Boiler water treatment is critical to ensure your boiler is operating as efficiently as possible. No proper water treatment program is complete without a thorough understanding of the feedwater setup.
Feedwater is water that is preheated and/or purified and then supplied to a boiler. This process will cause the feedwater to be converted into steam which will drive turbines. By using high-quality feedwater, your process will operate efficiently and safely, leading to lower fuel costs and longer equipment life.
A crucial step in preparing the feedwater for the boiler is preheating and deaerating the water in a separate vessel directly upstream of the boiler.
These vessels come in two main varieties: deaerators and feedwater tanks. Deaerators are pressurized vessels that have a designated deaerating apparatus in addition to its feedwater storage section.
Feedwater tanks are heated storage tanks that, unlike deaerators, don’t 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 injections. Given the many functions of this vessel, proper design and operation of the feedwater tank are 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. Read on for four of the most common design issues with feedwater tanks:
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 to 195 degrees Fahrenheit 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. Making sure there is an adequate heating supply and the control valve is functioning are the first steps to establish that 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 feed tank at 190 degrees Fahrenheit 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.
The cold water entering the feedwater tank should generally be 50 to 80 degrees Fahrenheit, which means it contains quite a bit of oxygen. How makeup is being fed (on/off or continuous) and where in the tank it’s fed can have a great impact on how easily the dissolved oxygen is liberated. Makeup should ideally be fed about three to six inches underneath the water line on the side of the tank, preferably through a steam sparger at a slow and continuous rate.
Adding makeup above the water line can cause a splashing effect, which 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.
On/off makeup control can also 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 also be completely consumed as a result.
KEY INSIGHT: 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.
Figure 1 – When the makeup is placed over the feedwater pump inlet, the system can short circuit and send cold water straight to the boiler
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 very 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 makeup sparger is recommended if the feedwater tank has more than one feedwater pump supply line.
Another consideration for continuous feedwater pumps is the associated piping. 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.
KEY INSIGHT: Deadheading is when a pump is operating with no liquid flow. When this happens, the pump will recirculate the fluid. This could cause increased temperature, damage to the pump, shortened equipment life, and even full system failure.
Figure 2 – Water needs time to deaerate before being sent to the boiler and recirculation lines should be returned under the water line to prevent reaeration.
Chemical injections into the feedwater tank is recommended to help protect it and give time for the oxygen scavenger to react. However, where the chemical is injected can influence 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 therefore will be treated prior to entering the boiler. On most feedwater tanks, the ideal place is in the middle of the tank underneath the water line.
Figure 3 – Untreated makeup water should pass through chemical treatment location to ensure water is treated.
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 including residence time, venting, condensate return, etc.
Do you have more questions about boiler water treatment or feedwater tanks? Our industry experts are ready to help!
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