Boiler Level Control

CASE STUDY DRUM LEVEL CONTROL: Water Cycle: The main purpose of most boilers is to heat water to steam. To accomplish this a series of tubes are placed in the boiler to confine the water and provide a large heat transfer surface for interfacing with the hot gases of combustion. The water in these tubes is turned to steam. A large heater is provided on each end of the tubes to distribute the water from Control: The object of the control system is to provide steam at a constant pressure, and to do so safely minimum cost. The control must be flexible enough to react to changes in the amount of steam required and provide steam at an optimum efficiency. The control must provide adequate feed water to exactly offset that drawn off as steam and last as blown down. At the same time the system has to provide adequate fuel to produce the heat needed to convert the water to steam an amount of to completely burn the fuel must be made available. And, off course, all these functions must be ordinate to provide not only maximum efficiency but also safety. The control scheme outs typical and includes sum of the important concepts. FEED WATER CONTROL We all look at the feed water portion of the control scheme. The aim of this part of the control is to maintain quite precisely the liquid level in the boiler drum to maximize the steam water separation maintaining the level is also important to assure that liquid is present in the boiler tubes. This assures the tubes will be cooled by the water as it boils and acts to protect them from over heating in addition, it assures that there will be the water circulation described pervasively the main elements of the control of the drum leveler the measurement of the liquid level in the drum and the control of the feed water flow valve. The obvious control would be to provide the drum level measurement to a level controller whose output regulate the feed water control valve (Single Element) this is not, however, a good solution here the primary reason this does not provide good control is that the level is a slow responding phenomenon that tends to integrate the results of the feed water flow in and steam out. That is a change in the feed water flow very long time to show up as a small increase in the level this is due to the large volume capacity of water in the boiler.

If the steam flow changes, this will affect the level, which in turn will control the feed water valve. As seen before, this is very slow and what we really want to do is to sense a change in steam flow and make a change to the feed water flow. We can do this by providing the steam flow measurement as a feed forward signal in the level controller in this way the feed forward signal from steam will effect the feed water valve for changes in steam flow while the level control accurately maintain the correct level by controlling the feed water to correct for losses and minor changes. This control system is called ” two element feed water control.” There is another phenomenon that happens in a boiler “shrink and swell” that we need to look at when there is a change in steam flow (consider an increased demand and resulting increase in steam flow) the pressure in the boiler and drum will immediately begin to be reduced as the steam is drawn off when these happens the vapor bubbles in the boiler will expand the displacing the water in the boiler which increases the liquid level in the drum calling for less feed water flow the level controller. This is off course, the wrong direction has the feed water flow should be increased to replace the increase the steam drawn off. When the cold feed water enters the drum it cools water in the boiler which condensates the steam bubbles which reduces the column and the reduces the drum level. Again the measurement going the wrong way that is, more liquid is resulting in the measurement indicating the lower level the shrink and swell phenomenon are transects from changes in steam flow and feed water and are relatively short lived. The phenomenon is over come by a Sophisticated scheme called the “three element drum level control system”. Here, feed water flow to the boiler is actually measured and controlled. The flow rate is based on a divine signal, which is the result of both the steam flow, and feed water signal is drum level controller. Now as steam flow increases feed water flow is increased and when flow is decreases less feed water flow is called for the drum level controller will maintain correct level by adjusting the feed water flow demand to correct for losses and minor changes

SINGLE ELEMENT CONTROL

TWO ELEMENT CONTROL

Steam flow DRUM LEVE L

L T

sp

F T

PV

LIC

M/A

DRUM LEVE L

Lt :level transmitter

Ft :flow transmitter Ff :feed forward control m/a:manual/auto pv:process variable sp:set point

L T

LIC

FF

M/A

feed water line control valve

F T

LI L F

C M/A T

F

DRUM LEVE L

THREE ELEMENT CONTROL

Steam flow

DRUM LEVE L

Feed water

F T L T Square root

Level transmitter

F T

pv LIC

Level indicating control

pv

FF

sp LIC

M/A

Ft :flow transmitter Ff :feed forward control m/a:manual/auto pv:process variable sp:set point

feed water line control valve

flow