Re: Boiler Water Level Control

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Posted by Gburns on October 31, 2001 at 06:22:20:

In Reply to: Boiler Water Level Control posted by Paul Eady on October 23, 2001 at 15:06:08:

As a marine engineer myself I know that the biggest advantage to shore side level control is that the building is not moving side to side and front to rear.

There is a significant difference to what was and is still done on in a marine plants compared with a modern shore side steam plant. Most shore side facilities with considerable size will utilize a DCS or other type computer based control. Almost all of these plants will utilize multiple transmitters sending a 4-20 mA signal. Some will offset the transmitter to compensate for the water/steam density at operating conditions while other plants will do the compensation in the DCS. The advantage of doing the compensation in the DCS is that it can be compensated with changing pressure where as doing the compensation at the transmitter level will only be accurate when at operating pressure.

Typically the level of water in the steam drum is maintained by the feedwater regulator utilizing either a three-element control strategy that anticipates changing conditions for accurate level control or a two-element system which only looks at level and feedwater flow. The three elements, or input signals, used by the control system logic are:

h Steam drum water level
h Steam flow and continuous blowdown
h Feedwater flow

The three-element control strategy maintains steam drum level by adjusting feedwater flow into the drum by anticipating changes in steam load and continuous blowdown flow. This signal is used as feedforward for the drum level controller. Because steam flows meters are only accurate within a certain operating range the two-element drum level control is used for low load operation. Other control schemes may incorpate a 4th and 5th element to compensate for shrink and swell, however, if the drum level compensation is done properly in the DCS this is typically not necessary.

: I am a Marine Engineer whose esxperience on steam plant is limited to 1950's technology. I am studying towards my Stage 5 Certification in Power and Plant engineering and need information on modern electronic boiler water level control. Any infomation on operaion of automated controls would be appreciated. Thanks.

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