ISO 20816 Vibration Limits Explained: Zones, Boundary Values and How to Apply Them

Somebody in your plant is quoting a vibration limit right now, and there is a fair chance it comes from a laminated chart printed fifteen years ago, drawn from a standard that no longer exists, applied to a machine class it was never written for.

The question behind the chart is legitimate. A pump reads 5.2 mm/s. Is that a problem? The honest answer depends on which standard applies to that machine, which machine group it falls into, how it is mounted, and what it read last month. Most arguments about vibration limits are really arguments about those four things.

This article covers the standard itself: ISO 20816, the successor to ISO 10816, in enough depth to use it properly. What changed in the transition, which part applies to which machine, the four evaluation zones, the actual boundary numbers, the rigid-versus-flexible support classification that shifts those numbers by more than 50%, and the parts almost nobody reads: shaft vibration limits, measurement conditions, and the change criterion that matters more than any absolute value.

We have covered how to diagnose faults from a vibration spectrum in our guide to vibration analysis for rotating equipment. This is the companion piece: not what the vibration means, but what the standard says you are allowed to have.

From ISO 10816 to ISO 20816: What Actually Changed

For decades, machine vibration evaluation lived in two parallel standards. ISO 10816 covered vibration measured on non-rotating parts, meaning the bearing housings and pedestals where you put an accelerometer. ISO 7919 covered shaft relative vibration, measured with proximity probes watching the shaft itself inside the bearing. Two documents, two sets of limits, for what is physically one machine.

The ISO 20816 series merges them. ISO 20816-1, published in 2016, sets out the general framework and cancels ISO 10816-1. The machine-specific parts have followed one by one, each replacing its 10816 and 7919 counterparts as it is published. The part most plant engineers need, ISO 20816-3:2022 for industrial machinery above 15 kW, replaces both ISO 10816-3:2009 and ISO 7919-3:2009 in a single document: housing vibration criteria in its Annex A, shaft vibration criteria in its Annex B.

Here is the practical news for anyone holding an old chart: the familiar velocity boundary values survived the transition. The zone boundaries in ISO 20816-3:2022 for housing vibration carry the same numbers as the 10816-3 tables that preceded them. What your laminated chart is missing is everything else: the shaft vibration criteria, the displacement limits for slow machines, the transient operation guidance, and the change criterion. The numbers were never the hard part of the standard. The conditions attached to them are.

Which Part Applies to Your Machine

Quoting "the ISO limit" without naming the part is where most misapplication starts, because the series is deliberately split by machine type and the limits differ between parts.

ISO 20816-1 is the general framework: measurement principles, evaluation criteria, definitions. It contains no boundary numbers for specific machines. If someone cites limits "from 20816-1", they are improvising.

ISO 20816-2 covers land-based gas turbines, steam turbines and generators above 40 MW with fluid-film bearings, running at 1,500, 1,800, 3,000 or 3,600 r/min. This is the utility power station document.

ISO 20816-3 is the workhorse: coupled industrial machinery with a power rating above 15 kW and operating speeds between 120 and 30,000 r/min, measured in situ. Its scope list includes steam turbines and generators up to 40 MW, rotary compressors, industrial gas turbines up to 3 MW, flexibly coupled electric motors of any type, rolls, mills, conveyors, blowers and fans. For most of a factory or a captive power plant, this is the document.

ISO 20816-4 covers gas turbines above 3 MW with fluid-film bearings, and further parts cover hydro sets, reciprocating compressor systems and gearbox acceptance testing.

Just as important is what Part 3 explicitly excludes, because these are the machines people most often force into its tables anyway. Rotordynamic pumps with the impeller mounted directly on the motor shaft have their own standard (ISO 10816-7, not yet migrated to the 20816 series). Reciprocating machines, screw compressors, submerged motor-pumps and wind turbines are all out of scope. A vertical multistage pump judged against the Part 3 table is being judged against numbers derived from a different machine population, and the verdict, in either direction, means little.

The Four Zones and What They Mean

ISO 20816 evaluates vibration severity through four zones, and the wording of each zone matters because it defines the action.

