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VLEC Managment / Day 3 Revision - Refrigeration and Cooldown
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{ "quizId": "day3_refrigeration_cooldown_revision", "courseId": "VLEC Managment", "title": "Day 3 Revision - Refrigeration and Cooldown", "subtitle": "BSM Maritime Training Centre", "passPercent": 70, "version": "1.0", "date": "2026-03-17", "questions": [ { "id": 1, "topic": "Learning Objectives", "question": "Which thermodynamic diagram is specifically highlighted as a learning objective for refrigeration cycle analysis?", "options": [ "Tide table", "Mollier diagram", "Cargo loading plan", "Hull stress chart" ], "answer": "Mollier diagram", "explanation": "The Day 3 objectives specifically mention learning to identify and interpret Mollier or pressure-enthalpy diagrams." }, { "id": 2, "topic": "Reliquefaction Functions", "question": "What is the primary function of the reliquefaction plant during normal voyage?", "options": [ "Heating ballast tanks", "Condensing boil-off gas to control tank pressure", "Generating inert gas", "Purifying lubricating oil" ], "answer": "Condensing boil-off gas to control tank pressure", "explanation": "The reliquefaction plant continuously condenses boil-off gas to maintain cargo tank pressure within design limits." }, { "id": 3, "topic": "Reliquefaction Functions", "question": "Which of the following is listed as an additional auxiliary function of the reliquefaction plant?", "options": [ "Main engine starting air charging", "Line clearing operations using hot gas mode", "Ballast pump priming", "Fresh water production" ], "answer": "Line clearing operations using hot gas mode", "explanation": "Additional reliquefaction plant functions listed include hot gas line clearing, vapor purging ashore, and inter-tank vapor transfer." }, { "id": 4, "topic": "Mollier Diagram", "question": "On a Mollier diagram, what does the vertical axis represent?", "options": [ "Absolute pressure in bar", "Compressor RPM", "Cargo tank volume", "Liquid density" ], "answer": "Absolute pressure in bar", "explanation": "The vertical axis on the Mollier diagram is absolute pressure, shown on a logarithmic scale." }, { "id": 5, "topic": "Mollier Diagram", "question": "What does the horizontal axis on a Mollier diagram represent?", "options": [ "Temperature in Celsius", "Enthalpy in kJ/kg", "Specific gravity", "Vapor pressure ratio" ], "answer": "Enthalpy in kJ/kg", "explanation": "The horizontal axis on the pressure-enthalpy or Mollier diagram is enthalpy, measured in kJ/kg." }, { "id": 6, "topic": "Direct Systems", "question": "In a direct reliquefaction system, what is used as the primary cooling medium for the cargo condenser?", "options": [ "Warm glycol", "Seawater", "Nitrogen", "Hot oil" ], "answer": "Seawater", "explanation": "Direct system reliquefaction plants use seawater as the primary cooling medium in the cargo condenser." }, { "id": 7, "topic": "Direct Systems", "question": "Why do warmer seas reduce direct-system efficiency?", "options": [ "They reduce cargo density only", "They require higher condensing pressures and more compressor work", "They stop seawater flow completely", "They increase glycol viscosity" ], "answer": "They require higher condensing pressures and more compressor work", "explanation": "Warmer seawater raises condensing pressure, reducing efficiency and increasing the compressor work required." }, { "id": 8, "topic": "Multi-Stage Compression", "question": "What is a key benefit of intercooling between compression stages?", "options": [ "It raises cargo tank pressure", "It reduces discharge temperatures and throttling losses", "It increases non-condensables", "It prevents all valve wear" ], "answer": "It reduces discharge temperatures and throttling losses", "explanation": "Intercooling between compression stages reduces discharge temperatures and reduces throttling losses." }, { "id": 9, "topic": "Cascade Systems", "question": "Why are cascade systems essential for fully refrigerated Ethane/Ethylene cargoes?", "options": [ "They operate only with steam heating", "They cannot be handled with direct seawater cooling systems", "They eliminate the need for compressors", "They do not require condensers" ], "answer": "They cannot be handled with direct seawater cooling systems", "explanation": "The presentation states that fully refrigerated Ethane/Ethylene cargoes require cascade systems because direct seawater cooling is not suitable." }, { "id": 10, "topic": "Indirect Systems", "question": "Indirect reliquefaction systems are essential for cargoes that cannot be mechanically compressed for what reason?", "options": [ "Electrical reasons", "Chemical reasons", "Navigation reasons", "Structural reasons" ], "answer": "Chemical