Boiler Level Control
A regulated supply of water must exist for a boiler generating steam. With the system in balance without leaks the flow of water to the boiler will be equal to the flow of steam. When there is a change in steam demand then the flow of water will have to be changed.
The water level is measured using a suitable device. This level is sent to the controller as a measured value. The controller compares this measured value against a set point. Any deviation from this set point caused a demand signal to be generated.
A single element controller is considered suitable for only the smalest of boilers. The reason for this is the effect of stepped load changes
Should there be a sudden demand for increase steam flow there will be a requirement to increase the firing rate. The increased firing rate increases the ebullition or bubble formation in the water and the volume of water increases or swells. This will be seen as an immediate increase in the water level.
For a single term controller the tendency will be to close the feed supply valve. However, the increased firing rate will generate more steam flow from the boiler and in reality an increase in water flow would be required
For a stepped load increase there will be tendency for the water level to rise due to swell as described. The Water level controller will give a reduced demand signal. The steam flow transmitter will detect an increase in steam flow and send an increase demand signal via a conditioner which adjusts this signal to suit the system. These two signals pass through a Summator. The resultant effect will prevent any immediate sharp change in the demand signal to the feed flow control valve.
Variations in feed pressure will vary the true feedflow rate to valve demand signal. For improved control an additional feed water flow element may be added. This is the case only when the cost and complexity can be justified due to improved plant efficiency and critical water level control.