Accelerometer mod. TA18-S. SIL Safety Report

Transcription

1 Accelerometer mod. TA18-S SIL Safety Report SIL005/11 rev.1 of Page 1 of 7

2 1. Field of use The transducers are made to monitoring vibrations in systems that must meet particular technical safety requirements according to IEC Operating limit values are shown in the specification sheets for the single models. 2. Acronyms and abbreviations Acronym Abbrev. English Description HFT MTBF MTTR PFD PFD AVG PFH AVG SIL SFF Low demand mode DCS LRV URV Hardware Fault Tolerance Tolerance of hardware faults of the device. The ability of a functional unit to perform the function requested even if an error or a deviation occurs. Mean time between two faults. Mean Time Between Failures Mean Time To Mean time between the moment an error appears in a Repair device and the time of repair. Probability of The probability of dangerous faults of a safety function Failure on Demand Average Probability The average probability of dangerous faults of a safety of Failure on function in case of need Demand Average Probability The average probability of dangerous faults of a safety of Failure per Hour function per hour Safety Integrity The international regulation IEC defines four socalled discreet integrity levels associated with Safety Level Integrity Levels (SIL 1 - SIL 4). Each level corresponds to an interval of probability per fault of a safety function. When the Safety Integrity Level increases on safety system, the probability that they will fail to perform the required functions decreases. Safe Failure Part of a non-hazardous fault, or rather a fault without the Fraction potential to bring the safety system to a hazardous or unallowed state. Low demand mode Type of measurement with low demand operation Type of operation of measurement in which the demand rate for a safety system is not greater than once a year, and it is not greater than twice the frequency of the repeated control. Distribuited Control Control system used in industrial applications to oversee System and monitor remote devices. Lower Range Value Zero value of the measurement field Upper Range Value Span of the measurement field SIL005/11 rev.1 of Page 2 of 7

3 3. Regulations in force Standard IEC parts 1-7: Functional safety of electrical/electronic/programmable electronic safety-related systems. 4. Other applicable documentation In addition to the SIL safety standards, it is necessary to consider the following documentation: Specification TA-18S All documents are available in Italian and English. The use of the product is responsible for complying with current laws and standards. 5. Terms and Definitions Term Dangerous faults Safety system Safety function Definition A fault with the potential to bring the safety system to a hazardous or non-functional state. System that can perform safety functions to reach or maintain a system in safe conditions. Function defined or performed by a safety system with the objective of reaching or maintaining a safe status of the plant, in consideration of a pre-defined hazardous event. 6. Safety function TA-18S transducers generate an electrical signal that is proportionate to the acceleration of a vibration with sensitivity of 100 mv/g, according to the specification sheet. Transducers perform their function by means of piezo-electric transducers, without using any software. No self-diagnostic functions are envisaged. Limits of precision and safety are reported in the specification sheets. SIL005/11 rev.1 of Page 3 of 7

4 7. Controls The safety function of the entire safety circuit must be assessed regularly according to IEC Assessment intervals are established in the calculations of the individual safety circuits of a plant. The owner is responsible for selecting the type of assessment and the time intervals for the designated period of time. Follow these instructions to verify the safety function of the transducers: 1-Switch on the transducer 2-Verify with transducer stopped that output bias is correct (about 10Vdc) 3-Verify simply by shaking the transducer that it is powered with a. c. voltage. Broken transducers must be sent to our assistance department, with indications of the type of fault and its possible cause. 8. Configuration The transducer is delivered already configured and tested according to the specifications indicated in the customer s order. Before commissioning the transducer as part of the safety function, check the configurations to make sure it guarantees the function of the safety system. Verify that the right transducer is installed on the right measuring point. Since there are no modifiable configuration parameters, the safety function is guaranteed by the original configuration. 9. IEC proven in use IEC A previously developed subsystem shall only be regarded as proven in use when it has a clearly restricted functionality and when there is adequate documentary evidence which is based on the previous use of a specific configuration of the subsystem (during which time all failures have been formally recorded, see ), and which take into accountany additional analysisor testing, as required (see ). The documentary evidence shall demonstrate that the likelihood of any failure of the subsystem (due to random hardware and systematic faults) in the E/E/PE safety-related system is low enough so that the required safety integral level(s) of the safety function(s) which use the subsystem is achieved For a device that can be considered as proven-in-use, it is necessary to keep in mind the volume of operating experience. TA-18S transducers were introduced on the market in the year 2003; since then, substantial revisions have never been done on the project. SIL005/11 rev.1 of Page 4 of 7

