Every time a blade on a fan or an impeller vane on a pump sweeps past a fixed obstruction, such as a cutwater, a diffuser, or a support, it produces a small pressure pulse. Those pulses repeat once per blade per revolution, so they land at a very predictable frequency: the number of blades or vanes multiplied by running speed. That frequency, called blade-pass or vane-pass frequency, is one of the most useful markers on a vibration spectrum for pumps, fans, and compressors, because problems with the flow through the machine tend to show up right there. Watching it is a direct window into the aerodynamic and hydraulic health of the machine.
Blade/vane-pass frequency in one line: Blade-pass frequency and vane-pass frequency are the vibration frequencies produced by the blades of a fan or the vanes of a pump or compressor impeller passing fixed points in the casing, equal to the number of blades or vanes multiplied by the running speed. They are diagnostic markers of aerodynamic and hydraulic forces, and changes in their amplitude or the appearance of sidebands around them flag problems such as excessive clearance, flow recirculation, or looseness in the flow path.
The physics behind these frequencies is straightforward. A rotating impeller or fan wheel carries a set number of blades or vanes, and as the wheel turns, each one in turn passes close to a stationary feature such as the pump cutwater, a diffuser vane, a compressor stator, or a structural obstruction in the flow. Every pass generates a brief pressure and flow disturbance, and because there are a fixed number of blades and each completes one full lap per revolution, the total number of these disturbances per revolution equals the blade or vane count. Multiply that by revolutions per minute and you have the frequency at which the pulses occur.
So blade-pass frequency is simply the number of blades times running speed, and vane-pass frequency is the number of vanes times running speed; the two terms describe the same idea applied to fans and blowers on one hand and pumps and compressors on the other. Because it is a whole-number multiple of running speed, it lands on a specific order line in the spectrum, well above the running-speed peak. Knowing the blade or vane count lets an analyst predict exactly where to look for it, which is what makes it such a reliable diagnostic landmark rather than a vague hump.
It is worth stressing that a modest, steady blade-pass or vane-pass component is normal, not a fault. Any machine that moves fluid past stationary features will generate some pressure pulsation at this frequency simply as a consequence of how it works. The diagnostic value comes not from its mere presence but from how it changes: a component that grows over time, or that sprouts sidebands, or that becomes dominant, is telling you the flow interaction has gotten worse than the machine's healthy baseline, which is the real signal an analyst is watching for.
A rising blade-pass or vane-pass amplitude usually points to a worsening interaction between the rotating element and the stationary flow path. On a pump, a growing vane-pass component often indicates that the gap between the impeller vanes and the cutwater or diffuser has changed, or that the flow through the machine has become disturbed, so the pressure pulse each vane produces as it passes the cutwater has become stronger. On a fan, a growing blade-pass component can reflect changed clearances or obstructions that make each blade's passage generate a larger disturbance. In both cases the amplitude at this frequency tracks the severity of the aerodynamic or hydraulic problem.
Operating the machine away from its best efficiency point is a common driver of these symptoms. A pump running well below its design flow can recirculate, with fluid sloshing back through the impeller rather than passing cleanly through, and that recirculation dramatically increases the pressure pulsation at vane-pass frequency. A machine forced far from its intended duty point can therefore show a strong vane-pass or blade-pass component not because anything is broken but because it is being operated in a regime that stresses the flow path, which is itself useful diagnostic information about how the machine is being run.
Sidebands are the other tell. When the blade-pass or vane-pass peak is flanked by smaller peaks spaced at running speed on either side, it indicates that the blade-passing event is being modulated once per revolution, which points to an asymmetry: an uneven gap, a damaged or fouled blade, or looseness that lets the interaction vary around the wheel. A clean single peak suggests a symmetric, uniform interaction, while a peak surrounded by sidebands suggests the wheel is seeing different conditions at different angular positions. Reading the presence and spacing of sidebands, alongside the amplitude trend, is how an analyst separates a general flow issue from a localized defect.
Because blade-pass and vane-pass frequencies are tied to running speed, watching them meaningfully requires knowing both the running speed and the blade or vane count, so the analysis lands on the right order rather than a fixed hertz value that drifts as the machine's speed changes. On variable-speed pumps and fans this is where order-referenced analysis matters, since a fixed-frequency spectrum would let the vane-pass peak wander. A monitoring system that carries the machine's running speed alongside its vibration can place the blade-pass or vane-pass component correctly and trend its amplitude even as the machine changes speed.
A platform that historizes the spectrum and the running speed together lets these components be trended as a routine part of monitoring rather than only checked during a special analysis. Merobix pulls the vibration spectra and the speed reference into a browser-accessible history, so the amplitude at blade-pass or vane-pass frequency can be tracked over time against the machine's healthy baseline, and a steady rise raises a flag before it becomes severe. Because process data such as flow and pressure can live in the same history, an analyst can also see whether a spike in vane-pass energy lines up with the pump being pushed off its normal operating point.
That combination of vibration and process context is where a SCADA history is especially powerful for these frequencies. A growing vane-pass component that coincides with the pump running at low flow tells a different story than the same growth at normal flow, and having both the vibration trend and the operating conditions in one record lets an operator distinguish an operational cause from a developing mechanical or clearance problem. For fleets of pumps, fans, and compressors across remote sites, trending blade-pass and vane-pass amplitudes against baseline and against operating conditions turns these predictable order lines into an early-warning indicator of aerodynamic and hydraulic trouble.
Multiply the number of blades or impeller vanes by the machine's running speed. For example, an impeller with a given number of vanes turning at a given speed produces vane-pass pulses at that vane count times the running speed, landing on a specific order line above the running-speed peak. Knowing the blade or vane count is essential, because it tells the analyst exactly where on the spectrum to look.
No. A modest, steady blade-pass or vane-pass component is normal on any machine that moves fluid past stationary features, since each blade or vane naturally produces a pressure pulse as it passes. The diagnostic concern is a change: a component that grows over time, becomes dominant, or develops sidebands indicates the flow interaction has worsened beyond the machine's healthy baseline. It is the change relative to baseline, not the presence of the peak, that matters.
Sidebands spaced at running speed on either side of the vane-pass peak indicate the blade-passing event is being modulated once per revolution, which points to an asymmetry around the wheel. This could be an uneven gap, a damaged or fouled vane, or looseness that lets the interaction vary with angular position. A clean single peak suggests a uniform, symmetric interaction, while sidebands suggest the wheel sees different conditions at different positions.
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