packing — air_leaks (pressure-decay)

Physics

An isolated branch or zone of compressed-air line, sealed off by a manual or automated isolation valve during a scheduled no-demand window (overnight, between shifts), holds a trapped volume of air at the line’s static pressure; if that zone has no leak, its pressure stays essentially constant (small decay only from temperature drift). A leak through a worn fitting, cracked tube, or failed quick-connect on the packing line’s pneumatic actuators bleeds that trapped volume down at a rate proportional to the leak’s effective orifice area, giving a measurable psi/min decay rate. A pressure transducer on the isolated zone, read during the scheduled no-demand window, computes that decay rate directly; this recipe reuses the energy detector’s generic excess-over-baseline math on the decay-rate signal instead of true kW, with the historically power-named fields carrying psi/min values (a learned-good zone shows a small baseline decay rate from thermal effects; a leak pushes the decay rate well above that baseline). The relevant dynamic is the pressure decay over the test window (minutes to an hour), not any fast valve transient; sampling the transducer at 0.1 Hz (once every 10 s) gives ample margin by Nyquist over that slow decay dynamic while still resolving the decay curve’s shape for a reliable rate estimate.

Sensor

• Spec: pressure transducer, 0-150 psi range, 4-20 mA / Modbus output
• Signal: scalar ([psi]), range n/a

Example parts

• Wika A-10 pressure transmitter (0-150 psi, 4-20 mA output)
• Ashcroft G2 pressure transmitter (0-150 psi, 4-20 mA output)
• ifm electronic PA3000 pressure sensor (0-145 psi, analog + IO-Link output)

Mounting

Install the pressure transducer on the isolated branch/zone of the packing line’s compressed-air supply, downstream of the zone’s isolation valve, at a convenient tee or gauge port. Depressurize and lock out the zone before cutting in the transducer’s process connection; confirm the isolation valve holds zero leakage at its seat by verifying pressure stability over a short test window before relying on the zone for leak-rate measurement.

Wiring

Transducer’s 4-20 mA analog output or Modbus output goes to an analog/serial input on the edge box; for a 4-20 mA loop, use a shielded twisted pair and ground the shield at the edge-box end only to avoid a ground loop with the packing line’s control cabinet.

Validation

isolate the zone during a scheduled no-demand window and record the detector’s learned baseline decay rate; then open a calibrated reference leak (a small needle valve bleeding a known small flow) on the isolated zone and confirm the detector raises an excess-decay event, closing it to confirm the detector clears on the following overnight run

Expected: detector flags the calibrated reference leak within open_after test windows and clears within close_after_clean windows of its removal, with the learned baseline decay rate consistent night-to-night within +/-20% absent any injected leak

Safety notes

• a qualified/licensed electrician must verify and lock out (LOTO) any electrical supply to the transducer’s transmitter electronics before wiring, using a CAT-rated meter appropriate to the transmitter’s supply voltage class
• depressurize and lock out the isolated zone (bleed residual pressure, verify zero pressure gauge reading) before cutting into the branch to install the transducer’s process connection; compressed air released suddenly from a cut line is a projectile and noise hazard
• confirm the manual/automated isolation valve is rated and correctly positioned to hold the zone’s full line pressure before relying on it for isolation; a partially-seating valve will read as a false leak and mask operators’ confidence in the test
• wear eye and hearing protection when bleeding down or working near the isolated zone

Cost

120–450 USD, difficulty 2/5, ~35 min install downtime.