The Dangers of Uncontrolled Gases In Your Steam System
One of the most serious and costly hazards that can befall your steam system is the buildup of air and noncondensible gases (usually carbon dioxide and oxygen). Left unattended, such accumulations accelerate corrosion and block flow, resulting in leaks, steam or water hammer, reduced heat transfer and, eventually, expensive repairs.
The real danger to your steam system is in the corrosive potential of these gases when combined with condensate. Together, they create caustic carbonic acid which can cause leaks at heat exchanger walls or tubes. It is also strong enough to eat away drain lines, leading to leaks in your steam fittings and condensate return lines.
Corrosion is often so severe that condensate discharged from a steam trap will turn bright red or dark brown from the dissolved iron. Under such conditions, pressure-retaining components within the heat transfer equipment cannot withstand the pressure of the system.
THE ROOT OF MANY PROBLEMS
Corrosive condensate is only one of several hazards caused by air and non-condensibles in steam systems.
System binding. Even though they are compressed, air and non-condensible gases inside a steam system still occupy volume and can displace steam and condensate. When system binding occurs, flow of steam and condensate into and out of the system can be blocked. Since the condensate cannot drain, it cools down. When it comes into contact with the hotter steam, it causes instantaneous condensation which unleashes severe velocity and pressure fluctuations within the system. Water is accelerated and impacts piping, elbows, fittings and valves in potentially destructive water or steam hammer. The result may be leaks, float collapsing and other component failures.
Energy costs. Most industrial or institutional steam systems use steam to transfer heat energy. Any reduction in the capability of the steam to transfer heat energy is a potential cost.
When steam is distributed and flowing through the piping system, steam pressure actually drops. Steam mains and branch lines are sized to distribute the steam without excessive pressure drop. To avoid the energy loss associated with steam pressure drop, these lines must be sized very carefully to avoid making the energy loss higher than normal. As steam pressure drops, so does the associated steam temperature. This could slow heat transfer, demanding more steam, increasing the pressure drop and wasting more total energy.
Air and non-condensible accumulations reduce heat energy transfer in almost the same way as a steam pressure drop by:
- reducing the effective steam temperature
- insulating the heat transfer surfaces
Temperature drop. Dalton’s law of partial pressures states that “the pressure of a mixture of gases is equal to the sum of the partial pressures;’ In the case of air and non-condensible gases mixing with steam, the gases will exert part of the pressure and the pressure exerted by the steam will be reduced. As we have seen, there is a direct relationship between the temperature and pressure of steam. As the pressure of steam decreases, the temperature also decreases-along with the efficiency of heat transfer.
Insulating effect. Air and non-condensible accumulations can also reduce heat energy transfer by insulating the heat exchanger. As steam flows within a heat exchanger tube, it moves from the center of the tube out toward the heat exchanger walls.
Since air and non-condensibles do not condense, they behave as relatively lazy gases and can be pushed along by the flowing steam within the tube. The general steam flow is toward the walls of the heat exchanger; the air and non-condensible gases accumulate at the walls and can form an insulating film.
SOURCES OF AIR AND NON-CONDENSIBLE GASES
Air and non-condensible gases, usually carbon dioxide and oxygen, cause the problems we’ve discussed. Air can enter a steam system during operation, usually at the heat exchanger. Non-condensible gases are introduced to the system through boiler feed water.
Steam systems are full of air at startup. As the steam enters the system, it condenses and will form high condensate loads. This condensate and steam mixture moving through the piping will force the air ahead of it into the far reaches of the system.
Since the end of the system is the heat exchange equipment and the steam trap, the ability of these components to deal with high volumes of air at very low pressures determines the effectiveness of the air removal. Pockets within heat exchangers will normally form at the last place the steam and condensate flow will reach. These air pockets are free to remain in the system unless steam flow or condensate flow sweep them away.
All boiler feedwater contains elements which are or can produce non-condensible gases when the water is boiled. These gases are transported into the system along with the steam. In addition to the gases in solution, all feedwater contains carbonates and/or bicarbonates which are converted in the boiler drum to CO2.
TECHNIQUES FOR REMOVAL AND CONTROL
Most industrial and institutional steam systems are designed to reduce intake and accumulation of noncondensible gases. Understanding the operating principles of this equipment and how to manage the system can eliminate recurring hazards.
