Quick answer
A rising pressure reading in a pneumatic conveying line while material throughput drops typically signals a blockage, excessive air leakage, or a change in material characteristics that increases flow resistance. Verify line isolation, inspect for obstructions, check air‑seal integrity, and confirm that the material feed rate and particle size distribution match the design assumptions. Once the root cause is removed, restore normal pressure and throughput and document the corrective actions.
Problem / Job
The plant operator observes that the pressure gauge on the main conveying line is climbing above its normal set‑point, while the mass flow meter downstream reports a decreasing throughput. The line is part of a cement grinding‑to‑storage pneumatic transport system that must maintain a steady feed to the kiln.
Evidence
- Pressure trend: steady increase of 0.5–1.0 bar over 15 minutes.
- Throughput trend: 10–20 % reduction in measured mass flow.
- No recent changes to air‑compressor output or pipe‑diameter.
- Visual inspection shows no external damage to the line.
Decision
Proceed with a systematic diagnostic workflow that isolates the pneumatic circuit, confirms air‑leakage, checks for internal blockages, validates material properties, and verifies control‑system set‑points before restarting the line.
7‑Step Diagnostic Workflow
- Secure the line. Initiate lock‑out/tag‑out (LOTO) on the main air‑supply valve and any downstream isolation valves. Depressurize the line using the designated vent valve and verify zero pressure before entering any confined space.
- Record baseline data. Capture the current pressure, airflow (CFM), and throughput values. Retrieve the most recent trend logs from the control system for comparison.
- Inspect for external leaks. Conduct a visual and acoustic survey of all couplings, flanges, and flexible hoses. Use a calibrated ultrasonic leak detector to quantify any air loss. Document any leak locations and repair them before proceeding.
- Check internal blockage. With the line isolated and depressurized, open access ports or inspection hatches. Use a flexible borescope to view the interior of bends, elbows, and the discharge chute. Look for material buildup, foreign objects, or collapsed pipe sections. If a blockage is confirmed, remove it using approved pneumatic or mechanical cleaning tools, following LOTO procedures for each tool.
- Validate material characteristics. Sample the conveyed cement powder at the feed point and at the discharge point. Verify particle size distribution, moisture content, and bulk density against the design specifications. A shift toward finer particles or higher moisture can increase drag and raise pressure.
- Confirm air‑flow set‑points. Review the compressor control panel and the variable‑frequency drive (VFD) settings that regulate air supply. Ensure the commanded airflow matches the design requirement for the current throughput. Adjust the set‑point only after confirming that the line is clear and the material properties are within spec.
- Re‑pressurize and monitor. Remove LOTO devices, open the main air‑supply valve, and gradually bring the line back to operating pressure. Observe pressure and throughput for at least 10 minutes. If pressure stabilizes and throughput returns to normal, document the corrective actions and close the work order. If the trend persists, repeat steps 3–6 focusing on secondary loops and downstream equipment (e.g., cyclones, filters).
Diagnostic Matrix
| Evidence | Likely Direction | Next Check |
|---|---|---|
| Pressure rising, airflow constant, throughput falling | Increased flow resistance inside the line | Inspect for internal blockage or material compaction |
| Pressure rising, airflow increasing, throughput unchanged | Air leakage downstream of the pressure gauge | Perform ultrasonic leak detection on downstream joints |
| Pressure stable, airflow constant, throughput falling | Material feed rate reduced or feed hopper empty | Check feed hopper level and feeder motor operation |
| Pressure rising, airflow decreasing, throughput falling | Compressor or VFD under‑performance | Verify compressor output pressure and VFD speed set‑point |
| Pressure rising, high moisture content in material sample | Increased drag due to moisture‑induced agglomeration | Dry material upstream or adjust dryer set‑points |
| Pressure rising after recent pipe replacement | Improper pipe alignment or reduced internal diameter | Inspect new pipe sections for deformation or incorrect fittings |
Acceptance / Verification Criteria
- Pressure gauge reads within the normal operating band documented in the line’s design data (verify against OEM or site‑specific set‑point).
- Airflow (CFM) measured at the compressor outlet matches the design flow for the current throughput; any deviation must be within the tolerance specified by the control system.
- Mass flow meter downstream shows a stable throughput that aligns with the production schedule and does not deviate more than the allowable variance recorded in the process specification.
- No audible or ultrasonic air leaks are detected after the line is pressurized.
- Material sample analysis confirms particle size distribution, moisture content, and bulk density are within the limits defined for the conveying system.
- All LOTO devices are removed, and the line is returned to service with a documented sign‑off from the responsible shift supervisor.
Common Mistakes & Repeat‑Failure Traps
- Skipping LOTO. Restarting the line before confirming zero pressure can cause sudden pressure spikes that damage seals or cause pipe rupture.
- Assuming the pressure gauge is accurate. Faulty or mis‑calibrated transducers can mask the true pressure trend; always cross‑check with a portable calibrated gauge.
- Neglecting downstream equipment. Cyclones, filters, or dust collectors can become clogged, creating back‑pressure that mimics a line blockage.
- Changing air‑flow set‑points without confirming line condition. Over‑pressurizing a partially blocked line can exacerbate pipe deformation or cause material blow‑back.
- Over‑relying on a single data point. Transient spikes in pressure or flow are normal during start‑up; use trend data over at least 10 minutes before concluding.
- Failing to re‑sample material after corrective action. Moisture or particle size changes can re‑appear after cleaning, leading to recurring pressure rise.
- Ignoring manufacturer‑specific recommendations. OEMs often provide unique clearance tolerances for bends and elbows; substituting generic parts can create hidden restrictions.
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