Understanding Feedwater Treatment

Feedwater Treatment of Steam Boilers

With any steam raising plant, the aim is to economically provide the correct quantity of steam to satisfy site demand, Safely, Effectively and Efficiently. Optimising the boiler to run at peak efficiency immediately reduces operational and production costs.

Efficient generation and distribution of steam starts with the management and control of the boilers and associated equipment.

The logical approach begins with the conditioning and storage of the boiler feedwater, through to the accurate measurement and control of water and total dissolved solids (TDS) levels within the boiler to the safe handling and heat recovery of boiler blow down.

Water level and TDS control have benefitted from advances in control technology but the approach to feedwater treatment has changed little over the last 50 to 60 years: this despite the fact that the majority of boiler failures are attributable in one way or another to water treatment.

UNDERSTANDING FEEDWATER TREATMENT

The chemical quality of water varies widely across the country and the recommendation for feedwater treatment is the work of specialist chemists.

THE SPECIFICATION AND APPLICATION OF WATER TREATMENT MUST BE STRICTLY OBSERVED IN ORDER TO OBVIATE THE SERIOUS CONSEQUENCES OF SCALE FORMATION, SLUDGE BUILD-UP AND WATER FOAMING.

A service contract with a feedwater treatment specialist involves regular inspections of your boiler feedwater and raw water supplies and recommendations regarding chemical dosing. However the responsibility of ensuring the correct day to day treatment of chemicals is the Boiler Engineers.

The aims of feedwater treatment are:-

1.        CORROSION PREVENTION

Corrosion in boiler plant is mainly due to the presence of gases dissolved in the water. These are carbon and oxygen which originate from:-

a)             The raw water supply.

b)              Introduction as a side effect during water treatment.

c)              As contaminants in the condensate.

They can be removed by mechanical and / or chemical deaeration. Chemically, carbon dioxide can be removed by the addition of alkalis such as caustic soda, sodium carbonate and sodium phosphate. Oxygen is controlled by the use of scavengers such as sodium sulphite or organic compounds.

2.        SCALE PREVENTION

The presence of hardness in boiler water inevitably results in scale build up on the heat transfer surfaces. Scale is a thermal barrier therefore, the temperature of heated surfaces increases. This can result in tube leakage and possibly tube plate and furnace damage. Build up of scale must therefore be prevented. Scale in boiler plant originates from the mineral salts which give rise to hardness in water.

Two types of hardness exist:-

a) Temporary or alkaline hardness due mainly to soluble calcium and magnesium bi-carbonates.

b) Permanent and non-alkaline hardness due mainly to soluble calcium and magnesium sulphates.

During boiling the temporary hardness breaks down to insoluble calcium and magnesium carbonates which precipitate out as scale onto all the heat transfer surfaces. No chemical change takes place in the permanent hardness salts but, upon boiling, the solubility decreases and if the boiling is prolonged so that the concentration in the water increases, beyond solubility limits, this too will precipitate out as scale. Removal of these salts is therefore of prime importance. They may be removed before the water reaches the boiler by chemical or phsyical-chemical methods i.e. by external treatment, or they may be removed in the boiler itself by injecting suitable chemicals which will precipitate out the salts in the form of sludge i.e. by internal treatment. Choice of external or internal treatment depends on the total hardness of the raw water and the quantity of condensate recoverable. In general, internal treatment may be used if the combined hardness of the raw water plus the condensate does not exceed 50 ppm. Hardness in excess of this will require external treatment to avoid excessive sludge in the boiler. The actual choice of treatment is detailed under Item 4 of this document.

3. PREVENTION OF CARRYOVER

Water may carry over from the boiler in the form of foam (foaming) or as actual flow of water (priming). This can be caused by:-

a)             High water level in the boiler.

b)             Excessive steam velocity across the boiler crown valve.

c)             Sudden and heavy load fluctuations.

d)             The physical and chemical conditions of the boiler water.

Carryover may also be aggravated by high concentrations of dissolved and suspended solids in the boiler water. It is recommended that a maximum dissolved solids content of 3500 ppm is maintained with a suspended solids content as low as possible. The concentration of solids is controlled by blowing down the boiler through the bottom drain valve therefore, from the point of fuel economy and boiler efficiency, the water treatment method should be one which introduces a minimum quantity of solids.

4. CHOICE OF TREATMENT METHOD

The most important factor in deciding the type of treatment is the quantity of condensate which can be recovered from the steam system. Condensate is distilled water and does not contain dissolved mineral salts (though it may contain corrosive gases in solution). Thus even hard waters may be treated by cheap internal means provided that they can be sufficiently diluted by pure condensate

Conversely more expensive external treatment may be necessary on fairly soft waters of about 50 ppm total hardness if condensate is not available. In deciding upon the method of treatment the quantity of condensate must be known so that the total hardness of the feedwater can be calculated. If the total hardness of the feedwater (i.e. raw water plus condensate) is less than 50 ppm internal treatment is usually possible. If however the total hardness exceeds 50 ppm external treatment is usually necessary.

5. INTERNAL TREATMENT

The purpose of this form of treatment is to precipitate out the hardness salts within the boiler as a free flowing sludge which can be removed by blowing down. Two distinct types of treatment are available.

a)      Inorganic, based principally on sodium carbonate and / or phosphate together with an oxygen scavenger.

b)      Organic, based on tannin extracts, which though they do not soften the water they promote sludge formation and absorb oxygen used in conjunction with an alkaline to protect against carbon dioxide corrosion. Organic treatment has the advantage that it tends not to increase the solid content of the boiler water to the same extent as inorganic.

6. RECOMMENDED BOILER WATER ANALYSIS

The following water analysis is generally considered suitable for shell type packaged boilers.

7.       GENERAL

Steam boilers are fitted with drain valves underneath the boiler shell. The blowing down of the boiler shell should be done as often as is dictated by the feedwater conditions, the schedule will be fixed by your feedwater treatment specialist.

It is recommended that a sample of the boiler water should be taken daily for testing and a record kept of the boiler water conditions.

As a general rule, the boiler should be blown down once every 8hr shift to remove sludge. This should be done by giving the drain valve a 5 to 10 sec. duration blow down. Short gulp blow downs are more effective in sludge removal than a long continuous blowdown.

Existing feedwater treatment systems presuppose that:-

·        feedwater temperature and quality are constant

·        Steaming rate and condensate returns are constant

·        Cold water make-up is constant

We all know that they vary day to day, hour to hour:

One test a day is it enough?      Or………… Is it time for Change? Click the Link to find out