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Extend SIL proof test intervals with new Liquiphant FTL51

Saving time and resources on proof testing with innovative technology

CABB AG in Switzerland attaches great importance to plant safety. That is why it has decided on a new model of the proven tuning fork level switch from Endress+Hauser. Operated in homogeneous redundancy, the new Liquiphant ensures reliable point level detection in a SIL3 safety loop. The device supports CABB AG`s new testing strategy for proof tests, which re-quires a comprehensive SIL proof test to be carried out with almost 100% proof test coverage (PTC) every three years.

Liquiphant FTL51 extends SIL proof test intervals ©Endress+Hauser

The new Liquiphant FTL51 in a SIL3 application with thionyl chloride.

Customer benefits

  • Shorter intervals in which the measuring devices are inactive reduce the plant's downtime.

  • The probability of systematic errors is reduced because the measuring device has to be removed and installed less frequently.

  • Thanks to the development in accordance with IEC 61508, the device can also be used directly in SIL safety circuits without the need for additional operational testing by the user

Michael Lemke, expert for functional safety at CABB

With Heartbeat Technology we verify the measuring instrument without process interruption. This saves a lot of time and resources.

Mr. Michael Lemke, Functional Safety Expert


According to Michael Lemke, functional safety expert at CABB AG, more and more SIF circuits (safety circuits with a defined SIL level) are being installed in the plant. Accordingly, the maintenance and testing effort increases. This is aggravated by the fact that the time windows for carrying out proof tests are becoming shorter and shorter, while the number of maintenance staff remains the same.

Endress+Hauser solution

The Liquiphant is used in a SIL3 application. It consists of four tanks, which are connected by pipes that communicate with each other. The medium used is thionyl chloride which is acutely toxic to humans. For this reason, the technicians remove the devices for a comprehensive test according to SIL guidelines under full protection. The tank must be emptied and cleaned for this purpose. In the past, the level switch had to be removed for testing every year.

By using Endress+Hauser measurement technology, this effort has been reduced significantly. The new Liquiphant FTL51 is used in 1oo4 architecture and homogeneous redundancy on the tanks. Each individual tank is therefore equipped with a new Liquiphant FTL51. Each of the measuring devices has a Bluetooth interface in conjunction with Heartbeat Technology. This technology makes it possible to verify the level switch within the SmartBlue app while the process is running.