Zone A is where the vibration of newly commissioned machines normally falls. The standard itself adds a note worth framing: the effort required to achieve Zone A can be disproportionate and unnecessary. Zone A is not a target for an operating machine. It is a description of a new one.

Zone B is acceptable for unrestricted long-term operation. A machine in Zone B needs nothing except continued monitoring.

Zone C is unsatisfactory for long-term continuous operation. The machine may generally be run for a limited period in this condition until a suitable opportunity arises for remedial action. Zone C is not "shut down now". It is "book the repair and stop pretending this is normal".

Zone D is vibration of sufficient severity to cause damage to the machine.

Note what the zones are for. The standard states plainly that the boundary values are not intended to serve as acceptance specifications; those are always subject to agreement between the machine manufacturer and the customer. For new or refurbished machines, acceptance has historically been specified in Zone A or Zone B, normally not exceeding 1.25 times the A/B boundary. So the zone table serves two distinct jobs: a benchmark at acceptance testing, and a coarse severity scale in operation. Confusing the two, for instance rejecting a ten-year-old machine because it is no longer in Zone A, generates work with no engineering content.

The Actual Numbers: ISO 20816-3 Boundary Values

Here are the housing vibration boundaries from ISO 20816-3:2022, expressed as broadband RMS velocity in mm/s, measured on the bearings, pedestals or housings, valid across a frequency range of 10 Hz to 1,000 Hz (extended down to 2 Hz for machines running below 600 r/min).

The standard splits machines into two groups by size. Group 1 is large machines: rated power above 300 kW, or electrical machines with a shaft height of 315 mm or more. Group 2 is medium machines: above 15 kW up to and including 300 kW, or electrical machines with shaft heights from 160 mm up to 315 mm.

For Group 1 machines on rigid supports, the zone boundaries are 2.3 mm/s between Zones A and B, 4.5 mm/s between B and C, and 7.1 mm/s between C and D.

For Group 1 machines on flexible supports, the boundaries rise to 3.5 mm/s, 7.1 mm/s and 11.0 mm/s.

For Group 2 machines on rigid supports, the boundaries are 1.4 mm/s, 2.8 mm/s and 4.5 mm/s.

For Group 2 machines on flexible supports, they are 2.3 mm/s, 4.5 mm/s and 7.1 mm/s.

The standard also publishes matching RMS displacement values alongside each velocity boundary, from 22 µm at the Group 2 rigid A/B line up to 140 µm at the Group 1 flexible C/D line. These exist because a pure velocity criterion under-protects slow machines: below roughly 600 r/min, a machine can carry a large, damaging displacement while the velocity number still looks respectable. For low-speed machinery the standard requires evaluation against both velocity and displacement, and whichever is worse governs.

Read the pattern in those numbers. That 5.2 mm/s pump from the opening paragraph is in Zone D if it is a 200 kW machine on a stiff grouted baseplate, and comfortably in Zone B if it is a 500 kW machine on a sprung foundation. Same reading, opposite verdicts. Anyone quoting a single universal limit in mm/s has flattened a two-by-two matrix into one number, and the flattening does the damage.

Not sure which group, which support class, or which part of the standard your critical machines fall under? Our maintenance and asset management team classifies the machine population, sets per-machine alarm bands and puts the paperwork behind them.

ISO 20816-3:2022 housing vibration zone boundaries, broadband RMS velocity in mm/s. The same reading can be Zone B or Zone D depending on machine group and support class. Source: ISO 20816-3:2022.

Rigid or Flexible: The Classification Most People Get Wrong

The support classification shifts every boundary by roughly 50%, and it is the input most often assigned by eye. The standard's definition is not about how solid the foundation looks. A support is rigid when the lowest natural frequency of the combined machine and support system is higher than the machine's main excitation frequency, which for most machines is running speed. If the natural frequency sits below the main excitation frequency, the support is flexible.

Two consequences follow that rarely make it onto the chart. First, the classification can differ by direction on the same machine. A machine can be stiff vertically, with its vertical natural frequency well above running speed, while the horizontal natural frequency sits considerably lower. The standard is explicit: in that case you evaluate each measurement direction against the support class that applies in that direction. A single machine can legitimately carry rigid limits vertically and flexible limits horizontally.

