DESCALING

To avoid scaling, ensure that always soft and properly dosed water is fed to the boiler. Descaling is not a part of routine maintenance, as descaling shortens the life of coil. Before attempting descaling make sure that boiler coil is really scaled up. If descaling is found necessary, never do it when the coil is hot.

Symptoms of a scaled coil

Measure the blow down time and compare it with the blow time recorded at the time of commissioning. When blow down time goes beyond twice the initial blow down time, it will be necessary to descale the coil. When scaling occurs the pressure gauge at the inlet of the coil will indicate a higher pressure than that observed during commissioning. Make sure that the rate of pumping of the boiler feed water (water consumption) is identical with that the time of commissioning to ensure that this does not missed one regarding the coil pressure drop. The blow down timing can vary considerably depending upon the resistance to flow offered by the piping between the blow valve to the point of drain. Blow down timing for comparison should, therefore, be the one recorded at the time of commissioning.

Descaling Procedure

Before descaling, please keep a record of the following:

Blow down time before and after descaling. Pressure drop across the coil with boiler running at rated pressure.Approximate number of hours the boiler has worked before descaling. In the procedure described below a separate pump is to be used for descaling. This is outlined below in three stages.

  • Connections
  • Descaling
  • Recommissioning

Connections

Take a separate pump motor set consisting of an electric motor and a single or multi-stage centrifugal pump (preferably acid proof) of 25 Liters/min to 50 Liters/min at a pressure at which water can flow through coil. While the ideal would be to employ an acid-proof pump, in view of its cost, a normal water pump will serve the purpose, provided it is carefully washed with an alkaline solution after pumping the descaling compound. Take an acid proof tank of a minimum 50 liters capacity. A large plastic bucket will do. Take polyethylene tubing and connect one end of the tube to the inlet of the pump with worm clips, the other end being dripped to the bottom of the bucket. Connect the delivery of the pump to inlet of the coil through the blow connection. Use MS pipes and fittings as pressure here will be more. Connect a pressure gauge in this line. Remove the copper piping connected to the steam pressure gauge and steam temperature combistat bulb from the steam header. Remove the steam safety valve also. Plug the openings. Connect polyethylene tube of required length to the piping of auxiliary steam valve and leave the other end free at the top of the bucket. Fix a filter mesh just under this end of the tubing to collect scale and dirt that might otherwise enter the bucket. Keep the main steam stop valve closed throughout the descaling operation. Make sure before starting that the non-return valve, connected between the boiler feed pump outlet and steam coil inlet, is functioning properly and that there is no danger of acid flowing back into the boiler feed pump, as it might damage the pump.

Discaling Operation

Fill the bucket with soft warm water. Start the pumping set and fill the boiler coil with soft water. After the coil is entirely filled with water, it will return to the bucket through the polyethylene tube. Take the mixture of descaling compound and water in the bucket. Dip a piece of pH indicating paper which is yellow in colour, into the bucket. It should turn red indicating acidit. Hold another piece of pH paper in the water emerging from the tube just above the bucket; it will normally show higher pH value (more greenish in colour) indicating that the alkaline salts from the coil are being carried in the flushing water. Continue the circulation for about 8 to 10 minutes, then add another two litres of descaling compound to the bucket and continue for a further 8 to 10 minutes checking periodically the change of colour  with pH paper. The process may have to be repeated until the return water has the same pH as the water in the bucket. During these operations the water pressure reading on the gauge fitted at the inlet of the coil should be checked to ascertain the variation of pressure with the progress of cleaning. As the coil is cleaned, this pressure comes down. Now throw away the acidic water in the bucket and add soft water to it. Let out the out coming water from the coil to drain and not into the buckey. Run the centrifugal pump to force all the contents out of the coil. keep adding soft water to the bucket and pomp it through the coil. The colour of the indicating paper when dipped in the water in the bucket will be slightly GREENISH, showing a slight alkalinity of 8 to 9 pH. On the other hand, the pH of the return water will be slightly acidic. Continue rinsing till the pH of the return water is the same as that of the soft water. Add about 1 kg of caustic soda or caustic potash to the soft water in the bucket. Put the return water connection back into the bucket and circulate this alkaline water through the boiler coil for 15 minutes to half an hour. This caustic solution completely neutralizes any acidic traces in the coil. Once again rinse the boiler coil with soft water for about 15 minutes and again check the pH of the water in the bucket and at the return outlet. A slight green color is to be expected. The coil is now descaled.

RECOMMISSIONING

Disconnect the temporary connections and reconnect the boiler to its permanent piping and tanks. Use the boiler feed pump to fill up the coil with soft water. Fire the boiler with the main steam stop valve closed and auxiliary steam valve open. Give a blow down. Carefully collect and examine the condition of the blow down; some rusty water might come out of the boiler. Continue steaming and blowing down till the blow down comes out clesn. Check and note the blow down time and pressure drop across the coil. Descaling operation is complete and the boiler can be put back in service.