Safety in a double pack

The author: Jessica Nentwich Product Management Analysis Transmitters, Endress+Hauser Conducta

SIL (Safety Integrity Level) – This means reducing risk to a tolerable level through error prevention and error monitoring. Thus, SIL instrumentation plays a key role in guaranteeing the safety of staff and ensures maximum safety for processes, plants and the environment. The TÜV-certified SIL2 analysis measuring point from Endress+Hauser offers this level of safety coupled with maximum measuring point availability, while also keeping costs to a minimum. The measuring point will initially be available with pH glass sensors and the digital Memosens technology with an inductive plug-in head. The SIL2 pH measuring point has not only been developed in accordance with IEC 61508, it also already complies with the latest amendments to the standard and is therefore in pole position for the future.

According to IEC 61511, proven-in-use and designed in accordance with IEC 61508 are equivalent ways of preventing systematic errors. However, the trend is clearly towards equipment that is designed in accordance with IEC 61508. For manufacturers, a design in accordance with IEC 61508 means longer development times, increased investment in documentation and, if necessary, certification by an external body, including audits. Operators benefit from this in many ways:

In the case of safety-related key data, operators must pay close attention to the assumptions on which these are based. Otherwise, a technical evaluation or even a comparison of different products is made more difficult. The requirements as per IEC 61508 and proven-in-use as per IEC 61511 are both lacking in simple practical implementation. Surprisingly low proof testing intervals are often used for the calculation, in order to achieve a specific SIL level. For example, an interval of one week means that the process shuts down once a week.

In general, good safety-related values such as the PFD value (probability of failure on demand) can be achieved by setting the initial assumptions very low. However, these assumptions are completely impractical for operators.

Endress+Hauser’s calculations for the Memosens pH analysis measuring point are based on realistic assumptions, for example a proof testing interval of one year, and no redundant use of electronic components to achieve lower PFD values. The underlying error rates of electronic components correspond to the useful lifetime as per IEC 61508–2 section 7.4.7.4 note 3. This amounts to 8 to 12 years for transmitters and cables and three years for the sensor.

The Liquiline M CM42 SIL device monitors its safety-related components during the boot process and permanently after that, to ensure that at all times safety requirements are being met and potential errors detected quickly. The system’s standard safety mechanisms which run continuously are as follows: Program sequence monitoring, a redundant logical measurement path from sensor to current output, precision monitoring, flash/ROM checks, RAM tests, redundant AD conversion, monitoring of sensor age, and much more.

Various procedures have been implemented in order to guarantee correct and safe parameter configuration:

Moreover, changes can be made only after the password has been entered. This means that the configuration cannot be changed accidentally or by unauthorised persons; in this way, operating errors are prevented in compliance with the specifications of Namur NE 79 and the new safety requirements of IEC 61508:20010. With the SIL2 device Liquiline M CM42, it is almost impossible to operate a measuring point without the required level of safety.

Overall, the measuring point implements the following safety functions: Reliable calculation and output of the pH value (with the help of reference values from calibration) at both current outputs, safe limit value monitoring, safe implementation of a two-point calibration, and on-site adjustment with the help of the display and DKD-accredited Endress+Hauser pH buffer solutions. Two-point calibration is a part of the safety function. This is because when calculating the pH values, the chosen reference procedure (in this case two-point calibration) has an important role to play. For the calibration and switchover to the safe operating mode of the Liquiline M CM42, the transmitter uses special methods to visualise these processes on the display while taking user interaction into account.

In this way, the SIL measuring point can be used like a normal measuring point and employed with the same operating sequences. The differences are hidden from the user, and easy and intelligible guidance is provided on the display. The Liquiline M CM42 can also be deployed directly for monitoring the pH limit value. However, the measuring point is mostly only used to ensure that the voltage value measured at the sensor is safely transmitted to the current output of the transmitter.

Memosens is equipped with complete galvanic isolation. This means that it is uniquely resistant to environmental influences such as moisture and its associated problems, namely corrosion and salt bridges. Unexpected measuring point outages caused by moisture are consigned to the past. This progress has been made possible by the inductive, non-contact transmission of data and energy.

In addition, digital sensor intelligence provides support through preventive maintenance, e. g. tag control, a calibration timer or operating hours meter for different medium characteristics. High measuring point availability is guaranteed thanks to the possibility of precalibrating sensors in the laboratory under constant conditions, while measuring continues in the process. This means that a sensor can be replaced in less than one minute. Calibrating sensors in the rain and cold at a height of several meters is a thing of the past.

Online-Info: www.cpp-net.com/2211437