Fig. 6 Pressure transient by sudden coupler detachment
In consequence, the oil velocity decreases earlier by the higher air pressure and the minimum volume is larger as shown in Fig.7. The peek is more flattened in the adiabatic change. The areas below the pressure lines seem almost the same each other, which correspond to the amount of impulse by the air to push the oil column back.
As the influence of above two assumptions on the results is small, the actual change can be estimated by this method.
3.3 A device to suppress pressure rise of waterhammer
As mentioned in the previous section, the pressure rise at the downstream of the valve might be suppressed by trapping some amount of air. However, the effect is not reliable. The proper amount of air depends on the conditions of velocity, pressure, and so on. It is also pointed out that an improper air inflow sometimes brings about severer pressure transients than that following the collapse of vapor cavity.
In order to suppress the pressure rise safely and detach the coupler instantly, a device for this particular system is proposed. Figure 8 shows the schematic figure. Instead of equipping valves at the coupler, two valves are equipped at the bottom of the loading arm as indicated in the figure.
When the ship begins to move away, the valve on the ship is closed first and the coupler is detached immediately.
The oil left in the loading arm is sucked into the undersea pipeline by the inertia of preceding oil. Then the main valve on the berth is closed and the sub valve is opened at the same time. As air flows in through the sub valve, the cavity pressure is kept around atmospheric. A high pressure peek, which begins after the oil returns and the cavity disappears, is suppressed by releasing some amount of oil into the emergency reservoir. A computed pressure transient in the system is shown in Fig.9. In this example, the opening area of sub valve is 15 % of the sectional area of loading arm. The proper area ratio between them depends on the operating condition. However, by choosing the opening area of sub valve large enough, the waterhammer of the system is suppressed safely for all the operating conditions. This system is also valid in the case where the loading arms are broken at the parts except the couplers when the ship moves away.
Fig. 7 Isothermal change and adiabatic change
