Sources for the content of this technical bulletin:
Spirax-Sarco, Armstrong, Lalonde-Systhermique, AWT - Association of Water Technologies and TGWT
Management of non-condensable gases in steam generation systems
The presence of non-condensable gases in a steam network reduces the steam pressure and therefore decreases the temperature. Energy transfers are difficult, so the pressure of the exchangers must be increased to obtain the same temperature. It therefore costs more fuel to heat the product. In addition, the presence of non-condensable gases increases corrosion, resulting in costs associated with overuse of anti-corrosion products and frequent repairs. These are avoidable expenses that justify the management of these incondensable gases.
What are non-condensable gases?
All non-condensable gases are grouped under the word "air". These gases are called "non-condensable" because a cryogenic temperature (about -150oC) is required to condense them.
Where do non-condensable gases come from?
All steam systems are necessarily full of air at start-up. Each time the system or part of the system is shut down, it fills with air. This can be explained by the fact that the condensation of the steam causes a vacuum in the network and all the defective joints, or the slightest leaks, allow air to enter.
The best steam systems, equipped with automatic drain valves to prevent freezing or destructive vacuum formation, allow air to enter the system. Air can also enter the system through vacuum breakers.
Finally, air can enter the system dissolved in the feed water. At 80 oC, water can dissolve an amount of air equal to about 0.06% of its volume. Carbon dioxide has a higher solubility, about 30 times that of oxygen. When water is heated in the boiler, the gases are released with the steam and carried into the distribution system. Since air is present everywhere, in all systems, it is necessary to provide for efficient removal of non-condensable gases throughout the system.
What are the impacts of non-condensable gases?
CORROSION
Some of the oxygen that enters a steam system through the air turns to oxide and rust with the metal in the system. The calcium (or magnesium) bicarbonate of water hardness is the main source of carbon dioxide (CO2) in a system. Water treatments remove calcium but leave the bicarbonate. The bicarbonate, following the chemical reactions that occur in the boiler, releases CO2 that spreads in the network with the steam. In contact with the water (condensate), the carbon dioxide degrades into carbonic acid which attacks the steel of pipes and equipment.
Oxygen is the main cause of corrosion in steam systems, but if carbon dioxide is also present, then the pH will be low, the water will tend to be acidic, and the corrosion rate will be high.
POOR HEAT TRANSFER
Air, including nitrogen and other gases, is the best insulator, or the worst conductor: 1/100 of an inch of air insulates as much as 11 feet of copper, 15.5 feet of steel or 1/5 inch of water. That's why all good insulation contains air. Knowing this, it is easy to see why it is very important to remove air that could concentrate on the heating surface of the heat exchangers.
The figure opposite shows how steam must pass through a layer of air before it condenses on the wall of an exchanger, which slows down the heating process.
In addition, the presence of non-condensable gases in an exchanger reduces the partial pressure of the steam and thus reduces the temperature. The pressure gauge indicates the total pressure in the exchanger, not just the partial pressure of the steam. If there are non-condensable gases in the exchanger, the actual temperature, which is determined by the partial pressure of the steam, will be lower than the temperature inferred from the pressure gauge, because this one indicates the total pressure. The following equation illustrates this phenomenon:
Gauge pressure
= total pressure (Pt)
= partial pressure of steam (Pv) + partial pressure of gases (Pg)
If there is no air in the steam, Pt = Pv. At a pressure of 100 psig, the steam table tells us that the temperature is 170 oC. If the steam contains 20% air, then 100 (Pt) = 80 (Pv) + 20 (Pg) and the temperature that corresponds to a steam pressure of 80 psig is 162 oC. Therefore, the exchanger pressure must be increased to 170 oC and heating the product to the desired temperature involves higher fuel costs.
AIR BEHAVIOUR
The graph opposite shows that at a constant pressure of 150 psig (10 bar) in a unit, the temperature decreases as the amount of air in the steam increases. At 0% air, the temperature is 180oC, while it decreases to 160oC when there is 50% air.
A consequence of this phenomenon is that the temperature in the exchanger is actually lower than the pressure gauge suggests. This is because the temperature is determined by the partial pressure of the steam only, not by the total pressure!
UNEVEN PRODUCT TEMPERATURES
If the non-condensable gases accumulate non-uniformly on the heating surface of an exchanger, the result is a non-uniform temperature on the product side of the surface. Removing non-condensable gases from heat exchangers can have a positive impact on product quality. This is especially critical with heat exchangers such as rotary dryers and hot plates, where an extremely uniform heating surface temperature is essential for a quality product. Temperature uniformity is also important for many liquid products.
