why would be -15 to +15 inches with center zero as the controller set point in steam level control as transmitter range is -30 to 0 inches of water column.

 

Scales are arbitrary representations. Where someone puts the zero on a scale depends on their interpretation of the situation. Just because the numbers are arbitrary, doesn't mean that the values are not meaningful numbers (nan = not a number)

It appears that whoever set it up thought in terms of 15" above empty as being zero, the nominal control point. Control setpoints above or below the 'nominal' use an algebraic sign and an value in inches above or below the nominal control point.

It's different than a setpoint value representing elevation above 'empty' where the same nominal control setpoint would be 15", but in the end, the the numbers are arbitrary scales representing something.

 

The scale presented to the operators should be something that is meaningful to them. For a level control on a boiler drum, one common reference point is the drum centre-line, with the range going + and - of that.

If the transmitter calibration is -30 to 0, it sounds like the range is related to the level tapping points.

 

the simple description

for a dp xmtr connection hi and lo, both connections are filled, but one leg has necessarily greater liquid head (say the equivalent of 30" off-set)...