When an air bottle is subjected for inspection:
(a) As a management engineer, why does internal inspection needed?
(b) When opened up what are the parts to be inspected?
(c) Write a procedure for preparing a starting air receiver for internal inspection.
(a)
- Air bottles maintained according to PMS. Internal inspection done once in every (5) years.
- The corrosion and pitting are formed with vapour coming out from compress air enter into the reservoir.
- Oil particle carried over with the compressed air from compressor. If oxidation of the oil occurs corrosion and pitting.
- If the air reservoir is adjacent to the shipside, often cooler than the other parts of the engine room, corrosion or pitting expected on the inside cold surface of the reservoir adjacent to the shipside.
- The moisture formed on the bottom and cold surfaces and causes the corrosion.
- Due to the above facts, internal inspection of air bottles is needed.
(b) Parts to be inspected
(1) Inspect all valves, such as stop valve, safety valve and drain valve for air tightness, corrosion, erosion, soundness of valve spindle and springs (hammer test)
(2) Inspect manhole door joint face door studs & nuts, radial clearance between door & the frame (1/16″ on diameter) for corrosion & erosion.
(3) Inspect internal corrosion and pitting especially
(a) at the top of reservoir
(b) the bottom where condensate accumulates.
(c) around the valve openings
(d) any cooler areas where corrosion and pitting usually occur.
(4) Conduct further internal inspection for
(a) pitting corrosion,
(b) fatigue cracking,
(c) laminations,
(d) indentations and
(e) localized bulging.
(c) Procedure for preparing an internal inspection
(a) Risk assessment, permit to work, enclose space entry permit to be done.
(b) Air bottle should be isolated and drain out until empty.
(c) Lock down or blank off the cross-connection pipe from remaining air bottle.
(d) After carried out all necessary safety precaution, air bottle manhole opened up and applied air circulation by blower.
(e) Tested the atmosphere in air bottle frequently and provided the second person at entrance of air bottle.
(f) Ensure the protective equipment and appliance are in normal working condition.
Test for air bottle if cannot enter.
- Large pressure vessels, perform internal and external inspection, are no need to perform hydraulic pressure test if the visual condition is good and no defect.
- If the pressure vessel which cannot be entered to conduct internal inspection, it must be hydraulic tested. The test pressure is 1.25 x working pressure and maintains a period of 10 minutes.
Air bottle
(a) What protective devices are fitted?
(b) Why protective coating required and what are basic properties of coating?
(c) What is precaution taken before examination of internal inspection?
(d) Why is it required to fit drain valve? Why should that valve be overhauled? Whenever the opportunity occurs?
(a) Protective devices
1. Safety valve: Set to open full flow with not more than 10% rise in accumulation pressure.
2. Fusible plug: To prevent serious pressure rise in case of fire near air bottle. Its melting point is about 150˚C and its discharge is led to outside of engine room.
Material composition = Bismuth 50%, Tin 30% and Lead 20%.
3. Atmospheric relief valve: Provided for back up of fusible plug. In case of engine room fire and when CO2 flooding is required. Open this valve before evacuating engine room.
4. Drain valve: Fitted at bottom of the reservoir to drain out oil, moisture and condensate.
5. Pressure switch: To control compressors start/stop and maintain air bottle pressure within safe limits.
6. Pressure gauge: Fitted to indicate the compressed air pressure in the air bottle.
(b) Protective coating
- It is required to prevent internal corrosion of air bottle. Condensation of moisture held in air as suspension and decomposition cause acid formation.
- Therefore internal surface should be coated with anti corrosive coating.
- The basic properties of coating are
- Anti-corrosive,
- Anti-toxic
- Anti-oxidation.
- Graphite suspension in water, Epoxy coating, Copal vanish and Linseed oil are mostly used.
(c) Precaution taken before examination of internal inspection
1. Risk assessment and work permit are to be carried out.
2. All mounted valves are closed tightly.
3. Isolate cross over pipe from another bottle.
4. Any liquid in the bottom of air bottle should be drained out and cleaned.
5. Well ventilate provided inside air bottle.
6. Use gas tight and safe portable light.
7. Attendant should be provided at the entrance.
8. Secure man hole at open position.
9. Supply ventilation at all time.
10. Breathing apparatus, portable fire extinguisher to be kept stand-by.
(d) Drain valve
- Oil and moisture carry along with compressed air and accumulate in air bottle.