Second, if the classification cannot be readily determined from drawings and calculation, the standard says it may be determined by testing, typically a bump test to find the natural frequencies. That is an afternoon's work with an analyser, and it is cheaper than either of the failure modes of guessing: applying rigid limits to a flexibly supported machine and chasing alarms on a healthy asset, or applying flexible limits to a rigid one and letting a real fault run inside a generous envelope.

Shaft Vibration Limits: The Other Half of the Standard

Because ISO 20816-3 absorbed ISO 7919-3, it also carries evaluation criteria for shaft relative vibration, measured with proximity probes as peak-to-peak displacement in micrometres. This half of the standard matters for machines on fluid-film journal bearings, where the shaft rides on an oil film and the real question is how much of the bearing clearance the vibration is consuming.

The shaft criteria are formulae rather than fixed values, because the allowable displacement falls as speed rises. The zone boundaries are inversely proportional to the square root of the maximum operating speed in r/min: 4,800 divided by root n for the A/B boundary, 9,000 divided by root n for B/C, and 13,200 divided by root n for C/D, each giving peak-to-peak micrometres. For a machine running at 3,000 r/min, that works out to roughly 88 µm, 164 µm and 241 µm.

The standard then adds the caveat that makes the formulae honest: the numbers mean nothing if they exceed what the bearing can physically accommodate. Where the calculated C/D boundary approaches the bearing's diametral clearance, the limits shall be reduced, and the standard's Annex C illustrates the principle with factors of 0.4, 0.6 and 0.7 times the clearance for the three boundaries. On a large turbine or compressor train, the bearing clearance, not the formula, is frequently the binding constraint. This is one reason turbine OEM acceptance criteria are often tighter than the generic standard, and why the standard defers to them.

Shaft relative vibration boundaries are formulae, not fixed values, and fall as speed rises. Worked at two common synchronous speeds. Source: ISO 20816-3:2022.

Using the Tables Properly: Conditions, Changes and Common Mistakes

The boundary values come with operating conditions attached, and stripping them off is the most common field error after misclassifying the support.

The limits apply at steady state. The zone boundaries are defined for steady-state operation at rated speed and load. They do not apply during start-up, shut-down or passage through resonance. For those transients the standard offers separate guidance: as a general rule, operation up to the upper limit of Zone C is acceptable for limited transient periods, and during run-up and run-down between 20% and 90% of rated speed, housing vibration up to 1.0 times the C/D boundary is tolerable. Tripping a machine because it brushed 6 mm/s while accelerating through a critical speed is a misreading of the standard, and so is ignoring 6 mm/s at steady load because "it does that on start-up too".

Measure like for like. Readings must be taken at the same locations, in the same directions, under approximately the same load and speed, or comparisons are meaningless. The standard requires locations and directions to be recorded with each result, which sounds bureaucratic until two technicians produce readings 40% apart from opposite sides of the same bearing.

The change criterion outranks the absolute value. ISO 20816 contains two evaluation criteria, and almost everyone stops at the first. Criterion I is the zone table. Criterion II says that a change in vibration magnitude exceeding 25% of the Zone B/C boundary is significant and shall be investigated, even if the machine has not reached Zone C, and explicitly whether the change is an increase or a decrease. For a Group 1 rigid machine, that means a shift of about 1.1 mm/s demands investigation regardless of where the machine sits in the zones. A machine that moves from 1.2 to 2.4 mm/s is still comfortably in Zone B and has also doubled its vibration; Criterion II is the part of the standard that refuses to be reassured by that.

This is the standard agreeing with what every experienced analyst already practises: the baseline and the trend carry more information than the table. The zone boundaries were derived from fleet-wide experience across thousands of machines. Your machine is not a fleet average. Its own history is the better reference, which is the same logic that drives the shift from calendar-based to condition-based work we set out in preventive versus predictive maintenance. And a broadband number, however well judged, still cannot tell you why: a rising trend at 5.9 mm/s is a fact, but only the spectrum tells you whether it is unbalance, misalignment or a bearing on its way out.

