate supervision and control accessories 4.1 Leakage Head valve or opposed head valve deterioration - Impossibility to execute maintenance in the supervisionand control accessories - Risk of accident Improperly isolate the supervision and control accessories Valve piston stiff Lack of squeeze in the closing of the valve Limit the pressure in case of circuit blockage Do not relief the pressure 4.2 Overpressure. High pressure in the exit of motorpumps Incorrect adjustment - Actuation of TRIP in thegenerating unit - Risk of disrupting the tubingand gaskets - Risk of break in themotorpump - Risk of