Quick answer
A change in preheater draft accompanied by temperature changes can indicate altered gas-path resistance, but does not by itself establish riser-duct deposits. First verify pressure taps, measurement locations, fan speed, damper position, gas flow, process duty and gas conditions against comparable healthy-operation data. Evaluate other restrictions, false instrument readings and control changes before selecting an inspection or cleaning intervention. Any access requires approved isolation and hazard assessment; after remediation, verify the operating point and duct pressure losses under comparable conditions.
Diagnostic scope
This article focuses on validating riser-duct pressure-loss evidence before considering an intrusive inspection. It differs from broader references about preheater deposit chemistry, cyclone blockages and ID-fan impeller fouling. A change in displayed draft alone does not establish either deposit formation or the location of a restriction. Compare calibrated instruments, correctly located pressure taps, gas flow and fan controls at equivalent operating conditions.
Localize the restriction before choosing a maintenance target
A draft trend is a measurement at a particular pressure tap, not a map of where resistance has changed. Use a verified pressure profile across identified gas-path sections at comparable flow, gas density and fan-control conditions. Where direct gas-flow measurement is unavailable or unreliable, record that limitation and avoid assigning a restriction to a specific component from draft alone.
| Suspected location | Evidence needed to distinguish it | Decision boundary |
|---|---|---|
| Riser duct | Verified pressure loss across taps that actually bracket the riser, supported by comparable gas-flow and operating data. | Consider a riser inspection only when the localized evidence and safe-access assessment support it. |
| Cyclone or another preheater section | A separately verified section pressure-loss change, with instrument and process effects checked. | Investigate that section; do not label it riser build-up merely because overall draft changed. |
| ID fan or its controls | Fan speed, damper position, measured operating point, vibration and mechanical evidence where appropriate. | Use the fan-specific diagnostic route; a power or current change alone does not prove fouling. |
| Pressure measurement system | Tap blockage, impulse-line condition, reference pressure, calibration and signal validation. | Correct the measurement problem before authorizing intrusive duct work. |
This article addresses riser-duct localization and verification, not the broader chemistry of preheater blockages, general duct-deposit maintenance or fan-impeller fouling.
Problem / Job
A preheater draft trend may change alongside temperature readings for several reasons, including gas-flow variation, fan-control response, pressure-tap faults, false-air ingress or deposits in the gas path. The diagnostic task is to verify the instruments and operating conditions, determine whether the evidence localizes increased resistance to the riser duct, and select an authorized maintenance response only when justified. No single trend pattern establishes the cause.
Evidence
- Pressure trends from identified, verified measurement points compared at equivalent process duty and gas-flow conditions; no universal draft-change threshold applies.
- Gas-temperature trends from verified sensor locations compared with gas flow, fuel input, false-air ingress and process conditions, rather than assuming a dedicated fan-outlet sensor or assigning a temperature deviation to blockage.
- Kiln-inlet temperature and fuel-rate trends assessed alongside operating load, gas composition and sensor validity.
- Inspection evidence documenting deposit location and extent, where access has been assessed and authorized.
- Fan electrical kW, current, speed, controls and vibration evaluated as supporting operating-point evidence, not proof of a duct obstruction.
Decision
Escalate to a controlled inspection only after validating instruments, comparing operating conditions and identifying evidence that reasonably localizes the restriction. Confirm the deposit’s extent and consequence before approving cleaning; use the plant’s isolation, access and engineering-review procedures. If performance does not recover, reassess other restrictions, control changes and measurement errors before considering component repair or replacement.
7‑Step Diagnostic & Remediation Workflow
- Data capture and trend analysis
- Export sufficient time-stamped draft, temperature, fan-power, fan-speed, gas-flow and control-position data to cover the onset of the deviation and a comparable healthy operating period; choose the time window based on process dynamics and data availability.
- Plot time-aligned draft, gas flow, fan speed and control position, marking operating transitions and instrument maintenance events. Compare like-for-like operating points before interpreting any deviation.
- Compare temperature and draft trends at matched process conditions, allowing for sensor lag and measurement uncertainty. Correlation alone does not establish deposit formation or the location of a restriction.
- Pre‑shutdown safety verification
- Notify operations control and confirm the plant is in a safe state for a controlled shutdown.
- Before any access, the responsible authorized personnel must identify and isolate all hazardous energy sources using the site-approved equipment-specific lockout/tagout procedure. This includes relevant fan and feeder drives, pneumatic or hydraulic actuators, stored mechanical energy, and connected process equipment capable of introducing hot gas or material. Qualified personnel must verify electrical isolation using approved testing methods where required; apply personal locks and tags and verify every isolation point before opening the duct. Do not treat a single motor disconnect as complete isolation.