- They can cause: starting air line explosion and
- Corrosion at internal surface of air bottle, air starting line system and pneumatic control valve
- Drain valve is provided at the lowest point of air bottle to drain out oil and moisture from bottle to prevent above defects.
- The valve overhauled at survey and opportunity occurs to maintain the optimum condition.
- Perfect air tight when shut to prevent air pressure drop at air bottle and excess running of air compressors.
(a) Sketch the Inert Gas System and why the Inert Gas System is used onboard?
(b) Define function of (1) Scrubber (2) Deck Water Seal (3) Pressure Vacuum valve
(c) Safety devices fitted in inert gas system
(a)
- Inert gas system is mostly used in tanker.
- For preventing back flow of cargo gases to machinery space, inert gas plant and inert gas distribution system together fixed and portable measuring instruments and control devices.
- Inert gas consists mainly of oxygen, Carbon dioxide, Nitrogen
- Inert gas produced either by burning diesel oil in a special air or water cooled generator or by washing and cooling waste boiler flue gases.
- Inert gas used only in fixed installation and needs large bored piping.
- Its function is essentially fire preventive by providing an inert atmosphere in the neighborhood of cargo which loaded, transported and unloaded under a blanket of inert gas.
- During passage, inert gas pressure maintained above cargo.
- After discharge of cargo and before tank washing, tank purged with inert gas.
- During purging oxygen content of gas emitted
(b)
(1) Scrubber
Functions-
1. To cool the flue gas
2. remove most of sulphur dioxide
3. To particulate soot.
(2) Deck water seal
1. To permit inert gas to deliver to deck main
2. When plant shut down, prevents any back flow of cargo gas even
(3) Pressure/Vacuum valve
One or more Pressure/ Vacuum valves or Liquid-filled P/V breaker are fitted to prevent excessive pressure or vacuum of the deck main and cargo tanks.
(c) Safety devices fitted in inert gas system
1. Combustion process alarm -Flame failure.
2. Fuel oil low pressure alarm
3. Combustion air low pressure alarm
4. Emergency stop
5. Scrubber water-level high alarm
6. Cooling water pressure High/ Low alarm.
7. IG blower motor failure alarm
8. O2 content high and high-high alarms.
9. IG outlet temperature high alarm.
10. Non-return valve on deck main supply line
11. Liquid filled Pressure/ vacuum breaker on deck main supply line.
12. Deck seal at main supply line on deck main and it is acting as non-return valve.
13. Deck main line pressure high/ low and Low Low alarms.
14. Deck seal water-level low alarm and pressure Low alarm
15. P/V valve for each tank
With reference to refrigerating system,
(a) Sketch and describe thermostatic expansion valve
(b) Sketch and describe high pressure cut out.
(a) Thermostatic expansion valve
- TEV controls flow of liquid refrigerant to evaporator, maintain constant super heat of refrigerant at evaporator outlet.
- Liquid refrigerant from condenser passes through T.E.V, pressure suddenly drops and partially evaporated.
- Flow of liquid refrigerant is controlled by movement of needle valve.
- Sensing bulb is connected to top of diaphragm with capillary tube.
- Sensing fluid (same as refrigerant used in the system), inside bulb expand or contract with change of cold room temperature and control flow of liquid refrigerant to evaporator.
- Actual pressure of refrigerant at end of evaporator is connected to underside of diaphragm with equalizing line.
- Thermal sensing fluid of sensing bulb transfers pressure equivalent to get superheat temperature of 6.6˚C.
- This pressure is balanced with equalizing pressure plus spring pressure.
- If superheat temperature falls, needle valve throttles flow.
- Reduce flow of refrigerant and maintain constant superheat temperature at evaporator outlet.
(b) High Pressure cut out
- Consists of bellow, main spring, adjusting screw, pivot, catch and switch arm.
- Bellow is connected by small bore pipe between compressor discharge and condenser.
- Bellow tends to expand by pressure and its movement is opposed by spring pressure which can be set by adjusting screw.
- During normal operation, switch arm is held up by catch and holds electrical contact in place.
