Question With Answer

1) About Cold & Hot corrosion

   Cold Corrosion:- It is formed because of sulphur content in fuel. SO2 when combined with water{Moisture in air, condensation due to low JCW temperature} gives H2SO4 which leads to corrosion.

Hot Corrosion:- It occurs due to increase in temperature (above 550 c) due to presence of vanadium & sodium in fuel oil.
  V+O2-------> V2O5     V2O5 get deposited  on valve structure. as it is highly corrosive it creates small grooves on the surface. When there is rise in temperature gases impinges on it, wire drawing occurs. thus valve leaks or breaks in-course of .

2) Function Of Roto caps

Roto Caps:- The rotocap is a mechanical means to rotate a valve during operation.

 Usually fitted to exhaust valves, rotation of the valve ensures that the valve is kept at an even temperature so it does not distort and leak by, and it helps  dislodge any build up of deposits on the valve and seat.

 The valve cover has an number of  inclined tracks machined into it, each containing a ball and spring. The balls run on a race; When the valve is closed, the belville washer locks the valve cover and spring carrier together, preventing rotation, and at the same time acts on the ball race, pushing it hard up against the balls, locking them half way up the inclined track and compressing the springs.

 Rotocaps stop operating below about 25% engine load. Contaminants in the lub.oil causing deposits in the inclined tracks  will stop the rotocap from working. Continued operation will then cause indentations in the tracks by the balls, which will render the rotocap beyond repair.

 It is normal to mark the top of the rotocap with a blob of paint so that it can be easily seen if they have stopped rotating. If this is the case, then they should be changed as soon as possible and overhauled.

 It should be noted that rotocaps rotate the valve as it opens. Although this will prevent distortion due to uneven temperatures, it does not lend itself to removing the build up of deposits on the. 

3) About Exhaust gas Economiser fire & how it occure 

 Exhaust gas Economiser:- 

• A flame appear at the economizer coil during running of main engine at sea while the EGE is put into service with circulation water passing through the coils, it is called economizer fire.
• It is actually cause by soot fire at the economizer coils.
• For the economizer coils fire, heat is already presented due to passing the gases of main engine.
• Air is available due to excess supply of scavenging air into the unit combustion chamber.
• Thick deposit of unburnt fuel, carbon residue (soots) are sticking at the economizer coils .
• Soots are formed from incomplete combustion and use of low grade fuel and high carbon content fuel.
• At the manoeuvring time, the more incomplete combustion may occur and at that time leaving flue gas velocity is very low, thus unburnt fuel can be adhering on the economizer coils.
• A flame is produced when air and fuel are proportionally mixed in the sufficient heating temperature. 

Indications of EGE fire:-

1. Smoke smell will get from the economizer.
2. Over heat at economizer body ( external casing of uptake.)
3. Heavy smoke and sparks will emit from the funnel.
4. Sudden unexpected increase in uptake gas temperature ( Abnormally high stack pyrometer reading)
5. Flame visible in the smoke indicator.

Prevention of EGE fire:-

1. To get complete combustion of fuel, maintain the main engine (ME) in optimum combustion condition.( governor, fuel injection pump, fuel injection valve, fuel injection timing, fuel condition, air cooler, turbocharger, do not run too long ME with slow speed)
2. Regular open up & cleaning smoke side of economiser depending upon soot accumulation. (water washing)
3. Regular soot blowing operation. Check & maintain soot blowing equipment.
4. Regular overhaul boiler burner, correct air fuel ratio & damper. 

 4) About Hydrogen Fires

  Hydrogen Fire: Hydrogen is flammable at concentrations between 4% and 75% in air, which is a very wide range compared to other common fuels.

• The hydrogen concentration could easily reach the lower flammability limit (4%) if there were a leak in a confined space with no ventilation. An outdoor leak would simply rise quickly and diffuse.
• Hydrogen burns with a pale blue flame that is almost invisible during daylight hours, so fires are almost impossible to see with the naked eye.
• Hydrogen fires have low radiant heat, so you can't sense the presence of a flame until you are very close to it (or even in it).
• Combustion can't occur in a tank that contains only hydrogen. Oxygen (or air) and an ignition source are required for combustion to occur.

Hydrogen Flame Detection:-

• Hydrogen burns with a pale blue flame that is nearly invisible in daylight. The flame may appear yellow if there are impurities in the air like dust or sodium.
• A pure hydrogen flame will not produce smoke.
• Hydrogen flames have low radiant heat. Unlike a hydrocarbon fire, you may not feel any heat until you are very close to the flame.
• Because of these properties, use a portable flame detector, such as a thermal imaging camera, when possible. If flame detection equipment is not available, listen for venting hydrogen and watch for thermal waves.
• Note that vent stacks are standard in storage facilities, and the ignition of venting gaseous hydrogen is common. Systems are designed to do this safely.
• Flame detectors may be installed in storage facilities and fueling stations. Listen and watch for audible or visual alarms.

