Process safety is a top concern for manufacturers with hazardous areas. I received a question on a blog post, Calibrating Coriolis Flowmeters In-Situ:
What is Emerson’s statement of proof test coverage (e.g. for SIS testing) for the in-situ tests described in this article?
I turned to Emerson’s Tom O’Banion on the Micro Motion Coriolis flow and density measurement team for his guidance. I asked if I could share it back with you. Here’s the Coriolis Flowmeter with Model 1700 or Model 2700 Transmitter – Safety Manual for SIS, IEC 61508 Functional Safety Assessment, and certificate.
Tom noted the following very important metrics regarding proof testing Coriolis meters. With a 4-20 mA loop check, an RTD check, and:
- Wet calibration, there is a 99% dangerous undetected (DU) coverage
- Smart meter verification (SMV), there is a 91% DU coverage
Here’s a table from the safety manual showing DUs for various proof test options:
The safe failure fraction (SFF) is greater than 93%, so these Coriolis devices are suitable in safety instrumented system applications up to SIL 3.
Tom stressed that smart meter verification has the advantage of extending proof test wet calibrations, which can be time-consuming and difficult to perform.
Here is some background on proof tests and smart meter verification from a whitepaper Tom co-wrote with Emerson’s Tim Cunningham, Allow Smart Meter Verification to Reduce your Proving and Proof-Test Costs. Proof test frequency is determined by reliability calculations for the given safety loop. The proof test must be performed at least as frequently as specified in the calculation in order to maintain the required safety integrity of the Safety Instrumented Function (SIF).
Smart Meter Verification is a non-intrusive methodology to verify flow tube stiffness. The verification can be done under flowing conditions, in-situ, with no interruption to the process measurements. This flow tube verification complements the long-term stability and linearity associated with Coriolis flowmeters. Flow tubes stiffness can be shown to directly correlate to the flow calibration factor. Verifying that the stiffness is unchanged from the factory baseline confirms that the FCF is still correct. Stable verification results suggest that the proving intervals might be extended.
You can visit the Micro Motion safety certified products page for more background on their use in process safety applications.