Reliable operation of the stall detection system starts with inspection and calibration

Howden's stall probe system keeps your fans free from operating in stall for extended periods and can avoid fan failure

Howden has listened to customers’ concerns about the calibration of the stall probes and has developed a stall probe calibration procedure. The calibration procedure ensures reliable operation of the stall probe and makes sure you are alerted in case of the fan stalling.

Axial fans running in stall can lead to catastrophic equipment failure, causing severe damage to equipment and endangering personnel.

Fans running in stall for a prolonged period of time can experience blade rupture with the result of costly replacement of major fan components as well as considerable downtime.

The Howden stall probe instantly detects the formation of a stall cell in an axial fan. Incorrect calibration or plugging (refer "Technical Bulletin 6: Howden Purge Air System") can result in the probes not detecting or warning of stall, leading to catastrophic failure because no warning is given, and the fan continues to operate in the stall region.

The stall probe detects whether a fan is running in stall or not, and correct inspection and calibration are required for optimum operation of the stall probe.

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Stall protection


  • An inspection should be carried out before calibration.
  • Stall probes must not be dismounted from the impeller casing when the fan is running.
  • Stall probe calibration must be carried out as detailed below and as according to the IOM manual.

How to inspect an axial fan stall probe

  • Ensure condensation in hoses or pipes is not trapped and can flow back into the fan
  • Inspect tubes for defects (looseness, bends, etc.)
  • Check for leakages
  • Inspect regularly (frequency according to the IOM manual)

Possible reasons for false alarms from a stall probe

  • Double sealing system
  • More difficult to assemble and align


  • Leakages in the stall system
  • Condensation
  • The nut on the stall probes are not tightened correctly
Stall probe for axial fans

How to calibrate the stall probe on VARIAX® fans

  • Calibration is recommended once a year.
  • A mechanical and visual inspection of the probe and associated tubing should be carried out prior to any calibration.
  • The ports and tubes must be clean and free from any blockage or damage.
  • The front surface of the probe where the measuring holes are located (1) must be flush with the inside of the fan impeller casing.
  • To reduce the risk of false alarms from the stall probes, the stall probes must be adjusted for the measured differential pressure across probe ports (1).
  • The adjustment should give a differential pressure as close to 0 (zero) as possible at a blade angle of typically 55º* with the fan running. Maximum acceptable deviation from zero is ± 250 Pa (±1 inWg). The exact angle of the stall probe of which no or little differential pressure is achieved depends on the design of inlet box guide vanes and the probe location to the guide vane.

* Refer to IOM manual for correct blade angle

Stall probe calibration procedure on VARIAX® fans

  1. The stall detection alarm/actions should be disabled in the DCS prior to any adjustment.
  2. Mark the connecting hoses 'high' and 'low' and disconnect the hoses from the stall probe.
  3. Check that the stall probe is connected to a device to measure the differential pressure across the connecting pipes (5) as adjustments are made. The measuring device must be able to measure both positive and negative pressure.
  4. With the fan operating, adjust the blade pitch to typically 55º*. If multiple fans are operating in parallel, they should all be adjusted to same blade pitch to avoid any possibility of fans being pushed into stall.
  5. Aligning the stall probe in such way, that the control surface (4) is parallel to the edge of the impeller casing. In this position, the separating plate (6) will be parallel to the fan rotation axis.
  6. Adjust the position by loosening the nut (3) and turning the probe back and forth until differential pressure reaches a satisfying value as near to zero as possible. Keep the probe in position when tightening the nut again.
  7. Once the desired calibration has been achieved, and the nut (3) is tightened, remove the measuring device, reconnect the pipe work to the stall probe so that the hose marked with 'high' is connected to the hole which the blades meet first (refer IOM manual).
  8. Enable the stall detection alarm in the DCS.

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