The following points concerning sensing devices are important to consider when designing alarm or shutdown systems:
• Shutdown sensors (pressure, level, flow switches, etc.) should be located such that they are readily accessible for routine testing and maintenance.
• A system shutdown device should be dedicated exclusively for shutdown purposes and should not serve in a dual capacity as a control device. A device for miscellaneous and pre-shutdown functions may serve in a dual capacity for control purposes.
• Sensors are usually connected directly to the process and therefore are often in contact with hot, corrosive, contaminating substances that can seriously affect sensor reliability. If point sensors of adequate design are not available, shutdowns may be connected to a signal from a dedicated transmitter. Then the problems of process interface can be handled by the transmitter, which can be monitored continuously. When using programmable controllers, transmitters can be directly wired to an analog card and the shutdown switch programmed in the software. Sometimes specially designed transmitters, such as flow, analysis, or interface level, result in a more reliable system.
• To reduce confusion and accidental shutdowns, consideration should be given to equipping sensors with only one contact dedicated to the shutdown system.
• Multiple microswitch contacts in sensors that are not snap acting will not actuate simultaneously. They can either shut down equipment without actuating the associated shutdown alarm, or activate that shutdown alarm without shutting down equipment. To reduce confusion and the chance of a shutdown occurring without any annunciation, interposing relays with multiple contacts driven by a single contact in the sensor are recommended. This assures simultaneous alarm and shutdown activation. Programmable controllers are ideal for providing software relays to do this.
• Dual setpoint devices should not be used for shutdown services except for vibration, analyses, or bearing-temperature instruments.