What is meant by erosion in boilers ?

• It is a mechanical attack on the metal surface which may be due to a disturbance in the flow of the fluid over the metal surfaces, resulting in a loss of metal. Suspended abrasive matters in the fluid can increase the rate of metal losses.

 What is meant by corrosion in boilers ?

• It is the deterioration of metals due to oxidation. The present of water in an acidic condition provides the electrolyte required for corrosion action.

 Two forms of corrosion

Direct chemical attack 

• It occurs when metal at high temperature comes into contact with air or other gases, resulting in oxidation or sulphidation of the metal.

 Electro-chemical action

• Galvanic action, this being set up when two dissimilar metals are placed in an electrolyte. The noble of the two metals form a cathode to the base metal which, forming the anode, is wasted away.

What is meant by caustic embrittlement in boilers?

• Caustic embattlement is a form of inter crystalline cracking, which results from a solution of sodium hydroxide or caustic soda, becoming more and more concentrated at the bottom of a crack or fissure (narrow opening) which may be the result of fatigue, in the boiler plate or furnace.
• The plate must be stressed, so that wastage take place at the bottom of crack, the plate weakens, the crack extends to expose new metal to the caustic action and thus it proceed.
• Caustic embattlement will only occur when there is a high caustic alkalinity that is when the ratio of NaOH to the alkalinity is high. To keep this ratio at the safe level the sodium sulfate to sodium hydroxide should be maintained above 2:5.
• The sodium sulfate comes out of solution in high sodium hydroxide concentration and by doing forms a protective layer on the surface of the plate.
• Also keep the lowest T.D.S level. (Total dissolved suspended & solid)

What is pH value for boiler water ?

• It is the logarithm of reciprocal of hydrogen iron concentration.

What do you do for preventing of oxygen in boilers?

• The oxygen-scavenging chemical which is used for deaeration the water are usually sodium sulphate or hydrazine.
• 2Na2SO3  + O2   =   2Na2SO4
• N2H4    + O2   =   2H2O +N2
• Excessive dosage of hydrazine could lead to steam and condensate line corrosion due to ammonia being produced as the excess hydrazine decomposed.
• In high pressure boiler, by using the sodium sulphite, the sulphite can break down to give hydrogen sulphide which can attack steel brass and copper.

If chloride is found too much in your boiler water what will you do ?

• Blow down frequently
• Reduce the boiler load to minimum.
• If highly contaminant shut down completely and wash out.
• Trace and find out the fault and remedies.

What do you understand  by composite boiler ?

• If exhaust gas and oil fired can be used the same time, it is termed the composite boiler.
• In this type a separate tube nest of tube for exhaust gas is provided, situated above the return tubes from the fire furnace.
• Uptake from the tube nest are separated.

What do you understand  by alternative boiler ?

• If the exhaust gas and the oil fire are arranged to be used only at a time, the exhaust gas boiler term as the alternative boiler.
• This unit can be oil fire heated by the exhaust gases alternatively.
• It required only one uptake.

What do you understand  by economizer boiler ?

• In this system a separate exhaust gas economizer is connected to an oil fired auxiliary boiler or an accumulator by means of piping and a set of circulation pump.
• Exhaust gas is used as heating medium and it has no steam space.

 What is meant of accumulation pressure in marine boilers?

• Accumulation pressure is the rise in boiler pressure which take place when the spring loaded safety valve lift due to the increase loading caused by further compression of the spring.

What is the purpose of accumulation pressure test ?

• To detect the safety valve is suitable for this boiler or not.
• To limit the rise in boiler pressure under full fire condition.

What is the procedure of accumulation pressure test ?

This test is carried out a new boiler or new safety valve.

1. Shut off feed water
2. Closed main steam stop valve.
3. Increase cut off pressure of boiler.
4. Bypass high pressure cut off of the boiler.
5. Arrange the boiler fire rate to a maximum.
6. Safety valve will be lift during the test.
7. The test is carried out as long as the water permits in the boiler.
8. Accumulation pressure should not exceed 10% of working pressure in the specified time.
9. Specified time is 15 mins for a smoke tube boiler and 7 mins for water tube boiler.

