Boiler blowdown tank

Boiler Blowdown Tank: Function, Design, and Application

The boiler blowdown tank is a device with a very simple, yet important and widely used function in heating systems. In essence, it is a tank that first brings the blowdown water from the steam boiler down to ambient temperature and then safely discharges it into the surrounding environment.

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Application of the Blowdown Tank

Blowdown (or ‘underwater purging’) in steam boilers is performed to remove water saturated with dissolved salts and suspended solids from the boiler. Therefore, the function of this device is to control the boiler’s output water.

The device’s operation includes reducing the water’s temperature and pressure to an acceptable level for system cleaning and discharge. These tanks are typically metallic. A blowdown tank is usually constructed from steel or alloy sheet. This tank operates within a closed loop and is considered a pressure vessel.

Given that during blowdown or boiler draining, in special conditions where the water is still at a high temperature, it cannot be directed towards the sewage system or an open discharge area in the environment. Therefore, this hot water is cooled in a tank. Consequently, underground or above-ground metallic tanks are used to reduce the temperature and pressure of the water discharged from safety valves or the steam boiler.

So, generally, blowdown tanks are used to remove dissolved solids and the sludge formed in the water discharged from the boiler. They are also intended to reduce TDS and prevent damage caused by system scaling and corrosion.

Blowdown Tank

What is Boiler Water TDS?

TDS is an abbreviation for Total Dissolved Solids, which indicates the amount of impurities present in the water. A high TDS level in hot water and steam boilers can cause operational problems.

Design Temperature and Pressure of the Blowdown Tank

The pressure of a blowdown tank is calculated based on the maximum working pressure of the steam boiler, with one-fourth of that value considered as the blowdown tank pressure.

Based on the pipe pressure and its size, the tank dimensions are determined. The pressure inside the tank should not exceed 0.35 bar gauge under any circumstances. The design pressure of the tank must be at least 25% of the steam boiler’s maximum working pressure, and the design temperature should be equal to or greater than the saturation temperature corresponding to the blowdown tank’s design pressure.

Dimensions of the Blowdown Tank

The dimensions of the boiler blowdown tank depend on the boiler pressure and the size of the blowdown pipe. The tank vent must be large enough so that the pressure inside the tank does not exceed 0.35 bar gauge. The tank volume should also be such that the temperature of the overflowed water does not exceed 43ºC.

Blowdown Tank Vent

To discharge steam from the steam boiler, a blowdown vent must be used to prevent water from exiting through the pipe along with the steam. A vent cap is present at the end of the vent pipe, and this pipe is straight.

Advantages of Using a Blowdown Tank

• Maintains the water TDS level within the permissible limit.
• Removal of contaminants from steam and production of impure steam. (Note: This point may require clarification, as blowdown primarily purifies the boiler water to improve steam quality rather than directly treating steam.)
• Prevents corrosion and scaling of steam boiler tubes.

Disadvantages and Notes of the Blowdown Tank

• Reduction of steam boiler temperature and pressure leads to decreased efficiency.
• If not properly maintained and scheduled, water blowdown can lead to reduced heat and pressure losses. (Note: This statement appears to be a mistranslation or error in the original Persian text. Improper or excessive blowdown typically increases heat and pressure losses, rather than reducing them.)