Shipping Industry

Two Term and Three Term Boiler Water Level Controller

This article discusses about how the water level of boiler is kept under control. Two terms and three terms are the controllers that are used in the boiler in order to keep all the parameters related to the steam flow, water feed etc. So, let’s discuss about them in detail.

Water level control is very important process that needs to be carried out on the water tube in order to protect it from getting damaged. As we know that water tube boilers have very less amount water for carrying out steam process, maintaining a correct water level is therefore prerequisite. Generally when we employ electronic circuit for measurement of water level, two terms are always kept in mind namely swell and shrink.

Now suppose the load on the boiler increases due to any reason like starting of the cargo pump etc. the steam pressure inside the boiler will decrease. As we know that the decreases in pressure will facilitate the formation of more steam and this will increases the water level to some extent due to expansion, and this phenomenon is called as the swollen. Now if you are relying on the conventional water gauge, then obviously it will try to decreases the water supply even though the water inside the boiler is going down.
Now suppose load on the boiler decreases and this will lead to increases in pressure of the boiler, this increase in pressure will be responsible for less steam formation and we the water level will go down due to increases in pressure. Again as per the conventional water level indicator, the water needs to feed even though the water in the boiler is sufficient. This decrease in water level is called as the shrink. Every cadet should know swell and shrink phenomenon in boiler.
To sort out these anomalies associated with conventional water level indicator, we two electronic system two monitor it, namely: Two terms water level indicator and three term water level indicator.

Two term Boiler water Level controller

Two terms controller comes very handy in controlling the ill effects caused due to the shrinkage or swell up in boiler. Actually in two term water controller another term know as steam flow is also controlled and thus two terms water level and steam flow are monitored. A differential pressure is fitted with the boiler and it measures any change in the pressure. If the pressure inside the boiler increases, that means we need to bring down the supply of water to the boiler.
Boiler water level controller, Gauge Glass, P+I controller
As we know that the flow of the steam through the boiler is not uniform, we need something to compile the ever changing signal. To compile this signal, we extract out the square root from the signal. The signal generated from both the pressure difference and the water level controller is fed to the compiler circuit which in turn regulates the feed water supply to the boiler.

 Three term Boiler water Level controller

Boiler water level controller, Gauge glass, P+I+D controller
Three term controller working is similar to the two terms controller for boiler but a third variable, that is the variable water supply control is also included in it. Differential relay output is fed to an averaging relay which compares it with the received drum level signal and gives the correcting signal to control valve.  Actuating signal is fed to the supply valve that controls the supply of the water.
In a normal condition the supply of fed water is equal to the steam produced by the boilers and it is kept under the control with the help of a differential relay that controls that keeps things under the normal condition.
It is always better to have three term water level controller in order to get the precise results while the boiler is in the working condition.
Hope you liked my article, please comments for any suggestions.
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Mark Eriscson
Love to write about topics related to maritime sector. Was born in sailor's family, so love to ride on those high seas. In free time i would like to go for a good sunbath and some soothing guitar music. Want to have my own boat one day.

4 Replies to “Two Term and Three Term Boiler Water Level Controller

    1. Thank You Nisanth, for your feedback. We would like to know what more can be added and what you did not able to understood.

      Thank You

  1. I understand how you control the level of the Boiler Drum using both 1 x 2 Element & 1 x3 Element controllers. But how is the Feed water pump controlled to cope with these changes in power demand and to respond to the varying loads that will effect the Drum water level. Does it speed up & slow accordingly? Does the Governor change the steam demand flowing to the steam turbine driven Feed pump? or is an electrical controlled pump controlled by a governor of P&I controller ? or an differential flow controller controls the Feed water pump ? I await your answerer.

  2. This post is riddled with errors.
    The “Terms” referenced should be “elements”. Ie- drum DP element, feed water Flow DP element, and steam flow DP element.
    “Term” is the action type/s of the level controller- proportional, or proportional and Integral (to reduce offset of the process variable from the setpoint). The derivative term is not used in boiling fluid level control as the measured error from the surface height would be too “noisy” or erratic.

    Power boilers usually have “3-element, two term” level control.

    Good luck with surviving in industry!

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