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Alkali Boil-out of Waste Heat Boilers

Alkali boil out of new boilers is necessary after erection and assembly, in order to remove all foreign matters from the boiler, steam drum and piping. In addition to this, a new boiler will usually contain grease or oil which must be removed by boiling out. A light film of non-conducting oil could cause overheating and damage to boiler internals.  To overcome this hazard boil out should be carried out in three principle steps i.e. physical washing, warm degreasing & pressurized boil out at pressure recommended by supplier (typically about 50% of operating pressure).

Alkali boil out is a case specific activity and the procedure should always be recycled with boiler vendor and only after his approval one should proceed. A typical way of carrying the boil out is described below and it is intended to give the reader an idea of resources required for the job and also the information it needs to collect before proceeding.
A typical split of work scope among various interfaces during the boil out activity is also given just to give an idea of how to manage this critical activity with a targeted approach.

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Fig. 1 Split of work for Alkali boil-out

Before the start of the activity,

1. It is necessary to remove or isolate (as necessary) all control accessories, including but not limited to the following:
a) All remote and local gauges should be replaced by tubular or other type glass or mica baking.
b) Level switches or transmitters
c) Safety relief valves
d) Silencer on the vent nozzle etc.

2. It is important to install temporary gaskets on manholes and other flanged connections. After completion of the activity and before start-up, remove them and replace by the actual gaskets.

3. It is important to leave the internal parts such as piping, impingement plates etc. in place. Some partition plates or inspection doors will be removed after the completion of the activity in order to be sure of the cleanliness of the internal shrouds in the steam drum.
Following chemicals should be available:
a) Calgonite                     3.0 kg per cubic meter of water
b) Caustic Soda               1.0 kg per cubic meter of water
c) Wetting Agent             ½  kg per cubic meter of water
d) Sodium Nitrate              ¼ kg per cubic meter of water
e) Sodium Sulfite              0.1 kg per cubic meter of water
f) Sodium phosphate        3.0 kg per cubic meter of water

4. The chemicals mentioned in (a) thru (d) can also be replaced with the following:
g) Soda Ash                    3.0 kg per cubic meter of water
h) Caustic Soda               3.2 kg per cubic meter
i) Wetting Agent             ½  kg per cubic meter of water

Boil-out chemicals are highly caustic. Caustic soda will produce a violent flash if introduced too fast in water. All chemicals should be premixed with water in a container before they are introduced into the drum. Wear goggles, rubber apron and rubber gloves at all times while mixing and handling chemicals. Warn all personnel to exercise due care. Use MSDS of these chemicals for more information.

5. It is important to have following systems available. The problem becomes even significant for under construction sites. For a typical Ammonia plant under-going pre-commissioning, following systems should be ready before boil out is started:
a) Functional power network
b) Off site & on site water treatment
c) Nitrogen Plant and instrument air system
d) Demin water system (the preheating system of DMW can be excluded)
e) BFW system, starting from de-aerator and all pre-heaters
f) IBD and CBD networks including flash vessels
g) Effluent disposal and associated piping system
h) Reformer, convection section
i) Secondary reformer loaded with catalyst after refractory dry out
j) Primary reformer loaded with catalyst and front end leak check performed. Primary reformer should be ready for firing (Primary reformer fuel gas).
k) HTSC loaded with catalyst
l) Nitrogen startup loop for primary reformer through HTSC

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Fig. 2 Waste Heat Boilers in Ammonia Plant

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Fig. 3 Plan view of WHB / steam drum assembly

Boil out - steps
Step-1 (Physical washing)
1. After making sure that all connections are tightly closed, the steam drum can be filled by starting BFW pump. It is important to over flow the boiler from vent nozzle just to be sure that all parts get washed.

2. Once filled with water, the system should be drained from low point drains and IBD lines of the boiler.

Step-2 (Warm Degreasing)
1. The steam drum should be filled to 100mm below normal operating level and the quantity of water used should be measured. One can also estimate the system hold up of fire tube boilers by using boiler data sheet (A comparison of empty shell weight & weight loaded with water).

