Level Control Systems

Boiler drum level control systems are widely used in industrial boilers to control the level of boiling water in boiler drums and to help ensure a continuous supply of steam.

If the level is too high, water can be carried over to the steam line, resulting in low dryness percentage of steam.

A level control system tightly controls the level in the event of an increase or decrease in steam demand regardless of the feedwater flow variation.

A tight water level control in the drum of a boiler can be achieved by using one of these systems.

  • Single Element Level Control System
  • Two Element Level Control System
  • Three Element Level Control System

Maintaining the correct water level in the drum is critical for many reasons. Too high water level means transport of water and impurities to the steam line and poor quality steam production. In addition, possible water hammer hits and damage to equipment can be very costly.

Too low water level will cause boiler deformation and even collapse due to overheating in the fire tubes. Normally, the water level in the drum is expected to be kept within 2 to 5 cm of the normal water level point and with some tolerance for temporary load changes.

Factors Effecting Drum Level         

In boiling conditions, steam bubbles are located below the water surface and have a volume. This situation may increase the water level in the boiler and cause an incorrect level information to be received. Another effect on the water level in the drum is the pressure.

The bubbles expand or shrink as the load demand changes. A higher steam demand will cause the pressure of the water in the drum to drop and the steam bubbles to expand to give the appearance of a higher water level than they actually are.

This imaginary high water level causes the feedwater to be closed at a time when more water is needed. The increase in water level as a result of the drop in drum pressure is called "swell". The decrease in the water level caused by the increase in drum pressure is called "shrink".

Drum Level Control Strategies

The single element level control system is a useful method for constant load conditions. However, it cannot respond quickly enough when load changes are more frequent, unpredictable, or severe.

More information should be input and processed to predict the amount of water to be added to the drum to compensate for sudden load changes. The addition of some information providing elements to the system (such as flow meter and transmitter) allows the controller to estimate the amount of water added to keep the drum water level within a specified range.

 

Single Element Level Control

This is a very simple drum water level control system. It requires a single analog input to operate and provides a single analog output in return. This is why it is called a single element drum level control. Because there is no relationship between the drum water level and the steam or feedwater flow, it can only be applied with a single feed pump in a single boiler providing a relatively stable load.

Its performance is ineffective compared to two element and three element level control systems. Example usage method is shown in the image below.

This system consists of a proportional signal coming from a level transmitter. This signal is compared with a previously set point and the difference found as the deviation value. This signal is passed to the feed water valve by the controller as a proportional output. Thus, the flow level of the feed water is adjusted by the valve.

Notes:

  • Only one analog input and one analog output are required.
  • Since there is no relationship between the drum level and the steam or feed water flow, it can only be used in single boiler - single feed pump structures with stable loads.
  • It will be insufficient in places where swelling may occur.

Two Element Level Control

This drum level control design is suitable for single drum boilers with constant pressure of the feed water.

The two element drum water level control system includes the same level element and feed valve used in the single element configuration, but has an additional steam flow meter element providing a density corrected (by the controller) mass flow signal to control feedwater flow,

The two-element drum water level control system can be applied to a single drum boiler where the feed water is at a constant pressure.

The presence of double control elements in such a system is to provide a constant drum level control.

Example usage method is shown in the image below.

Notes:

  • More accurate control of drum water level than a single element
  • Steam flow acts as a forward feed signal to allow faster level adjustments.
  • Can be applied to single boiler / single feed pump configurations with constant feedwater pressure.

 

Three Element Level Control

The three element drum water level control is suitable for the use of variable feedwater pressure or multiple boilers with multiple feedwater pumps.

In this design, three elements are used to control the level, steam and feedwater flow, respectively. This system offers much better and improved drum water level control than any other system.

For the best control, the correct flow values of both steam and feed water in terms of density must be maintained.

Example usage method is shown in the image below.

The 3 Elements Drum level control system is suitable to be used when there are more than one boiler and more than one pump or when the feed water is not constant in terms of pressure and steam consumption.

3 Elements Level Control System Components :

  • Level Element
  • Steam Flow Meter
  • Water Flow Meter and Feedwater Valve

The level element and steam flow meter correct the errors that cannot be measured in the system due to blowdown or leakage from the boiler pipes.

The feed water flow meter, together with the feedwater valve, responds quickly to changes in feed water demand.

In order to achieve optimum control, steam and feed water flow values should be corrected with density.

Notes:

  • The 3 Element drum level control system is an effective and accurate method for fluctuating steam loads.
  • Ideal where a system suffers from fluctuating feedwater pressure and flow rate.
  • A more advanced level of control is required.