reasons", "explanation": "Indirect systems are needed for chemically sensitive cargoes such as ethylene oxide and propylene oxide." }, { "id": 11, "topic": "Knockout Drum", "question": "Why is a liquid separator or knockout drum fitted ahead of the cargo compressor?", "options": [ "To increase seawater pressure", "To protect the compressor from incompressible liquid carryover", "To heat the vapor before compression", "To measure inhibitor concentration" ], "answer": "To protect the compressor from incompressible liquid carryover", "explanation": "Liquids are incompressible and can cause catastrophic liquid knock if they enter the compressor cylinder." }, { "id": 12, "topic": "Knockout Drum", "question": "During starting, why should the suction valve be throttled?", "options": [ "To avoid condensation within piping and compressor", "To increase shaft speed immediately", "To raise glycol outlet temperature above 60C", "To keep receiver level high" ], "answer": "To avoid condensation within piping and compressor", "explanation": "The operational note states that the suction valve should be throttled during starting to avoid condensation within piping and compressor." }, { "id": 13, "topic": "Cargo Compressor", "question": "What is a typical example of 3rd-stage ethane hot gas discharge conditions mentioned in the presentation?", "options": [ "5 bar and -20C", "12 bar and +40C", "28 bar and +135C", "36 bar and +10C" ], "answer": "28 bar and +135C", "explanation": "The presentation gives ethane 3rd-stage discharge as an example at about 28 bar and +135C." }, { "id": 14, "topic": "Condenser", "question": "What is the usual temperature relationship between condensate leaving the condenser and the cooling medium?", "options": [ "The condensate is 5-8C higher than the seawater or refrigerant temperature", "The condensate is exactly the same temperature as seawater", "The condensate is always 20C hotter than hot gas inlet", "The condensate must be below the cooling medium temperature" ], "answer": "The condensate is 5-8C higher than the seawater or refrigerant temperature", "explanation": "The condenser outlet condensate is typically around 5-8C warmer than the seawater or refrigerant temperature." }, { "id": 15, "topic": "Receiver", "question": "What is the function of the condenser accumulator or receiver?", "options": [ "To store lubricating oil for the compressor", "To collect condensed liquid and keep the condenser liquid-free", "To increase tank pressure during loading", "To separate nitrogen from cargo vapor" ], "answer": "To collect condensed liquid and keep the condenser liquid-free", "explanation": "The receiver collects condensed liquid so the condenser remains free of accumulated liquid and maintains maximum heat transfer area." }, { "id": 16, "topic": "Condensing Pressure", "question": "Which factor will generally lower condensing pressure?", "options": [ "Lower coolant temperature", "More condenser fouling", "Higher methane content in ethane", "Restricted seawater flow" ], "answer": "Lower coolant temperature", "explanation": "Lower seawater or refrigerant temperature reduces condensing pressure." }, { "id": 17, "topic": "Economizer", "question": "What is one major purpose of the economizer or intercooler?", "options": [ "To increase non-condensables in the system", "To cool suction gas between compression stages", "To block condensate return to tanks", "To replace the condenser receiver" ], "answer": "To cool suction gas between compression stages", "explanation": "The economizer provides suction gas cooling between compression stages and also sub-cools condensate." }, { "id": 18, "topic": "Operating Modes", "question": "What does NIC mean in reliquefaction operating modes?", "options": [ "Nitrogen intercooling control", "No inter-cooling", "Normal injection cycle", "Non-inhibited condensate" ], "answer": "No inter-cooling", "explanation": "NIC means no inter-cooling in a two-stage compression mode." }, { "id": 19, "topic": "Incondensables", "question": "Where do foreign or incondensable gases collect if they do not condense?", "options": [ "In the cargo tank bottom", "In the condenser accumulator above the liquid level", "In the glycol expansion tank", "Inside the manifold drains only" ], "answer": "In the condenser accumulator above the liquid level", "explanation": "Non-condensables collect in the condenser accumulator above the liquid level because they do not condense at cargo conditions." }, { "id": 20, "topic": "Incondensables", "question": "What is a likely consequence of excessive non-condensables collecting in the system?", "options": [ "Lower condenser pressure", "Higher pressure and possible compressor trip", "Automatic increase in cargo purity", "Improved reliquefaction efficiency" ], "answer": "Higher pressure and