Drum Level On-Off Control

On-off drum level control is one of methods to control drum level in steam boiler and power plant. Most of drum level control methods explained until now could be utilized to generate an on-off indication pertaining to level control. The many typical drum level control method is basically to get started the feedwater pump on low stage and enable it to operate till a higher water stage is achieved in steam boiler.

The devices which are used at on-off drum level control as following below:

- For perform flexible on-off transitioning drum levels, a steam boiler can uses capacitance probe. - For perform unchanging transitioning drum levels steam boiler can uses conductivity probes. - Steam boiler can uses magnetic transition using a developed-in hysteresis to control the level of float.

The method of on-off drum level control is most used commonly on steam boiler which has capacity of steam production under 4500 kg/h for the reason that it is the lowest costly choice. For steam boiler which has capacity of steam production above exceeding 4500 kg/h or 3 MW, the method of modulating drum level control should be installed.

It could be argued, on the other hand, in which the method of on-off drum level control isn’t perfect for control boiler drum level, simply because the fairly high rate of feedwater flow any time the pump is on decreases the pressure of steam boiler. This condition leads to the firing rate of burner to frequently fluctuate since the on and off condition of pump is switched.

Using a common case, it could be found by calculations which also using feedwater at temperature of 80°C, the firing rate of burner might must be 40% greater by using feed pump is on, when compared with feed pump is off. This specific constant deviation leads to:

- Decreased performance. - Temperature of steam boiler will be changes. - Use on the burner settings.

When high load of steam occur, the parameter of flow rate of steam can usually maximize water is carried over into steam, in addition to can usually create the levels of boiler water significantly unpredictable with the related hazard of small amount of water lockout, especially on multi-boiler constructions. Then again, the reality is still that on-off drum level control is really commonly utilized on medium or small scale boilers.

There are advantages and disadvantages the use on-off drum level control method. The advantages of this method are: low cost and uncomplicated, while the disadvantages of this method are: carry-over of water into steam tend to be more than other method, flow rate and pressure of steam can be changes rapidly, and every steam boiler needs its individual feed pump in the on-off drum level control method.

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