The curve opposite shows the heat transfer coefficient as a function of the percentage of air. For a given flow velocity, the more air in an exchanger, the poorer the heat transfer. Furthermore, the higher the flow velocity, the better the heat transfer is for the same percentage of air.
MANAGEMENT OF NON-CONDENSABLE GASES
The removal of non-condensable gases ideally results from a combination of mechanical and chemical methods. A four-step strategy is generally required:
1. Return as much condensate as possible;
2. If possible, replace boiler feed tanks with pressurized deaerators;
3. Apply an effective chemical treatment;
4. Install mechanical vents throughout the system.
1. RETURN AS MUCH CONDENSATE AS POSSIBLE
Since the condensate is already hot, deaerated and chemically treated, more of it is returned to the boiler will have a significant impact on costs. A higher proportion of returned condensate means less make-up water and, more importantly, less fuel consumption in the boiler.
2. REPLACE, IF POSSIBLE, THE FEED TANKS WITH THERMAL DEAERATORS.
Water exposed to air becomes saturated with oxygen, and the concentration varies with temperature: the higher the temperature, the lower the oxygen content. In conventional steam systems, the first step in eliminating non-condensable gases is to preheat the water in a boiler feed tank to remove the oxygen. Generally, the tank is maintained at a temperature of 85°C to 90°C and is equipped with a vent open to the atmosphere. This practice does not deaerate optimally and generates significant energy losses since the steam used to heat the water escapes through the vent. Whenever possible, it is preferable to replace the boiler feed tank with a pressurized deaerator in order to achieve a more complete elimination of non-condensable gases and greater energy efficiency.
If a liquid is at its saturation temperature, the solubility of a gas in the liquid is zero, but the liquid must still be shaken vigorously or boiled to be completely deaerated. This is what happens in the head of a deaerator where the water is separated into as many droplets as possible and surrounded by a cloud of vapor. This generates a very high surface-to-mass ratio and causes rapid heat transfer from the steam to the water, which quickly reaches saturation temperature. The dissolved gases are released and carried with the excess steam to a vent fixed on the top of the deaerator. The water thus degassed falls back into the deaerator tank. A layer of steam is maintained above the water surface to ensure that the air is not reabsorbed.
3. APPLY AN EFFECTIVE CHEMICAL TREATMENT
In most plants, the existing chemical treatment is appropriate. Historically, conventional solutions have not been mechanically optimal. There is a limit to what can be accomplished through chemical treatment alone, chemistry compensates to correct mechanical inefficiencies.
The mechanical removal of oxygen and carbon dioxide significantly reduces the consumption of corrosion control chemicals. Oxygen inhibitors will take care of removing the oxygen from the feed water; the greater the amount of oxygen remaining, the higher the consumption of inhibitors. And as for the carbonic acid formed by the presence of CO2, neutralizing amines, as well as ammonia, are effective in counteracting the effects of low pH by increasing the pH to a desirable range to reduce steel corrosion, ideally as close as possible to a pH of 8.5. The accumulation of carbon dioxide gas in certain areas of a steam/condensate system is often responsible for difficulties in adequately controlling the pH of the condensate.
4. INSTALL MECHANICAL VENTS (TRAPS) THROUGHOUT THE SYSTEM.
The use of a pressurised deaerator and chemical treatment is not sufficient to ensure the complete elimination of non-condensable gases in the network.
Inevitably, non-condensable gases will accumulate in the heat exchangers if nothing is done to prevent it. Eliminating non-condensable gases from the exchangers improves heat transfer and helps to ensure a uniform product temperature, in addition to not contributing to the accumulation of CO2 with the adverse effects mentioned above on the control of pH, and therefore, corrosion.
Traps (vents) can be installed at all strategic points of a steam network: on the piping and on all types of heat exchangers. The proper positioning of thermal eliminators depends on a variety of factors and requires a good understanding of air movement and its behavior in a steam network: this aspect of your steam installations requires the expertise of a firm specializing in steam networks.
Inevitably, non-condensable gases will accumulate in the heat exchangers if nothing is done to prevent it. Eliminating non-condensable gases from the exchangers improves heat transfer and helps to ensure a uniform product temperature, in addition to not contributing to the accumulation of CO2 with the adverse effects mentioned above on the control of pH, and therefore, corrosion.
Traps (vents) can be installed at all strategic points of a steam network: on the piping and on all types of heat exchangers. The proper positioning of thermal eliminators depends on a variety of factors and requires a good understanding of air movement and its behavior in a steam network: this aspect of your steam installations requires the expertise of a firm specializing in steam networks.
TGWT - Technical Support Team
info@tgwt.com