Introduction
The seawater system is a class A system according to Service Limit State (SLS). The system is made from reinforced fiberglass piping, containing a pump manifold with three pumps, each with a rated capacity of 5,500 m3/h. The manifold is connected to two vaporizers via two 1200 mm diameter buried pipes. The design pressure of the system is 8 barg.
A surge analysis is performed by DRG on the system according to the applicable ISO 14692 code for fiberglass systems. It is the future planning of the system to install a total of five pumps and four vaporizers. Therefore, a surge analysis is performed with the upset conditions of two and four pumps active. Emphasis is placed on avoiding the possible rise of vacuum during any of the scenarios.
Analysis
From the analysis, a pump stop within two seconds, occurring for more than one pump, was the governing case. Normally the inertia of the pump and driver made the pumps spin down in approximately 30 seconds.
A stop in 2 seconds was only possible if a mechanical problem occurred (bearing problem). More than one pump stopping in two seconds due to a mechanical problem was however considered unrealistic.
Results
In the case when the pump rundown due to power failure, the predicted pump spin-down time is large enough to prevent a vacuum from occurring.
In case one pump is running and stopped in two seconds the flow rates and velocities found were low and a vacuum did not occur because the inertia in the flowing water was limited and insufficient to produce substantial sub-atmospheric pressures. When more pumps were running and one pump was stopped in two seconds the three remaining pumps supplied sufficient pressure to prevent vacuum to occur.
“Installation of vacuum breakers necessary or not?”
Based on the results from the different scenarios it was concluded that there was no need for the installation of vacuum breakers.
