Feedwater tanks are heated storage tanks that do not have a specific deaerating section and operate at atmospheric pressure. In addition to being a feedwater reservoir, these tanks also act as condensate receivers, cold-water makeup locations and as a point for chemical injection.
Given the multitude of functions this vessel can have, proper design and operation of the feedwater tank is critical to ensure the boiler sees fully treated feedwater consistently. A properly designed feedwater tank will help reduce thermal shock and decrease oxygen scavenger usage. Avoiding the four most common design issues with feedwater tanks can help reduce problems with your boiler feedwater system.
1. Inadequate Heating of Feedwater
Boiler feedwater needs to be heated prior to entering the boiler to prevent thermal shock — and for the removal of oxygen from the water. Most feedwater tanks are equipped with a steam sparger, which heats the tank with steam from the boiler. The temperature of the feedwater tank should be maintained between 185 and 195°F (85 and 90°C) to reduce the amount of oxygen in the water. By increasing the temperature and reducing the oxygen content, you can greatly reduce your sulfite or oxygen scavenger requirements.
FIGURE 1. The oxygen content of water is greatly reduced as feedwater temperature increases, reducing oxygen scavenger requirements.
Making sure there is an adequate heating supply and the control valve is functioning properly are the first steps in making sure the feedwater tank is operating properly. A quick and easy check is to routinely take note of the vent discharge versus the temperature gauge. A well-heated feedtank at 190°F (88°C) will have noticeable — but not excessive — water vapor coming from the vent. Heating feedwater is one of the most cost-effective ways to remove oxygen and reduce chemical consumption.
2. Improper Cold-Water Makeup Design
The cold water entering the feedwater tank is generally 50 to 80°F (10 to 27°C), which means it contains quite a bit of oxygen. How makeup is being fed (on/off or continuous) and where in the tank it is fed can have a great impact on how easily the dissolved oxygen is liberated. Makeup should ideally be fed about 3 to 6” underneath the water line on the side of the tank, preferably through a sparger at a slow and continuous rate.
Adding makeup water above the water line can cause a splashing effect that reaerates the water. There are also risks with placing the makeup inlet over a feedwater pump supply line. Cold makeup water is denser than the heated water in the tank. Makeup water fed through the top can quickly sink to the bottom of the tank and inlet of the feedwater pump, causing cold untreated water to go directly to the boiler.
FIGURE 2. When the makeup is placed over the feedwater pump inlet, the system can short circuit and send cold water straight to the boiler.
On/off makeup control also can cause issues. If the makeup rate is too great when the feedwater tank calls for water, the feedwater temperature can lower faster than the steam can raise it. The oxygen scavenger residual in the tank could be completely consumed as a result. If the boiler calls for water when the tank is making up, the system runs the risk of sending feedwater with oxygen to the boiler, which can cause oxygen pitting and boiler failure.
3. Incorrect Feedwater Pump Location and Design
How the feedwater pump is installed in the system can have a great effect on how the system operates. The water at the inlet of the feedwater pump should be fully heated and devoid of oxygen. Feedwater tanks are rarely well mixed with uniform temperature and chemical composition throughout. The feedwater pump supply should be placed on the opposite side from where the makeup enters. This will give the cold makeup water the greatest amount of time to be heated and chemically treated before entering the boiler. The aforementioned makeup sparger is recommended if the feedwater tank has more than one feedwater pump-supply line.
FIGURE 3. Water needs time to deaerate before being sent to the boiler, and recirculation lines should be returned under the water line to prevent reaeration.
Another consideration is the feedwater pump piping for continuous feedwater pumps. Continuous feedwater pumps have recirculation lines to prevent deadheading, with the water being returned to the feedwater tank. This recirculation line should be plumbed below the water line. If the recirculation line is plumbed above the water line, it will reaerate the water and greatly increase oxygen scavenger usage.
4. Chemical Injection Location
Chemical injection into the feedwater tank is recommended to help protect the feedwater tank and give time for the oxygen scavenger to react. However, where the chemical is injected can have an effect on treatment quality. Chemicals should be injected underneath the waterline, preferably through a quill. The quill should be installed in a spot that bisects the location of the cold-water makeup and feedwater pump supply. This will ensure the cold-water makeup has to travel past the chemical-injection spot and thus be treated prior to entering the boiler. On most feedwater tanks, an ideal place is in the middle of the tank underneath the water line.
