A-12
Appendix A Maintenance and Troubleshooting of this Device
A3 Setting using a differential pressure gauge
A3-1 Clogging and pressure frequency index
The pressure frequency index may vary depending on clogging in the connecting pipe. The
pressure frequency index may either become smaller or larger due to clogging in the connecting
pipe. In differential pressure measurement, there is a connecting pipe on both the high pressure
and low pressure sides. When both pipes are clogged over a certain level, the pressure frequency
index will decrease. However, the index will increase or not change significantly as clogging
progresses. The reason for this is explained below.
In differential pressure measurement, the fluctuation in the differential pressure is used to calculate
the pressure frequency index (excluding the case where static pressure sensor is selected in the
Press Freq Index Sensor Selection). Differential pressure fluctuation is a combination of the
fluctuation on the high pressure side and the fluctuation on the low pressure side. Therefore, when
there are common components in the fluctuation on both sides, under normal conditions they
cancel each other out and the detected fluctuation becomes smaller than the original common
component. In such a case, if the balance between the fluctuation on the two sides changes, the
components that canceled each other out under normal operating conditions expand, and the
fluctuation becomes larger than under normal operating conditions. Therefore, when there is a
certain difference in the amount of clogging between the high and low pressure sides, that is, when
one side only is clogged, the pressure frequency index will be larger than under normal operating
conditions. In addition, depending on the relationship between the cause of increase in the index
and the cause of decrease in the index, the index value may not change even when clogging occurs.
Note that when both connecting pipes are clogged over a certain level, the cause of decrease in
the index will become stronger, so that the index value eventually will become small as clogging
progresses.
A3-2 Points to note regarding the diagnosis of clogging
When the pressure frequency index is used to diagnose clogging in the connecting pipe, observe
the cautions below.
When the fluctuation is very small or when the fluctuation frequency is low, clogging cannot
be diagnosed. The reason for this is that the pressure or differential pressure needs to include a
sufficient number of fluctuations to calculate the pressure frequency index accurately. Specific
examples are listed below.
• There is no fluid flow, or flow velocity is very slow.
• Fluid viscosity is high.
Variation in the pressure frequency index does not always mean that clogging in the connecting
pipe has occurred. The index may vary due to other factors, such as the following.
• A change in the operating conditions of the pump, compressor, etc. (ON/OFF, number of units,
RPM, etc.)
• Air bubbles entering the process piping or connecting pipe.
• A change in the viscosity of the process fluid
• Flow velocity variation caused by valve hunting
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