Steam Drum Level Controlling System Single element , Double element, Three element:-
Boiler drum level control system element single element , double element & three element Steam drum is very important part of any steam generation unit or boiler because steam drum is continuous feed the water to water wall panel for the steam generation and also help in the steam purification. And also protect the boiler tubes from the starvation during tripping of steam generation unit .Water-level controls continuously watch out the level of water in a steam boiler in order to control the flow of feed water into the boiler and to protect against a low water condition in any emergency which may expose the heating surfaces with consequent damage. The control may be float operated but modern plant will have conductivity probes like hydra-step. The probes will be fitted in pads or standpipes on the crown of the shell of hydra step or drum and enclosed in a protection tube which will extend to below the lowest water level. With water tube boilers the control of the water level needs to be precise and sensitive to fluctuating loads due to the high evaporative rates and relatively small steam drums and small water content.
Single Element control
In single element control using only the drum level is applied during the low load (< 20-25% boiler load). before we talk about the control theory of single element control , we tells about the (SHRINK and SWELLING) how to happened inside boiler drum ,whenever you startup firing the boiler during the cold startup .
Whenever shrink and swell effect happening in a steam drum let us consider a sharp increase of steam consumption. With the sudden increase in the steam demand the steam drum pressure drops immediately. With the sudden drop in the pressure inside the steam drum the steam bubbles in the water wall panel and the drum swell and results in a sharp momentary increase in the drum level. After the pressure stabilizes the drum levels behaves in a stabilized manner. This initial increase in the level is called swell and it is unique to the steam drum.
In similar manner when the steam demand reduces suddenly, the drum pressure rises immediately inside the steam drum and the steam bubbles in the water wall panel and also inside the steam drum shrink. This result to a sharp momentary decrease in the drum level. This initial decrease in the steam drum level is called shrink.
In single element control method is based on the output of a level controller cascades into a flow controller. Consider now the use of a conventional single element control strategy to control the steam drum level. As the drum level increases the controller reduces the feed water supply. And similarly, if the drum level decreases the controller increases the feed water supply. Let us assume that the steam consumption increases suddenly due to increasing in load .its result that swelling effect is happening inside the steam drum level will raise initially and then decrease. The controller will reduce the feed water supply initially. This will in effect reduce the water inventory and after the swell effect the water drum level will drum significantly.
Disadvantage of one element control system:-
Therefore we use only this system structure at (20 to 25 % load) , and we have to take care of drum level by controlling the start up blow off valves. So, to eliminate such this problem, another control scheme is applied to eliminate the drawbacks of one-element control at higher loads.
Double element control system:-
In double element level controlling the swelling and shrinking effect of boiler drum is taken care. Pressure of the drum varies according to steam flow. if load increases and drum pressure drops . In two element controller this effect is compensated by measuring steam flow along with drum level. Steam flow will give incorrect feed back if it is not companensated suitably according to stem pressure and temperature. Flow of steam has direct relationship with pressure and temperature. So in most of the causes measurement of steam flow is compensated by pressure and temperature. this type of controller is used in medium size boiler with moderate load variation.
Three element control system
Three level control system is best operated when boiler load > 20-25% boiler
In three element control system mainly three things are considered they are
– Drum level
– Main steam flow
– Feed water flow
This type of controller is widely used for controlling the drum level inside the boiler operation process. This controller can take care of any type of boiler and any type of load variation. Normally in this case the feed water flow and steam flow are pressure and temperature compensated. In two element controller, controller does not consider how much feed water is entering into the boiler. So the controller action is no smooth . an ideal controller should taken in to account how much feed water is flowing through the boiler , so that furthered corrective action can be initiated . Boiler feed water from feed pump is used for final steam temperature control. Sometime this flow is also considered in three element controller for smooth functioning.
In three element controlling situation, the steam flow rate should also be considered for drum level control. It can be done by adding the steam flow rate as a feed forward signal to the output of the level controller. So that the feed water flow is compensated for changes in the steam flow rate demand. With this strategy as the steam flow rate changes the demand for the feed water flow rate also changes in the right direction and minimizes the effect of shrink and swell on the drum level.
Let us suppose that the steam demand increases suddenly. As the steam demand increases the feed forward signal increases the feed water supply to the steam drum. Due to the swelling effect the level controller reduces the feed water supply. In three-element level control scheme changes the feed water supply appropriately and reduces the effect of swell on the drum level. Thus, the three-element level control strategy provides a more stable drum level control.
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