Valve Communication Solutions. Safety instrumented systems

Transcription

1 Safety instrumented systems

2 Safety Instrumented System (SIS) is implemented as part of a risk reduction strategy. The primary focus is to prevent catastrophic accidents resulting from abnormal operation. 2

3 Definitions SIS Safety Instrumented System An automatic system to reduce the level of risk when a hazard condition occurs. An SIS can be made up of several SIFs. SIF Safety Instrumented Function A safety function with a specified safety integrity level which is necessary to achieve functional safety (IEC 61511). SIL Safety Integrity Level A number from one to four that defines the required risk reduction an SIF needs to lower risk to an acceptable level. SIL 1 is the least stringent and SIL 4 is the most stringent. Typically SIL 4 is not used in process control. 3

4 Definitions Logic solver A logic device, like a PLC, that accepts inputs and generates outputs but is designed and approved to IEC A logic solver is separate from the process PLC or DCS and contains the logic to perform the SIF. Proof test A test to uncover undetected failures within the SIF that would prevent the safety function. An example would be to completely cycle an emergency shutdown valve to make sureall elements are still operational. Partial Stroke Test (PST) A test in which a valve is moved off its open seat (typically to 85% Open), checked for functionality, and opened up again. A PST is performed between proof tests to make sure the major elements of a SIF are still operational. This will decrease the average PFD without shutting down the process like a proof test will. DCS network Safety controller 4

5 Definitions MTBF Mean Time Between Failure The statistical approximation of how long a number of units will operate before a failure can be expected. PFD Probability of Failure on Demand The chance that the safety system will fail when called on to operate. RRF Risk Reduction Factor The reciprocal of the average PFD or PFD avg. To calculate the risk reduction needed to lower the risk in a SIF to an acceptable level (consistent with the defined SIL level), the following formula can be used; RRF=(Unmitigated Risk)/(Tolerable Risk). 5

6 SIS system SIF 1 SIF 2 SIF n 6

7 Main elements of an SIF system 1. Sensors to detect process anomalies Flow meters Level controls Valve monitors Pressure sensors etc. 2. Logic solvers to receive inputs, evaluate and drive appropriate outputs (typically separate from operating system) 3. Final control elements take action to drive process to safe state. Safety shut-off valves Control valves Pumps/motors 7

8 Elements of SIS system 8

9 Determining risk PFD = Probability of Failure on Demand is the chance that the safety system will fail when called on to operate. More critical failure event - - lower acceptable system PFD PFD system = PFDsensors + PFDlogic solver + PFDfinal element Safety Integrity Level (SIL) defines the bounds of the PFD RRF = Risk Reduction Factor = 1/PFD End user determines SIL rating for plant Determining risk SIL PFD RRF to ,000 to 100, to ,000 to 10, to to 1, to to 100 9

10 SIL determination Extent of damage (D) Slight injury Severe injury Exposure time (E) Frequent Hazard avoidance (A) Some possibility Death Several deaths Continuous Limited possibility Factors are fed into a table to determine potential damage consequences leading to a SIL level determination. Probability of occur (P) Low Moderate High Higher SIL valves 10

11 PFD determination Certified compliant by a third party Evaluated under fault conditions; errors safe or unsafe? Safety manual with device limitations Proven in prior use (similar applications) Mean time between failures (MTBF) Values are well documented; 1/MTBF=PFD PFD example 200 units are operating continuously and there are 15 failures over 2.5 years. What is MTBF? What is PFD? Safety instrumented systems PFD determination 200 units x 2.5 yrs/15 units = 33.3 years MTBF = 33.3 years x 8760hrs/year = 292,000 hours PFD = 1/MTBF = 1/292,000 hours PFD = = 3.4 x

12 Special note on MTBF MTBF is not life expectancy!! MTBF represents the statistical approximation of how long a number of units should operate before a failure can be expected Lifetime is the period of time between starting to use the device and the beginning of wear out. 12

13 Safety loop failure sources Final control element failures Valve/actuator sticks Incorrect selection or sizing Solenoid valve fails Final elements Logic solver Sensors 50% 8% 42% 13

14 PFD reduction Redundant safety sensors and final control elements Greater reliability components Proof testing the safety system (entire system is tested) Partial stroke testing the final control elements - -safety shut-off valves Diagnostic elements evaluating system components for potential problems 14

15 Partial Stroke Testing (PST) benefits Reduce sticking of valve and actuator Demonstrate performance Reduce PFDavg and improve reliability of safety system! As PFD increases over time, proof testing reduces PFD to near the original number PST is done more frequently, does not require the system to shut down, and reduces the average PFD Because a PST is not as comprehensive as a proof test, it does not reduce the PFD as a proof test would. The PFD still increases slightly over time. PFD Ave PFD 1 Ave PFD 2 Proof testing PST testing Proof testing only Proof testing and PST 15

16 Emergency shut-down applications (ESD) Axiom Expeditor overview The Axiom Expeditor could be used to perform Partial Stroke Testing (PST) in a Safety Instrumented System (SIS) to increase reliability and decrease the Probability of Failure on Demand (PFD). 16

17 Emergency shut-down applications (ESD) Axiom Expeditor installation 17

18 Safety related product documents 3 rd party SIL certificate Test report Safety manual 18

19 Quartz now with TUV SIL Certificate Most Quartz models are covered Type A Device Up to SIL 3 Redundant sensors to improve SIL Level See Safety Manual and certificate for details 19