Blowdown control system

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April 2006 PLCs, DCSs & Controllers

Because water is such a good solvent, boiler feedwater can contain a high level of dissolved salts and minerals, even if a feedwater treatment plant is installed. Water treatment can chemically modify the form of the dissolved solids, but will not usually remove them completely, and can even increase the total dissolved solids (TDS) level.

As steam is evaporated these dissolved solids are concentrated in the boiler, increasing the TDS level. If the TDS is allowed to get too high then carryover of the boiler water with steam will occur. This can cause serious damage to the steam system through water hammer, corrosion and deposits on heat transfer surfaces. Deposits in control valves and steam traps can build up rapidly, causing operating problems. Product contamination and reduced heat transfer rates may affect productivity.

In order to limit the TDS to the level recommended by the boiler maker, it is normal practice to periodically drain off, (or 'blow down') boiler water and allow it to be replaced by relatively low TDS feedwater. Excessive blowdown is of course very expensive in terms of lost energy and water treatment chemicals.

The Spirax-Sarco BCS3 automatic control system is available with one of three types of controllers to solve these problems. The system minimises the amount of blowdown required, reducing energy and water treatment costs. It also avoids excessively high TDS levels and associated carryover. The BCS3 system is suitable for shell and water tube boilers and can be linked to energy/building management systems. The system provides automatic control of boiler TDS and avoids carryover problems due to high TDS levels. Remote indication of the operating conditions via the high TDS alarm and 0 to 20/4 to 20 mA outputs are offered. Probe cleaning plus compensation for scaling on the CP32 conductivity probe gives accurate TDS control.

The BC3200 is a wall-mounted controller with a four-digit LED display and two pushbuttons for simple commissioning, calibration and operation. It operates on 230 V or 115 V, 50-60 Hz selected by an internal switch. Selectable ranges such as ppm or µS/cm are available to suit all types of boilers.

With the same features as the BC3200, the BC3210 controller has an enclosure for mounting in a boiler panel. The BC3100 is a DIN rail or chassis plate-mounting controller with LED indication of the blowdown status, and potentiometer adjustment of set point, calibration and alarm levels.

For more information contact Richard Ransome, Spirax-Sarco, 011 929 6344, [email protected]

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Blowdown control system

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