Boiler Blowdown Control

Before boiler feedwater is passed into the boiler.

It must be chemically treated to remove the corrosive elements that may be present and would ultimately corrode the boiler as well as affect the quality of steam required within a process.

Chemicals entering the boiler via the feedwater must be removed from the boiler. Failure to do so can result in the boiler system suffering from scale formation, corrosion, brittle and cracking metal, carry-over and foaming.

Therefore a proper chemical balance must be maintained within the boiler itself.

Instantaneous manual system

The simplest implementation of blowdown control is an instantaneous manual system that is operated once per shift to reduce the boiler total dissolved solids (TDS) to a sufficient level well below the boiler specified maximum limit. The TDS are then allowed to build up during the next shift until they reach the maximum level again.

A TDS test should be carried out prior to blowdown so that the time can be adjusted to reflect changes in average boiler load conditions which may occur on a day-to-day basis.

Advantage:

• Easily implemented with relatively low sensor outlay.

Disadvantage:

• Load fluctuations are not taken into account. Heat recovery from blowdown is expensive and difficult.

Automatically timed control

Figure 2 (above) shows a simple semi-automatic system where a timer is used to control blowdown for short periods according to a pre-set schedule. Again, with this system, daily testing of the boiler is necessary so that the timing schedule can be adjusted to take into account changes in boiler and system operation.

The system can be made fully automatic by installing a TDS monitoring facility as pictured in Figure 3. This will override the timer in the event of variation from the desired TDS level.

Disadvantage:

• Standard fully open/closed valve provides coarse control

Continuous control

Continuous blowdown systems are preferable where heat recovery is required. In its simplest form, such a system consists of a valve, adjusted after regular boiler water testing. The valve position is determined from the boiler pressure, TDS levels and the blowdown rate required.

As shown in Figure 4, a control module is used to modulate the blowdown valve using inputs from a TDS detector located in the cooled blowdown sidestream. For this system to operate correctly, cooled blowdown must flow continuously over the detector.

Advantage:

• Smaller and cheaper heat recovery plant
• Possibility of recovering up to 50% of the heat in the blowdown