An unloading line was being commissioned for use with cyclohexane. This line featured two ship loading arms located at the end of a jetty, and from these loading arms, the unloading line ran through to two storage tanks.
During transient upset scenarios such as valve closures, pump start-ups, or shutdowns, pressure surges could occur within the system. These pressure surges could potentially result in excess pressures exceeding the pressure rating of the pipe. Additionally, the pressure surges in the unloading line would lead to net forces acting on the piping, resulting in increased stresses and loading on the restraints. It was necessary to ensure that the unloading line conformed with the EN13480 design code during these transient upset scenarios.
Analysis
The pressure surges due to the transient upset scenarios were modeled using the transient one-dimensional fluid modeling package BOSfluids. The pressure profiles in the unloading line were used to determine the net (unbalanced) forces acting upon the piping.
These forces were then applied to a CAESAR II piping model of the system. The dynamic solver within CAESAR II was used to calculate the stresses arising in the unloading line as a result of the pressure surges.
Results
It was shown that by extending the closure time of the valves, the magnitude of the pressure surges could be kept within the pressure rating of the pipe. It was also recommended that a two-stage closure profile, within which the last part of valve closure is very slow, be implemented for one of the valves.
The dynamic piping analysis showed that excessive stresses would result in the piping system due to the pressure surges. To ensure conformance with the EN13480 piping code, it was necessary to add additional restraints to remove some of the degrees of freedom. The engineering firm worked with the client to help ensure that these new supports were proposed in realizable locations.
The end result was an unloading line that conformed with the EN13480 for all of the transient upset scenarios.
Key Takeaways
- Pressure surges from valve operations and pump transients can cause excessive stresses in piping systems.
- Transient modeling software like BOSfluids can predict pressure profiles during upset conditions.
- Dynamic piping analysis in CAESAR II can evaluate resulting stresses against design codes like EN13480.
- Adjusting valve closure times and adding pipe restraints are potential mitigations for excessive surge stresses.
- Close collaboration between transient analysts and piping designers is crucial for code-compliant designs.