Deaeration. Deaerators are designed to do exactly what the name implies remove gases from incoming boiler feedwater. As we saw earlier, CO2 goes into solution with water when the temperature is decreased. Deaerators are designed to heat feedwater up over an extended surface area to give the CO2 and oxygen an opportunity to come out of solution where they are under very low pressure and can be easily vented. Returned condensate, often laden with CO2, is also typically de-aerated.
Thermostatic vents. Thermostatic steam traps (particularly bellows type) can be used as automatic air vents on heat exchange equipment. Air in the system tends to be lighter than steam and does not condense so it gets pushed to quiet zones by the flowing steam. At these locations, the thermostatic device senses the temperature reduction caused by the air accumulation and vents it. Batch process cookers, large shell and tube heat exchangers, and large steam coils should incorporate automatic air vents to eliminate these air accumulations.
Steam traps. Steam traps should discharge condensate from a process application at or near saturation temperature. Selection of the traps that back up or sub cool condensate will accelerate carbonic acid corrosion cause steam leaks, reduce heat energy transfer and possibly increase maintenance.
Subcooling traps are typically thermostatic traps which are designed to back up condensate. These traps may be of thermal expansion design, balanced pressure bellows, bimetal, wafer or diaphragm type. How much these traps will subcool depends mainly on the mechanical characteristics of the type of trap. The degree of temperature drop will also depend on the steam pressure and condensate load.
It’s important to locate steam traps properly because one of their functions is to vent the air and non-condensible gases. When installing steam traps, follow the ABCs of trap location:
- ACCESSIBLE for inspection and repair
- BELOW drip points whenever possible
- CLOSE to the drip point
Steam traps are like thermostatic air vents in that they do not reach into the system to draw out air and noncondensibles for venting. They will vent only whatever reaches them and for this reason, unless they are located so they have an opportunity to see air and non-condensibles, they will not vent them. A properly located nonsubcooling steam trap can usually take care of the lower quiet zones within a heat exchanger since the traps will vent air also. Properly sized nonsubcooling traps, such as inverted bucket traps and float and thermostatic traps, will help maximize heat energy transfer in the system.
CO2 and oxygen corrosion can also be a major cause of problems in steam traps with small orifices. If copper or iron products of corrosion are in the condensate flowing to the steam trap (as they are in many older systems), they go into solution in the carbonic acid. When they pass through an orifice of small size to a lower pressure, the condensate flashes and these corrosion products can be deposited as oxides downstream of the orifice. This may plug very small orifices. Even when larger orifices are used, plugging can occur in the outlet piping of the steam trap.
Insulated condensate returns, when condensate is at a higher temperature, corrosion is slowed because CO2 goes into solution best in cooler condensate. Condensate should be maintained as hot as possible in the return system to minimize the carbonic acid formation.
Chemical treatment. The proper makeup water treatment is essential to remove as much CO2 as possible from the boiler feedwater. However, even with proper deaeration and alkalinity control of makeup water and condensate return, it’s impossible to eliminate all CO2 from the system. At some point the condensate will become corrosive. The addition of amines to the system can be helpful in preventing corrosion.
Uncontrolled air and non-condensible gases in your steam system can cause corrosion, water hammer, heat transfer and drainage problems in both the steam system and your steam heat exchange equipment. Therefore, a program to minimize gas introduction rates, to vent gases where they accumulate, to drain condensate before it subcools, to prevent cooling of condensate return systems and to minimize the corrosive effects of carbonic acid should be implemented.
A complete action plan would entail:
- Study of your deaerator piping design and existing piping practices
- Analysis of your major heat exchange equipment for air vent inclusion and piping practices
- Analysis of your steam trap selection and piping practices
- Analysis of your maintenance records on heat exchanger tube bundles and steam coil repairs and replacement for persistent corrosion or water hammer problems
- Survey of your condensate return lines for adequate insulation
- Analysis of your chemical treatment practices and problems.
Safeguarding your steam system against the corrosive effects of air and non-condensible gases is a key element in system longevity and the avoidance of downtime. For more information about this important subject contact Campbell-Sevey.