Finding meaningful and accurate failure data and failure rates is one of the key challenges of SIS engineering. According to IEC 61511 2nd edition:
“The lack of reliability data reflective of the operating environment is a recurrent shortcoming of probabilistic calculations” (11.9.3 note 2).
Ideally, everyone implementing SIS would have a large database of high quality, locally sourced, prior use data. In reality, engineers often have to draw from a variety of sources of failure rate data to accurately model a system.
The list below is not exhaustive, but it should provide a good starting point toward finding the required reliability data for any SIS design where prior use data is unavailable. It is a good idea to cross-reference multiple sources to understand the potential uncertainties in the data. Some more sophisticated data sources will even provide uncertainty estimates with their data.
Caution about Failure Data
Common issues to look out for include failure data that:
- Is based on manufacturer warranty return data. These may under-report failures.
- Is based on high demand or accelerated life testing (e.g. B10 testing). These may be based on different wear-out failure modes than encountered in process applications.
- Is based on theoretical FMEDA studies. These studies may assume ideal environmental or process conditions and neglect real-world failure modes (e.g. plugging, corrosion).
- Is reported as PFD or PFDavg rather than failure frequency. The basis of the PFD calculation (e.g. test interval, test coverage, etc.) needs to be thoroughly understood.
- Is significantly lower than data for similar devices, especially for SIL certificates. There is a wide range of quality in the certification world. Strong claims should be backed up with strong data, preferably from real-world application.
- Omits portions of the device or interface. For example does the valve include the actuator? the solenoid? Does the transmitter include the impulse lines?
- Has an unclear or inconsistent definition of failure. For example, I recently looked at Pressure Relief Valve (PRV) failure data, and I found that my four data sources had four different definitions of “failure”.
- Does not specify or specifies a very short useful life. A failure rate based on a 5 year useful life is meaningless if the equipment is expected to be in service for 15 years.
Useful Life Caution
I would like to reiterate the last point about useful life. Useful life has historically been been poorly understood and inconsistently observed in the SIS community. Maybe I will cover it in detail in a future post. For now, consider that IEC 61508 states that probabilistic calculations based on constant failure rates are “meaningless” when components are used beyond their useful life.
None of the above issues imply that any of the data sources are incorrect or invalid, but the basis of the failure rate data must be understood to determine if the data is applicable for a particular application. Even when perfectly applicable data is unavailable, it is often possible to adjust the available data to better reflect the application.
SIL Certificate Databases
SIL certificates are a convenient source of data, but should be view skeptically and cross-referenced with other sources since they may sometimes omit certain components from the analysis or make unrealistic assumptions about operating conditions.
In some cases, it may be easier to find SIL certificates directly from the manufacturer. I am just listing a few popular vendors who happen to have centralized locations for many of their certificates. Keep in mind that in addition to SIL Certificates, the product Safety Manual will often have more detailed information. Watch out for manufacturers that don’t have safety manuals; that’s a strong sign they do not understand IEC 61508 / 61511.
Other Web Resources for Failure Data
Miscellaneous other free resources
- SILSafeData.com (exida) (good sanity check for other data sources)
- Failure Rate and Event Data for use within Risk Assessments (UK HSE)
- Process Equipment Reliability Database (CCPS) (members only)
- MIL-HDBK-217 (I purposely do not link to this because it has been shown to be unreliable and you should not use it)
- Non-Electronic Parts Reliability Data NPRD-2 (1981) (a very old version of NPRD, but it is free!)
- Handbook of Reliability Prediction Procedures for Mechanical Equipment (no failure rates here, but lots of failure rate models)
- Weibull Reliability Database (what if your failure rate is *gasp* not constant?)
Nuclear Industry Failure Data
The nuclear industry has a large volume of failure rate data that is generally applicable to the process industries
- Component Reliability Data for Use In Probabilistic Safety Assessment (IAEA-TECDOC-478 – 1988)
- Survey of Ranges of Component Reliability Data for Use in Probabilistic Safety Assessment (IAEA-TECDOC-508 – 1989)
- Generic Component Reliability Data for Research Reactor PSA (IAEA-TECDOC-930 – 1997)
- Industry-Average Performance for Components and Initiating Events at U.S. Commercial Nuclear Power Plants (NUREG-6928) (excellent resource)
- Industry Performance of Relief Valves at U.S. Commercial Nuclear Power Plants through 2007 (NUREG-7037)
- Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications (NUREG-1278)
Unfortunately, not all data is free. However, some of these resources may be free to you via the Internet, depending on company subscriptions, copyright scruples, etc.
- SINTEF PDS Data Handbook (highly recommended)
- OREDA Handbook (don’t really need it if you have the PDS book, IMHO)
- Safety Equipment Reliability Handbook
- Guidelines for Process Equipment Reliability Data, with Data Tables
- Guidelines for Safe and Reliable Instrumented Protective Systems (some data)
- Design of Reliable Industrial and Commercial Power Systems (IEEE 493-2007)
- Reliability Data for Pumps and Drivers, Valve Actuators, and Valves (IEEE 500-1984 P&V)
- SIF SIL Evaluation Techniques (ISA TR84.00.02-2015 Part 1) (small amount of user data)
- Electric components – Reliability – Reference conditions for failure rates and stress models for conversion (IEC 61709:2017)
There are many commercially available SIS software packages that have built-in failure rate data. I am not covering these here, as many of them obtain their data from one or more of the sources listed above. Software tools and built-in data are convenient, but all of the same caveats apply. Data must be thoroughly understood to ensure it is used meaningfully. Beyond data, it is also critical to understand how the software is using the data. Perfect failure data applied to the wrong model is still meaningless!
OTHER GUIDANCE ON FAILURE DATA
These resources do not provide failure rate data, but rather provide guidance on how to use and interpret data from other sources.
- Guide to finding and using reliability data for QRA
- Field Failure Data – the Good, the Bad and the Ugly (Goble)
- Reassessing Failure Rates (Generowicz)
- Modifying ‘SIL’ Certified Equipment Failure Rates on the Basis of Deployment (Dearden)
- Updating Failure Rates and Test Intervals in the Operational Phase […] (Hauge et al)
- Provision and Updating of Estimates of Reliability Parameters […] (Bjartnes)
- A Hierarchical Bayesian Approach to IEC 61511 Prior Use (Thomas) (my own contribution)
A common theme among much of the guidance literature is that failure rates are uncertain. The goal of searching for failure rates should not be to cherry-pick the data until we find the lowest failure rate. We should evaluate all of the available data, estimate the uncertainty, and judge the applicability to our own specific applications and environments. Later, the data should be updated based on actual experience in the application. By the way, the Bayesian framework is an excellent way to accomplish this initial analysis and future updating.
I hope this post is helpful to you. Please add any resources I may have missed in the comments.