- Verify isolation of all connected gas, heat, material, electrical, pneumatic and stored-energy sources. Confirm residual pressure, temperature and atmospheric hazards are within documented safe-access limits; assess confined-space entry, gas testing, ventilation, standby and rescue arrangements where applicable. Do not assume a generic vent valve exists or that venting alone makes entry safe.
- Physical inspection of the riser duct
- Prefer remote inspection when feasible. Do not open an inspection hatch or remove an access panel until authorized personnel have verified isolation, residual pressure and temperature, atmospheric conditions, falling-material hazards and the relevant access permit. Assess confined-space classification and required rescue arrangements before any entry; follow the site-approved procedure and OEM limitations.
- Where the approved access plan permits, use a suitable remote camera or borescope to document coating, meal deposits, clinker-like accretions or other material in the riser. Do not infer deposit composition from appearance alone; evaluate representative samples only under an approved sampling and exposure-control procedure. The appropriate inspection device depends on actual temperature, dust, access geometry and the equipment rating.
- Record deposit location, approximate extent, distribution and any evidence of narrowing or loose material. Assess unstable accretions and falling-material hazards before approaching or disturbing deposits; photographic evidence alone does not establish the resulting pressure loss.
- Determine cleaning method
- Select a cleaning method only after confirming deposit composition, adhesion, access hazards and equipment limitations. Do not assume a universal deposit-thickness trigger or compressed-air pressure; compressed-air use can mobilize hazardous dust and dislodge hot material, so it requires a documented risk assessment and an approved engineered procedure.
- For adhered clinker or scale, have competent maintenance personnel select a plant-approved removal method after assessing refractory integrity, deposit stability, tool access, dust exposure and the risk of falling material. Do not introduce water onto hot refractory or deposits without an engineered procedure addressing thermal shock, steam generation and process contamination.
- Document the chosen method and any OEM‑specified limitations.
- Execute cleaning
- Before cleaning begins, competent personnel must review the selected tool’s operating instructions and the site-approved work method, establish exclusion zones for falling material and flying debris, assess dust and heat exposure, and verify equipment-specific energy isolation and pressure controls. Select respiratory, eye, face, hand, hearing and other protection from the documented hazard assessment; PPE does not replace engineered controls. Stop work if conditions differ from the approved assessment.
- During offline internal cleaning, keep the fan and all connected hazardous energy sources isolated and verified safe; live fan power or draft readings are not appropriate cleaning-control criteria. Monitor personnel exposure, debris movement and the cleaning equipment’s own operating limits under the approved permit.
- Collect debris for disposal according to environmental regulations.
- Maintenance closeout and controlled recommissioning
- Document the completed cleaning, removed material, equipment condition and any remaining deposits. Confirm that tools and personnel are accounted for, access covers and guards are secured, and required inspections and permits are closed under the site’s procedure.
- Only authorized personnel may release isolation locks in accordance with the approved lockout/tagout release procedure. Verify readiness of connected process equipment and restore services in the prescribed sequence; do not restart while personnel are exposed to the duct or fan.
- Restart the applicable kiln/preheater induced-draft fan or other gas-flow equipment only under the site- and equipment-specific commissioning sequence. Confirm the actual fan arrangement, interlocks and connected process readiness; then allow gas flow, controls and process conditions to stabilize before evaluating performance.
- Stabilized performance verification and monitoring
- Record pressure readings and section pressure losses, gas temperatures, flow, fan speed, damper position, true electrical input power and process duty at documented stable operating points.
- Compare the verified readings with the approved process envelope and a matched healthy baseline, accounting for throughput, gas density, fan control position and instrument condition; investigate deviations rather than requiring exact historical agreement.
- Trend draft, temperature, gas flow and fan electrical load for a site-approved observation period. If further inspection is justified, perform it only after a new approved isolation and access assessment; document the operational significance of any residual deposits.
