Methods of Treatment
- Dissolved gasses of oxygen, and CO2
It is one of the jobs of the deaerator to remove the dissolved gasses in the returning condensate. So if there are consistently high quantities of dissolved gasses in the system it is best to check that the DA is operating correctly.
If however the gasses continue to form, the best method of reducing them is by using a blowdown valve. Some Chief Engineers prefer to leave the boiler on constant blowdown, and I have used this method myself with the provisos,
1. The DA is operating satisfactory
2. The amount of make-up feed is monitored
3. Tests for O2 and CO2 are carried out every watch
4. The steam from the blowdown valve can be used in a feedwater heater.
Acid and alkalinity are used to describe properties of fluids, their levels being measured in units of pH from 0 to 14, a pH of 7 being neutral.
Feed water in ships boilers are best maintained at an alkalinity around 8.5pH, as we go below this figure, the acidic water will cause corrosion of the watertubes and internals whereas a reading above will cause excessive coating of scale on these components.
Sodium Phosphate of Soda (NaPo4) can be used to adjust the alkalinity of boiler water.
This is caused by high calcium and magnesium content in the feedwater system. Hardness will cause scaling on the watertubes and internal fittings. It can be controlled by using carbonates, phosphates and certain types of polymers. Carbonates should be used sparingly as they can promote the formation of CO2 with the attendant risk of corrosion.
- Total Dissolved Solids (TDS)
Dissolved solids build up gradually being left behind in the boiler water as impurities.
Strict control of the TDS is imperative as a build-up can cause carry-over of water from the steam drum. This is not only detrimental to the turbines but the sensitive auxiliary equipment and instrumentation and should be avoided at all costs.
This falls into the TDS category but it is worthwhile having a closer look is causes and reduction. A rapid rise in salt PPM indicates a leak of seawater into the feed system, usually from the condenser or process coolers.
Control is achieved again by using the constant blowdown valve and regular topping up of the feed system with make-up feed water.