FIGURE 4. Untreated makeup water should pass through chemical treatment location to ensure water is treated.
In conclusion, feedwater tank design can have a great impact on boiler operation and chemical usage. While these are the most common issues with feedwater tank design, it is by no means a comprehensive list. Many other design factors can affect boiler operation such as residence time, venting and condensate return. Working with a knowledgeable manufacturer can help you avoid designing problems into your feedwater tank design or chemical setup.
Using Aluminum Boilers With Other Metals
Alloys of aluminum are being implemented in heat transfer applications, including hot-water boilers, more frequently due to aluminum’s high conductance of heat. However, aluminum is much more expensive and highly reactive when compared to standard materials used in these applications, such as steel.
While the use of these alloys can provide improvements to heat transfer efficiency for hot-water boilers and heat exchangers, a proper water treatment program tailored to minimize corrosion of aluminum while maintaining protection of other metals within the system is critical. The decision to implement such equipment must be made by balancing the potential savings through reduced utility consumption or shortened cycle times against the increased risk of system damage and premature failure due to the reactive nature of aluminum alloys.
The most critical factor to protecting aluminum within a water system is maintaining pH within the stable range for aluminum.
Aluminum forms a naturally protective barrier of aluminum oxide on the metal surface, but this oxide film is only stable in a pH range of approximately 4.0 to 8.5. Outside of this range, the oxide film can begin to degrade. Once the film degrades, even in a small area, the exposed aluminum beneath will corrode much more rapidly, resulting in what is known as pitting. This pitting is much more aggressive under alkaline conditions, which is typical of most water systems that have been treated with a standard corrosion-inhibitor blend. This is because steel and most other metals commonly found in water systems are more stable and protected from corrosion under more alkaline conditions.
Thus, to maintain protection of the entire system, the pH must be controlled within the stable range for aluminum while maintaining an adequate level of corrosion inhibition for any other materials in use.
As with any water system, corrosion inhibitors are required to minimize corrosion rates on the metals within the system. However, when systems contain aluminum, the inhibitor to be used must be selected with careful consideration to ensure that all materials in use are protected.
Any inhibitor selected must be buffered to near-neutral pH, and, dependent upon the pH of the raw water being used to make up the system volume, additional buffering solution may be required to bring pH to within a stable range, typically pH 5.0 to 8.0. There are many corrosion-inhibitor chemistries that will provide adequate protection from corrosion for steel and other metallurgies while maintaining pH within the acceptable range to minimize corrosion of aluminum.
There are additional considerations if the water system also contains copper, brass or any other yellow metal. The protection of these materials is critical to maintaining corrosion resistance of aluminum. Any corrosion of these materials may result in copper ions being released into the water, which, when contacting any aluminum surface, will result in severe pitting.
Therefore, appropriate inhibitors must be present to prevent the corrosion of these materials to ensure protection of both the yellow metals and the aluminum.
As previously mentioned, aluminum is a highly reactive metal, being one of the more anodic metals on the galvanic series. As such, it is important to maintain galvanic separation between aluminum within the system and any other metal, particularly those that are more cathodic than aluminum, namely copper and brass alloys.
If aluminum is directly connected to other metals, a galvanic cell will form that will result in rapid corrosion. Aluminum, acting as the anode, will experience significant degradation and loss of material, eventually leading to failure.
Steps should be taken to minimize or eliminate the use of copper and brass alloys in a system that contains aluminum wherever possible, or at the least, ensure that these metals are galvanically separated from the aluminum. This can be achieved using dielectric fittings, or other means of separation, such as a buffer of PVC pipe or other non-reactive material between the aluminum and other metals.
The decision to implement aluminum boilers — or other heat exchange equipment containing aluminum alloys within a water system — must be made in a thoughtful way. While aluminum may provide potential savings through improved heat transfer efficiency, this must be balanced with the requirements of the metallurgy throughout the system. Great care must be taken to control water chemistry, particularly pH, to ensure any efficiency gains are not negated by damage to other system components.