- If pressure becomes excessive, bellow expands and pushes switch arm catch.
- Upper end of catch slips to right and releases the switch arm, electrical contacts break and compressor is stopped.
- This cutout cannot be restored until trouble has been rectified. Reset switch by hand after rectified the fault.
(a) Compare the advantages and disadvantages of Plate type heat exchangers to Shell and tube type.
(b) Identify with reasons, factors which contribute to failure of multi-tubular heat exchangers.
(c) For what reasons plate patterns of the Plate type heat exchangers are corrugated.
(a) Advantages and disadvantages of Shell and tube type heat exchanger
Advantages
1. extended heat transfer using fins or baffles
2. defective tubes easily plug
3. less expensive than a plate type using same materials
Disadvantages
1. Fixed size (only 30% excess capacity)
2. Requires spare tube stack
3. Cause any leakage contamination
4. Difficult to clean (Inside-mechanical cleaning, Outside-chemical cleaning)
Advantages and Disadvantages of plate type heat exchanger
Advantages
1. Compact design and less space required
2. Easy and efficient cleaning
3. Lighter in weight.
4. no velocity limit.
5. Easy choice of flow pattern.
6. Heat transfer capacity increase y adding more plates n pair
7. High heat transfer coefficient as corrugations create high turbulent
8. Risk of leakage between liquids is eliminated by using Nitrite rubber gaskets.
9. Titanium plate is ‘Self-Healing’; it has extremely resistance to corrosion and erosion.
Disadvantages
1. Plates are of high cost
2. Temperature and pressure limitation
3. Defective gaskets are difficult to remove.
4. Scratches or careless markings can encourage erosion which leads to perforation of the plate and contamination.
(b) Factors which contribute to failure of multi-tubular heat exchanger
(a) Tube failure due to
(1) material defects
(2) long age
(3) lack of regular maintenance
(4) careless tube cleaning
(b) Baffle plate failure due to
(1) material defects
(2) long age
(3) lack of regular maintenance
(4) excessive flow speed
(c) Division plate failure due to
(1) material defects
(2) long age
(3) lack of regular maintenance
(4) excessive flow speed
(5) zinc anodes are wasted
(6) protective paints are wasted
(d) Sealing failure due to
(1) long age
(2) careless maintenance
(3) temperature control excess than limit
(c) Reasons for plates of Plate type heat exchanger are corrugated.
1. Corrugate plate increased strength
2. Corrugated plate pattern, increases heat transfer area and turbulent flow
3. Turbulence flow prevents deposit and fouling on the plates.
4. Get more heat transfer area than plane surface.
With reference to the plate type heat exchanger
(a) State the causes of plate leakage
(b) State the likely result of excessive tightening of plate pack.
(c) Describe maintenance works required to obtain effective heat transfer.
(a) Causes of plate leakage
1. Deterioration of gaskets due to harden and loss of elasticity of joints.
2. Damage of corrugated plate and gasket due to incorrect cleaning method, uneven tightening
3. Temperature and pressure higher than limitation (max. 100°C and 14 bar)
4. Improper assembly of gaskets (or) foreign particles between seat and gaskets during assembly.
5. Carrying bolts lock nut loosens.
6. Distortion and vibration of foundation.
(b) Result of excessive tightening of plate
1. Seal damage leading to water leakage.
2. Deformation of plate affect turbulence of flow.
3. Narrow space between two plates, decrease amount of fluid flow and reducing cooler efficiency.
4. Shorter life of plate due to stress.
5. Inter-mixing of fluids and contamination occur due to gasket failure.
(c) Maintenance works required to obtained effective heat transfer
1. Whole unit cleaned without dismantling plates, by forcing cooling water in reverse flow.
2. After dismantling, plates cleaned by brushing with perlon brushes and fresh water. After brushing, rinse with fresh water.
3. Do not over tighten to prevent nitrite rubber gaskets damage. Tightening done as per maker instruction.
4. Do not allow high temperature liquid passing through cooler without cooling medium.
5. If S.W is used as cooling medium, sea strainer cleaned regularly to prevent insufficient S.W flow due to blocked strainer
6. Jacket C.W or cooling medium F.W tested and treated regularly to prevent scale formation.