Diagnostic Matrix
| Evidence | Likely Direction | Next Check |
|---|---|---|
| Draft becomes more negative at a recorded measurement point | Possible fan-control, process-flow or gas-path resistance change; cause and location unconfirmed | Verify pressure-tap condition and reference, fan speed, control position, gas flow and section-by-section pressure loss before authorizing duct access. |
| Exhaust gas temperature rises above normal | Possible changes in heat exchange, combustion, false-air ingress, gas flow or sensor response; a duct restriction is not established by temperature alone | Verify the temperature sensor and measurement location; compare fuel rate, process load, false air, gas flow and the section pressure-loss profile at matched operating conditions. |
| Kiln inlet temperature drops while fuel rate is steady | Possible changes in heat balance, gas flow, fuel distribution or instrumentation; the cause is not localized | Confirm the kiln-inlet measurement, combustion and process duty; review gas-flow evidence and correctly referenced pressure losses across individual preheater sections. |
| Fan electrical input power differs from a matched healthy operating point | Operating-point or control change possible; duct restriction not established | Check actual gas flow, density, fan speed, damper position, drive settings and segment pressure losses before considering mechanical inspection. |
| Visible deposits at an approved inspection location; composition not yet confirmed | Deposits are present, but their operational significance and whether they restrict flow are not yet established | Document deposit position and extent, correlate with validated pressure-loss evidence, and obtain an approved maintenance decision before cleaning. |
| No deposit observed at the inspected location, but draft remains abnormal | An accessible inspection point does not exclude deposits elsewhere. Instrument error, fan-control response, fan condition or another gas-path restriction also remains possible; the cause is not yet localized. | Revalidate pressure taps and references, fan speed and controls, actual flow and section pressure losses. Extend inspection only where the pressure profile and an approved safe-access plan justify it; assess fan condition when supported by evidence. |
Acceptance & Verification Criteria
Following maintenance closeout, authorized isolation release and a controlled restart, assess the following measurable criteria under stable, documented operating conditions before approving unrestricted normal production. Operating measurements cannot be verified while the fan remains isolated; apply the site’s startup limits and hold points throughout recommissioning:
- Draft stability: Compare verified pressure readings and pressure loss across defined sections at matched gas flow, fan speed and process duty against site-approved operating limits and a documented healthy baseline; no universal pressure tolerance or cycle count applies.
- Temperature profile: Assess outlet and kiln-inlet temperatures against the approved process envelope, calibrated instrumentation and comparable operating conditions, including throughput, fuel rate, false-air ingress and gas flow; avoid a universal temperature tolerance.
- Power consumption: Verify motor and drive operating limits, and compare true electrical input kW at a matched fan operating point. Removing resistance can change flow and power in either direction depending on the fan curve and control response; do not impose a universal percentage tolerance.
- Pressure differential: Measure pressure loss between correctly located taps spanning the specific suspected riser or preheater section. Do not confuse fan inlet-to-outlet pressure rise with preheater gas-path pressure loss; compare results at equivalent gas flow and density with site-approved references.
- Vibration: Assess fan-bearing vibration against the installed machine’s documented alert and alarm limits, measurement conditions and comparable healthy baseline. An alarm-free reading alone does not establish mechanical health or confirm that a riser restriction has been removed.
- Physical inspection: Where a follow-up inspection is justified and safely authorized, document the location and extent of residual deposits and evaluate their effect on flow, refractory condition and safe operation; no universal acceptable thickness applies.
If an acceptance criterion is not met, keep the affected equipment under the site’s approved operating restrictions, investigate the cause and obtain the required engineering and operations authorization before returning to normal service. Do not repeat intrusive work without a new risk assessment and approved isolation.
Common Mistakes & Repeat‑Failure Traps
- Opening the riser without complete isolation: Opening a duct hatch or access panel before equipment-specific hazardous-energy isolation and verification can expose personnel to hot gas, residual pressure, falling deposits, material inflow or connected moving equipment. Use the approved isolation, atmosphere-testing, access and rescue controls before any intervention.
- Inadequate data capture: Relying on a single data point instead of a trend can conceal intermittent deviations whose causes require confirmation using time-aligned process, control and instrument data.
- Unapproved water cleaning: Introducing water to hot deposits or refractory can cause thermal shock, steam generation and material instability. Do not use water unless an equipment-specific engineered cleaning procedure expressly authorizes it and controls the associated hazards.
- Unverified cleaning effectiveness: Removing accessible surface material does not prove that the relevant flow restriction has been addressed. Document residual deposits and compare section pressure loss, gas flow and operating conditions after recommissioning before declaring the intervention effective.
- Assuming cleaning proves the cause: If draft remains abnormal after duct cleaning, repeat the instrument, gas-flow and control-position checks and compare pressure loss by section. Investigate the fan, including impeller fouling or mechanical condition, only when supporting operating evidence justifies that inspection.
- Restarting without readiness checks: Do not energize equipment until guards, access points, personnel clearance, isolation release and permits have been verified under the approved procedure. Assess draft, temperature and fan operating conditions after controlled restart and stabilization, rather than claiming operating readings can be verified while the fan is isolated.
- Missing documentation: Failing to record cleaning method, debris type, and post‑cleaning measurements makes trend analysis difficult for future incidents.
Addressing these pitfalls supports more reliable fault diagnosis and maintenance decisions. Any reduction in repeat failures or improvement in equipment service life must be demonstrated through plant maintenance and operating records rather than assumed.
Related Infinity guides
Build a stronger cement maintenance library
The Infinity for Cement Organization technical package contains 900+ technical files. It is a one-time $249.99 purchase, with instant download immediately after payment and lifetime updates forever at no additional cost.