All the methods of level detection described so far can be used to produce an on/off signal for level control. The most common method of level control is simply to start the feedpump at a low level and allow it to run until a higher water level is reached within the boiler.
• With a float level control, a magnetic switch with a built-in hysteresis or dead-band will be used.
• With conductivity probes, two probes are necessary, (pump on and pump off) which will give fixed switching levels.
• A capacitance probe can be used to give adjustable on/off switching levels.
In the UK, on/off type control is almost universal on boilers below about 5 000 kg/h steam generation rate because it is the least expensive option. (In Australia and New Zealand, standards state that for boilers exceeding 3 MW (typically 5 000 kg/h), modulating control must be fitted).
It can be argued, however, that this type of on/off control is not ideal for boiler control, because the relatively high flowrate of ‘cold’ feedwater when the pump is on reduces the boiler pressure.
This causes the burner firing rate to continuously vary as the pump switches on and off.
Taking a typical example, it can be shown by calculation that even with feedwater at 80°C, the burner firing rate may have to be 40% higher with the feedpump on, than with the feedpump off.
This continuous variation causes:
• Wear on the burner controls.
• Temperature cycling of the boiler.
• Reduced efficiency.
• A ‘saw-tooth’ type steam flowrate as depicted by the chart recorder shown in Figure 3.17.2.
If steam loads are high, the variable steam flowrate will tend to increase water carryover with the steam, and will tend to make water levels increasingly unstable with the associated danger of low water level lockout, particularly on multi-boiler installations.
However, the fact remains that on/off control is very widely used on boilers of small to medium output, as defined above, and that many problems associated with steam boilers operating with large swings in load are due in part to on/off level control systems.
Summary of on/off level control
• Good for boilers on stand-by.
• Each boiler requires its own feedpump.
• More wear and tear on the feedpump and control gear.
• Variable steam pressure and flowrate.
• More boiler water carryover.
• Higher chance of daily operating problems under large load swings.
In this type of system the feedpump runs continuously, and an automatic valve (between the feedpump and the boiler) controls the feedwater flowrate to match the steam demand.
When operating correctly, modulating control can dramatically smooth the steam flowrate chart and ensure greater water level stability inside the boiler.
For modulating level control, the following methods can be used to sense the water level:
• Floats with a continuous signal output.
• Capacitance probes.
• Differential pressure cells.