Procedure of safety valve setting under steam pressure ?

Setting of safety valves:

1. Take standard pressure gauge (approved by surveyor) for accuracy.
2. Fill up water up to ¼ of gauge glass level, and shut main steam stop valve, feed check valve.
3. Without compression rings, hoods and easing gears, reassembled the safety valves with spring compression less than previous setting.
4. Raise the boiler pressure to desired blow off pressure.
5. Screw-down spring compression nuts of any lifting valves, until all are quite.
6. Arrange to have the desired steam pressure
7. Adjust each valve in turn:        Slacken compression nut until the valve lifts. Screw-down compression nut sufficiently enough, so that when the valve spindle is lightly tapped, valve return to its seat and remain seated. Measure gap between compression nut and spring casing. Make a compression ring equal to this gap, and insert under compression nut. Gag the spindle of this safety valve, to prevent opening, while remaining valve is being set.
8. Remaining valve is again set and insert compression ring.
9. Remove gag and retest both valve to lift and close together.
10. Cap, cotter and easing gear to be refitted
11. Caps and cotter pins padlocked to prevent accidentally altering the setting.
12. When the surveyor satisfied the setting pressure, easing gear should be tested.
13. All safety valves set to lift at not greater than 3% above approved working pressure (design pressure).

How do you take action if one of the passages of gauge glass is chocked ?

• Steam and water cocks and passages in the gauge can be cleared while the boiler is still steaming.
• To do this, shut the steam and water cocks and open the drain cock.
• Remove the check plug opposite the obstruction.
• Insert the cleaning plug. Screw in the plug with small hole about 5 mm diameter, drilled through it in place of the cleaning plug.
• Insert into this hole a rod of such a size that held by a gloved hand, it can be easily moved without being stuck.
• Then open the chocked and push the rod through to clear the blockage.
• When clear, open drain to prevent a build up of pressure and only a small amount of steam will blow past the rod, the glove protecting the operator from injury.
• Then close the cock and replace the normal cleaning plug. The gauge glass can be tested now and if satisfactory return to service.
• Do not carry out this operation on a plate type glass on a high pressure boiler.

What are the causes of boiler furnace blow back ?

1. Insufficient purging time
2. Accumulation of oil in furnace from the leaky burners
3. Boiler tubes and uptake have full of soot deposits.
4. Air registers control not operating for the high flame mode.
5. Too little air
6. Insufficient oil temperature

How to keep boiler not in service ?

For fire tube boiler out of service for short period

• The boiler must be completely filled with alkaline water.
• The boiler must be topped up periodically and any air in the system must be got rid off.
• Regularly test the boiler water and keep the alkalinity in the range of recommended value.

If the boiler is to be taken out of service for long period

• It should be drained completely and open up.
• Dried out by means of heater units.
• Then the trays of quick lime (moisture absorbent material) should be place internally in suitable positions.
• Blanks should be fitted to the pipe connections in the event of steam being maintained in other boiler and blow down line.
• The quick lime should be renewed at least once every two months.

What is the purposes of boiler water treatment ?

• To prevent scale formation ( Trisodium phosphates is used)
• To remove trace of oxygen (sodium sulphide or hydrazine is used )
• To give alkalinity and minimize corrosion.(sodium hydroxide is used )
• To reduce risk of caustic cracking ( sodium sulphate or sodium nitrate is used)

What is the difference between safety valve and relief valve ?

Safety valve

1. Directly open the design lift at set pressure.
2. Can open manually by easing gear.
3. Setting pressure is just above the working and not more than 3% above the approved working pressure.

Relief valve

1. Setting pressure is 10% above working pressure .
2. Opening is proportion to the increase in pressure.
3. Can not be open manually.

What are the markings on safety valve ?

1. Manufacturer’s name
2. Serial number
3. Inlet diameter
4. Operating pressure
5. Discharge capacity
6. Safe working pressure
7. Blow off pressure

 

Reference:

Oral Guide by – MIN ZAR TAR