A new off-loading system is under consideration for an ammonia storage plant, designed to transfer ammonia (NH3) from a jetty to three storage tanks. This system will feature piping predominantly sized at DN350, intended to handle a design flow rate of 1000 tonnes per hour. The ammonia will be transported at -33°C, its boiling point at atmospheric pressure. The study involves a static stress analysis, surge analysis, and dynamic stress analysis to evaluate the system’s performance under various upset conditions that could generate significant transient surge loads.
Analysis
The system model was developed using initial isometric and overview drawings. Static and dynamic stress analyses were conducted using CAESAR II v5.2 software. This CAESAR II model also served as the basis for the surge analysis, performed with BOSfluids software. The surge model included the existing piping connected to all three tanks.
Surge Analysis
For continuity, some existing piping was incorporated into the stress model. Although these lines were not analyzed for stress, their inclusion was necessary for a comprehensive assessment of the new piping. The new system comprises a large bore import line and a smaller circulation line used for cooling the import line when inactive.
Three upset conditions were analyzed:
- Rapid Closure of the ERC Valve: The Emergency Release Control (ERC) valve closes quickly to prevent ammonia spillage if a ship drifts away from the jetty.
- Displacement Pump Trip: A trip in the ship’s displacement pump was considered.
- Closure of the Tank Valve: This valve closes when the fluid level inside the tank drops below a preset threshold.
Results
The analyses yielded the following results:
- A preliminary static stress analysis was conducted according to the piping code EN13480 for both the import and circulation lines.
- With the modifications to routing and support arrangements proposed by DRG, static stresses remained within the allowable limits of EN13480, given a design pressure of 25 barg and a temperature of -40°C.
- The most significant impact on the system was observed during the closure of the tank valve while ammonia was being imported at maximum flow rate, resulting in unbalanced forces.
- This impact can be mitigated by employing a bilinear closing curve, which achieves 75% of the cutoff within the first 25% of the total closing time.
- The line sizes for certain tanks are insufficient, leading to excessive pressure loss and restricting flow rates.
- Based on the dynamic response study, additional modifications to the system were recommended. After incorporating these changes, all dynamic stresses were found to be within acceptable limits.
