{"id":52413,"date":"2021-01-03T12:09:45","date_gmt":"2021-01-03T06:39:45","guid":{"rendered":"https:\/\/ye1o34zhk8.onrocket.site\/?p=52413"},"modified":"2023-10-17T15:19:13","modified_gmt":"2023-10-17T09:49:13","slug":"sil-verification","status":"publish","type":"post","link":"https:\/\/instrumentationtools.com\/sil-verification\/","title":{"rendered":"Basic Terms used in SIL Verification"},"content":{"rendered":"\n

This article elaborates on the basic definition and use of HFT, SFF, and PFDavg terms which are widely used during the SIL verification process.<\/strong><\/p>\n\n\n\n

IEC 61511 defines the safety life cycle where in SIL verification is part of phase 4 (SIS design and engineering). <\/p>\n\n\n\n

Before this step Hazard and risk analysis, allocation of safety functions to protection layers, SIS safety requirement specification phases are completed.<\/p>\n\n\n\n

SIL (Safety Integrity Level)<\/h2>\n\n\n\n

A quantitative target for measuring the level of safety in a process. <\/p>\n\n\n\n

Defining a target SIL level<\/a> for the process should be based on the assessment of the likelihood that an incident will occur and the consequences of the incident.<\/p>\n\n\n\n

HFT (Hardware Fault Tolerance)<\/h2>\n\n\n\n

HFT is the ability of equipment to continue to perform the required function in presence of faults or errors. <\/strong><\/p>\n\n\n\n

HFT of device indicates the quality of safety system.<\/p>\n\n\n\n

HFT is N<\/strong> means N+1 faults could result into loss of entire safety function.<\/p>\n\n\n\n

HFT is 0<\/strong> means 1 fault can cause loss of entire safety function <\/p>\n\n\n\n

(e.g. 1oo1 pressure transmitter used in SIF). Loss of this transmitter will result in the loss of the entire safety loop.<\/p>\n\n\n\n

HFT is 1<\/strong> means 2 faults can cause loss of entire safety function <\/p>\n\n\n\n

(e.g. 1oo2 voting)<\/p>\n\n\n\n

Following table illustrate the HFT of various voting<\/a> configuration. So the HFT of XooY = Y-X<\/p>\n\n\n\n

\"HFT<\/figure>\n\n\n\n

Table 1 : <\/strong>HFT and Voting correlation table<\/p>\n\n\n\n

Please be aware that HFT is not synonyms to redundant devices. 2oo2<\/a> configuration is also redundant but fault-tolerant.<\/p>\n\n\n\n

Higher HFT number will help to achieve higher SIL level of equipment.<\/p>\n\n\n\n

SFF (Safe Failure Function)<\/h2>\n\n\n\n

SFF is basically measure of effectiveness of built-in diagnosis of device.<\/strong><\/p>\n\n\n\n

Any failure that occurs would be of two types:<\/p>\n\n\n\n

    \n
  1. Safe failure (\u03bbS <\/sub>)and <\/li>\n\n\n\n
  2. Dangerous failure (\u03bbD<\/sub>). <\/li>\n<\/ol>\n\n\n\n

    Further, this failure can be detected by means of diagnosis or remain undetected. Be afraid of Dangerous Undetected failure (because it is neither safe nor detected by any means of diagnosis)<\/p>\n\n\n\n

    Safe failure fraction<\/strong> is the ratio of safe failures(\u03bbs <\/sub>=\u03bbSD<\/sub> + \u03bbSU<\/sub>), plus dangerous detected failures(\u03bbDU<\/sub>), divided by the total failure.<\/p>\n\n\n\n

    \"Safe<\/figure>\n\n\n\n

    Higher the SFF means higher the built-in diagnostic coverage of device, this will help to claim reasonably high SIL level of device.<\/p>\n\n\n\n

    Architectural Constraints<\/h2>\n\n\n\n

    Architectural constraints are limitations that are imposed on the hardware selected to implement a safety instrumented function<\/a>, regardless of the performance calculated for a subsystem (e.g PFDavg). <\/strong><\/p>\n\n\n\n

    \"Safe<\/figure>\n\n\n\n

    Table 2 : Architectural constraints for type A subsystems \u2013 Route 1H<\/strong><\/p>\n\n\n\n

    \"Hardware<\/figure>\n\n\n\n

    Table 3 : Architectural constraints for type B subsystems \u2013 Route 1H<\/strong><\/p>\n\n\n\n

    Type A devices<\/h3>\n\n\n\n

    Type A devices are considered to be \u2018simple\u2019 devices with known failure modes. <\/p>\n\n\n\n

    Examples:<\/strong> <\/p>\n\n\n\n