5) About Blow Down in Boiler 

 Why We Blow Down a Boiler:-

            A boiler blowdown is water that we as building engineers, Navy guys, homeowners purposely discharge from a boiler. We do this to avoid the concentrated build-up of impurities inside the boiler. These impurities and byproducts appear due to the continuous evaporation of steam. 

          Isn’t boiler water pure Steven? It’s chemically treated but not pure. Kind of like the water that comes out of your faucet but different. If you were to take a pot, fill it with water, and boil it off.Peeking into the bottom of the pot, you’d probably find some sort of brown/black, maybe smelly, crusty stuff. Impurities from the water that couldn’t boil away.

           Water has the ability to capture tiny, miniscule pieces of solids that may be present in equipment such as boilers. Piping and components like valves are never manufactured or sealed completely perfectly. There will always be some amount of particulate or impurity floating around in boiler water. Then we add in the high temperatures and pressure.

The repeated evaporation of water, over and over again, concentrates these dissolved impurities. This can show up in many forms – from a hard, calcium like scale to a soft sludge that accumulates in the bottom, or mud drum, of the boiler. Scale, sludge, impurities – they all reduce the ability for the boiler to transfer heat. This then reduces efficiency of your equipment.

How to Blow Down a Boiler:-

Water is discharged out of the boiler due to the steam pressure inside. Two bottom blowdown valves are often used to minimize erosion. The sealing valve is generally opened first and closed last. Both are opened rapidly and fully to minimize erosion on the seat and disk faces. Care is taken to avoid trapping scale or rust particles within the valve by reopening a valve to flush the particles out.

The Steps

1. Check boiler water level.
2. Check position of bottom blowdown valves – should all be closed.
3. Wear any necessary ppe (personal protective equipment) baby.
4. Check boiler fire rate – re-think blowing down.
5. Open the valves closest to the boiler first (usually butterfly or ball valves.)
6. Open the last valve in the piping (usually some type of throttle valve.)
7. Open or close this last valve as needed to maintain water level in the boiler.
8. Keep all valves open for the specified time (10 seconds to 30 seconds usually.)
9. The duration of the bottom blowdown depends on what conductivity range is best for your boilers.
10. For example, if your conductivity is at 3200 micromhos, and the recommendation is 3000, you wouldn’t have to blow down as long. But what if your reading came out at 6000 micromhos? Longer blowdown right?
11. Close the throttling valve.
12. Close the quick acting valves you opened first.
13. Crack open the throttle valve to flush any pressure or sediment then close.
14. Check boiler water level and fire rate.
15. Ensure proper operation of the feedwater pump for that boiler or make sure that water level is being made up in the boiler.

6) Crankcase Relief Door

Crankcase explosion:-

• It is an explosion that occurs in the crankcase because of the mixture of oil mist and air in the ratio that is within the range of flammability.
• In addition there must be a source of high temperature energy sufficient to initiate combustion. This source is called hot spot.

Causes of crankcase explosion:-

• The normal content of crankcase is air.
• In this air there are oil globules (droplets) formed by the mechanical atomization of the oil as it s sprayed from the edge of the bearing and other places and as it is thrown about and churned by the quickly moving parts.
• If a hot spot occurs the oil particles in the neighborhood will evaporate.
• This evaporation may cause the formation of a quantity of white condense oil mist in the cooler regions.
• The continual generation of heat at hot spot vaporization may proceed a space until the ratio of vapour to air lies within the range of flammability.
• If hot spot can provide the necessary heat for ignition of vapour, a primary explosion may occur.

Prevention of crankcase explosion:-

• To minimize the formation of explosive mixture breather pipe or exhaust fan fitted on crankcase.
• To prevent the formation of hot spot in the crankcase, the bearing should be in correct running clearance, lubricating oil should be adequately supplied to bearing , chain with sprocket wheels and other running parts.
• The piston with rings and cylinders should be in safe working limits.
• Good fitting and efficient locking of working parts.

Preventive measure of crankcase explosion

1. There should be adequate cooling of the engine.
2. Proper purification and analysis of lube oil.
3. Lube oil filter to be changed over & cleaned as per schedule.
4. Ensure proper cylinder lubrication by checking the condition of piston, piston rings and liner through scavenge ports.
5. Clean scavenge space as per schedule & drain scavenges space regularly.
6. Maintain the stuffing box gland sealing in good condition.
7. All running gears maintenance & checks to be carried out as per PMS.
8. Be alert & rectify for any abnormal noise in crankcase.
9. All safety trips & alarms fitted on M/E to be tried out for satisfactory.
10. Proper watch on all running gears temperature & pressure to be maintained.
11. Blow through all sampling tubes of Oil Mist Detector (OMD) regularly.
12. Zero adjustment & sensitivity of OMD to be checked regularly.
13. Check for oil leakage at crankcase explosion relief doors & check for the operation by hand.
14. Check flame trap for cleanliness and intact condition.

Testing of crankcase relief valve:-

• When engine is stopping, after removing the flame trap and press up the valve disc and check the action of opening and closing.
• The valve must be opened smartly and closed positively and rapidly.
• The valve must be oil and gas tight.
• When the engine is running we must only check the oil leakage that shows the v/v is seal or not.