Understanding the API 684 Standard

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Portable balancer & Vibration analyzer Balanset-1A

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Optical Sensor (Laser Tachometer)

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Dynamic balancer “Balanset-1A” OEM

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API 684 (American Petroleum Institute Standard 684: “API Standard Paragraphs Rotordynamic Tutorial: Lateral Critical Speeds, Unbalance Response, Stability, Train Torsionals, and Rotor Balancing”) is a comprehensive tutorial document that explains how to perform rotor dynamics analysis on turbomachinery. Unlike a prescriptive specification, API 684 is educational: it teaches engineers how to calculate critical speeds, predict unbalance response, assess rotor stability, evaluate torsional vibration, and establish balancing requirements for the equipment covered by other API standards (API 610 pumps, API 617 compressors, API 612 steam turbines).

1. Definition: What is API 684?

API 684 serves as the theoretical foundation and analytical guide that underpins the vibration requirements written into the various API equipment standards. Where those standards state what a rotor must achieve, API 684 supplies the how and why — the analysis methods, the derivation of acceptance criteria, and the troubleshooting approaches that rotating-equipment engineers and analysts rely on. Its tutorial style means it reads as much like a textbook as a standard, which is precisely why it is so widely used for training.

2. Standard Structure

API 684 is organised into three tutorial parts, moving from lateral behaviour through torsional behaviour to balancing.

Part 1: Lateral rotor dynamics

• Critical speed analysis: methods for calculating lateral critical speeds and the associated mode shapes.
• Unbalance response: predicting the vibration a rotor will produce in response to a given unbalance distribution.
• Stability analysis: assessing how much margin a rotor has before self-excited instability sets in.
• Modelling techniques: finite-element and transfer-matrix methods for building the rotor-bearing model.
• Acceptance criteria: how to judge whether the analysis results are acceptable.

Part 2: Train torsional analysis

• Torsional natural frequencies: calculation methods for the whole shaft train.
• Forced response: prediction of twisting vibration from known excitation sources.
• Transient analysis: startup, shutdown, and fault conditions such as short-circuit torques.
• Acceptance: stress limits and separation margins for safe operation. The broader discipline is covered in torsional analysis.

Part 3: Rotor balancing

• Balance criteria: application of ISO balance quality grades (G-grades) — now defined in the modern ISO 21940-11 series, which superseded the older ISO 1940-1.
• Shop balancing: procedures and tolerances for the balancing machine.
• Field balancing: in-situ correction methods on the assembled machine.
• Flexible-rotor balancing: the special considerations that apply when a rotor bends near or above its critical speed.

3. Key Concepts Addressed

Several recurring design rules sit at the heart of the document:

• Separation margins: operating speed should sit at least 15–20% away from any critical speed, with larger margins demanded of low-damping systems. API 684 gives guidance on evaluating whether a margin is adequate.
• Damping requirements: minimum damping levels for safe operation, expressed through log-decrement criteria and limits on the amplification factor at each critical speed.
• Stability criteria: analytical methods for predicting the onset of instability, determining the threshold of stability, and selecting bearings that keep the rotor stable.

4. Practical Application Across the Machine Life Cycle

API 684 is used at three distinct stages.

Design phase

The manufacturer performs a rotor dynamic analysis using API 684 methods, predicts the critical speeds and unbalance response, documents the work in the vendor submittal, and the purchaser reviews and approves it before metal is cut.

Commissioning

Once the machine runs, the measured behaviour is compared against the predictions: critical speeds are checked to fall within the predicted range (typically ±15%), the unbalance response is confirmed to match the calculations, and Campbell diagrams are validated against the as-built rotor.

Troubleshooting

When a problem appears in service, API 684 guidance helps diagnose it: the rotor dynamic model is updated from test data, proposed modifications are evaluated, and their effects are predicted before any change is implemented.

5. Relationship to Other API Standards and Its Value

API 684 is the connective tissue between the equipment standards. API 617 (compressors) references it for rotor dynamics requirements and specifies that analysis must follow API 684 methods, with acceptance criteria derived from its principles. API 610 (pumps) bases its critical-speed, rotor-dynamics, and balancing requirements on the same document. API 612 (steam turbines) calls for both lateral and torsional analysis per API 684. It also complements the machinery-protection requirements of API 670, which governs the monitoring instrumentation that watches over the rotor in service.

The value of this shared foundation is threefold. It gives the industry a standardised approach — a common methodology that yields consistent analysis quality and lets purchasers compare vendors on equal terms. Its tutorial format makes it a genuine educational resource and training tool, not merely a list of requirements. And because it is built on decades of proven practice, it incorporates hard-won lessons learned, reducing the risk of inadequate analysis and improving equipment reliability.

6. Who Performs the Analysis and What They Deliver

The analysis is carried out by equipment manufacturers (OEMs), engineering contractors, specialised rotor-dynamics consultants, and end-user engineering departments — work that requires dedicated software and experienced analysts. A typical deliverable package includes the rotor dynamics report, Campbell diagrams, unbalance-response predictions, a stability analysis, torsional analysis where applicable, and recommended operating-speed ranges. While API 684 governs the deep analytical work on critical turbomachinery, the everyday verification it ultimately calls for — confirming the actual unbalance response and trimming a rotor in its own bearings — is exactly what a portable two-channel analyser such as the Balanset-1A does on site, measuring 1× amplitude and phase and balancing to an ISO 21940-11 grade.

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