General API 670 requirements for machinery protection systems

The American Petroleum Institute’s standard for machinery protection systems (API Standard 670) is a widely recognised industry standard, used actively by regulators and operators around the globe.

The API 670 standard describes the minimum requirements for a machinery protection system (MPS) that measures shaft and casing vibration, shaft position, shaft rotational speed, piston rod drop, overspeed, compressor surge and/or critical machine temperatures. In order to reduce the chances of misinterpretation, API 670 also includes detailed guidelines that apply when designing, applying, testing and maintaining machinery protection systems.

Note: This article references API 670, 5th edition (November 2014) to highlight and discuss some of the general design specifications described in section 4 of this standard. Bold type is used to refer to corresponding sections and section numbers (4.x) are added where useful.

Effective machinery protection system strategy

Whether generating power, driving process equipment, compressing or pumping fluids, the failure of critical production assets can have catastrophic, expensive or just time-consuming outcomes. In general, the most important considerations are the safety of personnel, the possibility of substantial machine damage or production losses that lead to a financial impact. To help avoid such events, a machinery protection system with appropriate features and functions (4.11) should be installed in order to protect a machine and its immediate environment. The MPS shall generate alarms (with alert and danger levels) in a timely manner when unwanted situations occur so that corrective actions can be undertaken. In the case of danger alarms (typically safety-related), MPSs commonly use a relay to trigger a system-level control system, such as a turbine control system, in order to automatically initiate the safe shutdown (“trip”) of the machine.

For a machinery protection strategy to be effective, the measurements must be accurate, precise and reliable. Accordingly, API 670 lists accuracy requirements (4.5) for different measurement chains across different temperature ranges for normal system operation or for testing. The reliability (4.17) of the MPS, as well as its system security, safeguards, self-tests and diagnostics (4.16), are other essential considerations for the uninterrupted, continuous operation required over long periods of time in industrial applications.

Segregation concept

Independence and separation of systems, components or parts is an important underlying design concept throughout the standard. For example:

  • Segregation (4.8) requires that the MPS shall be fully separate from any other systems, such as another protection system, control system or condition monitoring system (CMS).
  • Within the MPS, all machinery protection loops are typically hard-wired (wireless communication is not permitted here) and interconnections to other devices in the machine's auto-shutdown loop are implemented via system output relays (4.12).
  • A problem with one measurement chain, input channel or signal processing shall not affect any other channel. This applies to system power supplies (4.10) as well as to individual sensor power sources. To mitigate for such failures, MPSs often use redundant power supplies (4.11).
  • Digital communication links (4.13) to systems outside of the MPS, such as a CMS, shall not affect machinery protection functions. This also applies to buffered (“raw”) analog outputs, even in the event of a short-circuit on these outputs.
  • Similarly, interchangeability (4.6) implies that it shall be possible to physically and electrically replace components of the MPS in-situ, without going outside of the measurement accuracy requirements.

Environmental requirements

Machinery protection systems are commonly deployed in industrial or other harsh environments where specific system enclosures and environmental requirements (4.9) are necessary to comply with different area classifications. Such environments can provide challenging temperature ranges (4.1), humidity (4.2), shock (4.3) conditions and the need for chemical resistance (4.4). Therefore, measurement chains are often required to be constructed using corrosion-resistant materials. While cabling is required to be protected using rigid conduits and junction boxes as described in section system wiring and conduits (4.14), which also helps to minimize the effects of the electromagnetic interference (EMI) commonly found in industrial environments. Examples include the segregation of signal and power wires, or the use of shielded cables.

Other electrical issues, such as grounding (4.15) can adversely affect the electrical signals. The impact of incorrect or absent grounding ranges from noise and interference to a worst case scenario where electrical leakage through the chassis causes damage to instrument components or even a personal injury.

API 670 also mentions project definition and execution requirements related to the scope of supply and responsibility (4.7) that concern stakeholders from system vendors to owners.

To conclude

An understanding and appreciation of the general API 670 requirements outlined above is important because API 670 is one of the most widely applied international standards for machinery protection.

For us here at Meggitt vibro-meter, it is paramount that are our products and instrumentation are fully compliant with API 670. However, everyone involved in the design, selection, installation, operation or documentation of such systems can benefit from this important reference that is full of collective knowledge and good engineering practice.

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