A new level of sophistication

Automatic level controls in the boilerhouse can provide process companies with a solution to tighter safety expectations and cost restraints on manpower. Murdo MacDonald, technical manager of Spirax Sarco investigates

One solution to this problem is to use the latest automatic level control technology. Providing continuous monitoring and measurement of boiler water levels, level controls are ideal for ensuring safe boiler operation with minimal manpower.

The primary function of level controls is to ensure sufficient water is fed to the boiler to meet demand. If the water level falls too low, the heating surfaces could become exposed and the boiler would overheat. Too high, and water could be sucked into the steam, leading to poor quality steam that could impair heating or production efficiency.

To control the water level in the boiler, you must first be able to measure it. Unfortunately, this is not always straightforward. Even at low, steady steam generation rates, when conditions are most stable, there is still considerable water movement and turbulence.

The job of the modern level control probe is to accurately measure the amount of water in the boiler under these conditions.

There are two main types of electronic level probe, conductivity and capacitance. Conductivity probes are cut to length during installation to give the required pump on/off switching levels and level alarms. They work by measuring the change in resistance between the boiler water level and the probe tip.

Capacitance probes on the other hand provide continuous level detection, comprising an immersed conductive rod insulated from the fluid. The capacitance between the rod and the earthed vessel changes according to the length of probe immersed in order to provide an accurate measure of fluid level. A typical modern system will comprise one or more level probes linked to level controllers, which operate valves or pumps to control the feedwater supply to the boiler.

A common method of ensuring sufficient feedwater to meet steam demand is through on/off level control, with a feed pump used to maintain the boiler at the required level.

Using conductivity or capacitance probes, on/off level control provides a cost-effective solution for many applications where there is a steady load, such as small boilers or fluid feedtanks.

In larger applications where demand varies however, on/off control can cause variation of the steam output rate. Adding cold water leads to a fall in the boiler water temperature, resulting in cycling of the steam output and reduction in the boiler pressure, increasing the risk of boiler lockout.

A more effective method is modulating control, where the water level is maintained by a proportional controller acting on signals from a capacitance probe. The controller regulates a valve in the boiler feedwater line, with a continuously running feed pump.

Although standard single element control systems such as these will meet the majority of applications, they will often be inadequate for large boilers subjected to very sudden increases in steam demand.

The solution is to use two-element control, utilising a steam meter to measure the boiler’s output and ensure the feedwater flowrate is matched to steam demand.

Under certain conditions where boiler feedwater pressure varies considerably, three-element control, comprising outputs from both a steam meter and a water meter, can be used to smooth out operation.

As well as maintaining safe levels of water in the boiler, level control systems provide a variety of alarms to warn operators of high or low water levels. These alarms will automatically shut down the boiler when dangerous levels are reached.