Contactless, currentless, safe

The author: Thomas Münstermann Head of Process Safety Department/ Product Manager, Rembe Safety + Control

Companies and facility operators are faced with major challenges when it comes to process safety. Legal requirements, regulations and approvals, internal specifications and potential risks to employees and the environment increasingly force the operator’s hand. Pressure vessels and systems, especially, have to be protected against inadmissible loads. In this respect, bursting discs are considered a reliable and maintenance-free protection concept for all branches of activity. As well as these protective systems, safe alarms are also rapidly gaining in significance in process technology. Most new installations are equipped with additional sensors to transmit an emergency stop signal to the safety system of the entire facility if required. This avoids potentially high consequential damage and long downtimes. Such damage can easily add up to six or seven figures.

For more than forty years now, Rembe Safety + Control has been very successfully helping to avoid this enormous damage and minimise the follow-up costs, amongst other things using mechanical pressure relief systems and suitable burst sensors. In many cases, bursting discs are designed with integrated or retrofitted detection systems which have a classic electrical interrupt contact. If an emergency occurs within the facility, the bursting disc is triggered, snapping the sensor cable. The current flow is interrupted because the wire has been destroyed and the event is signalled to the process control system. This method of monitoring is reliable and has been established for decades. In the process, however, the actual signal transmitter (cable) is damaged beyond repair. Like the bursting disc itself, the cable must be replaced each time an alarm is tripped, leading to extra but avoidable costs.

A classic burst sensor is installed in direct contact with the bursting disc. In critical processes with a low response pressure, the additional burst resistance (burst sensor) can negatively affect the system’s properties and significantly reduce process safety. By comparison, there are a whole series of contactless processes such as magnetic, inductive or capacitive proximity sensors which have no influence on the burst-ing behaviour. This advantage is at the expense of increased sensitivity to electrical and magnetic interference signals as well as natural or induced vibration. The outcome: false alarms which restrict their use in many areas.

There are no solutions at all available in the market for systems with very high leakage requirements and very small nominal widths, which adds an element of uncertainty to the production process. To fill this gap, Rembe Fibre Force cooperated with its affiliate Rembe Safety + Control to develop an optical safe alarm, the SLR90. This contactless and currentless design is ideal for difficult and critical environmental conditions to satisfy the requirements in chemical engineering, atomic structures, special constructions and facility construction.

A narrow-band, specially modulated, optical signal is sent to the bursting disc to detect even the smallest changes due to corrosion, leakage or bursting. This status information is transmitted in an isolated state to an easy-to-operate optoelectronic converter unit, then converted to an industry-standard useful signal such as 1 to 5 V, 4 to 20 mA or for different bus systems (including Profibus, Ethernet/IP, Ethercat, DeviceNet) and forwarded to the safety system in real time. Apart from continuous status information, the SLR90 optical safe alarm function supplies a command signal as an alarm contact whenever the bursting disc responds, to enable the system to be stopped in an emergency. The de-energised signal is insensitive to electromagnetic interference and as such ideal for highly corrosive or explosive atmospheres.

No specialist or prior knowledge is required when the optical unit is used in the field. The burst sensor can be installed and adjusted by any experienced electrician. The optoelectronic converter unit can be mounted easily into any switch cabinet on a DIN rail and integrated directly in the process control system as a plug & play component. The SLR90 is positioned on the side facing away from the process and indicates even very low response pressures reliably. Operating temperatures of up to 350 °C at the signal transmitter are likewise possible using a special version. As a result, high-temperature processes for which the market currently has no – or virtually no – solution can also be monitored.

The simple modular design of the SLR90 makes it possible to react quickly and individually to customer-specific requirements and adjust the burst sensor to extreme conditions. For example, the SLR90 can be inserted into a client’s piping system for ultra-pure media downstream of the screw-in bursting disc unit. If a second screw-in unit is used, the space required for the burst sensor is less than 40 mm. Retrofitting in the bursting disc holder or screw-in unit is an option in almost all installation scenarios.

Unlike classic interrupt contacts, the SLR90 does not need to be replaced if it bursts. This results in a longer life, less unscheduled maintenance, fewer downtimes and lower costs. In addition, the SLR90 can be incorporated into an existing system as a separate, autonomous burst sensor to achieve a higher safety integrity level (SIL), for instance if IEC 61508 or IEC 61511 is stipulated. The aim is to ensure a reliable process by monitoring all components.

In particular, bursting discs with a small nominal width (¼” to DN 25) and minimal leakage rate, including reverse acting bursting discs, could not previously be integrated into the process control technology because a suitable so-lution was lacking. For the first time, the SLR90 enables these discs to be indicated. The safety of the system is increased while simultaneously reducing the potential risk to staff.

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