Boiler C**trol (Feed water C**trol

Khaled Hamza · #1 12-24-2010, 06:40 PM

Boiler C**trol Theory (Feed water C**trol)
The feed water is c**trolled to keep the drum level .

Water-level c**trols c**tinuously m**itor the level of water in a steam Boiler in order to c**trol the flow of feed water into the boiler and to protect against a low water c**diti** which may expose the heating surfaces with c**sequent damage. The c**trol may be float operated but modern plant will have c**ductivity probes. The probes will be fitted in pads or standpipes ** the crown of the shell or drum and enclosed in a protecti** tube which will extend to below the lowest water level.
With watertube boilers the c**trol 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 c**tent.

(1) **e element c**trol
**e element c**trol using **ly the drum level is applied during the low load (< 20-25% boiler load).
before we talk about the c**trol theory of **e element c**trol , we have to show the (SHRINK and SWELLING) phenomena which is happened at boiler drum ,whenever you start firing the boiler:-

To illustrate the shrink and swell effect in a steam drum let us c**sider a sharp increase of steam c**sumpti**. With the sudden increase in the steam c**sumpti** the steam drum pressure drops immediately. With the sudden drop in the pressure the steam bubbles in the water wall 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 c**venti**al manner. This initial increase in the level is called swell and it is unique to the steam drum.

Similarly, when the steam c**sumpti** reduces suddenly, the drum pressure rises immediately and the steam bubbles in the water shrink. This leads to a sharp momentary decrease in the drum level. This initial decrease in the steam drum level is called shrink.

In a c**venti**al **e-element c**trol strategy the output of a level c**troller cascades into a flow c**troller. C**sider now the use of a c**venti**al **e-element c**trol strategy to c**trol the steam drum level. As the drum level increases the c**troller reduces the feed water supply. And similarly, if the drum level decreases the c**troller increases the feed water supply. Let us assume that the steam c**sumpti** increases suddenly. Due to the swelling effect the steam drum level will rise initially and then decrease. The c**troller will initially reduce the feed water supply. This will in effect reduce the water inventory and after the swell effect the water drum level will drum significantly.

This is a disadvantage of **e element c**trol system , therefore we use **ly this system structure at (20 to 25 % load) , and we have to take care of drum level by c**trolling the start up blow off valves. So, to eliminate such this problem , another c**trol scheme is applied to eliminate the drawbacks of **e-element c**trol at higher loads.

(2) Three element c**trol
Three element c**trol using the followings is applied during the normal operati** (> 20-25% boiler load )
- Drum level
- Main steam flow
- Feed water flow

In order to handle the situati**, the steam flow rate should also be c**sidered for drum level c**trol. It can be d**e by adding the steam flow rate as a feed forward signal to the output of the level c**troller. Hence, the supply of 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 directi** and minimizes the effect of shrink and swell ** the drum level.

Now, let us assume that the steam c**sumpti** increases suddenly. As the steam c**sumpti** increases the feed forward signal increases the feed water supply to the steam drum. Due to the swell effect the level c**troller reduces the feed water supply. The net effect of the three-element level c**trol scheme changes the feed water supply appropriately and reduces the effect of swell ** the drum level. Thus, the three-element level c**trol strategy provides a **** stable drum level c**trol.

bigleve2100 · #2 08-12-2011, 06:09 AM

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