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Ratio calculations derived from two
flows are often implemented using regulatory PV datapoints
in the form "Ratio = X1 / X2". Under steady process
conditions the results are satisfactory. However analogue
input data points can read negative values and under fault
or process start-up conditions false readings can be obtained.
For example if we consider X2 in the
above example. As the value of X2 decreases the calculation
result becomes larger. If X2 becomes negative the calculation
result steps from a large positive value to a negative value.
This results in false data being made available to the operator
in the form of incorrect indications and alarm annunciation.
If the ratio calculation is being used as an input to a
control loop violent control valve movements would be experienced.
This scenario can be disguised by clamping
the analogue input values to the ratio calculation at zero.
Inevitably in the future these clamps will be removed for
maintenance or calibration and the whole design relies that
someone will remember to replace the clamps.
G&L's solution is to use the following
fortran style calculation:
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C1 = X1/MAX(0,PVEULO,X2);
Ratio = MID3 (0,C1,C1)
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Where:
C1 is a temporary calculation value
X1 is an analogue input value to the ratio calculation
X2 is an analogue input value to the ratio calculation
PVEULO is the lower range limit of the ratio calculation.
This calculation divides
X1 by the maximum value of either 0, PVEULO or X2.
As the value of X1 decreases the calculation result tends
towards zero, if X1 becomes negative the calculation result
continues to decrease as a negative value.
If X2 is negative or is a bad PV the value is clamped
by the algorithm to either 0 or PVEULO, which ever is
the highest.
Therefore the calculated
value can not step change from a large positive value
to a negative value. If analogue input X1 fails the calculated
value will indicate 0% FSD, enunciating the low alarm.
If analogue input X2 fails the calculated value will indicate
100% FSD enunciating the high alarm. Because the fail
values are defined as either 0% or 100% FSD the operator
can instantly recognise an instrument problem and even
which instrument is faulty.
This is an example
of how G&L uses all facilities available to provide
a securely engineered system.
These calculations are entirely
original, developed by G&L Engineering.
© 2000 G&L Engineering Ltd.
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