Deaerators

We can offer refurbished vessels at considerable savings over new process.

These vessels have very little wear or corrosion and usually come with stand, gantries and ladder.

Why gases need to be removed from boiler feedwater

Oxygen is the main cause of corrosion in hotwell tanks, feedlines, feedpumps and boilers. If carbon dioxide is also present then the pH will be low, the water will tend to be acidic, and the rate of corrosion will be increased. Typically the corrosion is of the pitting type where, although the metal loss may not be great, deep penetration and perforation can occur in a short period.

Elimination of the dissolved oxygen may be achieved by chemical or physical methods, but more usually by a combination of both.

The essential requirements to reduce corrosion are to maintain the feedwater at a pH of not less than 8.5 to 9, the lowest level at which carbon dioxide is absent, and to remove all traces of oxygen. The return of condensate from the plant will have a significant impact on boiler feedwater treatment – condensate is hot and already chemically treated, consequently as more condensate is returned, less feedwater treatment is required.

Water exposed to air can become saturated with oxygen, and the concentration will vary with temperature: the higher the temperature, the lower the oxygen content.

The first step in feedwater treatment is to heat the water to drive off the oxygen. Typically a boiler feedtank should be operated at 85°C to 90°C. This leaves an oxygen content of around 2mg / litre (ppm). Operation at higher temperatures than this at atmospheric pressure can be difficult due to the close proximity of saturation temperature and the probability of cavitation in the feedpump, unless the feedtank is installed at a very high level above the boiler feedpump.

The addition of an oxygen scavenging chemical (sodium sulphite, hydrazine or tannin) will remove the remaining oxygen and prevent corrosion.

Operating principles of a pressurised deaerator

If a liquid is at its saturation temperature, the solubility of a gas in it is zero, although the liquid must be strongly agitated or boiled to ensure it is completely deaerated.

This is achieved in the head section of a deaerator by breaking the water into as many small drops as possible, and surrounding these drops with an atmosphere of steam. This gives a high surface area to mass ratio and allows rapid heat transfer from the steam to the water, which quickly attains steam saturation temperature. This releases the dissolved gases, which are then carried with the excess steam to be vented to atmosphere. (This mixture of gases and steam is at a lower than saturation temperature and the vent will operate thermostatically). The deaerated water then falls to the storage section of the vessel.

A blanket of steam is maintained above the stored water to ensure that gases are not re-absorbed.

Water distribution

The incoming water must be broken down into small drops to maximise the water surface area to mass ratio. This is essential to raising the water temperature, and releasing the gases during the very short residence period in the deaerator dome (or head).

Breaking the water up into small drops can be achieved using one of the methods employed inside the dome’s steam environment.