Correct boiler water treatment ensures a long service life for boiler

Oxygen in the boiler water is the Number One enemy of the boiler plant, closely followed by seawater and oil.

Severe corrosion of boiler convectiontubes on the water side Corroded boiler furnace dome and convection tubes

Various treatment options

A variety of boiler water treatments are effective to deal with the ultimately hazardous problems

of scaling, corrosion or sludge in a boiler plant. The remedy depends of the type of boiler plant and

problem.

The options range from multichemical treatment to single-treatment/one-shot products and from

manual to automatic water treatment systems. Reduced manning levels create a demand for easier

methods and procedures because engine room staff has less time to undertake traditional water analysis and treatment.

Easy - but not fail proof

Fairly simple methods of boiler water analysis involving combined treatment chemicals are available.

Such systems are often referred to as single-treatment or "one-shot" systems and are based on two simple tests: the pH value and the conductivity of the boiler water.

One-shot systems generally require a supply of high quality make-up water and an optimized condensate and feed water system (far from common on all plants).

Unfortunately, the advantages are minor in comparison with the results when such support conditions are not satisfied. In many cases, serious corrosion damage is subsequently found in the boiler.

Boiler corrosion and oxygen

One of the most common reasons for boiler corrosion remains the action of dissolved oxygen in the

make-up water and feedwater. In many procedures, excessive oxygen content is unfortunately merely treated as a secondary factor based on "good" experience and/or results measured on an optimized plant during a test trial.

Multi-chemical water treatment

The use of a multi-chemical boiler water treatment system incorporating an oxygen-binding component which can be controlled separately, based on the analysis of remaining dissolved oxygen or the

amount of component in excess.

The multi-chemical treatment system has the further merit of better individual control with reduced

chemical usage.

Good boiler water treatment is dependent not just on the chemicals but also on the daily maintenance of the condensate and feedwater systems, as well as the boiler itself. It is important that the condensate returns in its full amount and that, if possible, the hot well temperature is kept at 85-95°C for deoxygenizing and that the boiler is regularly skimmed and blown down. The crew must still ensure skilled and efficient monitoring and maintenance of the complete system.

A feedwater deaerator, whether pressurized or under vacuum, is generally better than traditional

open hot wells. Deaerators underwrite closed installations into which only a small amount of

air/oxygen penetrates.

Additionally, it is possible for a larger amount of the heat to be returned to the system instead of

cooling the condensate in a cooler before it enters an open hot well.

In terms of boiler water treatment, the best water condition is obtained if the majority of the

condensate can be led directly to a deaerator since the pressure, temperature and mechanical design give better possibilities for water deaeration.

AWT - Automatic water treatment systems

Automatic water treatment (AWT) systems can take care of many of the daily tasks and reduce the

crew's work load. Furthermore, most systems can log water conditions at frequent intervals, enabling records to be stored on floppy disks for subsequent easy assess.

A multi-chemical treatment method should be adopted with an AWT system, and the system should incorporate an oxygen probe for measuring and controlling the amount of dissolved oxygen in the boiler water.

Seawater and oil contamination

Attention is drawn to the damage caused by contamination from seawater and oil. Both promote heat transfer resistance on the water side of the boiler, causing the steel structure facing the flame and flue gas to be heated beyond its design point. Structural damage/rupture will occur when the contamination becomes sufficiently high, resulting in a leaking boiler.

Condensate and feedwater system layout

Owners and yards should consider the layout of the condensate and feedwater system at the newbuilding design stage with attention to the following aspects:

The proposed type of boiler water treatment

Number and location of chemical injection points

Manual or automatic water treatment

Installation point for salinity and oil-detection equipment

Design of cascade tank

Hot well or deaerator system

Temperature control of the hot well

Sodium sulphite can be used instead of hydrazine; the allowed excess in that case being 30-60

ppm. When other types of oxygenbinding agents are applied, it is recommended that their excess should be measured to confirm that no oxygen is dissolved in the boiler water. A value of <0.02 ppm is advised if the content of dissolved oxygen is measured directly.

Recommended boiler water charactistics for boilers working at pressures up to 20 bar

Appearance clear and free of mud Chloride content <100 ppm Cl

"P" alkalinity 100-150 ppm CaCO3

Total alkalinity <2 x "P" alkalinity

pH at 25°C 10.5-11.5

Hydrazine excess 0.2-0.5 ppm N2H4

Phosphate excess 20-50 ppm PO4

Specific gravity (20°C) <1.003 kg/m3

Conductivity (25°C) <2000 micromhos

Oil content nil

Feedwater characteristics recommended for boilers

Appearance clear and free of mud

Hardness 0-0.5 ppm CaCO3

Chloride content <15 ppm

pH at 25°C 8.5-9.5

Oil content nil