Water Treatment is Key to Maintaining Boiler Efficiency
Boiler or steam production is one of the most advanced applications for water treatment technology. Boiler system operators and engineers strive for the highest efficiency possible from their equipment, and they have learned that poor water quality has a direct impact on boiler efficiency and longevity. Some boilers return a portion of the condensed unused steam or “condensate return” to the boiler feedwater. Since most boilers use more water than the condensate return can provide, the water added to meet the boiler’s feedwater requirement is referred to as “make up” water. Boiler feedwater thus consists of the condensate return and the make up water. Boiler water treatment involves three primary concerns: carryover, deposits, and corrosion.
Feedwater Concerns
Carryover is a boiler industry term for a condition where boiler water is found in the steam lines. It is usually caused by foaming or a mechanical problem such as water escaping around a damaged baffle. Any carryover water found in the steam will degrade steam quality and could cause corrosion in the steam lines. Scale Deposits from calcium carbonate are common in low-pressure boilers and are a threat to any water-using process equipment. Scale deposits form on boiler tubes and interfere with the transfer of heat through the tubes which can cause tube metal failure and, ultimately, severe damage or failure. The maximum safe range for tube metal for low-pressure boilers is 900 to 1000 degrees F (480-540 C). Corrosion is another major concern. Most water treatment professionals recognize corrosion as metal returning to its ore form. Corrosion can attack metals such as iron over a large area or produce very small pinpoint holes, returning iron to iron oxide. Besides the boiler system, corrosion can also affect the pre-boiler systems and condensate return lines. Corrosion results from low pH, stresses, or excessive amounts of dissolved oxygen and carbon dioxide in the water. Where there is excessive dissolved oxygen in the feedwater corrosion is actually accelerated by low pH and elevated temperature. These conditions can exist in almost any boiler application. Corrosion prevention consists of removing dissolved oxygen from the feedwater, maintaining a slightly alkaline feedwater, and keeping the boiler tubes clean. When treating corrosion in the boiler feedwater, do not simply remove dissolved oxygen as your only defense to slow down or stop corrosion.
Basic boiler water treatment consists of three recognized methods: internal,
external, and blow down.
Internal Conditioning: When properly applied, chemicals will react with the feedwater hardness, control the effects of corrosion, remove dissolved oxygen, and prevent carryover. In some cases, internal and external treatments are used on boiler feedwaters. Boiler operators use blow down in conjunction with internal treatments.
External Conditioning: Several processes can improve external water quality, including precipitation, ion exchange, deaeration, evaporation, and reverse osmosis (RO). The process in which coagulants are used to remove suspended solids is referred to as clarification.
Chemical Precipitation: This is used to reduce hardness and suspended solids and a common type of this is lime-soda softening.
Deaeration: This removes dissolved oxygen prior to boiler feedwater entering the boiler system. A portion of the steam is vented which carries off most of the dissolved oxygen from the feedwater.
Evaporators: This unit is used when steam is abundant as a source of heat. The process involves preheating the boiler feedwater into a relatively pure vapor form. It works well where the dissolved solids in the feedwater are very high.
Ion Exchange: For softening and deionization (DI) of boiler feedwater has been used extensively with very positive results. In boiler water treatment total hardness and total dissolved solids need to be reduced which is where ion exchange is used.
Demineralization: A term which can be used interchangeably with deionization. Both refer to the elimination of dissolved solids through ion-exchange.
Reverse Osmosis (RO): This method lowers costs associated with operation of the DI plant by substantially reducing the total dissolved solids prior to the DI system. This lengthens the periods between regeneration and/or tank exchanges and increases the DI resin’s life expectancy. Controlling the boiler is of utmost importance in any steam generation energy saving program. Below are some ways to improve boiler efficiencies:
- Reducing excess air Recovering waste heat from blowdown
- Preheating combustion air Installing an economizer
- Stopping dynamic operation
- Switching to lower cost fuel
- Reducing scale deposits Reducing boiler pressure
- Reducing blowdown Operating at peak efficiency