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12-6 Understanding Limits
Example 2. Consider a case in which a tank is being
pumped full of liquid. The tank is designed to hold 3000
gallons. When the tank level drops to the 500 gallon
point, it is desired to fill it. If it should ever drop to 100 gal-
lons, a dangerous condition could result, and the process
must be shut down immediately. Further, when the tank
has filled to the 2500 gallon point, the pump should quit.
If the level should get to 2900 gallons, an alarm should
warn the operator that the pump has not shut down prop-
erly and the tank could overflow. Because of slopping
around of the tank liquid, a 2% noise limit (0.02 x 3000 =
150 gallons) is safe.
First, the 2900-gallon point is considered to be a dan-
ger point, and we want a limit to be fired if that level is
exceeded. That means that the upper-upper limit is a
high limit. We will call it Limit 1. Since it is a high limit, the
hysteresis is negative.
Limit 1 Setpoint = 2900
Limit 1 Hysteresis = -150
Next, consider the shutdown that must occur if the
level drops below 100 gallons. A low limit energizes
below the setpoint, so this lower-lower limit must be a low
limit. Therefore, hysteresis will be positive. In order to
have the limits in order from most gallons to least gallons,
call this Limit 4.
Limit 4 Setpoint = 100
Limit 4 Hysteresis = 150