Drum level indication

I told that when a drum level control indicator scale for 30 inch span would be from - 15 to 15 inches, why it would not be from 0 to 30 inches?
Thanks for help
 
Generally, It is a good practice to have the transmitter output match what the operator would see in the gauge glass of the drum. i.e. Normal water level is zero with +/- 15 inches of water level.
 
Generally, It is a good practice to have the transmitter output match what the operator would see in the gauge glass of the drum. i.e. Normal water level is zero with +/- 15 inches of water level.
OK, but why the normal water level would not be 15 inches with 0 to 30 inches of the total level?
Thanks for your help
 
OK, but why the normal water level would not be 15 inches with 0 to 30 inches of the total level?
Thanks for your help
Low water level can damage boiler tubes and high water level can cause carryover damage to the equipment driven by steam. The boiler designers deemed normal to be zero and also gave parameters for low and high water level conditions based on boiler design. The normal water level reference is on all boiler designs to give a common reference rather than confusing and differing levels for each boiler. It is easy to remember that zero is normal water rather than a marine D type B&W boiler is 15 and a large power plant Foster Wheeler boiler is 17. By the way, 15 and 17 are made up values. Accepting that normal water level is zero is much easier.
 
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