Stages of cleaning process of Boiler:
1. Cold and Hot water flushing
- Water temperature - 80 to 85 °C
- Velocity - 0.2 to 1.5 m/sec
- Flushing period - 15 to 20 min.
Flushing process will be repeated until outlet water conductivity reached 30 microsemen/cm.
Amonia (0.1%) and hydrazine (100ppm) then added in DM water, recirculated and drained to waste.
2. BOILER ALKALI BOIL OUT
PRECHECKS:-
1.
The boiler
shall be ready in all aspects for steaming to the design capacity and steam parameters.
2.
Ensure drum internals including screen boxes are fixed and welding is completed before alkali
boil out.
3.
Close all
sampling points and instrument tapings
at sources. Temporary sampling line for alkali boil out is recommended to avoid choking due to loose mill scale, debris, etc.
4.
Keep ready
the following chemicals
:-
•
Trisodium phosphate (Na3PO4) 4000 ppm
The quantity for each boil out shall be estimated
4 x water volume of boiler
(QM^3 x 4 Kgs)
•
Sodium Carbonate
(NaCO3) 4000 ppm.
The quantity for each boil out shall be estimated
4 x water volume of boiler
(QM^3 x 4 Kgs)
PROCEDURE FOR ALKALI BOILOUT:-
1.
The boil out
is to be carried out in three stages and fresh chemicals are to be charged
before each stage.
2.
Fill the
boiler with DM water (100 mm below manhole door level). Dissolve chemicals
in DM water (use plastic
containers) and pour into the drum through manhole door. (Do not put solids
in drum).
3.
Light-up the
boiler and raise the steam pressure gradually to 5 kg/cm² as per standard operating procedure and maintain
the pressure for minimum 12 hours.
4.
Blow down once in two hours for 2 minutes to be given.
5.
Samples are to be taken for every two hours and analysis is to be carried out to determine the following.
1. pH
2. Total alkalinity
3. Oil
6.
Ensure the
pressure is maintained in the boiler for each stage of alkali boilout as follows :-
For 20 kg/cm² pressure boiler:
|
Duration
|
|
I Stage
|
5 Kg/cm² |
12 Hours. |
II Stage
|
7 Kg/cm² |
12 Hours. |
III Stage
|
10 Kg/cm² |
12 Hours. |
|
|
|
|
For 44 kg/cm²
to 68 kg/cm²
line pressure boilers
|
Duration
|
|
I Stage
|
7 Kg/cm² |
12 Hours. |
II Stage
|
10 Kg/cm² |
12 Hours. |
III Stage
|
20 Kg/cm² |
12 Hours. |
7.
Each stage
of boiling can be terminated when the content of oil in the boiler water maintains a steady
value. However final reading of oil should be 5-10 ppm. Decision for termination will be
based on requirement for specific boiler.
8.
For each
stage, after termination of boil out procedure, shut down the boiler and allow the unit to cool down gradually.
9.
Drain the boiler
after the drum pressure reaches atmospheric pressure. Water jet shall be used
to clean the headers for each
stage.
10.
After
complete draining of boiler, clean the complete unit by filling the water and drain it for at least one time for each stage.
Repeat the fill and drain until free from Alkalies.
The hand hole pipes of all headers preferably
at bottom most point to be cut and
inspected for any foreign
material and cleaned using water jet.
11. After
inspection and cleaning re-weld the end caps and hydro test the boiler to working pressure.
· Remove
drum internals.
· Super
Heater filled with ammonia and hydrazine (200ppm), pH should maintenance 10.
· Add
ammonia bi fluoride (0.5 w/v) and pressure 4 to 5 Kg/cm2.
· Add
acid HCl ( 6% concentration) and
pressure 3 to 4 Kg/cm2. Period 1 to 1.5 hours.
· Test
sample every 15 to 20 min.
· Acid
cleaning will completed when acid concentrations and iron content reached to
constant value.
· System is
drained under N2 cover. N2 Capping protect metal surfaces from oxidation. Pressure
should be 0.5 to 1.0 Kg/cm2.
1st stage passivation:
· System filled
with DM water at temperature 90 to 95 °C.
· Add ammonia
and hydrazine (200 ppm) and pH should be 10.
· Circulation
the water for 20 hours and system is drained out.
· Drum and header
inspection and cleaning.
2nd stage passivation:
· Drum internals,
mounting, fittings are fitted back.
· Boiler filled
up to normal operating level with 200 ppm hydrazine and pH 10.
· Boiler lite
up and pressure maintained 40 Kg/cm2 for 24 hours.
· Ammonia and
hydrazine injected to maintain residual hydrazine 25 ppm.
· After 24 hours
boiler cooled and at 80 °C system will drained.
STEAM BLOWING:
Steam blowing is an established practice for the purpose of
physically removing any substance
which remains deposited in the super heater and associated pipe lines. These substances are mainly scales and loose
minerals that might have been entrapped during
manufacture, storage and erection at site. Steam blowing is to be
carried out only for boilers with super
heater.
PRINCIPLE:
The principle
of steam blowing is to give thermal
shock and dislodge
the scales. These
scales will be subsequently
driven out by the dynamic
velocity of the expanding steam.
PROCEDURE: THE METHOD OF STEAM BLOWING
IS AS BELOW:
1.
Operate the boiler and raise the boiler
pressure above 40 Kg/cm².
2.
At 40 kg/cm² steam pressure (line), slowly open the main steam stop valve by maintaining the drum level between 55% and 60%; blow the steam to atmosphere till the steam pressure drops down to 25 Kg/cm².
CAUTION: Do not allow the water to carry over (i.e. the stop valve
should be opened in such a way) when the target plate is fixed since the
water particles will give confusive results of
the plate condition.
3.
Slowly reduce
the firing rate and slump the
boiler. The main steam stop valve
shall be closed when the boiler pressure reaches 25 Kg/cm².
4.
After one hour, the boiler shall be restarted
with one compartment and the bed temperature shall be maintained at 800°C.
The steam blowing
shall be once again started
when the pressure
reaches 40 Kg/cm².
By the above procedure, the scale adhering
to the contours get cracked
due to temperature difference and get
removed to subsequent blows.
Temporary discharge piping which lead the steam to the atmosphere
shall be welded down - stream
of main steam line must be properly supported and anchored.
OPERATIONAL PRECAUTIONS:
1.
During steam
blowing operation, the unit must be brought up much slower while all equipments
are checked and expansion
movements are monitored closely.
2.
During blowing,
water level in drum gauge glass may fluctuate very widely. Feed regulating station must be checked for proper remote operation. Sufficient quantity of D.M water availability must be
ensured.
3.
Temporary
discharge piping which should be at least equal to the diameter of piping to which it is connected must be
well supported to withstand reaction forces during
steam blowing.
4.
Discharge
piping should be so diverted that personnel and equipments are not endangered. The area should be cordoned
off.
5.
For welded type of NRV, the internals shall be removed to avoid damage during
blowing. Similarly the orifice plate or flow nozzle
and thermowell shall be removed
to avoid damage during blowing.
DETERMINATION OF STEAM CLEANLINESS:
2.
The
cleanliness shall be determined by locating the area of highest concentration of the plate. Count the recognizable
impressions i.e. 0.2 mm depth in 1 cm² located
area.
3.
The piping
can be termed cleaned if less than two particles impressions of the considered 1 cm² area shown and if the
remaining area does not show any individual rough impression.
1.
For the
first blowing process it is not recommended to use target plate whereas for further
cycles always the clean side of
the target plate should be used.
2.
The pipe lines which are not pre-cleaned are expected to take blowing
cycles twice than the pre-cleaned pipe lines.
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