Boiler water treatment

Boiler water treatment

Each Chief Engineer should be given the name of the company of the treatment products which has the contract furnishing chemicals and service for his particular vessel. It is important that each vessel have the booklet of instructions for the treatment used.
Contracts with the company should provide that they will not only furnish the necessary chemicals as requisitioned but that their service engineer will meet each vessel upon arrival in stated ports, collect water samples, test same soonest possible and give a report. This report should include the analyse is of the water samples taken as well as recommendations of actions.
The Chief Engineer must see to it that the form provided with the chemical company for recording the feed water and boiler water tests is completely filled out each day of the voyage and that this report is turned over to the chemical company's representative when attending the vessel.
In order to counteract the effects of contamination, boiler feed water is treated chemically, to control (1) scale formation in the water sides of boilers, (2) corrosion, and (3) foaming and priming, and it is treated mechanically and by heating to reduce the dissolved oxygen present in the water.
The purpose of the boiler water treatment is:
1 To neutralize acid forming salts in boiler water, i.e. maintain a slightly alkaline condition.
2 To provide scale removing and scale preventing chemicals.
3 To precipitate impurities into a sludge which can be removed by bottom and surface blows, and
4 To prevent corrosion by oxidation.

APPLICATION OF BOILER WATER TREATMENT CHEMICALS TO THE BOILER
The importance of the correct method of applying treatment chemicals cannot be over-stressed.
Additions of hydrazine to the feed water into the feed system must be continuous and sufficient quantity to remove the last traces of oxygen from the feed water and to leave a small excess.
The excess being sufficient to give a small residual in the boiler water to produce sufficient ammonia, by decomposition of the hydrazine in the boiler, to raise the pH of the condensate to over 8.5.
Continuous application by means of a suitable dosing unit, direct injected into the feed line immediately after the F.W. pump.
The importance of continuous, 24 hours per day, seven days per week, injection of hydrazine and amine should be reflected in the installation of reliable chemical dosing units. The use of unreliable equipment can mean no hydrazine or amine additions when the unit is out of action. This can lead to the onset of corrosion in the boilers and subsequent repair costs far outweigh the cost of a reliable chemical dosing pump.
Other chemicals shall be injected in the same way.

MECHANICAL DE-AERATION
Mechanical de-aeration to reduce oxygen in the feed, is standard practice for all boilers. The need for it, however, depends on various factors, e.g. plant loading, boiler pressure and the evaporation rate. It is advisable to reduce the oxygen content of the feed water to under 0.02 mg/l for boilers up to 40 bar. With modern de-aerators these levels are easily attainable, if installed.
At atmospheric feed water system with open condensate- filter - hot well- tanks, the temperature shall be kept as near boiling as possible, also when large quantities of make up water is added. Heating coils in the tank as well as condensate temperature control and alarm should be provided.
Regular testing for oxygen content of the feed
water is necessary.

CHEMICAL DE-AERATION
As much of the dissolved oxygen as possible must be removed by mechanical de-aeration and supplementary chemical treatment used only for the last traces of oxygen. The feed temperature should be maintained as high as possible in the feed tank to reduce the oxygen content of the feed. In open feed tanks, the temperature shall be at least 85°C (or higher).
The chemicals used for removal of the residual oxygen from the feed water are:
1 Hydrazine )
The hydrazine (N2H4) must be continuously dosed into the feed water to react with all the oxygen present and to produce a small reserve of hydrazine in the boiler water. The normal level of hydrazine reserve to be maintained in the boiler water is between 0.1 and 0.3 ppm N2H4.
'he chemical reaction with oxygen can be expressed
N2H4 +O2=2H2O + N2
As the products of reaction are harmless - nitrogen as and water, hydrazine treatment does not increase the dissolved solids content of the boiler water.
The excess hydrazine in the boiler breaks down to give ammonia which provides suitable alkaline conditions in the steam condensate system. The reaction can proceed in two ways:
A 3N2H4---4MH3+N2
B 2N2H4--- 2NH3+N2+H2
Hydrazine for trace oxygen scavenging is normally recommended for boilers operating above40 bar but it can also be used in low pressure boilers where corrosion has been experienced and it is necessary to avoid any increase in boiler dissolved solids.
Sodium sulphite
Sodium sulphite is frequently used for removal of traces of oxygen from boiler feedwater. It can be used in marine boilers operating at pressures up to 40 bar. Catalysed sodium sulphite is used in most cases, as the reaction time with oxygen is extremely fast even at ambient temperature.
In marine practice, particularly for low pressure shell type boilers, it is usual to add the catalysed sodium direct to the boiler and not continuously into the feed water. This reduces the amount of sulphite required as most of the oxygen flashes off with the steam and only the small amount in the boiler water has to be removed by the sulphite. This, in turn, limits the increase in boiler dissolved solids resulting from the formation of sodium sulphate.
To ensure removal of all oxygen, a small sulphite reserve is maintained in the boilers, the level of excess
sulphite varying according to pressure and type of boiler and whether or not the feed water is de-aerated.
Hydrazine, apart from its comparatively slow reaction with oxygen at ambient temperatures and at a pH of under 9.0 has certain advantages over catalysed sodium sulphite, as follows:
A The use of hydrazine does not contribute to any increase in boiler dissolved solids and this is an important factor in high pressure boilers or in low pressure boilers where any additional dissolved solids must be avoided.
B The use of hydrazine results in a protective magnetic iron oxide film (Fe3O4) on the boiler meta! surfaces which acts as an additional protection against oxygen corrosion.
C The excess hydrazine in the boiler breaks down to produce ammonia which has beneficial effects in raising the pH of the steam/condensate system and thus affords some protection to the system by neutralising any carbon dioxide.

BOILER BLOW DOWN
Boiler blow down is an essential requirement for complete control of the boiler treatment. A reduction in the boiler water concentration of dissolved an suspended salts is required to meet the limits set for control of treatment.
An automatically operating continuous blow down control might also be fitted when required.
The frequency and duration of blow down is indicated by the test results from the boiler water conductivity which shows the total content of soluble salts collected into the boiler from the feed water and the treatment. It is also equal to the specific gravity, i.e. measurement of the total salt content may be done by a specific gravity meter.
The conductivity is measured in micro Siemens per centimetre (µS/cm) and the approximate relationship between the salt content and the conductivity is as follows:
S=K x C, where
S=salt content, mg!!
K=0.6 for boiler water and 0.7 for condensate
C=conductivity uS/cm.

The pH value indicates if the sample water is acidic, neutral or alkaline.
The following table shows the characteristics of water solutions over the pH-scale.
Since boiler water shall be alkaline in nature, it is important to follow the treatment makers' recommendations to keep the pH within the limits stated.
It is to be noted that large quantities of make-up water from evaporators containing dissolved CO2 might cause a decreasing pH in the condensate.
The CO2 dissolved into the make-up water will follow to condensers by the steam flow.

BOILER WATER TREATMENT PRODUCTS

Except for above mentioned chemicals for oxygen scavenging the boiler water treatment products are mostly special chemicals with combined effect. Apart of those Trinatriumphosphate Na3PO4 and sodium NaOH may be used for increasing the alkalinity and as softener.
There are several makers of boiler water treatment products where a few makers could be noted as examples:
Drew
VECOM
Nalfioc Ltd
UNITOR
Maritech
Automatic sample tests and dosing equipment will be recommended as a complement.
"One shot" type treatment products may be used in system with total balance only and with addition of a separate oxygen scavenger.