Boiler water treatment is critical to ensuring your boiler is operating as efficiently as possible and no proper water treatment program is complete without a thorough understanding of the feedwater setup. 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.
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 make up 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. Below are four of the most common design issues with feedwater tanks:
1. Inadequate Heating Of Feedwater
2. Improper Cold Water Make Up Design
On/Off make up control can also cause issues. If the make up 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. 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
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.