I. Location: First draw an "air map," then determine the "meter coordinates."
A:One meter before and after the dryer—use volume difference to lock in regeneration air consumption;
B: One meter on the workshop inlet main pipe—compare with the total of the individual meters to locate pipeline leaks;
C: Separate ring installation for key air-consuming equipment (injection molding, spraying, CNC)—facilitates calculation of "single-item air consumption."
Incorporate the "why install" explanation into the drawings to ensure a data loop for later audits; otherwise, even the most accurate tables become mere "numerical decorations."
2. Straight Pipe Section: 10D before and 5D after is the "passing grade," not the "recommended value." Compressed air velocity is 20–40 m/s; a 90° bend can introduce an additional 15% error. If only 5D is available on-site, prioritize insertion-type differential pressure or thermal type, and install a honeycomb rectifier to reduce the error to within ±2%. Remember: Insufficient straight pipe section cannot be compensated for by any principle.
3. Direction: Gas "flows upward," liquid "flows downward." When the vertical pipe flows upward, droplets fall back due to gravity, preventing water accumulation on the probe. If horizontal installation is only possible, the sampling port should be opened at the 3 o'clock position on the side of the pipe; pressure sampling at the 6 o'clock bottom is prohibited, otherwise condensate can fill the transmitter within 24 hours.
4. Meter Size: Not "Meter Size Matches Pipe Size," but "Range Priority."
Example: A DN80 pipe has a maximum flow rate of only 150 Nm³/h. If a DN80 vortex meter is selected, the lower limit signal of 12 Nm³/h will be cut off, and the night shift data will be directly zero. Correct approach: Select a DN50 thermal meter, with a range of 3–300 Nm³/h and a pressure loss of only 1 kPa, which is both accurate and economical.
5. Water and Oil Pretreatment: Without water removal, even the best meter will "slowly commit suicide."
ISO 8573-1 requires oil mist <0.1 mg/m³ and particles <0.1 μm. 70% of on-site conditions cannot be met, therefore:
Thermal type: Install a 5 μm sintered filter on the probe sleeve, and ultrasonically clean every 3 months;
Differential pressure: Add a 1 μm stainless steel filter screen before the pressure tap, and drain the condensate tank daily;
Vortex: An oil removal filter must be added, otherwise, after 6 months, the vortex generator will be covered with sludge, and the K coefficient will drift by more than 8%.
6. Pulsation buffer: During loading/unloading of piston machines and frequency converters, pressure pulsations of 0.2–0.5 Hz will appear in the pipeline. If the vortex frequency falls within the pulsation zone, the instantaneous flow rate will "jump wildly."
Low-cost solution: Add a 0.5 m³ buffer tank, and the pulsation amplitude can be reduced to ±2%; add 3 s damping to the meter, and use a 30 s moving average for the DCS, and the data will instantly "flatten."
7. Electrical: 220V and 24V should not share a common ground; 4-20mA and inverter wiring should be separate. The electromagnetic environment of the compressed air station is complex. Inverter harmonics can cause a 0.5mA ripple in the 4-20mA range, resulting in a 3% fluctuation. Shielded twisted-pair cable must be used, with single-end grounding; RS485 communication lines should be run through steel conduits, with a distance of ≥30 cm from power cables, and crossing at a 90° angle.
8. Reverse Check Valve: Preventing "Meter Reversal"
When the pipeline pressure is higher than the air compressor outlet at night, gas backflow occurs, and the vortex street accumulates a negative value. Adding a low-pressure-drop check valve (opening pressure 3 kPa) before the meter will ensure that the accumulated amount only increases and does not decrease, avoiding the embarrassment of a negative gas consumption at the end of the month.
9. Insulation and Drainage: Transmitters can freeze and crack in just one hour during northern winters.
Differential pressure condensate tanks must have 5 cm of rubber-plastic insulation and a 10 W/m self-regulating heating cable; the heating probe should be tilted downwards at 5° to allow condensate to flow back to the main pipe by gravity, preventing "ice cones" from bending the probe.
10. Labeling and Filing: Making "Installation" Traceable
For each meter installed, take photos to record:
A: Length of the straight pipe sections before and after installation (measuring tape + mobile phone watermark camera);
B: Pressure tap location (clock method);
C: Filter model and condensate tank drain valve brand.
Upload the photos to the CMMS system to generate a QR code, which is then affixed to the meter casing. If data anomalies are found three years later, scanning the code will restore the original site, avoiding situations where "the person has changed and there is no way to verify the information."
The purchase cost of a compressed air flow meter only accounts for 15% of its total lifecycle cost; installation and maintenance are the major expenses!
