A Practical Troubleshooting Case Study
In industrial steam measurement systems, discrepancies between sub-meters and the main flow meter are not uncommon. One particularly confusing situation is when the branch flow meter shows a stable flow rate, while the main meter continuously indicates zero flow.
Is this a sensor failure?
A wiring problem?
Or is the issue hidden in system configuration?
This article presents a real on-site case study and explains how the root cause was identified.
1. Field Problem Description
At a thermal power plant of a petrochemical enterprise in Zibo, Shandong Province, saturated steam was supplied to three downstream users.
The steam metering system was configured as follows:
Main steam pipeline:
DN100 pipe
One orifice plate flow meter installed on the main line
Branch pipelines:
Three vortex flow meters installed on downstream branches
During operation:
Two vortex meters were no longer in service
One vortex flow meter was operating normally
Indicated flow rate: approximately 300 kg/h
However, the main orifice flow meter displayed zero flow for a long period, even though:
Temperature indication was normal
Pressure indication was normal
Installation position complied with standards
This raised a key question:
Why does the sub-meter show 300 kg/h while the main meter shows zero?
2. Instrument Configuration Overview
The main orifice flow meter was designed with:
Maximum flow rate: 0–10 t/h
Differential pressure range: 0–16 kPa
The vortex flow meter on the branch line showed:
Steam pressure: 0.81 MPa
Flow rate: 300 kg/h
From an installation perspective:
Differential pressure transmitter mounted on a horizontal pipe
Positive and negative impulse lines routed upward
Condensate pots installed at equal heights
Root valves were gate valves
No obvious installation error was observed.
3. Initial Diagnosis: Is the Instrument Faulty?
At first glance, the situation suggested a possible failure of the differential pressure transmitter.
However, after reviewing the configuration parameters, one critical setting attracted attention:
Low-flow cutoff (small signal suppression): 100 Pa
This parameter turned out to be the key.
4. Root Cause Analysis: Small Signal Cutoff
For differential pressure flow meters, low-flow cutoff is commonly applied to prevent unstable readings at very small differential pressures.
In this case:
Maximum flow rate (qₘₐₓ): 10 t/h
Low-flow cutoff differential pressure: 100 Pa
Based on calculation, this cutoff value corresponds to a flow rate of approximately:
790 kg/h
This means:
Any actual flow below 790 kg/h is treated as “zero” by the system
The real branch flow of 300 kg/h was entirely filtered out
Therefore:
The main flow meter was not faulty —
the actual flow was simply below the configured cutoff threshold.
5. Why Was the Cutoff Set Too High?
For standard single-range differential pressure flow meters, industry experience shows that:
A cutoff of 2% of maximum flow (2% qₘₐₓ) is generally reasonable
However, in this application:
The selected maximum differential pressure (ΔPₘₐₓ = 16 kPa) was relatively small
As a result, the corresponding cutoff differential pressure became extremely low
Even a slight imbalance in condensate pot height could significantly affect measurement
For example:
A height difference of only 0.7 mm between condensate pots
Could already generate a false differential pressure signal
This makes accurate low-flow measurement very difficult.
6. Importance of Proper Differential Pressure Range Selection
If a higher differential pressure range had been selected, for example:
ΔPₘₐₓ = 60 kPa
Then:
The cutoff differential pressure corresponding to 2% qₘₐₓ would increase to about 24 Pa
Low-flow resolution would improve significantly
Small signal suppression would be much easier to control
In short:
Choosing an excessively small ΔPₘₐₓ greatly reduces low-flow measurement accuracy.
7. Special Consideration for Non-Standard DP Flow Meters
For some non-standard differential pressure flow meters, such as:
Elbow flow meters
Annubar (averaging pitot tube) flow meters
The maximum differential pressure cannot be freely selected.
For example, when measuring saturated steam at 0.7 MPa using an averaging pitot tube:
At a maximum velocity of 30 m/s
ΔPₘₐₓ may be only about 1,620 Pa
In such conditions:
A condensate pot height difference of just 1 mm
Can generate a false flow reading of nearly 8% of maximum flow
This highlights how sensitive DP steam measurement systems can be.
8. Key Engineering Conclusions
This case demonstrates several important lessons:
Zero reading does not always indicate instrument failure
Low-flow cutoff settings must be reviewed carefully
Differential pressure upper range selection is critical
Small installation deviations can significantly affect low-flow accuracy
Steam flow measurement requires system-level consideration, not only sensor selection
Conclusion
When discrepancies appear between main meters and sub-meters, engineers should not immediately suspect hardware faults.
Instead, attention should be directed to:
Flow range design
Differential pressure range selection
Low-signal cutoff configuration
In many cases, the issue lies not in the instrument itself, but in parameter design choices made during the initial engineering stage.