A client designed a foam piping system that covers the three tanks in a tank pit in the harbor. The firewater system is directly connected to the general water supply. The main pipeline passes through the foam house where the water is mixed with foam. A manifold divides the mixture over three different branches covering the north, central, and south areas of the tank pit. These rings are equipped with multiple deluge nozzles that expel the foam to the site.
For this project, the team at DRG performed a surge and a pipe stress analysis (static and dynamic) to evaluate whether the system complied with the pressure allowable requirements according to the EN13480 code. In addition, the static stress and the effect of the unbalanced forces of the surge event have also been checked by this code.
Modeling the system
The first step in the analysis is capturing the geometry and characteristics of the foam system including nozzle and pipe specifications. This step has been done using BOSfluids based on information provided by the client. Furthermore, ow parameters for a homogeneous fluid and air-water mixture have been defined to perform a flood and drain analysis.
Figure 1 | Overview of the firewater system at the tank pit.
Below is a zoomed-in view of the supply line including the deluge valves. The site’s general water supply connection is modeled with a pressure boundary condition. The mixing unit is included as equipment to account for its pressure loss.
Figure 2 | Close-up of the foam house and the connection to the main water supply.
Surge Analysis
The surge analysis showed that it takes just under 30 seconds from the moment the deluge valves are opened for the foam mixture to reach the deluge nozzle at the furthest point of the branches. As the system is empty, air will be pushed out of the branches through the nozzles first. The branches at the end continue for a few meters after the last deluge nozzle, so the air that is trapped in this last section will have no choice other than to compress and mix with the water. Upon mixing water and air, the wave speed is changed drastically.
The figure below shows that a fraction of only 1% results in a 90% drop in the wave speed. To account for this reduction and not be overly conservative, a different fluid has been specified at the ends of the foam lines: bubbled water, which has a very small gas content, hence a lower wave speed. The pressure wave reflecting at the end of the system will be moving at this wave speed. With this approach, it was found that the maximum pressure in the system complies to the EN13480 code
Figure 3 | The wave speed in a water-gas mixture as a function of the gas fraction.
Stress Analysis
For the stress analysis, the model could be directly imported from BOSfluids to CAESAR II, including all its supports and nodes. The supporting consisted of U-bolts that were mounted on steel HEA-140 beams, at different heights, depending on the location of the support within the tank pit. The generic flexibility of the steel beams was also incorporated in the analysis, which is especially relevant for the dynamic analysis.
Multiple load cases were set up and both the effects of wind and the unbalanced forces arising from the pressure surge were included. Due to its location, seismic loads do not apply.
Using the initial support arrangement, the stress analysis showed that the maximum observed stress is caused by an occasional load involving wind loads and the dynamic forces when the deluge valves are opened. For this load case, a maximum stress has been observed that corresponds to 76.1% of the allowable.
Conclusions and recommendations
The new firewater system covering the tank pit has been subjected to a surge and stress analysis. The results showed that both the peak pressures arising from the opening of the deluge valves, as well as the maximum (static and dynamic) stress are within the limits that are prescribed by the EN13480 code. As such the team at DRG has recommended that no modifications to the system are required.
