The importance of the boiler feedtank, where boiler feedwater and make-up water are stored and into which condensate is returned, is often underestimated. Most items of plant in the boiler house are duplicated, but it is rare to have two feedtanks and this crucial item is often the last to be considered in the design process.
The feedtank is the major meeting place for cold make-up water and condensate return. It is best if both of these, together with flash steam from the blowdown system, flow through sparge pipes installed well below the water surface in the feedwater tank. The sparge pipes must be made from stainless steel and be adequately supported.
Feed Tank Operating temperature
Feedtank operating temperature is probably the most important parameter to consider for the following reasons:
- To minimise dissolved gases
It is important that the water in the feedtank is kept at a high enough temperature to minimise the content of dissolved oxygen and other gases.
Oxygen Content of Water vs Temperature
If a high proportion of make-up water is used, heating the feedwater can substantially reduce the amount of oxygen scavenging chemicals required.
This is shown in the example below:
Chemical Savings associated with reducing dissolved oxygen in Feedwater
Assumptions: sodium sulphite dose rate = 8ppm/ppm oxygen
usual to add 4ppm extra to maintain reserve
catalysed sulphite is 45% active
boiler rating 10000 kg/hr
operating period 6000 hr/yr
a) Feedwater at 60°C
Oxygen content at 60°C = 4.8ppm
Sodium sulphite required = [4.8 x 8] + 4 = 42.4ppm
At 45% active product required = 42.4 x 100/45 = 94.2ppm
Annual usage = 10000 x 6000 x 94.2 / 1000000 = 5653 kg/yr
b) Feedwater at 85°C
Oxygen content at 85°C = 2.3ppm
Sodium sulphite required = [2.3 x 8] + 4 = 22.4ppm
At 45% active product required = 22.4 x 100/45 = 49.8ppm
Annual usage = 10000 x 6000 x 49.8 / 1000000 = 2988 kg/yr
Reduction in chemical usage = (5653 – 2988) / 5653 x 100 = 47%
- To avoid damage to the boiler itself.
The boiler undergoes thermal shock when cold water is introduced to the hot surfaces of the boiler wall and its tubes. Hotter feedwater means a lower temperature difference and less risk of thermal shock.
- To maintain the designed output.
The lower the boiler feedwater temperature, the more heat is required in the boiler to produce steam. It is important to maintain the feedtank temperature as high as possible, to maintain the required boiler output.
- Cavitation of the boiler feed pump
Caution: very high condensate return rates (typically over 80%) may result in excessive Feedwater temperature, and cavitation in the feed pump.
If water close to boiling point enters a pump, it is liable to flash to steam at the low pressure area at the eye of the pump impeller. If this happens, bubbles of steam are formed as the pressure drops below the water vapour. When the pressure rises again, these bubbles will collapse and water flows into the resulting cavity at a very high velocity.
This is known as ‘cavitation’; it is noisy and can seriously damage the pump.
To avoid this problem, it is essential to provide the best possible Net Positive Suction Head (NPSH) to the pump so that the static pressure is as high as possible. This is greatly aided by locating the feedtank as high as possible.
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