Running machines against limits someone photocopied in 2009? Our industrial engineering services cover vibration surveys, support classification, baseline programmes and root cause analysis on turbines, pumps, compressors and generators.

Frequently Asked Questions

Is ISO 10816 still valid, or has it been fully replaced? The series is being replaced part by part. ISO 10816-1 was cancelled by ISO 20816-1 in 2016, and ISO 10816-3, the part most industrial plants used, was replaced together with ISO 7919-3 by ISO 20816-3 in 2022. A few 10816 parts, such as 10816-7 for rotordynamic pumps, remain current until their 20816 successors are published. A chart or contract citing 10816-3 still matches the current velocity tables, but the citation is out of date and the shaft vibration criteria are missing.

What is an acceptable vibration level in mm/s? There is no single number, and any answer without qualifiers is wrong. Under ISO 20816-3, unrestricted long-term operation (Zone B) extends to 2.8 mm/s for a medium machine on rigid supports, and to 7.1 mm/s for a large machine on flexible supports. The acceptable level for your machine depends on its power rating, its support classification and, above all, its own established baseline.

Do the ISO 20816-3 limits apply to pumps? Not to most of them. Rotordynamic pumps with the impeller mounted directly on the motor shaft, or rigidly attached to it, are explicitly excluded from Part 3 and covered by ISO 10816-7, which defines its own categories and values. A pump with a separate bearing frame, flexibly coupled to its motor, can fall under Part 3. Checking the exclusion list before quoting the table is thirty seconds well spent.

Are the limits RMS or peak values? The housing vibration limits are RMS velocity over the 10 Hz to 1,000 Hz band. Many portable meters and older charts display peak or peak-to-peak values, which for a pure sinusoid run about 1.4 and 2.8 times the RMS value respectively. Comparing a peak reading against an RMS table flatters nothing and condemns healthy machines; it is one of the most frequent unit errors in the field.

What about machines below 15 kW? They sit outside the scope of ISO 20816-3, which starts at 15 kW. In practice, small machines are cheap to replace and rarely justify individual evaluation against a standard; a sensible programme monitors them by exception and spends its analytical effort where the consequence of failure lives.

When do I use ISO 20816-2 instead of 20816-3? Part 2 applies to land-based steam turbines, gas turbines and generators above 40 MW with fluid-film bearings running at 1,500, 1,800, 3,000 or 3,600 r/min, which is to say utility-scale turbine-generator trains. A 20 MW industrial steam turbine belongs to Part 3; a 100 MW unit at synchronous speed belongs to Part 2. Gas turbines between 3 MW and 40 MW fall under Part 4.

Choosing the right part of ISO 20816 before quoting any limit. Most disputes about acceptable vibration are really disputes about scope. Sources: ISO 20816-1:2016; ISO 20816-2:2017; ISO 20816-3:2022; ISO 20816-4:2018.

A Table Is Not a Condition Monitoring Programme

ISO 20816 is a genuinely useful standard, provided it is used for what it is: a fleet-experience severity scale with carefully stated conditions, not a verdict machine. The engineers who wrote it say so themselves, twice, in the text: the boundary values are guidelines, not acceptance specifications, and a significant change demands investigation even when the absolute level does not.

So the working method is this. Classify each machine correctly by part, group and support, by calculation or bump test rather than by eye. Record the boundary values that actually apply to it. Establish a baseline under repeatable conditions. Then run the plant on trends and Criterion II, keeping the zone table as the coarse severity scale it was designed to be. The plants that get burned are almost never the ones that lacked a chart. They are the ones where the chart substituted for measurement.

Across four decades of turbine, generator and rotating equipment work, from steam turbine overhauls in Nigeria to root cause analysis at White Nile Sugar in Sudan, the pattern repeats: the standard tells you where the machine sits today, but only a baseline and a trend tell you where it is going.

Arguing about whether a reading is acceptable? Talk to our engineering team. We will classify the machine against the correct part of the standard, measure it under the right conditions, and give you a defensible answer instead of a laminated one.