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.
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. Deaerator optimization has already been discussed here, therefore in this blog, we will be discussing proper Feedwater Tank Design.
Figure 1 - A feedwater tank for a 400 HP 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
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 185F and 195F 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 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 190F will have noticeable water vapor coming from the vent, but not excessive. Heating feedwater is one of the most cost effective ways to remove oxygen and reduce chemical consumption.
Figure 2 - The oxygen content of water is greatly reduced as feed water temperature increases, reducing oxygen scavenger requirement.
2. Improper Cold Water Make Up Design
The cold water entering the feedwater tank is generally 50F to 80F, which means it contains quite a bit of oxygen. How make up 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.
Make up should ideally be fed about 3-6 inches underneath the water line on the side of the tank, preferably through a sparger at a slow and continuous rate. Adding make up above the water line can cause a splashing effect which reaerates the water. There are also risks with placing the make up inlet over a feedwater pump supply line. Cold make up water is denser than the heated water in the tank. Make up 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 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.
Figure 2 - When the make up is placed over the feedwater pump inlet, the system can short circuit and send cold water straight to the boiler
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 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 make up enters. This will give the cold make up water the greatest amount of time to be heated and chemically treated before entering the boiler. The aforementioned make up sparger is recommended if the feedwater tank has more than one feedwater pump supply line.
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.
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.
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 make up and feedwater pump supply. This will ensure the cold water make up has to travel past the chemical injection spot, and thus being 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 4 - Untreated make up 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 such as residence time, venting, condensate return, etc. Please contact your Klenzoid representative if you have concerns about your feedwater tank design or chemical setup!