Modifying SIL-Certified Equipment Failure Rates on the Basis of Deployment

To Identify Estimates of Equipment Failure Rates, You Must Calculate the Probability of Failure on Demand

By Harvey T. Dearden

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Table 2: Vulnerability Level for Logic Solvers and Non-Field equipment
Logic Solvers & Non-Field Equipment
Vulnerability Level
Reduced
Standard
Increased
Inherently secure design (i.e., not dependent on added security control measures) e.g. Soild State Logic, Safety PLC Benign environment - well within capability, IP rating not critical to suitability, not subject to environmental excursions Benign duty - well within capability or derated
Secure Access e.g., Relay System/Trip Amp in secure environment; locked cabinet/room authorized personnel access only Benign environment- well within capability, IP rating not critical to suitability, not subject to environmental excursions Benign duty - well within capability or derated
Standard PLC OR Relay System/Trip Amp with unsecured access OR Non-benign environment OR Non-benign duty

 

All sorts of qualifications might be introduced into the analysis of vulnerability, but given the levels of uncertainty in so many aspects of functional safety, there is limited value in refining the analysis. The important thing here is to recognise that the "raw" figure supplied by the vendor is not likely to be representative of the actual performance of the equipment in the field, and it is prudent to make some allowance in recognition of the influence of deployment.

Comparison of Field and Certificate Data

As a means of establishing a suitable allowance, we may compare nominal field values with vendor figures and identify the difference as being due to deployment. Table 3 compares some generic field values for field equipment, with nominal values representative of those typically found from vendor certification.

Table 3: Comparison of Field and Certified Values for Mean Time Between Dangerous Failures
 
Nominal MTBFd values (years)
Field Database
Vendor Certificate
Deployment
Press Tx
150
1000
175
Level Tx
115
140
600
Flow Tx
225
900
300
Temp Tx
150
2900
160
Remotely Operated Valve
115
200
265
Solenoid Valve
125
200
340

 

From consideration of these "ranging shots" and other database values, it is here suggested that a nominal figure of 300 years MTBF for dangerous failures be adopted in respect of "standard" vulnerability deployment.

From inspection of a range of database values, and as a matter of judgement, a factor 3.0 shift in values for field equipment is here suggested in respect of different categories of vulnerability. It is acknowledged there is little enough science here, but having identified that equipment will be more or less vulnerable depending on the circumstances of the deployment, it is appropriate to make some corresponding allowance, and the above is suggested as a starting point. Ultimately, the figures assumed should be validated by monitoring of the ongoing performance of the equipment.

Logic solvers and non-field equipment typically operate on benign duties in benign environments, but there will be some remaining vulnerability to the likes of voltage spikes, environmental control failures, electromagnetic and electrostatic effects, as well as unauthorized or misjudged interference. Given this residual vulnerability we may look for correspondence of the deployment element of standard vulnerability logic solvers and reduced vulnerability field equipment. In nominating the same value as for reduced vulnerability field equipment, the implication is that these residual influences represent 33% of the contribution to standard field equipment vulnerability, which does not appear unreasonable.

Suggested figures for dangerous failure rates due to deployment are given in the table below.

Table 4: Dangerous Failure Rates for Deployment.
Equipment Type
Vulnerability
Vendor Certificate
Deployment
Sensors
Reduced
900
0.0011
Standard
300
0.0033
Increased
100
0.01
Logic Solvers & Non Field Equipment
Reduced
2700
0.00037
Standard
900
0.0011
Increased
300
0.0033
Final Elements
Reduced
900
0.0011
Standard
300
0.0033
Increased
100
0.01

 

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