Why we balance to G2.5 for API 673 service when the industry accepts G6.3

“Dynamically balanced” on a fan quotation can mean a measured, recorded number — or it can mean nothing at all. The word only becomes a commitment when it carries a grade, a speed, and a residual-unbalance figure you can read off a test record. For general industrial fans we balance to G6.3. For API 673 critical service we hold G2.5 as our house balancing standard — and the reason is arithmetic, not marketing.

The G-grade is a number, not an adjective

ISO 1940-1 (now ISO 21940-11) defines balance quality as a G-grade: the product of a rotor’s permissible specific unbalance and its angular velocity, expressed in mm/s. Lower G means finer balance. The grade on its own is abstract — what the balancing machine actually measures, and what the report actually states, is a permissible residual unbalance in gram-millimetres. That number falls out of the grade, the rotor mass and the service speed:

• e_per (g·mm/kg) = 9549 × G / N — permissible specific unbalance, where G is the grade (mm/s) and N is speed (rpm).
• U_per (g·mm) = e_per × rotor mass — the total tolerance, split between the two correction planes.

One rule governs the whole calculation: N is the maximum speed the wheel can see in service — the VFD ceiling frequency, not the nominal motor speed. Plug in a lower N and you silently inflate the tolerance.

The worked example: same wheel, two grades

Take a shrouded impeller of 120 kg running at a maximum 1,480 rpm. At the general-service grade G6.3, e_per = 9549 × 6.3 / 1480 = 40.6 g·mm/kg, so U_per = 40.6 × 120 ≈ 4,877 g·mm total. Re-run the same wheel at G2.5 and the tolerance collapses:

120 kg wheel · 1,480 rpm max	G6.3 (general)	G2.5 (critical)
Specific unbalance e_per (g·mm/kg)	40.6	16.1
Total residual U_per (g·mm)	4,877	1,936
Per-plane share (g·mm)	~2,439	~968
Correction mass at 600 mm radius (g)	~8.1	~3.2

G2.5 is roughly 2.5× tighter than G6.3 (the ratio of the grades themselves), and the residual you are allowed to leave in the wheel drops from about 4,877 to about 1,936 g·mm. At a 600 mm correction radius that is the difference between chasing an 8 g imbalance and a 3 g one — well inside the resolution of a properly calibrated soft-bearing machine, but only if you set out to hold it.

Two planes, because a wheel is not a disc

A real impeller distributes its mass along the shaft axis, so correcting in a single plane can leave a couple that still shakes the bearings. Production balancing is therefore two-plane: measure and correct at both faces, and the per-plane residual — roughly half of U_per for a symmetric wheel between bearings — must sit at or below its share before the wheel passes. Correction weights are welded or mechanically fixed per the impeller material rules; nothing is tacked on where it could detach into the airstream.

Why API 673 service gets the tighter grade

API 673 (3rd Ed.) covers special-purpose fans for petroleum, chemical and gas service, and it does not invent its own balance limit — it invokes ISO 1940-1 directly. That is the honest chain: the standard points at the residual-unbalance method above, and critical-service fans are held to a finer grade because the vibration they can tolerate over years of continuous duty is far lower. To be precise about what we are claiming: G2.5 is our balancing house standard for API-class and critical-service wheels, verified on the shop machine and recorded — it is not, and we do not present it as, a witnessed API 673 mechanical run-test. Two different assurances; we are careful not to conflate them.

The report that proves it

None of this counts unless it is written down. Our balancing report travels with the fan and states: impeller mass, service speed, specified grade, the U_per calculation, initial and final residual unbalance per plane, correction masses and their positions, the balancing machine ID and its calibration-due date, and operator plus QA signatures. A wheel that will not come inside tolerance is quarantined, not shipped “close enough.” If you are specifying a critical-service fan, the three questions worth asking any supplier are: what grade, at what maximum speed, and may I see the residual on the record?

Balance is a state, not a permanent property — it is lost to erosion, to one-sided deposit, and to a cleaning crew that knocks off a load-bearing weight. That is why we re-balance after any rotor work before restart, and why field trim-balancing is judged against ISO 20816 vibration zones. If a fan in service is shaking, first prove it is imbalance and not resonance — see our notes on field vibration diagnostics and resonance versus imbalance. And because N drives the whole tolerance, it pays to nail the service speed down first when specifying the duty point.

Talk to us about balancing to G2.5 →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.