For safety instrumented functions (SIFs), proof testing is the process of identifying undetected dangerous failures in the sensor, final control element and logic solver which comprise the SIF. Dangerous failures are those that could put the safety instrumented system (SIS) in a hazardous or fail-to-function state. If undetected, in the case of level measurements, this could lead to an overfill and spill, with potentially disastrous consequences.
Regular proof testing is an essential requirement for all components in a SIF per the IEC 61511 global safety standard. The effectiveness of a proof test in finding these undetected failures is the proof test coverage. Exida defines proof test coverage as:
…a measure of how many undetected dangerous failures are detected by the proof test.
Manual proof testing combined with automatic diagnostics can increase this coverage. A whitepaper, Remote Proof-Testing Capabilities–Latest generation of level monitoring devices improves safety and efficiency, shares how in-process partial proof testing can be performed on liquid level measurement applications using non-contacting radar and vibrating fork level detector sensors.Emerson’s Teddy Tzegazeab contributed his expertise in level measurement and functional safety for this whitepaper. Teddy described the difference be a full and partial proof test:
A full proof-test returns the probability of failure on demand (PFD) average back to or close to the instrument’s original targeted level. A partial proof-test brings the PFD average back to a percentage of the original level.
Two methods are used to perform full proof test with level measurement devices.
…the level in the vessel can be raised to the activation point of the instrument being tested, providing functional proof that the instrument still works. The danger of this approach is that if the instrument is a critical-high or high-high level sensor for overfill prevention, and it does not activate during the test, a spill is likely, which could constitute a safety risk.
…remove the instrument from the vessel and insert the device in to a bucket filled with the product. This method may require the process to be taken offline, which may interrupt the overall production process, and manpower will be required to run the test.
Partial proof testing is performed to extend the time interval required to perform full proof testing. Partial proof testing:
…involves the level switch or transmitter being tested to ensure that it has no internal problems and all functions are operating properly. In partial proof-testing, the level instrument remains installed, and testing is done through a function test performed either locally, typically via a push button or magnetic test point, or initiated remotely via a command transmitted from the control room.
For atmospheric bulk liquid storage tanks, the API 2350 standard requires:
…all overfill prevention systems that are required to terminate receipt must be tested annually, while the high-high sensor alarm must be tested semi-annually. Additionally, continuous level sensors should be tested once a year, and point level sensors semi-annually.
Via a HART communications protocol command coming remotely from an operator control center, the Rosemount 2140 vibrating fork level detector enters a test mode, and:
…cycles the output through wet, dry and fault states, then returns into normal operation. If the partial proof-test detects an issue, it is reported on completion of the test.
For Rosemount 5408 non-contacting radar level transmitters, operator perform remote proof testing:
…by inputting a straightforward sequence of settings and commands from their interface.
Read the whitepaper for a greater understanding of how this remote partial proof testing works, considerations for guided wave radar (GWR) installations, and how this helps companies reduce risk and errors, save time, and increase safety and efficiency.
You can also connect and interact with other level measurement and functional safety experts in the Level and Safety Instrumented Systems groups in the Emerson Exchange 365 community.