5 From the sale data and the data regarding repair history, it is clear that 2044 units were sold in the five-year period To calculate faults, only the operative hours during the guarantee period are taken into consideration, because after this, they are no longer reported. Operative hours total 17,905,440 and this amount is deemed sufficient, considering the low complexity of the transducer and the use in SIL 2 applications. The following faults were shown from the operation experience: Year Systemic Faults Random Faults Faults Total Faults non measurable* Measurable Total 8 *Faults due to falls or mechanical collisions, for example Since there is no evidence that all faults in the guarantee period were reported, we assume that only 70% of these were regularly reported, thus calculating an estimated number of faults at 8/0.7=11.42 which is then rounded up to 12. From this, we obtain a failure rate of 6.70E-7 [1/h]. Since IEC requires the calculation with confidence limit of 70% we obtain 8.69E-7 [1/h]. IEC The documentary evidence required by shall demonstrate that the previous conditions of use (see note) of the specific subsystem are the same as, or sufficiently close to, those which will be experienced by the subsystem in the E/E/PE safety-related system, in order to determine that the likelihood of any unrevealed systematic faults is low enough so that the required safety integrity level(s) of the safety function(s) which use the subsistem is achieved. NOTE The conditions of use (operational profile) include all the factors which may influence the likelihood of systematic faults in the hardware and software of the subsystem. For exemple, environment, modes of use, functions performed, configuration, interfaces to other systems, operating system, translator, human factors. TA-18S transducers have always been used in similar applications under similar environmental conditions. Therefore, if they are used within the parameters listed in the specifications, the clause is complied with. IEC Where there is any difference between the previous conditions of use and those which will be experienced in the E/E/PE safety-related system, then any such difference(s) shall be identified and there shall be an explicit demonstration, using a combination of appropriate analytical methods and testing, in order to determine that the likelihood of any unrevealed systematic SIL005/11 rev.1 of Page 5 of 7

6 faults is low enough so that the required safety integrity level(s) of the safety function(s) which use the subsystem is achieved. Since there is no difference between the previous uses of the TA-18S transducer and the expected conditions of use, the clauses is considered complied with. IEC The documentary evidence required by shall establish that the extent of previous use of the specific configuration of the subsystem (in term of operational hours), is sufficient to support the claimed rates of failure on a statistical basis. As a minimum, sufficient operational time is required to establish the claimed failure rate data to a single side lower confidence limits of at least 70% (see IEC , annex D and IEEE 352). An operational time of any individual subsystem of less then one year shall not be considered as a part of the total operational time in the statistical analysis (see note). NOTE The necessary time, in term of operational hours, required to establish the claimed rates of failure may result from the operation of a number of identical subsystem, provided that failures from all the subsystem have been effectively detected and reported (see ). If, for example, 100 subsystem each work fault-free for h, then the total time of fault-free operation may be considered as 1,000,000 h. In this case, each subsystem has been in use for over a year and the operation therefore counts towards the total number of operational hours considered. As mentioned in point , the number of operative hours is deemed sufficient, due to the relative simplicity of the transducer and the use for SIL 2. Also, a confidence limit of 70% is considered. IEC Only previous operation where all failure of the subsystem have been effectively detected and reported (for example, when failure data has been collected in accordance with the recommendation of IEC ) shall be taken into account when determining whether the above requirement ( to ) have been met. As mentioned in , considering unrealistic the hypothesis that all faults were reported, only the guarantee period was considered, estimating that the reports made accounted for 70% of the actual failures. These precautionary assessments lead us to believe that the clause was complied with. IEC The following factors shall be taken into account when determining whether or not the above requirements ( to ) have been met, in terms of both the coverage and the degree of detail of the avaible information (see also 4.1 of IEC ): a-the complexity of the subsystem; b-the contribution made by the subsystem to the risk reduction; SIL005/11 rev.1 of Page 6 of 7

7 c-the consequence associated with a failure of the subsystem; d-the novelty of design. All factors listed in the standard were considered in this assessment. IEC The application of a proven-in-use safety related subsystem in the E/E/PE safety related system should be restricted to those functions and interfaces of the subsystem which meet the relevant requirements (see to ). NOTE The measures to are also applicable for subsystem which contain software. In this case it has to be assured that the subsystem performs in its safety related application only that function for which evidence of the required safety integrity is given. See also of IEC The TA-18S transducer does not contain software and is used exclusively in the conditions of the present proven-in-use report. Therefore, the clause has been complied with. 10. Conclusions The following table shows the SIL levels in the various conditions of PFD AVG and PFH AVG : Safety Integrity Level (SIL) Average Probability of Failure on Demand PFD AVG Average Probability of Failure per Hour PFH AVG SIL x x 10-8 SIL x x 10-7 SIL x x 10-6 SIL x x 10-5 Having obtained a PFH AVG value equal to 8.69E-7 it is deemed to correct to use this device for SIL 2 applications. Mandello del Lario, 03 February 2010 CEMB S.P.A. Instrumentation Division Manager ( Enrico Coti Zelati) SIL005/11 rev.1 of Page 7 of 7