1. Feedwater Valve Selection: Although the feedwater valve must be designed so that it will flow the required water for full boiler steaming capacity, the flow rate from blowdown and the relief valves opening must also be factored into the sizing of the feedwater flow control valve. It is important not to oversize the valve too much to accommodate blowdown and pressure relief. A good rule of thumb is to size a flow control valve for a pressure drop of 10-20% of the supply pressure, but make sure that the feedwater pumps are designed to provide the appropriate pressure to overcome the pressure drops.
2. Control Strategy Selection: Many heating boilers for multifamily apartment buildings or campuses can adequately maintain water level by using single element feedwater control because the steam demands do not swing rapidly as is the case with process steam loads. In these simplified cases, a single element feedwater control would simply use a level transmitter to open or close a feedwater valve. With process steam loads for places like hospitals, food processing facilities, etc.., it is important to be able to rapidly change loads to respond to steam demand. Therefore, it is important to utilize either a 2 or 3 element feedwater control strategy. In some cases, process steam loads may also require a watertube boiler in place of a firetube boiler due to the differences in water volume in the boiler.
3. Blowdown Control: When steam bubbles are formed and leave the surface of the water in the boiler, solids are left behind. These solids may either sink to the bottom of the boiler or be suspended in the water (known as Total Dissolved Solids, TDS). With increased TDS, the surface tension of the water to steam interface changes which can increase the resistance for steam bubbles to escape. This results in “swelling” which can cause carryover or surging water levels. Implementing a continuous blowdown control that blows down water near the water/steam interface helps to solve this issue.
4. Proper Water Level Setpoint: When operating a boiler, it is of utmost important to know the NWL (normal water line) of the boiler which is dictated by where the upper most tubes reside. This is different for each boiler, so the boiler manual must be consulted. If the water level is too low, a dangerous condition could occur, but if the water level is too high, surging water levels or carryover could be the consequence. Carryover or water could also damage downstream equipment like steam turbines.
5. Boil-out: When boilers are manufactured, oil and grease get left behind in the waterside of the boiler. Before commencing continuous operation of a boiler, a boil-out sequence must be accomplished to remove these oils from the boiler so that it does not impact the water level control. Consult the boiler manufacturer for the proper procedure.
6. Firing Rate Control: While it is important to ensure a rapid firing rate respond to changes in steam demand, it is also important to make sure that the feedwater control can keep up with the firing rate control. If the burner ramps to high fire too quickly, and the feedwater control isn’t adequately tuned or has enough control elements, the feedwater may either overshoot or undershoot the water level setpoint and result in a nuisance lockout.