A comprehensive stress and surge analysis was performed for the firewater piping system of an LNG plant located in South America. The system, constructed using Glassfiber Reinforced Epoxy (GRE) pipes, underwent a dynamic response analysis to account for the largest and most critical unbalanced forces.
Surge Analysis
The existing firewater system comprised two pump connections: a fresh water pump and a seawater pump. During normal operation, the fresh water pump supplied the firewater, while the seawater pump served as a backup in case of failure or high demand. A transient flow analysis was conducted using BOSfluids®, a software package developed by DRG for transient flow analysis.
The pressure and flow distribution were calculated for steady-state conditions and four critical transient scenarios:
- Switching from the fresh water pump to the seawater pump
- Closing all hydrants and monitors
- Simultaneously opening the LNG sprinklers (largest flow)
- Tripping both the fresh and seawater pumps subsequently
These scenarios were identified as the most critical for generating the highest and lowest pressures and the largest unbalanced loads in the system.
Stress Analysis
The piping system, primarily buried, included several connections to above-ground fire hydrants and corresponding equipment. System routing and pipe properties used for the analysis were based on data provided by the client. Stresses were assessed in accordance with ISO 14692, the standard for GRE piping design. The stress analysis was performed using CAESAR II pipe stress software, incorporating the unbalanced loads calculated with BOSfluids® to conduct the dynamic stress analysis, essential for assessing fatigue.
Results
For the given design conditions, several modifications were recommended to limit static stresses within allowable envelopes. These recommendations included additional laminate reinforcements on top of the GRE wall thickness or the application of flexible materials. The static stress analysis was supplemented by considering the unbalanced loads resulting from the transient flow pressure distribution in the system.
The maximum dynamic stresses were combined with the static operational stresses and assessed against the ISO 14692 long-term occasional envelope. The analysis concluded that none of the stresses exceeded the occasional allowable limits.
Conclusion
This detailed analysis ensured the firewater piping system’s integrity and reliability by identifying and addressing the most critical scenarios and stress points. The recommendations and modifications proposed will help maintain the system’s operational safety and efficiency.
