Yokogawa EJX440A - Manual (Page 45)

<4. Diagnostics>
4-5
IM 01C25T01-01E
 A/B Blocking Detection
“A Blocking” and “B Blocking” indicates the result
estimated from blockage degree based on the
difference of the high- and low-pressure-side
fluctuation values. Ratio fDP, SQRT (fDP / Ref
fDP) is used to detect A/B blocking. Ref fDP is the
average value of the sum of squares of differential
pressure fluctuations under normal condition.
As the value of
Ratio fDP exceeds the value of
Lim fDPmax, EJX gives basically an alarm of “A
Blocking”. On the other hand, if this value is below
the value of Lim fDPmin, EJX gives an alarm of “B
Blocking.
As a high- or low-pressure-side blockage
progresses, fDP increases. Therefore, “A Blocking”
with a differential pressure transmitter indicates
that a single-side impulse line is plugged for a
differential pressure transmitter. As the both-side
blockages progress simultaneously, fDP decreases.
Therefore, “B Blocking” with a differential pressure
transmitter indicates that both-side impulse lines
are plugged.
NOTE
A single-side impulse line blockage may
generate “B blocking” under the condition where
the fluctuation amplitude is much different
between high- and low-pressure sides.
With a transmitter to measure pressure or tank
level, “B blocking” only is detected.
 H/L Blocking Detection
EJX differential pressure transmitter enables
to detect both-, a high-, or low-pressure-side
blockage. The blockage degree characterized
by a comparison of high-pressure side and low-
pressure-side fluctuation values, BlkF, is used to
detect it. The value changes within a range of –1
to +1. As BlkF approaches +1, the high-pressure-
side blockage progresses. On the other hand, if it
approaches –1, the low-pressure-side blockage
progresses.
1
0
1
0
F0406.ai
BlkF
Blockage detection for
high-pressure side
Threshold
Blockage Progress
BlkF
Blockage detection for
low-pressure side
Threshold
Blockage Progress
-1 -1
Figure 4.2.2 Relation between Blockage Progress
and BlkF
The each threshold value to detect the high- or low-
pressure-side blockage is set to Lim BlkFmax or
Lim BlkFmin.
 L Side Blocking Detection
BlkF is preferentially used to "L Side Blocking"
detection. If BlkF cannot be used, Ratio fSPl,
SQRT ( fSPl / Ref fSPl) is used to "L Side Blocking"
detection. Ref fSPl is the average value of the sum
of squares of low-pressure-side static pressure
fluctuations under normal condition.
As the value of
Ratio fSPl is below the value of Lim
fSPlmin, EJX gives an alarm of "L Side Blocking".
On the other hand, if this value exceeds the value of
Lim fSPlmax, EJX gives an alarm of “Large Fluct
L”.
 H Side Blocking Detection
BlkF is preferentially used to "H Side Blocking"
detection. If BlkF cannot be used, Ratio fSPh,
SQRT ( fSPh / Ref fSPh) is used to "H Side
Blocking" detection. Ref fSPh is the average value
of the sum of squares of high-pressure-side static
pressure fluctuations under normal condition.
As the value of
Ratio fSPh is below the value
of Lim fSPhmin, EJX gives an alarm of "H Side
Blocking".
On the other hand, if this value exceeds the value of
Lim fSPhmax, EJX gives an alarm of “Large Fluct
H”.
 Large Fluctuation Detection
When a pump or compressor starts, the large
fluctuation is generated as process condition
changes rapidly. This phenomenon affects process
fluctuation measurement; so correct blockage
detection is not performed.