Automatic Blowdown in Steam Boilers
Blowdown, is the process of removing some of the boiler water from the boiler drum in order to reduce the amount of dissolved or suspended solids.
The concentration of solid particles entering the boiler together with the feed water, increases considerably with the evaporation of the boiler water.
Solid particles dissolved and suspended in the boiler water are important for the control of the boiler water quality. The TDS level should remain within the limits specified by the boiler manufacturer and applicable guidelines. For this purpose, a certain amount of boiler water is discharged continuously (surface blowdown) and periodically (bottom blowdown).
High amount of dissolved solids (TDS) can cause many problems such as water carryover and scale formation:
The scale is mainly caused by the presence of calcium and magnesium salts (carbonates or sulphates), which are soluble in heat, or a very high concentration of silica relative to the alkalinity of the water in the boiler.
The scale layer is caused by the precipitation of impurities coming from the feed water directly in the boiler on the heat transfer surfaces, or from the substance suspended in the water, which settles on the metal and becomes hard and sticky. Evaporation in a boiler causes an increase in the concentration of impurities. This prevents heat transfers and leads to overheating in the boiler pipes. The less heat they transmit on these surfaces, the more hazardous there is.
Blowdown cannot eliminate the problem of formation of scale layer alone, but it can keep the boiler water conductivity and quality at the desired level. It helps to prevent the formation of the scale layer by chemical dosing of the feed water.
No mechanical treatment should be performed on the scale formed surfaces. Chemical flushing should be done.
The high concentration of dissolved and suspended solid particles not only prevents heat transfer, but also causes the boiler water to foam. This can lead to the transport of solid particles and water along with the steam to the pipeline. Water Carryover causes both the quality of the steam to decrease and can cause great damage to the fixtures in the installation.
Automatic Surface Blowdown (TDS Control) System
Automatic Surface Blowdown Control system measures the conductivity in the boiler water with the conductivity probe and compares it with the maximum conductivity value set on the controller and tries to keep the conductivity of the water in the boiler constant by opening and closing the continuous blowdown valve. This is the most efficient system for surface blowdown in steam boilers.
If the conductivity (µS / cm) value is above the maximum allowable (set) conductivity value, the boiler water is blowdown until the conductivity value falls below the set value. If the boiler water conductivity value is below the set value, the blowdown valve maintains its closed position.
Automatic Bottom Blowdown System
Oil, rust, dirt and salts that do not dissolve in water, which may come from the installation, settle to the bottom of the boiler and form sludge. The automatic bottom blowdown system is used to discharge this sludge that accumulates at the bottom of the boiler. The bottom sludge will prevent heat transfer in the boiler and cause inefficient operation. This can cause mechanical collapse and even explosions. The recommended bottom blowdown time is 3-5 seconds every 8 hours.
At the set time and time interval in the controller, the bottom blowdown valve opens and the sludge settled at the bottom of the boiler clears. The feature that distinguishes Vira BK 4000 controller from ordinary timers is that it can be received an alarm if the valve remains open or closed.
Not: BS4-T Automatic Surface Blowdown System and DB2 Automatic Bottom Blowdown System are suitable for working in steam boiler plants according to TRD 604, EN 12952 and EN 12953.