Introduction to Marine Boiler Water Dosing
Regular testing of a ships boiler feed water should be carried out to determine the extent of chemical dosing required to maintain the boiler’s efficiency in supplying high pressure superheated steam to the main and auxiliary steam turbines.
When I was a young engineer sailing on ESSO oil tanker steamships, I was responsible for the boiler water treatment, being the senior watch keeping engineer on the 12-4 engine room watch.
In those days I tested the boiler water regularly using solutions and litmus papers that changed colors. These seem to have been replaced by simply operated and efficient digital hand testers. Following the boiler water testing, the required chemicals were injected at several points into the boiler and feed water supply piping.
Impurities in the boiler feed water were also controlled using the boiler blow-down valves, although this necessitated making up the water from the boiler feed water supply tanks.
These tanks were continually topped up by the evaporators and scrutinized by the Chief who was always telling me my boilers were using too much water.
Anyway I diverge; this is an article on marine boiler dosing with chemicals to mitigate the impurities in the feed water. We begin with a look at the common impurities and adverse conditions found in boiler feed water, going on to examine the effect on the boiler water tubes and internals from these impurities.
Boiler Water Impurities
- Sodium Hydroxide
Too high a concentration of sodium hydroxide will induce cracking to the water tubes and boiler internal surfaces.
- Magnesium Sulphate
Reduces to magnesite and forms scale.
Can pass from boiler into through steam lines into turbines where it can coat the blades.
- Bicarbonate of Lime
Forms lime scale.
- Sulphate of Lime
Forms lime scale.
- Dissolved Gases
The main gases are oxygen and carbon dioxide. Dissolved oxygen will form rust, and dissolved carbon dioxide will form carbonic acid. Both these actions will induce pitting in boiler water tubes and internals.
- Dissolved Solids
Dissolved solids are left behind after the evaporation of boiler water, sneaking up gradually. They can cause water carry-over from the boiler steam.
- Sodium Chloride
Sodium chloride is the salt content in seawater, so an increase in this indicates a leaky condenser.
Excess hardness will cause internal scaling of the water tubes, boiler fittings, and internal surfaces.
- pH Values
A low pH value indicates acidity which will cause the pitting of water tubes and boiler fittings and internal surfaces. A high pH value indicates alkalinity that can induce foaming and carry-over of boiler water.
Reference Web: Gutenberg - boiler feed impurities.
Chemical Treatment of Marine Boiler Water Impurities
Treatment of boiler feed water can be divided into two categories; internal treatment using chemicals, and external treatment such as deoxidization, using the Deaerater. (D/A)
Listed below are the common boiler feedwater impurities and conditions along with the chemicals and practices used to control them, internally and externally.
I have drawn up a table of these impurities and conditions along with the effects and treatments, it can be found in the Image Section at the end of the article.
1. Calcium Carbonate – soda ash (Sodium Carbonate)
2. Dissolved Oxygen – sodium sulphate
3. Dissolved Solids – blow-down boiler water
4. Dissolved CO2 – amines such as morphaline
5. Sulphate of Lime – Barium Carbonate
6. Magnesia – Barium Carbonate
7. Bicarbonate of lime – Carbonate of Soda
8. Carbonate of Soda – Barium Carbonate
9. High ph Acidity – alkalinize
10.Oil and grease – Iron alum
- gutenberg - boiler feed treatment
- lenntech - chemical treatment of boiler water impurities
- vogtpower - boiler feed treatment
Methods of Marine Boiler Water Dosing
The chemical manufacturers recommend the methods of adding the chemicals to the boiler and boiler feed water lines, along with the required doses following boiler water testing. However, standard chemical injection is carried out by using a high pressure metering pump, supplied by a mixing tank usually located above the pump.
It is important to use the lid of the tank not only to prevent any spillages, but also to prevent the dilution of the oxygen scavenging chemical by over-exposure to air.
The pump can be operated continuously, pulsed, or for a specific period. Chemicals for different purposes are injected to the boiler, the feed pipes, or auxiliary equipment as recommended by manufacturer.
Boiler feed softening chemicals should be fed into the feedwater supply just before the check valve, or into the water drum to enable reactions to take place outside the steam generating areas.
Chemicals used to combat dissolved gases, scale formation, and corrosion should be injected into the feed water system.
If the optimum purity of the boiler feed water is obtained, the metering pumps can be adjusted to a continuous supply of chemicals. I personally never achieved this state of euphoria on the old steamships I sailed on, in what now must seem like the dark ages to today’s seagoing engineers.
Reference Web: scribd - injection of chemicals