Single element drum level control is one of methods to control drum or boiler level control. The level control of water/steam is commonly discussed in one among 3 main methods such as single element drum level control, two element drum level control and also three element drum level control. The use of one of these methods is dependent on the certain boiler dimensions (economical) and load changes (design).
Single element drum level control can be probably the most basic method. In this method drum level is calculated employing a single measurement tool as well as offers a control transmission to the feedwater regulator with immediate connection to the existing running drum level. This method is applied in each feedwater control and ON/OFF methods.
Figure 1: Single Element Drum Level Control
(Source: Courtesy of Honeywell)
The single element drum level control method is exclusively efficient for small steam boilers having reasonably constant requirements and also gradual to quite small load alterations. This is mainly because the impact of swell and shrink which leads to an inappropriate preliminary control response, that can bring on under filling down or over filling up of steam drum.
Because steam need grows, there’s a preliminary decreasing of steam drum pressure leading to an unnatural increase inside drum level since the steam bubbles increase and enlarge the water level of drum. This happening directs a wrong control transmission to decrease feedwater flow, if actually the feedwater flow must be greater to manage amount balance.
On the other hand, on a reduction of steam need, there’s a preliminary increasing of pressure of steam drum that serves to decrease the drum level through decreasing steam bubbles and also diminishing the level of drum water. This directs a wrong transmission to maximize feedwater flow if actually it must be reducing to manage amount balance.
Operations encountering immediate or significant load alterations, can certainly lead to ‘phasing’ impact of swell and shrink leading to the controller of water level to get rid of command of drum level along with cause annoyance small water trips or even excessive carry-over and also water priming.