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| | | | | | | | | | | | | | | | | | | | | | INTRODUCTION TO API STANDARD 670 | | | | | | | | | | The American Petroleum Institute has developed a range of standards to assist users in the procurement of standardised systems of equipment for various applications. One standard particularly relevant to the monitoring of rotating equipment is the API Standard 670 covering Machinery Protection Systems, which has become heavily adopted by industry. The standard details the following measuring techniques:- • Casing Vibration • Radial Shaft Vibration • Shaft Axial Position • Shaft Rotational Speed • Piston Rod Drop • Phase Reference • Overspeed • Critical Machinery Temperatures The standard also covers the transducer & monitoring equipment requirements as well as installation, commissioning and documentation. The majority of the above techniques have been discussed in previous sections. The standard details a clear set of mechanical and electrical properties for both accelerometers and eddy current proximity probes. Adherence by manufacturers to these specifications assist the user in finding several sources for the same component (form, fit and function), as well as a robust product designed for the monitoring of heavy industrial machinery. Parameters detailed include; sensitivity, dynamic range, operating temperature range, accuracy, mechanical mounting options, connector and cabling standards, immunity to shock, etc. The Sensonics range of vibration transducers and eddy current proximity probes both conform to the API 670 standard. In terms of the industrial monitoring system philosophy, the focus is of course on protection, detailing the appropriate measurement technique and associated system elements to form a multi-channel system with high integrity, minimising the effects of single point failures. The standard generally specifies that a single circuit board failure shall not affect more than two channels of measurement. The key concepts are as follows. | | | | | | Sensonics 'Sentry' Machine protection System conforms to the API 670 standard. A typical Sentry rack configuration is pictured below. | | | | | | | | | | Sentry Machine Protection System For each of the measurement options detailed opposite, not only is the equipment specification important but also the installation technique. The majority of the complexity and difficulty involves the transducer mounting and cabling. Each application is normally unique, the 670 standard details various standard approaches that can be implemented. Important elements to consider include access to the transducer for servicing or replacement, termination to a suitable local junction box, the selection of armoured cable with or without additional conduit for either isolation or mechanical protection, cable segregation, grounding techniques etc. Protection Systems Incorporating Voting To build on the good practice underpinned by the API 670 standard, some applications require extreme reliability and also industrial safe failure in the event of a fault. Where the system has a direct impact on safety, IEC 61508 can be applied to determine the required 'Safety Integrity Level' or SIL rating. The rating can be between 1 and 4, with 4 being the most stringent. The 61 508 standard defines a methodology for the protection system from concept through to decommissioning, the complete life cycle. To determine system suitability for such an application, the backbone of the analysis is a failure mode and effect analysis; carried out to determine the probability of a failure on demand as well the percentage of safe failure modes. This will determine the SIL rating. Voted systems can provide enhanced system reliability in combination with a high degree of failure detection, therefore quite suitable for SIL rated applications, which may be a requirement because of a commercial impact (ie unnecessary shutdown) rather than safety alone. Two out of three majority voting has already been discussed with respect to turbine overspeed protection. It is possible to implement more complex schemes through the logical combination of many measurement parameters, such as a turbine high vibration trip system where pedestal bearing vibration alarms are processed to only trip under a specific combination, thus reducing the effect of spurious events and transducer failures. Further voting can be added to the alarm processing hardware to ensure complete system robustness, any voted decision is positive detection of a system failure. | | | | | | | | | | Integral display indicating measurement & status Monitoring resolution of 2% full scale Adjustments for alarm set points and scaling Channel fault monitoring Buffered transducer outputs Digital output of measured variables and channel configuration Isolated / non isolated 4-20mA per channel Alert and danger setpoints for each channel Relay for each setpoint with programmable delay Alarm defeat for system integrity testing Power supply protection | | | | | | | | | | | | | | 19 | | | | | | | | | | | | | | | | | | | | | |
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