Boiler Feed Water System

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) Sizing of Boiler Feed Water Tank


A properly sized feed water system will have a tank adequately sized to feed your boiler and pumps selected to deliver that water at the correct rate and pressure.


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 usable 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).


B) Sizing of Boiler Feed Water Pump

There are three areas that must be considered. 

i)  The correct flow rate in GPM

ii) The correct pressure needed 

iii) NPSH (net Positive Suction Head)

i)  Calculating the flow rate in GPM

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.

ii)  Calculating the Required Pressure

The next step is to determine the proper discharge of the pump. ASME code requires that you furnish feed water 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.

iii)  Calculating Net Positive Suction Head

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.

This is the minimum absolute pressure at the suction nozzle at which the pump can operate. To avoid pump cavitation, the NPSHA of the system must be greater than the NPSHR of the pump. In other words, the available NPSH must be higher than the required. We have always sized our deaerator stands to be two feet higher than the NPSH needed for the pump selection. Remember, the water level in the storage tank adds to the safety margin.

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.

Pump capacity depends upon evaporation rate of the boiler. A safe figure for an on-off application would be 2 times the evaporation rate of the boiler. With a modulating level control, a factor of 1.3 times the evaporation rate plus recirculation is recommended.

Courtesy: http://www.mckenziecorp.com