Revamps 2025 Issue

Safety time summary – HP spray water failure

Safety time summary – total spray water failure

MP steam temperature to reach 280°C MP steam temperature to reach 400°C

6 seconds 3 minutes 47 seconds 5 minutes 6 seconds 1 minute 19 seconds

MP steam temperature to reach 280°C

TTT PST

44 seconds 6 minutes 56 seconds 5 minutes 1 minute 12 seconds

TTT

Pip e wall at B/L to Reach 280°C

Pipe wall at B/L to reach 280°C

PST

Time delay selected

Av ailable SIF response time

IPF-ST

Avai lable SIF response time

IPF-ST

Table 3

Table 4

Steam temp.

460

500

Instant trip initiated

Pipe wall

Higher trip setting

temperature

TBD

450

Downstream system design temp Lower trip setting

400

280

Steady state operation at 230˚C

350

Pipe wall exceeds design temperature

230

300

Total loss of HP spray water occurs

IHP steam temperature MP steam temperature Pipe wall temperature at B/L

250

Time

Available SIF response time

TTT

200

5 10 15

50 55

Process safety time

Time (mins)

Figure 6 Total spray water failure

Figure 7 Total spray water failure dynamic simulation results

Dynamic simulation: total spray water failure In this scenario, the system is operating at steady state and 230°C when there is a total loss of spray water (see Figure 6 ). The temperature rapidly increases, passing through the lower set point temperature and then the pro- posed higher set point temperature of 400°C. The higher set point initiates an instant trip and acts to isolate the sys- tem. The SIF must act before the pipe wall temperature at the battery limit reaches 280°C. Figure 7 shows the results of a dynamic simulation for the total spray water failure scenario. The failure occurs five min - utes into the dynamic simulation run. Therefore, all measured times must subtract this value. The results are tabulated in Table 4 . As previously calculated, an estimate of the SIF response time is 15-60 seconds. Therefore, the calculated IPF-ST should be sufficient, but this would need to be con - firmed by the instrumentation engineer. Conclusion PST calculations are an essential part of instrumented safety system design on process plants. However, the key param - eter for the design of any particular SIF is the available SIF response time, also referred to as the IPF-ST, which refers to the time between a trip set point being reached and the system reaching failure or maximum permitted conditions. On many modern projects, safety time analysis is performed through dynamic simulation modelling. It is important to consider the response time of the sens- ing element, logic solver, signal transmission, and the final element in the SIF response time. In some systems, sensing

elements have inherent delays that can significantly impact the SIF response time. Unnecessary process trips can cause significant operational disruption, especially where long shutdown and start-up sequences are required. In this case study, dynamic simula- tion was used to demonstrate that by utilising a time-delayed lower set point trip, the safety system remains robust, but the opportunity is created for the control system to recover from a less severe failure event within a specified time delay and avoid an unnecessary shutdown of the steam plant, which could lead to significant disruption of refinery operations. The operability of the plant was therefore improved without com - promising the integrity of the safety system. Acknowledgements The author would like to thank the following people for their valuable contributions to this paper: Caleb Walker (Piping Engineer, Fluor Ltd) and Paul Garlick (Principal Process Engineer, Fluor Ltd) References 1 British Standards, BS EN 61511-1:2017+A1:2017 Functional Safety – Safety Instrumented Systems for the Process Industry Sector, 2017. 2 N. A. H. H. J. V. Paul Garlick, A Matter Of Time: Why Is Process Safety Time Important? , GPA Autumn Conference and AGM, 2019. 3 American Society of Mechanical Engineers, ASME B31.3-2020 Process Piping – ASME Code for Pressure Piping, New York, USA, 2021. Charlie Gould is a Process Engineer at Fluor with 10 years of experience in oil and gas, refining, chemicals, heat and power, carbon capture, and blue hydrogen projects. He holds a master’s in chemical engineering from the University of Surrey, UK, and is a chartered member of the IChemE. Email: charlie.gould@fluor.com

Revamps 2025