Boiler Alkali boil out is carried out to remove material such as lubricants, oil and rust. During manufacturing seamless steel tubes, mill scales are bound to be formed, and some of these can remain inside. During fabrication and erection of the pressure parts, some amount of oil and lubricants can also. get into the tube surface apart from tube weld slag and other materials.
Before commencing alkali boil out, the boiler is flushed with hot and cold water to remove loose debris and sand. During this process, the drum internals should not be in position and care should be taken to avoid water into the super heaters system.
Prerequisites alkali boil out of the boiler
the following prerequisites/inputs have to be ensured.
- Mechanical completion of boiler including auxiliaries and piping.
- Completion of hydraulic test activities and refractory dry out.
- Readiness of electrical and instrumentation controls and adequate illumination in and around boiler house. (Electrical power for motorized valves and gauge glass illuminator.)
- Availability of DM water through boiler feed water pumps.
- Readiness of sample collection system and availability of required chemicals for the alkali boil out.
- Boiler ready for light up with thermal expansion provisions.
- Availability of lab facilities to test the water. (pH and oil detection)
- Readiness of chemical dosing system.
- All the debris is removed from Boiler and all the access doors in the boiler should be closed.
- Good working drum level gauge glass with illuminator assembly, Boiler drum pressure gauge, safety /relief valves with gags or hydrostatic plugs are removed, CBD, IBD and all drain valves in good working conditions, water level control and associated alarms, cooling water piping and air piping, furnace pressure gauge and Temperature indications for flue gas, feed water and steam line.
- During alkali boil out, transmitters connected to steam drum and steam piping is isolated to avoid any damages to transmitters due to alkali.
- Blow down and drain connections are terminated to safe location.
Alkali out Chemical quantity per blowing should be available Tri sodium phosphate, Caustic soda and Detergent.
Procedure for 1st Stage boil-out
Fill the specified quality feed water 50mm below the steam drum manhole rim. Exact quantity of chemicals is to be dissolved in DM water externally in a container, and the concentrated solutions so prepared are to be poured in to the drum through the manholes. The entire quantity of chemical has been dissolved and poured in to the drum. Close the manhole and fill the drum to the normal level. Increase slowly the heat input to boiler to raise pressure to 30psi (2.1 kg/cm2 (g). A good flow of steam from air release valves will be noticed. Up on seeing this, close the air release valve.Continue increasing the pressure to 10kg/cm2 (g). Maintain pressure at l0kg/ cm2 (g) for a period of 16 hours. This will ensure rapid circulation of water through boiler. Give blow down at every 2 hours interval from water wall drains for one minute each. Extinguish the fire and blow down the water from the boiler manifolds individually for a period of 2-3 minutes each.Allow boiler to cool down naturally. When pressure comes to 2.5kg/cm2 (g) with manifolds closed, blow down steam drum water, until water level falls below gauge glass. Following this, the entire boiler water can be blown down as quickly as possible using all manifold drains and drum blow down drains. This way evacuate the water before any dirt or oil has time to settle down. When pressure falls to 30 psi (2.1 kg/ cm2 ) open air release valves.With blow down and drain valves open wash the drum, Manifolds, and tubes with high pressure clean water to wash out any remaining sediment.
Procedure for 2nd Stage boil-out
Follow the above procedure, and increase the pressure up 20kg/ cm2 (g) and carry boil-out operation by maintaining this pressure for a period of 12 hours. The pressure of 20kg/ cm2 (g) is to be maintained by controlling the fuel firing and modulation of the start -up vent. During this process normal water level in drum is to be ensured. Operate IBD for 10 seconds every one hour. Blow down through low point drains by opening the valves, one by one but one at a time for one minute interval each mean while ensure the drum level. Blow down the water wall drains every two hours for one minute. Blow down the system at an interval of every 2 hours. After every blow down operation collect drum water sample and analyze for phosphate and oil content. Replenish phosphate concentration through HP dosing system as and when the residual value goes less than half of the concentration maintained at the start of the process.Samples are taken from the blow down and analyzed for pH, alkalinity, oil content and conductivity. The quantity of oil and grease found in the boiler water determines the duration of the boil-out. Continue the boil-out operation till the oil content in the sample collected show less than 5ppm.
Completion of Boil-out
When analysis confirms the oil presence in the samples is equal to or less than 5ppm the boil out operations treated as completed.For confirmation it is suggested to collect 3 samples within 5 minutes of each other and check the consistency of the results.
Post Boil-out Operation
Operate each blow down and drain valve successively for a few seconds to discharge any accumulated sludge. Repeat several times, meanwhile maintain drum level. Allow the boiler to cool down naturally. When the drum pressure comes to 2kg/cm2 (g) open all air release valves. When the drum pressure drops to atmospheric, drain the boiler by opening all valves on drain headers. Fill the boiler up to the drum to the required level and back flush the super heaters into the drum till the sample collected from super heater off take tube mouth in the drum shows a low conductivity and high clarity. After completion of Alkali boil out and boiler is considerably cooled, flush the boiler with DM water till the system is free from alkali solution (i.e.) pH value of effluent should be equal to that the DM water used for flushing.
After boiler has cooled down completely, open all manhole doors and remove all manifold inspection caps to inspect the internal surfaces. Re-weld the end caps on headers as per recommended procedures and carry out necessary NDT and stress reliving as applicable. Accumulated dirt and sludge should be scrapped out in drum, manifolds and tubes washed through high pressure clean water. Examine all heating surfaces and gas passes of boiler for excessive
soot deposits resulting from low firing rates employed in Boil out. Clean manually the soot deposits. Check all refractory for any cracks. Check for any ash accumulation in any area. Check that expansion joints are set correctly and that expansion clearances are adequate.
NOTE – Boiler water contains chemicals, which are hazardous for human safety and hence safe disposal is necessary.