possible compressor trip", "explanation": "As foreign gases accumulate, condenser accumulator pressure rises and compressors may trip on high pressure." }, { "id": 21, "topic": "Vent Gas Disposal", "question": "Which vent gas disposal method is described as the preferred option for most operations?", "options": [ "Ship's vent mast", "Return to cargo tanks", "Vent gas cooler", "Compressor crankcase vent" ], "answer": "Vent gas cooler", "explanation": "The vent gas cooler is preferred because it recovers condensable vapors before venting and minimizes cargo loss." }, { "id": 22, "topic": "Compressor Design", "question": "What type of cargo compressor is used in the presentation's cargo system overview?", "options": [ "Oil-injected screw compressor", "Reciprocating oil-free piston compressor", "Centrifugal blower", "Rotary vane compressor" ], "answer": "Reciprocating oil-free piston compressor", "explanation": "The cargo compressor is described as a reciprocating, oil-free piston type with dry cylinders." }, { "id": 23, "topic": "Glycol Cooling", "question": "What glycol mixture is recommended for compressor cooling?", "options": [ "100% ethylene glycol", "50/50 nonethylene glycol and freshwater", "50/50 seawater and oil", "Freshwater only" ], "answer": "50/50 nonethylene glycol and freshwater", "explanation": "The presentation recommends a 50/50 weight mixture of nonethylene glycol and freshwater, with corrosion inhibitor." }, { "id": 24, "topic": "Trips", "question": "At what abnormal high discharge temperature is the cargo compressor listed to trip?", "options": [ "90C", "120C", "175C", "250C" ], "answer": "175C", "explanation": "One of the listed compressor trip conditions is abnormal high discharge temperature of 175C." }, { "id": 25, "topic": "Refrigerant Compressor Start-Up", "question": "How long should the oil separator heating element be activated before starting refrigerant compressors?", "options": [ "About 10 minutes", "About 30 minutes", "About 2 hours", "About 6 hours" ], "answer": "About 2 hours", "explanation": "The start-up sequence specifies heating the oil separator for approximately 2 hours before start." }, { "id": 26, "topic": "Cargo Compressor Start", "question": "Before starting a cargo compressor, to what temperature should the compressor be warmed by glycol circulation?", "options": [ "5C", "20C", "35C", "60C" ], "answer": "35C", "explanation": "The normal starting procedure says not to start until the compressor temperature reaches 35C." }, { "id": 27, "topic": "Monitoring", "question": "What suction-side condition must be maintained to avoid liquid knock during continuous operation?", "options": [ "Suction temperature lower than dew point", "Suction temperature higher than dew point", "Suction pressure always negative", "Suction line completely full of liquid" ], "answer": "Suction temperature higher than dew point", "explanation": "The suction-side gas temperature must remain above the corresponding dew point to avoid condensation and liquid knock." }, { "id": 28, "topic": "Cooldown", "question": "What can happen if cold cargo is introduced into the cargo system without prior cooldown?", "options": [ "Nothing significant if pressure is low", "Rapid vaporization and severe thermal stresses", "Immediate increase in inhibitor concentration", "Automatic pump alignment correction" ], "answer": "Rapid vaporization and severe thermal stresses", "explanation": "Without cooldown, cold cargo can vaporize rapidly causing pressure rise and severe thermal stresses that may damage the tank." }, { "id": 29, "topic": "Cooldown Objective", "question": "What is the stated target at the tank bottom during initial cooldown?", "options": [ "Within 10C of the anticipated cargo temperature", "Exactly ambient temperature", "At least 20C above cargo temperature", "Below seawater temperature" ], "answer": "Within 10C of the anticipated cargo temperature", "explanation": "The primary objective is to reduce steel temperature to as close as practical to cargo temperature, within 10C at the tank bottom." }, { "id": 30, "topic": "Membrane Tanks and Pumps", "question": "If IBS pressure decreases too fast during cooldown, what action is recommended?", "options": [ "Shut all nitrogen valves permanently", "Open the bypass of the nitrogen supply valve and control IBS pressure manually", "Increase spray rate to maximum regardless of trend", "Start deepwell pumps immediately at full speed" ], "answer": "Open the bypass of the nitrogen supply valve and control IBS pressure manually", "explanation": "When IBS pressure drops too quickly, manual control using the bypass may be necessary to maintain adequate pressure." } ], "courseFolder": "VLEC Managment", "quizFolder": "Day 3 Revision" }
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