Two Element Drum Level Control

Two element drum level control is one of methods to control boiler especially on drum level. There are 3 methods to control drum level as follow:

-          Single element drum level control

-          Two element drum level control

-          Three element drum level control

The method of two element drum level control is appropriate for operations with average load shifts and rates, and also it could be applied on almost any sizing of steam boiler. This method employs the two parameters, steam flow and drum level to amount balance the requirement of feedwater.

Figure 1: Two Element Drum Level Control
(Source: Courtesy of Honeywell)

Drum level is calculated and the malfunction among the preferred set level along with the real control level is delivered to a instructional math as one among two procedure parameters. Steam flow is calculated and included to the instructional math as the following procedure parameter. The outcome of the instructional math is the manage output to the control valve of feedwater.

Considering that steam flow can be quite dynamic, the outcome of this method is it's going to sense the increase or drop in load requirement previous to the drum level starts to alter. The method subsequently provides or takes away control output to strengthen the response of controller of drum level on the control valve of feedwater. Along with because steam flow is generally greater adjustable it might very easily ignore the trim impact of measurement of drum level on small load alterations, covering a accurate reaction to the requirement change. Throughout steady load the controller of drum level affects the control valve of feedwater and operates to trim the level to the wanted set level.

This method provides 2 side effects that must be considered. First similar to the single element drum level control method, the two element drum level control is unable to adjust load trouble or pressure within the feedwater source, since it is not a calculated in this method. The second consideration is the two element drum level control is unable to remove phasing connections among drum level and feedwater flow since simply the reasonably slow procedure of the drum level can be managed. That second concern can bring on sub-cooled water drum on a huge raise in need by permitting too much feedwater to get into the drum with no thought to the abilities of thermal dynamic on steam boiler.