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LEV testing - fan decline
An LEV system rarely fails with a bang. Far more often the fan - the heart of the system - loses performance slowly: a slipping belt, a fouled impeller, worn bearings, each shaving a little off the airflow. Nothing looks broken, the fan still turns, and control quietly slips below what is needed without anyone noticing.
The short answer
The most dangerous LEV failure is not the fan that stops - that gets noticed and fixed. It is the fan that keeps turning but slowly loses performance: a slipping or worn drive belt, dust and grease building up on the impeller, worn bearings, all sapping the airflow bit by bit. The system still runs and still looks like it is working, but the airflow at the hood has fallen below what is needed to capture the contaminant. Because the decline is gradual and invisible, nobody notices - which is exactly why a periodic thorough examination and test, measuring the actual airflow, is what catches it.
The failure you notice vs the one you don't
There are two ways an LEV fan can fail, and they are not equally dangerous. The obvious one is the fan that stops - a motor burns out, a belt snaps, the fan simply does not run. That failure gets noticed almost immediately, because the extraction visibly stops, and it gets fixed. It is a problem, but a self-announcing one.
The dangerous failure is the other kind: the fan that keeps running but slowly loses performance. The motor turns, the fan spins, the system hums along and looks entirely normal - but the airflow it is producing has quietly dropped. Because nothing has visibly broken, nobody investigates. The extraction looks like it is working, so it is assumed to be working. Meanwhile the airflow at the hood may have fallen below what is needed to capture the contaminant, and exposure is no longer being controlled. The slow failure is the one that catches people out.
What saps a fan's performance
Several things degrade a fan's performance gradually. A drive belt, on a belt-driven fan, stretches and wears over time, so it slips - and a slipping belt means the fan turns slower than it should, moving less air. The impeller - the bladed wheel that actually moves the air - collects dust and grease from the extracted air, and a fouled impeller is both heavier and less aerodynamically efficient, so it moves less air for the same effort. Bearings wear, adding friction and further reducing performance.
None of these announces itself. A belt does not suddenly slip by a lot; it gradually slips more as it wears. An impeller does not foul overnight; it builds up over months. Bearings wear slowly. So the fan's performance declines steadily and quietly, each factor taking a little more off the airflow. The result is a system that is still running, still looks fine, but is moving materially less air than it was - and materially less than it needs to. And because each cause is gradual, the decline is impossible to spot by eye or ear from one day to the next.
Why gradual is dangerous
The danger in a gradual decline is precisely that it is gradual. If the airflow halved overnight, someone would notice the difference - the pull at the hood would obviously be weaker. But when the airflow drops by a few per cent a month over a year, no single day feels different from the one before, and the slow slide passes unremarked. By the time the airflow has fallen far enough to matter, the change from any given day to the next has been too small to register.
And what matters is a threshold: capturing the contaminant needs a certain airflow at the hood, and below that the system stops controlling exposure. A fan whose performance has declined can drift below that line while still running and still looking fine. The staff are then being exposed, but nothing signals it - the system is on, the fan is turning, and the failure is invisible. This is why the slow fan failure is more dangerous than the dramatic one: the dramatic one gets fixed, while the slow one goes on exposing people precisely because it does not look like a failure.
What catches it
Because the slow fan failure is invisible to the eye, catching it needs measurement - and that is exactly what a thorough examination and test does. The test measures the actual airflow the system is producing and the capture velocity at the hoods, and compares them against what the system needs to control the contaminant. A fan that has declined shows up as measured airflow that has dropped below the benchmark, even though the system looks and sounds fine. The instruments see what the eye cannot.
This is one of the strongest arguments for periodic testing: it is the mechanism that catches gradual decline before it becomes a real exposure. A slipping belt gets identified and replaced; a fouled impeller gets cleaned; worn bearings get renewed - and the airflow is restored. Between tests, routine maintenance and simple checks (belt tension, impeller cleanliness, any change in the pull at the hood) help, but the definitive catch is the periodic measured test. It converts an invisible, dangerous slow failure into a measured fact that can be put right.
The takeaway
An LEV fan usually does not fail suddenly - it fades. A slipping belt, a fouled impeller, worn bearings each sap a little airflow, and the decline is gradual, quiet and invisible. The system keeps running and keeps looking like it works, while the airflow at the hood slips below what is needed to capture the contaminant. The dramatic failure gets noticed and fixed; the slow one goes on exposing people precisely because it does not look like a failure.
So the slow fan failure is the one to guard against, and the guard is measurement. A periodic thorough examination and test measures the actual airflow and capture velocity and compares them to what the system needs - catching a declined fan that looks entirely normal. Backed by routine upkeep between tests, that measured check is what stops a gradually fading fan from quietly becoming an uncontrolled exposure. The fan that hums along looking fine is not proof of control; the measured airflow is.
Questions
Gradually, not suddenly. A slipping or worn drive belt, dust and grease fouling the impeller, and worn bearings each sap a little airflow over time. The fan keeps turning and the system looks normal, but it is moving materially less air than it should - a slow, invisible decline.
Because a stopped fan gets noticed and fixed, while a fading one keeps running and looking fine. The gradual airflow loss passes unremarked day to day, so the system can drift below the airflow needed to control exposure without anyone realising the staff are no longer protected.
Chiefly three things: a drive belt that stretches and slips as it wears, an impeller that collects dust and grease and so moves air less efficiently, and bearings that wear and add friction. Each is gradual, so the decline is steady and hard to spot by eye or ear.
By measurement. A thorough examination and test measures the actual airflow and the capture velocity at the hoods and compares them with what the system needs. A declined fan shows up as measured airflow below the benchmark, even though the system looks and sounds completely normal.
Not reliably by eye - that is the problem. Routine checks help: belt tension, impeller cleanliness, and any change in the pull at the hood. But because the decline is gradual and the system keeps running, the definitive catch is the periodic measured test, not casual observation.
The cause is put right - the belt is replaced, the impeller cleaned, worn bearings renewed - and the airflow is restored to what the system needs. The test converts an invisible, dangerous slow failure into a measured fact that can be corrected before it becomes a real exposure.
Our thorough examination measures the actual airflow and capture velocity your LEV is producing - catching a fan whose performance has quietly declined while the system still looks like it works.
