The blower was not vibrating. The bellows was bolted solid.

At-a-Glance

The Setup

Shot-blasting is a hard duty. The line has to move a firm, steady volume of air against real system resistance — here, 6,500 CMH at 350 mmWC on a 15 HP drive — and it has to do it inside somebody else's factory, next to people. Our customer was the equipment OEM; their customer was the plant. The blower was ours. The installation, the ductwork and the foundation were site work.

That layering is the whole story. When a fan misbehaves at a site two steps removed from the works, it is the most visible object in the room and therefore the first thing blamed. Very often it is only the messenger.

The stakes were not comfort. The end-plant was holding the OEM's commissioning certificate over the noise — so a blower already built, tested and dispatched now stood between our customer and the close of their contract.

The Complication

The site's report was specific and, on its face, damning: the blower was audibly over 85 dB, where the plant expected below 75 dB "at worst case." Eleven photographs came with it. Noise plus vibration plus a held certificate is the classic signature of a fan fault — imbalance, a bearing gone, an impeller fouled or damaged in transit.

The obvious suspect was the rotating assembly, and it was wrong twice over. Nobody at site had reported a bearing failure, an impeller defect or a motor fault — noise and vibration were the symptoms, and the cause was simply being assumed. And the "<75 dB" figure the plant measured against had never been agreed with anybody: an expectation that travelled downstream, not a specification that travelled upstream.

The Diagnosis & Fix

We took the complaint the day it landed and issued our fan troubleshooting checklist to site — a structured elimination sequence rather than a guess, and the fastest containment available when the fan is a long way from the works. In parallel, we worked the photographs. The order of elimination matters:

1. Ruled out a mechanical failure of the rotating assembly — no bearing failure, impeller damage or motor fault was reported by site, and none was visible.
2. Ruled out an out-of-spec fan on noise — the approved GA drawing carries a stated sound level of 92 dB. The level actually reported from site was 89 dB: inside the figure the customer had approved. The complaint was being measured against 75 dB, a number that appears nowhere in the agreed documents.
3. Found the real defect — in the installation, not the machine. Plainly visible in the very first site photograph: the transit restraint links on the flexible (bellows) joint had never been removed. They were still bolted on, with the fan running.

A flexible joint exists to decouple. Leave the shipping restraints fitted and it cannot float — it becomes a rigid steel bridge from the fan into the ductwork and the building, short-circuiting the exact isolation it was installed to provide. Structure-borne vibration goes straight into the plant, and the plant gets louder. A commissioning-stage miss: nothing to do with how the blower was built or balanced.

The Result

Stated honestly, because the record supports this much and no more:

• We proved the acoustic complaint was a specification mismatch, not a defect — 89 dB reported against a 92 dB approved-drawing figure. The "<75 dB" target was set downstream of the agreed value; it was never a contracted number.
• We found a concrete physical installation fault from the site's own photographs, on day one — the flexible-joint transit links still fitted while the blower ran.
• We issued the corrective action: remove the links, re-run the troubleshooting checklist, revert with feedback.
• We changed our own process: "bellows / flexible-joint transit restraints removed" is now a hard, photo-signed line item before a fan is energised; restraints get a bright remove before operation tag at works; and the guaranteed sound level is stated explicitly on the GA and confirmed against the end-user's expectation at order stage.
• What the record does not confirm: there is no post-correction vibration or noise re-measurement, no re-test against ISO 14694 limits (rigidly-mounted BV-3: 4.5 accept / 7.1 alarm / 9.0 mm/s shutdown), and no written confirmation that the certificate was released. The file stands as action taken, outcome not confirmed — and we would rather say so than claim a resolution we cannot show you.

The Takeaway + Call to Action

Two cheap checks settle most "the fan is vibrating" calls before an engineer books a flight. One: walk the installation and confirm every transit restraint, bellows link and shipping bracket is off — a photograph will usually find them. Two: open the approved drawing and read what sound level was actually guaranteed. An expectation that was never agreed is not a failure; it is a communication defect, fixed at order stage, not at site.

We service centrifugal fans and blowers of any make — vibration and noise investigation, balancing to ISO 21940, performance verification tested to the IS 4894 / ISO 5801 / AMCA 210 method, bearing and impeller work, and commissioning support. If a fan on your line is being blamed for something the ductwork is doing, we will tell you so.

— Jitamitra Electro Engineering · Technical Services

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