Industry safety standards
Important influential institutes within each industry create standards to improve safety across the industry they operate in. The International Electrotechnical Commission (IEC) is the primary institute that does this for all electrical, electronic and related technologies, also known as “electro technology”. The American Petroleum Institute (API) is the institute that does this for the oil and natural gas industry. An important term is Process Safety time (PST) which in short means the time available between a moment in time that a failure occurs and the actual hazardous event caused by the same failure. Both of these institutes issued standards regarding Safety Instrumented Systems (SIF) and determination of PST which we explain in this article and are important to anyone involved.
IEC 61508; PST: Time between a failure, that has potential to give rise for a hazardous event
IEC 61508:2010 defines Process Safety Time (PST) as “Period of time between a failure, that has the potential to give rise to a hazardous event, occurring in the EUC [equipment under control] or EUC control system and the time by which action has to be completed in the EUC to prevent the hazardous event occurring”.
IEC 61511; PST: Time between failure and the occurrence of the hazardous event
IEC 61511:2003 Part 2: defines PST as “the time period between a failure occurring in the process or the basic process control system (with the potential to give rise to a hazardous event) and the occurrence of the hazardous event if the safety instrumented function is not performed”.
API 556; The interval between the initiating event and unacceptable process deviation and the hazardous event
API 556 second edition, 2011: “the interval between the initiating event leading to an unacceptable process deviation and the hazardous event”
CCPS; The time period between a failure occurring in the process or its control system and the occurrence of the hazardous event
According to the CCPS Guidelines for Safe and Reliable Instrumented Protective Systems Process Safety Time (PST) is: “the time period between a failure occurring in the process or its control system and the occurrence of the hazardous event.”
Graphical interpretation
Refer to figure below for graphical illustration of the Process Safety Time definition:
PST is an estimation
PST is not specified but rather estimated, calculated or measured (Potentially). PST is a system dependent and hence relates to the behavior of the process and process equipment or process control system within the context of a specific unmitigated hazard scenario.
PST is unique
PST is necessarily unique to each system under study and for the cause-consequence pair, even when multiple initiating events may eventually lead to the same consequence.This is because each initiating event has the potential to impact process dynamics in different ways. In another words PST of related but separate scenarios will not necessarily be equivalent.
PST as the first step
Determination of PST is the first step in identifying the time potentially available for all protection layers to respond and will be useful in specifying the required response time of each. Calculation/determination of PST could be carried out during Safety Integrity Level (SIL) review meeting or afterwards by an experienced process or process safety engineer. For some systems an experienced operator or vendor could help a lot.
SIF response time: Time required to bring the process to a safe state
The SIF response time is the time required by a SIF to bring the process to a safe state. This time is a summation of sensor(s) time to sense a parameter, logic solver to process this change in parameter value and sends signal to SIF final element and time required by final element to act. This time could be calculated when vendor data is available. Normally SIF response time calculation is by instrumentation department and after vendor is chosen and their documents and data received.
Calculating PST & SIF response
Calculation of PST and SIF response time could sometimes be a tricky and time consuming task that might even need dynamic simulation using HYSYS. Of course it could be done qualitative or quantitatively.
Typically, a Safety Instrumented Function (SIF) is designed not to allow the process to go excursions beyond equipment or process safe design limits, this means that PST also can’t exceed safe design envelope and will not have a set point outside safe design envelope.
Explanation about the graph on why the process safety is not starting at t₀:
The Graph shows that deviation of process parameter starting at t₀, however the Process Safety Time (PST) is calculated further in time, when we pass the point that could potentially make a hazardous event. In the standard’s words: “give rise to hazardous event”.
Consider the case that we are protecting a downstream piping system against high temperature. The design temperature of the downstream piping is 100°C. The setting of the high temperature switch/trip is 90°C. The Process Safety Time (PST) is the time that takes after the high temperature switch activates (90°C) and till the temperature reaches 100°C design temperature (potential dangerous situation).
Of course the intention of the high temperature trip is to prevent reaching 100°C, but we need to assume if this switch doesn’t work (failure on demand) how much time is available to reach the hazardous event (e.g. failure of piping system because of the high temperature and pressure combination).
See part 2, part 3, part 4 and part 5 for PST calculation methodology.
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