Three element drum level control is designed to correct the concerns of phasing even now found in the two element drum level control and single element drum level control method, a 3rd element on feedwater circulation is included to control method of drum level. In that method the instructional math result of the controller on two element drum level control is streamed down to a 2nd controller of feedwater flow to react as a remote set level.
|Figure 1: Three Element Drum Level Control|
(Source: Courtesy of Honeywell)
The controller of feedwater is a rapidly performing circulation controller that utilizes feedwater flow as the procedure parameter and steam flow as the set level. Therefore for each pound of steam pass leaving steam boiler single pound of feedwater is put in. This cycle provides finalized control on the valve of feedwater. Since the remote set level via the two element drum level control variations by using drum level changes and steam flow because of blowdown or various other small losses, the controller of feedwater sets the output to control the required feedwater flow to maintain the drum level especially about level condition, heat balance and mass/amount balance.
In three element drum level control, unmounted tuning of every controller permits for quite accurate drum level control no matter of steam need and also feedwater affects. Then again it truly is pretty necessary to get the thermodynamic of the steam boiler’s recuperation rate into consideration if tuning the controller of feedwater. In case the operator fail to perform this and arrange the controller totally reset too quick on the controller feedwater the operator may accidentally sub-cool the steam drum and also begin a cyclic phasing steps similar to which found in two element drum level control method. Perform the instructional math it will make the steam boiler more secure.
The three element drum level control method could simply manage significant and fast load alterations mainly because it's related the mass/amount balance among the feedwater flow to this method and also the steam flow coming from steam boiler. This method is a must on several boilers giving the similar supply process and feedwater header, because of the changes in the offered feedwater flow to anyone steam boiler, although several boilers are running. This can be regardless to steam boiler dimensions.
Furthermore in case the steam boiler is put through to immediate or unstable need variations like in a batching procedure, the method of three element drum level control has the ability of matching this requirement with no operator trim guidance and correction.