YOKOGAWA DPharp EJXC40A, Руководство пользователя

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<7. HART Communication>
7-34
IM 01C25W05-01EN
■ Determination of Pres 1 Side Blocking and
Pres 2 Side Blocking
In differential pressure measurement, it is possible
to detect which side, high pressure side or low
pressure side, of the impulse line has been blocked.
To determine blockage on one side of the impulse
line, the BlkF value is used.
BlkF is the blockage level expressing the relative
blockage of the high pressure side and low
pressure side of the impulse line. The value varies
in the range of –1.0 to 1.0. The value approaches
1.0 when a blockage occurs in the high pressure
side and –1.0 when a blockage occurs in the low
pressure side.
1
0
1
0
F0715.ai
BlkF
High pressure side
block detection
Threshold
Blockage progress
BlkF
Low pressure side
block detection
Threshold
Blockage progress
-1 -1
Figure 7.3.3 Relationship between blockage
progress and BlkF
When the BlkF value exceeds the Lim BlkFmax
value, “Pres 1 Side Blocking (high pressure side
blockage)” is detected.
When the BlkF value falls below the Lim BlkFmin
value, “Pres 2 Side Blocking (low pressure side
blockage)” is detected.
■ Determination of Pres 2 Side Blocking
The BlkF value is prioritized in the determination
of low pressure side blockage. However if the
BlkF blockage level is insufficient, Ratio fP2—the
square root of fP2 relative to Ref fP2 or SQRT (fP2/
Ref fP2)—is used in the determination of the low
pressure side blockage.
Ref fP2 is an average of the sum of squares of the
low pressure side pressure fluctuation acquired
during steady state operation. It is a reference value
used in the comparison with the value obtained
during diagnostic execution (fP2).
When Ratio fP2 falls below the Lim fP2min value,
“Pres 2 Side Blocking” is detected.
On the other hand, when Ratio fP2 exceeds the Lim
fP2max value “Large Fluct Pres 2 (low pressure
side pressure fluctuation error)” is detected.
■ Determination of Pres 1 Side Blocking
The BlkF value is prioritized in the determination
of high pressure side blockage. However if the
BlkF blockage level is insufficient, Ratio fP1—the
square root of fP1 relative to Ref fP1 or SQRT (fP1/
Ref fP1)—is used in the determination of the high
pressure side blockage.
Ref fP1 is an average of the sum of squares of the
high pressure side pressure fluctuation acquired
during steady state operation. It is a reference value
used in the comparison with the value obtained
during diagnostic execution (fP1).
When Ratio fP1 falls below the Lim fP1min value,
“Pres 1 Side Blocking” is detected.
On the other hand, when Ratio fP1 exceeds the Lim
fP1max value “Large Fluct Pres 1 (high pressure
side pressure fluctuation error)” is detected.
■ Determination of Large Fluctuation
If the process status changes drastically such
as when pumps or compressors start, pressure
fluctuation suddenly increases. If the pressure
fluctuation is too large, it will affect the blockage
detection results. If “Large Fluct Pres 2” or “Large
Fluct Pres 1” occurs, you need to check the process
status to determine whether the impulse line
blockage detection is operating properly.
The threshold for detecting abnormally large
pressure fluctuation is set in Lim fP2max and Lim
fP1max. As these values are set sufficiently large
for detecting large pressure fluctuations, they
seldom need to be changed.
7.3.2.2.2 Combination of the Reference Value
and Blockage Detection Result
■ Diag Applicable
When all the appropriate reference values are
measured, the transmitter can detect four types of
blockages (one-side blockage, both-side blockage,
low pressure side blockage, and high pressure side
blockage) and fluctuation errors.
If some of the reference values are invalid, the
combination of blockages that can be detected is
limited. The following figure shows the combination
of blockage detection functions that can be used.