During boiler operation, foam is generated on the evaporation surface in the upper part of the steam drum. When foam accumulates to a certain extent, it can cause the steam and water levels in the drum to become indistinguishable, forming a chaotic state where water contains steam and steam carries water. This phenomenon is called priming.
When priming occurs in a boiler, the steam quality significantly deteriorates due to moisture carryover and contamination. This can lead to the following adverse consequences:
1) Reduced boiler efficiency, wasting fuel and water. The presence of moisture in the steam increases the steam’s humidity, which inevitably increases water consumption; high humidity also lowers the temperature of superheated steam, inevitably leading to excessive fuel consumption. Practice has proven that when the moisture content in steam reaches 1%, water consumption increases by 1.5%, fuel consumption increases by 1%, and it can lower the superheated steam temperature by as much as 10°C.
2) Causes pipe blockage and mechanical damage. Moisture and impurities (salts, sludge, suspended solids, etc.) in the steam can precipitate and form salt scale on the superheater tube walls, blocking and corroding steam pipes. Impurities carried into equipment such as steam engines and steam pumps disrupt normal lubrication and can easily cause wear and other accidents.
3) Incorrect water level indication, leading to unexpected accidents.
There are many factors that cause priming, such as boiler construction, operating conditions, and water quality, but it is primarily caused by dissolved and suspended substances in the water leading to the formation of large amounts of foam.
As boiler water evaporates and concentrates, its salt content gradually increases, but until it exceeds a certain value, it does not produce large amounts of foam. This can be seen from the steam quality (Figure 10-9). The sodium content in the steam (which can indicate moisture carryover and contamination) is proportional to the salt content in the boiler water. The inflection points A1 and A₂ represent the limiting values of salt content. Exceeding these values can cause a significant deterioration in steam quality due to priming.
Generally speaking, foam does not form when the dissolved salt content in the boiler water is low. However, an increase in dissolved salts signifies the concentration of the boiler water. As dissolved salts increase, the inevitable suspended solids (grease, sludge, etc.), organic matter, and alkaline substances (NaOH or Na₃PO₄) in the boiler water will also increase correspondingly. This leads to two consequences:
1) It increases the viscosity of the boiler water, making it difficult for small steam bubbles in the water layer to merge into larger bubbles, thereby increasing bubble stability. Small bubbles rise more slowly, causing the boiler water volume to expand, which raises the water level, reduces the steam and water space, and makes steam-water separation difficult.
2) The aforementioned impurities are easily adsorbed onto the surface of steam bubbles, increasing the strength of the bubble water film. When bubbles rise from below the water layer to the water-steam interface, they do not burst immediately. Because the bursting speed of these bubbles lags behind their rising speed, they continuously accumulate at the steam-water interface, forming a foam layer. When the foam layer becomes very thick, it causes the steam and water interfaces to become indistinguishable, resulting in priming.
Moisture carryover with the steam sent from the steam drum is one reason for steam contamination. As boiler pressure increases, the steam’s own ability to dissolve certain salts also increases. For example; saturated steam at pressures of 30~40 /cm² has a significant ability to dissolve silicic acid, and at even higher pressures, it can also dissolve certain sodium salts and alkalis. Therefore, the higher the boiler pressure, the higher the requirements for water and steam quality.
Measures to prevent priming and ensure steam quality in boiler operation management are as follows:
1) Maintain stable boiler load (evaporation rate), pressure, water level, etc. When the boiler load increases, the number of steam bubbles in the water space also increases, and the kinetic energy of the bubbles is greater. This makes the boiler water more prone to foaming, leading to moisture carryover with the steam. When the boiler pressure drops suddenly, the boiling point of the water decreases, causing the boiler water to boil violently and produce a large number of steam bubbles. The bursting of these bubbles produces numerous small water droplets, which can also cause moisture carryover. The actual water level in the steam drum is usually higher than that indicated by the water level gauge, especially when the ship is pitching and rolling, making false water levels more likely. An excessively high water level obviously reduces the steam and water space, making steam-water separation difficult and increasing the amount of moisture carried over with the steam. Therefore, special attention must be paid during boiler start-up, banking, or when the ship encounters rough seas. Changes in load, pressure, etc., should be made slowly, and the water level should be carefully observed and adjusted promptly.
2) Strictly control the total dissolved solids (TDS) content and alkalinity of the boiler water. When the TDS content or alkalinity exceeds the specified limits, blowdown must be performed immediately to reduce their concentration. The cause must be identified and eliminated.
3) Prevent the ingress of grease, cooling system leaks, and the entry of corrosion products into the boiler. The reliability of filtration devices should be checked regularly and they should be cleaned promptly.
4) Sometimes, as an emergency measure, an appropriate amount of foam inhibitor, such as high molecular compounds like distearyl ethylenediamine or polyoxyalkylene glycol diether, can be added to the boiler water. The inhibitory capability of foam inhibitors is stronger in the presence of organic matter such as tannin and lignin.
