One of the most important factors in keeping your boiler on-line is to keep enough water in it. Otherwise the boiler will shutdown on a low water condition. This is especially true with firetube boilers that are fired automatically. That is why it is so important to size a feedwater system so that it has the capability of maintaining the proper water level in your boiler.
A properly sized feedwater system will have a tank adequately sized to feed your boiler and pumps selected to deliver that water at the correct rate and pressure.
CALCULATE THE STORAGE TANK NEEDED
In most cases ten minutes of water should be readily available for your boiler. One boiler horsepower = 34.5 lbs/hr of steam (or water) from and at 212o F. We also know that one-gallon of water weighs 8.37 lbs. To calculate the storage tank needed use the following formula:
BHP X 34.5 ÷ 8.337 lbs ÷ 60 min. X 10 = minimum useable capacity in gallons.
For example, if you have a 500 HP boiler the calculation will be as follows -
500 x 34.5 ÷ 8.337 ÷ 60 X 10 = 345 gallons
Now, it stands to reason that you can't operate your tank totally flooded so you have to allow for some extra room. A safety factor of 1.5 is generally the accepted rule of thumb.
We then take the 345 gallons and multiply by 1.5 to get 517.5 gallons and choose a tank size of 500 gallons (one of the standard tank sizes available).
Keep in mind that your boiler system requirements might demand a larger reserve especially if you have a process steam load that returns large slugs of water intermittently. In this case you may need a larger tank.
The next step is to select the correct pump for your application.
There are three areas that must be considered.
The correct flow rate in GPM
The correct pressure needed
NPSH (net Pump Suction Head).
To calculate the flow rate in GPM, use the following formula:
BHP X 34.5 ÷ 8.337 ÷ 60 X 1.5 = gpm
(Please note that the 1.5 is, once again, a safety factor.)
For the example we have been using the calculation will look like this:
500 BHP X 34.5 ÷ 8.337 ÷ 60 X 1.5 = 52 gpm
Another quick rule of thumb is that 1/10 of a gallon is needed for every boiler horsepower. So a 500 HP boiler will need a pump capable of delivering 50 gpm.
An 800 HP boiler will need an 80 gpm pump.
The next step is to determine the proper discharge of the pump. ASME code requires that you furnish feedwater to your boiler at 3% higher than what the relief valve setting is on the boiler. In addition, you must take into consideration any pressure drops between the pump and the boiler. This would include any valves and piping.
For this example we will say that our relief valve is set at 150 psig and there is a 5 lb. pressure drop. The calculation will look like this:
150 x 1.03 + 5 lb drop = 160 psig required.
The last piece of the puzzle is the correct NPSH, net positive suction head. This is the amount of liquid, in feet, required at the pump suction to prevent cavitation and insure the pump is working correctly. This will help determine the tank stand height you will need
To chose the correct NPSH refer to the pump selection tables. These tables are based on the pumps having the lowest possible NPSH needed. This is done to ensure the lowest tank stand and thus reduce the overall height of the boiler feed system. NPSHA is the feet available under the tank and NPSHR is the feet required by the pump. Check the pump curve for the NPSHR and then add one foot.