2. Assuming ‘ X ’ m3 of water in the system measured above following quantities of chemicals should be dissolved in demin water
a) Calgonite             3.0  x  ‘ X ‘   kg
b) Caustic Soda                  1.0  x  ‘ X ‘   kg
c) Wetting Agent     ½    x  ‘ X ‘   kg
d) Sodium Nitrate   ¼    x  ‘ X ‘   kg

3. The chemicals can be dosed in steam drum in small batches through any flanged connection on steam drum or by any other appropriate means.

4. Adjust the water level in steam drum to 100 to 150mm above the drum center line & keep the drum vent open

5. Heating of water inside the system should now be started.

6. In a typical ammonia plant, nitrogen circulation through the fired primary reformer should be started to heat the drum contents. The same can also be achieved with live steam injection provided boiler has a distribution pipe for steam.

7. Control the heating rate in the steam drum to recommended value, typically 20OC/hr for ammonia plant WHBs.

8. The temperature in the steam drum should be maintained at 90-95°C for a period of 48 hrs.

9. After 48 hrs stop heating and let the system cool. Drain the system from low points or from IBD lines.

10. An additional wash cycle can also be carried out using clean demin water.

Step-3 (Hot pressurized boil out)
1. Fill the steam drum to 100mm below normal operating level and measure the quantity of water used.

2. Dissolve following quantities of chemicals in demin water and add them to steam drum
a) Sodium phosphate                 3.0  x  ‘ X ‘   kg
b) Caustic Soda                           0.2  x  ‘ X ‘   kg
c) Wetting Agent              ¼     x  ‘ X ‘   kg
d) Sodium Sulfite                        0.1  x  ‘ X ‘   kg

3. Adjust the water level in steam drum to 100 to 150mm above the drum center line & keep the drum vent open.

4. Heating of boiler can now be started. In case of live steam injection level control will become difficult due to condensation of steam so it is better to heat the drum contents using actual heating medium.

5. Control the heating rate in the steam drum to recommended value, for typical ammonia plant WHB the heating rate should not exceed 30OC/hr.

6. Close the drum vent when a pressure of 1.0 kg/cm2g is reached.

7. Continue heating till the boil out pressure recommended by the manufacturer is reached in the drum. Steam exhaust can be controlled by a vent on steam system.

8. Once the pressure is ~ 10bar, open the vent fully and depressurize the boiler. Make up the water level subsequently. This should be repeated 3 to 4 times.

9. Finally pressurize the boiler to boil-out pressure recommended by manufacturer, typically 50% of normal operating pressure and hold. Control the drum pressure with a vent on steam header. Boil out process will produce sludge inside due to chemical reactions which now needs to be removed by periodic draining cycles. 

10. After every four hours, drain the system from CBD line, when the water level reaches to CBD line draining should be done by IBD line till low level cut out. Collect samples of drained solution at the start and finish of draining. After taking the two samples make up the level with clean demin water.

11. The samples collected should be analyzed for pH (9-10), turbidity & total phosphate content.

12. Completion of boil out shall be determined when phosphate depletion has stopped inside the drum and phosphate content of two samples collected during a given draining cycle become the same for several draining cycles and the solution is optically clean.

13. After completion of boil out cool the entire system slowly. Drain the system while fairly warm.

14. Re-fill the system with clean demin water and drain it. Repeat this at least 3-4 times and continue until the pH and the conductivity of drained-off water becomes equal to fresh water entering the system. It is to be noted that the difference of temperature between the rinsing water and metal surface of boilers should not exceed a certain limit recommended by boiler supplier, typically 60°C for ammonia plant WHBs.

15.Final draining should be done under nitrogen and system should not be allowed to come into contact with air when it is wet. Inspect the internal areas of the steam drum carefully and remove dismountable panels to ensure that no trace of oil remains, and that all foreign material, dirt, scale, welding spatter and metal chips have been removed from the drum. These heavier particles will be found in the boilers and down comer pipes and in pocket locations around baffles.

16. If no trace of oil can be found, the system is ready for operation.  If oil traces are found, boil out needs to be repeated,

17. In case of delay in start up the entire system should be preserved in accordance with supplier guidelines, for typical ammonia plants preservation is done under positive nitrogen pressure.

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Waseem Haider
Senior Process Engineer with Engro Fertilizers Limited. He has experience in Engineering, Project Management & Process Hazard Analysis. In his current capacity, he is acting as a technical support interface of Operations team for Pre-Commissioning & Commissioning of World's Largest single train Urea Plant.

 

 

 

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