Demonstration of a stall detection system for induced draft fans

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Abstract
Because aerodynamic stall is a major potential cause of mechanical failure in axial fans, effective stall-warning techniques have had wide application for many years. The study assesses the application of stall-detection to a class of heavy-duty fans for induced draft service in coal-fired power plants. With a multi-MW installed power, this class of fans operates at constant rotational speed, utilising variable-pitch in motion blades to adjust fan duty point. This load modulation system, in the presence of blade erosion or fouling, could result in fans becoming prone to stall as a consequence of the reduced pressure capacity. This article reports on a novel methodology, first proposed for the early detection of stall in low-speed axial-flow fans for tunnel ventilation. With respect to the original concept, which was based on the analysis of hydrodynamic pressure as measured on the fan ducting, the investigation demonstrates the potential of processing far-field pressure signals in identifying stall pre-cursors, and in so doing, establishing the stall incipience. This acoustic stall-detection system is characterised in a laboratory environment exploring the fan operating envelope. Time-resolved pressure measurements are carried out with pre-polarised pressure transducers fitted into (a) the fan casing over the fan blades (near field) and (b) in the inlet plenum 1 diameter upstream of the fan rotor (far field). The evolution of the rotating stall for the fan under scrutiny is first presented. The rationale of the signal processing technique is given, together with a sensitivity study on its metrical parameters. Finally, the signal visualization technique is used to compare the patterns related to the presence of stall in the pressure signals recorded in the near- and the far field.

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