The main purpose of the water level controller is to bring the drum up to level at boiler start-up and maintain the level at all time. A sudden decrease in this level may uncover boiler tubes, causing them to become overheated and damaged. Boiler high water level may interfere with the process of separating moisture from steam(i.e. carryover of saturated steam) within the drum, thus reducing boiler efficiency and carrying moisture into the turbine.
Before going further let’s go through the feed water system for the boiler. Two types of feed water system are shown here:
- In this system, two Feedwater pumps are there(one is standby for other). A three-valve is provided which controls the amount of feed water entering the boiler. When a large quantity of water is required, because of a sudden increase in steam demand, this valve will direct water to the boiler. And if feed water is not required, this three-way valve will recirculate water through feed pump. This system is usually required in large boilers, where the requirement of steam varies with operation e.g. for turbine use. In this system feed, the water pump will run continuously.
- In this system, two Feedwater pumps are there(one is standby for other) and no three-way valve. This system controls the water level of the boiler with the starting and stopping of the pump. The interval of starting pump will depend on steam demand if steam consumption is more feed water pump will start frequently. It is used for small boiler, where the steam requirement is less and steady.
All explanations and diagrams below are according to the feed water system having a three-way valve. The system 2 having no three-way valve, the level controller will control the starting and stopping of the feed water pump. There is three different types of Boiler water level controller. Anyone can be used depending upon steam system requirements:
- Single element drum level control
- Double element drum level control
- Three element drum level control
Elements are the input signals, which dictates how much water is required for a stable boiler water level. There are three elements which we use in this system:
- Boiler water level float(or drum level transmitter) signal
- Mass flow rate signal of steam
- Mass flow rate of feed water
Single Element Drum Level Control:
This is the simplest form of drum level control and used only for small boilers, where the production of steam is less and constant.
This consists of a proportional signal(element) coming from the drum level transmitter. This signal is compared to an input setpoint of water level and the difference between these two is a deviation value. This signal is an input valve for the controller which will generate corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flows into the boiler drum.
Salient features of Single Element Drum Level Control:
- Only one analog input and one analog output required
- It is applied to single boiler & single feed pump configurations with relatively stable loads since there is no relationship between drum level and steam- or feedwater flow
- Possible inadequate control option because of the swelling effect of Boiler
But this one element water level control system is unable to control the level in big boilers or boilers where the steam production is fluctuating rapidly. Due to a sudden increase in steam load will produce a drop in the pressure in the steam drum, because, the firing rate cannot increase fast enough to match the steam production rate at the new demand level. When the drum pressure drops, it has a dramatic effect on the natural convection within the boiler. A small fraction of the saturated water in the boiler to immediately vaporize because of the drop in drum pressure, producing a large amount of boil-up from most of the tubes in the boiler. During the transient, most of the tubes temporarily become risers (which takes water & steam from water drum to steam drum). Because of this, the level in the steam drum rises.
However, this rise in level is actually an opposite response to the load change. In simple terms, the boiler water level should decrease when the steam production rate increases. At that time the total steam draw rate has gone up, the net flow of water to the boiler needs to increase because the total mass of water in the boiler is falling. However, the level controller senses a rise in the water level of the steam drum, therefore controller calls for a reduction in the flow of feed water to the boiler.
This inverse response to a sudden load increase is dynamic swell and known as Swelling of Boiler. The opposite effect will take place, when there is a sudden drop in steam consumption, This effect is called as Shrinking of Boiler. At shirking of the boiler the water level transmitted by the level indicator is less than actual, therefore counteraction by feed valve controller will be to increase the feedwater flow.
Double Element Drum Level Control:
The two-element drum level controller applied to a single drum boiler where the feedwater is at a constant pressure. The two elements of this control system are made up of the following:
Drum Level Element: a proportional signal coming from the drum level transmitter. This signal is compared to a setpoint of water level and the resultant is a deviation value. On the basis of this signal, the controller will generate corrective action in the form of a proportional value.
Steam Flow Element: a mass flow rate signal from flow sensor on steam outlet line (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to fluctuation in the amount of steam consumption(load change).
Working as explained in Three Element Drum Level Control, except the part of the feed water flow transmitter
Salient features of Double Element Drum Level Control:
- Efficient control of drum level than with only one element
- Faster level adjustments
- Can best be applied to a single boiler & single feed pump configurations with a constant feedwater pressure
Three Element Drum Level Control:
This above-defined control system is not suited for a steam system where consumption is very high and fluctuating. The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers (as in case of tankers) and multiple feedwater pumps or where the feedwater has variations in pressure or flow(if connected with turbines etc. sudden rise and fall of the load are very frequent). The three-elements are made up of the following:
- Drum Level Element: a proportional signal coming from the drum level transmitter.
- Steam Flow Element: a mass flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes.
- Feedwater Flow Element: a mass flow rate signal on the feed water pipe, responds rapidly to variations in feedwater demand.
Any difference between steam mass flow and feedwater mass flow into the drum is corrected by the level controller. This difference between these flow rate can arise from
- Blowdown of boiler
- Leaks in the steam or sudden increase in steam consumption
- Feedwater pressure or flow fluctuations
Working of Three Element Drum Level Control:
The signals from the 3 elements can be transmitted to indicators and /or recorder if desired, also a hand/auto station enables the operator to the controller the position of the feed valve remotely if required.
The elements steam flow, feed flow and level indicator, each of which sense out a pneumatic signal proportional to the change in its measured variable. The relay compares the two signals from the steam flow and feed water flow transmitters and if these are in the direct ratio e.g. 1:1, the system is stable. But if no continuous blowdown, it transmits a signal to the P+I controller corresponding to the desired level of the water in the drum. This signal is compared with the measured valve being received by the controller from the level transmitter, any deviation will result in the change of controller output to the valve position.
Considering an increase in steam flow this would lead to a reduction in drum pressure and rising water level. When the steam flow increases the feed valve will be open to increase feed flow despite the fact that the water level has risen. However, the water level will gradually return to its described valve and a new boiler throughput exist. If the control system where signal element i.e level control only, then upon steam flow increasing and water level rising the feed valve will close in.
In order to achieve optimum control, both steam and feedwater flow values should be corrected for density, i.e. why square root converter is provided.
Salient features of Three Element Drum Level Control:
- The three-element system provides tighter control for drum level with fluctuating steam load. This system is ideal for fluctuating feedwater pressure or flow.
- More sophisticated level of control
Enhanced three element drum level control
Nowadays this system is in use, it is more responsive than the above one. The enhanced three-element drum level control module includes the standard three element level components with the following improvements:
- The three-element mode is used during high steam demand. In case of failure of the steam flow system, it switches to the two-element mode and also the module falls back to single element level control if the feedwater flow measurement fails or if there is a low steam demand.
- The drum level can be controlled from up to three independent transmitters and is density compensated for pressure within the boiler drum.
Salient features of Enhanced Three Element Drum Level Control:
- More accurate control through a choice of control schemes. Drum level maintained on the failure of steam or feedwater flow measurements
- This system includes an additional level control loop Boiler drum level control