Auxiliary Boilers – Marine Engineering Knowledge
What is foaming in auxiliary boilers and how to prevent it ?
It is the formation of thick layer of steam bubbles on the top of the water surface inside the boiler due to:
- High concentration of impurities.
- By the animal or vegetable fats in feed water carry over from the oil heaters.
- Increase in level of dissolved & suspended solid TDS level.
- Increase in water level.
To prevent foaming, surface or scum blow down should done frequently to expel any floating impurities for the boiler and no lube oil should be allowed to enter the boiler.
What is boiler priming and how to prevent it ?
It is condition in which large amount of water are carried along with the steam into the steam line. It is caused by:
- Excessive foaming
- Improper amount of steam space
- By a sudden rush of steam such as is produced when steam stop valve is suddenly opened.
To prevent priming, never keep the water level too high. Open steam stop valve slowly.
What action do you made take in case of foaming and priming ?
- Scum blow down
- Reduce boiler fire rate
- Check whether boiler chemical added are in excess
- Detect the source of contamination
- In case of bad priming the boiler may have to be taken out of service, shut down.
Note: For contamination due to oil, auxiliary boiler have to be chemically cleaned.
What is the boiler cool down procedure ?
- Change the fuel oil burning system from HO to DO and then shut down the burning system.
- Stop feed p/p close feed check valve.
- Drain down the boiler after allowing it to cool down.
- If no sufficient time to do this, lower the boiler pressure to 3 to 4 bar.
- Shut the main steam stop valve.
- Open the ship side valve then open the blow down valve.
- Banging noise will appear when boiler is empty.
- Close the boiler blow down valve and ship side valve.
- Then release the steam pressure through safety v/v by means of easing gear.
- When pressure is off, open the air vent and the boiler to cool down.
Boiler Opening up and Inspection Procedure
Follow as procedure above, after making sure no vacuum in it, first:
- To remove top man hole door, slacken the dog holding nuts but do not remove them until first broken the joint
- Remove nuts and dogs and take out the door.
- The bottom door can be removed after warning personnel to keep clear of the top door.
- Make ventilation before entering. Do not allow naked light near the boiler.
- Preliminary internal inspection carried out before cleaning, to check scale deposits and any special points.
- Plug orifice to blow down valve to prevent choke, place guards over the manholes landings to prevent damage.
- Carried out cleaning and internal works.
- When all works completed, a full internal examination must be carried out
- Cleanliness, all openings are clear, water level gauge connection clears from deposits
- All internal pipes and fittings have been replaced correctly and securely attached,
- Remove plug from the blow down valve orifice
- The face of manhole doors and landings inspect to clean and undamaged.
- Replace manhole doors by using new joints.
- Operate all boiler mountings. Open air vent cock and fill the boiler with water to sufficient level.
Describe the boiler start up procedure from cool condition ?
The boiler is carried out firing from cold condition to normal working pressure condition very slowly to avoid thermal stress.
- Check the boiler blow down valve in close position.
- Shut the main steam stop v/v.
- Open the air vent cocks.
- Open the feed check valve and pumping up to ¼ of gauge glass level.
- Start the force draft fan with dampers, open correctly to purge the furnace and combustion space of any foul gas.
- Light the burner after closing the recirculating valve.
- Normally allow the fire to burn for 5-minutes and stop for 15 mins. This step continues until steam come out.
- When steam coming out from the air vent close the air vent (at 1 to 2 bar pressure).
- Rise up the working pressure step by step slowly. (Allow the fire to burn for 30 mins and stop the fire for 10 min.)
- When the steam pressure reached is working pressure drain the steam line. (to avoid water hammer)
- Main steam stop valve open slowly (crack opening)
How to take action if gauge glass showing low water level ?
- If water level has not yet dropped completely out the sight glass, water may be put into the boiler.
- If water drops completely out of sight glass, check another sight glass, if both disappear water; do not add water until the boiler is cool enough to prevent any possible damage due to rapid cooling of over heated plate.
- The fire must be immediately stopped.
- The main steam stop valve must be closed.
- Blow down and cool down the boiler.
- Check leakage, drum outside, Located the cause of trouble and make necessary repair.
- Enter the boiler after it has cooled and examined any possible damage.
- After repair, water fill up slowly and restore to normal operation.
- If no damage occurred, inject the water slowly into the boiler and restore it to operation.
What are the causes if gauge glass shows low water level ?
- One gauge glass defective
- Boiler tube crack and leaking
- Feed water regulator jammed
- Failure of feed water pump (Air in feed water line or pump)
- Level controller malfunction
- Steam consumption is too much
What is caustic embrittlement? How to prevent it ?
- It is inter crystalline fracture. It is cause by high concentration of caustic soda and the material under stress. The stress corrosion cracks follow the grain or crystal boundaries of the material and failure.
- Sodium sulphate or sodium nitrate is used for the prevention of caustic embrittlement.
- It can be found in highly stress area in boiler. Such as tube and tube plate connection, riveted head, seam and boiler mountings.