For safe and efficient boiler operation, a constant level of water in the boiler drum is required to be maintained.
Drum Level Control Systems are used extensively throughout the process industries and the Utilities to control the level of boiling water contained in boiler drums on process plant and help provide a constant supply of steam. If the level is too high, flooding of steam purification equipment can occur. If the level is too low, reduction in efficiency of the treatment and re circulation function. Pressure can also build to dangerous levels.
A drum level control system tightly controls the level whatever the disturbances, level change, increase/decrease of steam demand, feedwater flow variations.
In the process industries, boiling water to make steam is a very important procedure. The control of water level is a major function in this process and it is achieved through a water steam interface established in a cylindrical vessel called the drum which is usually lying on its side and located near the top of the boiler. Providing tight water level control in a drum is accomplished by utilizing one of three types of drum level control: single-element, two-element, or three-element. This article discuss only about 3-element drum level control.
It handles loads exhibiting wide and rapid rates of change. Plants which exhibit load characteristics of this type are those with mixed, continuous, and batch processing demands. It is also recommended where normal load characteristics are fairly steady; but upsets can be sudden, unpredictable and/or a significant portion of the load. In the figure below, the control scheme for three-element drum level control.
Steam flow is measured by the steam flow transmitter (FT-1), its signal is fed to the feedwater flow computer (FY-3) after processing through the square root extractor (FY-1). The drum level is measured by the level transmitter (LT-1) and its signal is transmitted to the drum level controller (LC-1). In the drum level controller, the process signal is compared to the drum level set point, where a required corrective output signal to maintain the drum level is produced. This corrective signal is sent to the feedwater flow computer (FY-3). The feedwater flow computer combines the signal from the two variables, and produces an output signal to FIC-2. Feedwater flow is measured by the transmitter (FT-2). The output signal of the feedwater flow transmitter is linearized by the square root extractor, (FY-2). This signal is the process variable to the feedwater controller (FIC-2) and is compared to the output of the feedwater flow computer (FY-3) which acts as set-point. The feedwater flow controller (FIC-2) produces the necessary corrective signal to maintain feedwater flow at its set-point by the adjustment of the feedwater control valve (FCV-1). All of the work necessary to compensate for load change is done by the feed-forward system (i.e. a pound of feedwater change is made for every pound of steam flow change). The drum level portion of the control scheme is used only in a compensating role. Despite low-to-moderate volume/ throughput ratio and a wide operating range, it is expected the drum level will be maintained very close to set-point. Achieving this requires use of the integrating response and reset in both the drum level and feedwater controllers.
FT-1, FT-2 and LT-1 may be the same type of transmitter.Auto/Manual transfer of the feedwater control valve is accomplished via FK-1.This means stocking only one type of transmitter in the case of a transmitter failure. In order to understand this, consider an example as illustrated below. In this case, I have ignored that the flow transmitter is of square root and it is taken as linear. So please ignore the square root extractor in the below explanation for simplicity.
I have also ignored the complex calculation in the controller and flow meter station and controllers regarding error calculation. Also I have considered this to be a proportional controller. Integral and differentiating controllers are considered if the load is varying abruptly, rapidly and over a period of time. Here I am avoiding time variable (integral and differential part) for simplicity of presentation.
Consider the drum level is at a stable level of 30% with feed water flow of 30% of its span or range. Now the load of the steam is stable and is supplying a stable load flow of 30% of its range. Now the control valve is having 30% opening. This is a stable condition and needs no controlling.
Consider steam consumption is increased by 20% from 30%. Now the steam flow is 50% as shown in step 1.The level of the steam drum is still 30%. These two values are fed to the FY 3. This feedwater flow computer FY3 uses a summing relay and controller, subtracts the level value 30% (in percentage) from the steam flow value 50% (in percentage) as shown in step 2. Thus 50%-30% gives us a value of 20%. This value of 20% acts as a set point to feedwater flow controller (FIC-2).Also the feed water flow transmitter output FY 2 is having 30%. The feedwater flow controller (FIC-2) uses a summing relay and controller which adds the output from FY2 and FY3. Thus the output of FIC 2 will be 20% + 30% = 50% as in step 3. This 50% acts as a command to the control valve to open 50% as in step 4, thus more feed water enters the steam drum to compensate for the increase in load as in step 5.
Consider steam consumption is decreased to 20% from 30% as shown below. Now the steam flow is 20% as shown in step 1.The level of the steam drum is still 30%. These two values are fed to the FY 3. This feedwater flow computer FY3 uses a summing relay and controller which subtracts the level value 30% (in percentage) in percentage from the steam flow value 20% (in percentage) as shown in step 2. Thus 20%-30% gives us a value of -10%. This value of -10% acts as a set point to feedwater flow controller(FIC-2).
Also the feed water flow transmitter output FY 2 is having 30%. The feedwater flow controller (FIC-2) uses a summing relay and controller which adds the output from FY2 and FY3. Thus the output of FIC 2 will be -10% + 30% = 20% as in step 3. This 20% acts as a command to the control valve to close to 20% as in step 4, thus less feed water enters the steam drum to compensate for the decrease in load as in step 5.
Thus in every case the level of the steam drum is maintained at a safe level.
Please note that in actual industrial applications the op-amp and micro controller based controller works in an entirely different manner. This article is made simple for getting a basic knowledge of three element control. I have also ignored the integral and differential part of PID controller