The client commissioned a surge analysis for the Waste Water (WW) system at the Rotterdam terminal. The analysis aimed to quantify transient pressures and unbalanced forces generated by various hydraulic events: pump trips, pump startups, pump switchovers, and the closure of two key butterfly valves. The analysis was performed using the BOSfluids transient flow modeling package, with the goal of ensuring the WW system’s compliance with the ASME B31.3 code and recommending operational or design mitigations to avoid excessive surge pressures and unbalanced forces.
System and Model Description
The WW network comprises several pump stations equipped with 4, 3, and 2 pumps. The system routes effluent waste water through a series of aboveground and buried pipes, terminating at a main waste water reservoir. The model included an 118-meter buried pipeline segment to represent pressure wave reflections and frictional losses up to the system’s battery limit.
The WW system included piping with nominal diameters ranging from DN100 to DN350. The considered fluid was water at a temperature of 20 °C. The pumps were modeled according to their rated performance curves. Boundary conditions were defined by a fixed suction pressure of 0.08 barg and a downstream reservoir pressure of 4 barg.
Surge Scenarios Assessed
Five principal scenarios were analyzed for the WW system:
- Valve Closure: Evaluated for varying closure times to determine pressure surges and compliance with the ASME B31.3 allowable.
- Pump Trip: Simulated as a simultaneous loss of all operational pumps, reflecting a power blackout event.
- Pump Startup: Investigated for sequential and simultaneous startups of two pumps, both from a static system and with one group already running.
- Pump Switchover: Modeled as a rapid transition between standby and operating pumps within each group, with a 5-second overlap period.
- Steady-State Conditions: Provided baseline performance data for calibration and comparison to transient events.
Key Results and Analysis
Steady-State Performance
Under normal operation the total system flow was 2030 m³/h, and the pressure at the battery limit was maintained at 4 barg.
Valve Closure Events
Closure of the valve in 8 seconds produced peak pressures of 14.9 barg, exceeding the ASME B31.3 allowable (13.3 barg). Extending the closure time to at least 13 seconds limited the pressure surge to 12.0 barg. The highest unbalanced force in the system was 9.1 kN in the long straight section downstream. The design pressure of the piping (10 barg) was exceeded in several locations during these events, although the maximum transient did not surpass the code allowable when mitigated closure rates were used.
Pump Trip
Simultaneous tripping of the pumps caused a rapid pressure drop, propagating a negative pressure wave through the system. The minimum pressure did not fall below the water vapor pressure, thus cavitation was avoided. Nevertheless, the event generated the highest recorded unbalanced force (45.9 kN) in the system, again in the long pipeline sections downstream.
Pump Startup
Sequential startup of the four pumps resulted in a maximum pressure of 9.0 barg, which is lower than the allowable limit. Simultaneous sequential startup of the seven pumps produced a higher maximum surge of 11.8 barg. This exceeds the design pressure but remains below the ASME B31.3 allowable. The most severe unbalanced force during pump startups was 45.9 kN, located in the long straight (buried) section.
Pump Switchover
Switchover events produced modest pressure variations that were deemed to not be severe.
Conclusions and Recommendations
Analysis confirmed that, with recommended mitigations implemented, the WW system would comply with the ASME B31.3 instantaneous pressure limit of 13.3 barg. However, transient pressures exceeded the 10 barg piping design pressure at multiple locations during several scenarios, highlighting the need for a mechanical pipe stress analysis.
Key recommendations included:
- Closing the valve in no less than 13 seconds.
- Verifying that all equipment and piping in the network can withstand pressures exceeding 10 barg, especially in surge-prone sections.