What Does Blowdown Drum Mean?
A blowdown drum is a collection vessel or tank designed to receive a discharge of liquids or slurries from process vessels or other equipment.
These drums are maintained at very low levels or kept completely empty, and are equipped with relief devices to vent gasses or uncondensed vapors. They serve an important function in process control, as they allow substances to be released in a safe manner, preventing injury and property damage.
Blowdown drums are also known as blowdown tanks or blowoff tanks.
Safeopedia Explains Blowdown Drum
Blowdown is the process of removing water from a boiler using a blowdown valve. This valve removes the dissolved solids from the boiler water and flushes them through the boiler exit port.
Blowdown is associated with fuel-fired boilers common in industry, with heat exchangers, and in cooling towers in industrial processes that use water. This process maintains boiler water parameters within prescribed limits in order to minimize corrosion, erosion, carryover, and other problems.
The Purpose of a Blowdown Drum
Any corrosive element must be removed, including chemicals that can corrode the boiler and affect its steam quality. If not, the boiler system can suffer from scaling, corrosion, brittleness, cracking, foaming, and other issues.
Blowdown control can help achieve this, and the process involves activating the blowdown valve mechanism situated on the boiler drum. A small percentage of the boiler water containing dissolved solids and undissolved sediments is drawn out from below the boiler water's surface and collects in the blowdown drum.
To maintain the chemical balance in the boiler, the quantity of chemicals removed through blowdown must be equal to the quantity of chemicals that enter through the feedwater. Excessive blowdown can lead to inefficient boiler plant operation, as heat is lost every time water is expelled, which is directly related to the cost of fuel.
Implementing Blowdown Control
Instantaneous Manual System
A manual system is operated once per shift to reduce the total dissolved solids (TDS) in the boiler and maintain them at acceptable levels. The TDS is then allowed to build until the maximum level is reached again.
This type of system can be easily implemented, with a relatively low sensor outlay. However, it does not consider load fluctuations and heat recovery from blowdown is both difficult and expensive.
Automatically Timed Control
This type of system uses a timer to control blowdown for short periods, according to a preset schedule. By installing a TDS monitoring facility, the system can be made entirely automatic, overriding the timer in the event of variation from the desired TDS level.
This method is preferred where heat recovery is required. It consists of a valve adjusted after regular boiler water testing. The position of the valve is determined by the boiler pressure, TDS levels, and the required blowdown rate. Blowdown valve modulation is done using a control module with inputs from a TDS detector.
Blowdown Drum Size
The National Board of Boiler and Pressure Vessel Inspectors recommend using a tank with twice the volume of the water that will be removed from the boiler "when the normal water level is reduced by not less than four inches."