ALKALI BOIL OUT & PROCEDURE

ALKALI BOIL OUT

PROCEDURE FOR ALKALI BOIL OUT

1.0 OBJECTIVE
The purpose of alkali boil out is to remove from the boiler internals the deposits like oil, grease, mill scale, etc., resulting out of manufacturing, fabrication and erection procedures. Such deposits in the boiler tube walls severely restrict heat flow across the tube walls resulting in failure due to overheating.

2.0 PREREQUISITES

In order to commence alkali boil out of the boiler, following prerequisites/inputs have to be made available:

2.1 Mechanical completion of boiler including auxiliaries and piping.

2.2 Completion of boiler hydro test and other commissioning activities including refractory dry out.

2.3 Readiness of Electrical Equipment, Controls and Instrumentation.

2.4 Adequate illumination in and around the boiler house.

2.5 Electrical power for motorized valves, gauge glass illuminator, etc.

2.6 Demineralised water through boiler feedwater pumps.

2.7 Readiness of sample collection system.

2.8 Chemicals required for the process.

2.9 Full readiness of Boiler for lighting up and its thermal expansion.

2.10 Water testing facilities including pH and oil detection.

2.11 Readiness of chemical dosing system.

2.12 Inspection and mechanical cleaning.

2.13 Make a visual check of the entire boiler unit and remove all debris.

2.14 Remove drum internals (If required) from the steam drum as specified by TBW engineer.

2.15 After visual checks, close all access to the boiler unit internals.

2.16 Controls, interlocks and other requirements

Following minimum controls and requirements are necessary for boiler operation.

2.17 Good working Drum level gauge glass with illuminator assembly.

2.18 Boiler drum pressure gauge.

2.19 Safety/relief valves with gags or hydrostatic plugs removed.

2.20 CBD and all drain valves in good working order.

2.21 Water level control and associated alarms.

2.22 Cooling water/air piping.

2.23 Furnace pressure gauge.

2.24 Temp.indications for bed, flue gas, feed water, and steam line.

2.25 Safety interlocks.

During alkali boil out, transmitters connected to steam piping and steam drum will be kept isolated to avoid any damages to transmitters due to alkali.

3.0 PROCEDURES

3.1 Ensure that blowdown and drain connections are terminated to a safe location.

3.2 Ensure that all dampers are in good working condition and “OPEN” “CLOSE” positions are marked correctly.

3.3 Ensure that the feedwater pumps operate satisfactorily.

3.4 Fill specified quality feed water to NWL by keeping all vents open. Drain the water completely through the bottom drain, keeping the header outlet valve fully open. With this, the boiler gets rinsed.

3.5 Fill the unit with the specified quality of water up to the drum level approximately 200 to 300mm below the manhole opening. (Fill the drum through normal feedwater connections). DM Water should be used to boil out the operation. Add some hydrazine in the Deaerator storage tank to have 1/2 PPM of hydrazine traces in boiler water.
3.6 For alkali boil, 2500ppm of hydrous trisodium phosphate (Na3PO4 12H20), 1000ppm anhydrous disodium phosphate (Na2PO4) and 225ppm wetting agent (surfactant) will be used. The wetting agent is added to enhance the process of degreasing.

3.7 Exact quantity of chemicals is to be carried in dry form onto the drum level platform. The chemicals are to be dissolved in DM water externally in a container near the drum manhole in batches. The concentrated solutions so prepared are to be poured into the drum through the manhole. When the entire quantity of chemical has been so dissolved and poured into the drum, close the manhole. Raise the water level in the drum up to the light-up level (2nd port in the gauge glass).

3.8 BOILER LIGHT UP

3.8.1 After ensuring the above, light up the unit and raise boiler pressure very slowly to closely monitor the thermal expansion of the pressure parts and no case pressure raising should not exceed the cold start-up pressure rising curve.

3.8.2 Ensure that the normal water level is maintained in the drum through out the boil out of the operation.

3.9 DURATION

Raise boiler pressure to 50% of working pressure (Drum) or 40kg/cm2 whichever is lower. Carry out the boil out operation by maintaining this pressure for a period of 12 hours. The pressure is to be maintained by controlling the fuel firing and modulation of the start up vent. During this process normal drum level to be ensured.

3.10 BLOWDOWN

On attaining approx, 5kg/cm2(g) drum pressure, quickly operate the intermittent blowdown for 10 sec. Restore drum water level to N.W.L. Allow drum pressure to raise up to a value which is 50% of operating pressure or 40kg/cm2 (g) whichever is lower. Ensure that the water level in the drum is replenished each time. Leave the CBD valve slightly open to prevent choking of sample line and sample cooler. Operate the IBD for 10 seconds once every hour.

With due care considering the firing conditions inside the combustor Each bottom header drains to be operated once in every hour for 30 sec one by one to remove the sediments from the bottom headers. It is advised to stop the firing or reducing the firing to very minimum to avoid any hot spots in the pressure parts which may done in consultation with the commissioning Engineer.

At the end of 12 hours of boil out operation, a sample of boiler water is tested for oil traces, to determine the endpoint of the process. Oil ppm should preferably be less than 5ppm.

If the oil levels are not coming down even after adequate draining, If required chemicals may be added to drum through the HP dosing system.

After stopping the firing, operate all the bottom header drains for 30 – 45 sec to remove the sediments from the headers.

4.0 COMPLETION OF BOILOUT

4.1 Boiler water sample is analyzed for presence of oil. When analysis confirms that oil present in the sample, is within limits (=< 5 PPM) the boil out operations treated as complete.

Rinsing to be done to match the boiler pH very close to feed water pH.

For confirmation it is advised to collect 3 samples within 5 minutes of each other and check for consistency of results.

4.2 To neutralize the high pH drained water from blowdown or from rinsing during blow out process, a neutralization pit to be installed and the boiler drain lines to be temporarily connected to that. To neutralize before draining into the trench, acid can be added and mixed by a stirrer or by air bubble.

5.0 POST BOIL OUT OPERATION

5.1 Box up the boiler and allow the unit to cool by natural cooling. When the drum pressure drops to 2 kg/cm2 open the drum air vent & startup vent.

5.2 When the drum pressure drops to atmospheric, drain the boiler by opening all valves on drain headers.

5.3 After draining the boiler, cut the inspection caps in all bottom headers. Inspect drum and bottom headers for any sediment and for cleaning and flushing.

In case of in bed evaporator coils both in bed coils, bottom and top header end caps and inspection caps need to be cut and inspected. It is to be ensured that all the drain holes in the baffle plates to be clean and free from any blockage / foreign materials like weld slag/welding consumables.

5.4 Open the steam and water drum manholes and remove the sediments (if any).

5.5 Backflushing of superheaters and Flushing of the drums downcomer, supply pipes convection tubes thoroughly.

5.6 Re weld the end caps on headers as per the recommended procedures.

5.7 After welding radiography (if req.) Post weld heat treatment (if req.) hydro test of boiler to be completed to the maximum working pressure.