At BP Geel, a new extension of the fire protection system experienced operational failures during start-up due to surge effects. A similar incident with line AFW-023025 had previously prompted a surge analysis, leading to the current study focused on the adjacent foam system, line AFW-023024. The foam line, running nearly parallel to the fire water system, is designed to deliver a water-foam mixture by opening control valve AHV-2208. Recognizing the analogous design and operational principles between the foam and water systems, BP Chembel NV commissioned a surge analysis to evaluate transient flow phenomena, specifically to ensure that surge pressures and unbalanced loads remain within safe operational limits.
The surge analysis discussed here is part of a dual-study approach, with dynamic stress analysis to be reported separately. The principal objectives were to determine optimal valve operation strategies—minimizing surge pressures and unbalanced loads during transient events—while ensuring the system could be primed within a maximum start-up time of 240 seconds. Additionally, the analysis sought to identify and address other potential surge-related vulnerabilities.
Analysis Approach
The analysis was conducted using BOSfluids, a transient flow simulation tool well-suited for evaluating the effects of operational events such as valve actuation and pump operations. When the deluge valve is actuated, water rapidly fills the previously air-filled pipework, displacing the air towards the discharge monitor and generating a moving water-air interface, or “plug front.” Significant unbalanced forces are generated at elbows, especially near the system’s start where velocities are highest. As the water front advances, frictional losses slow it down. Upon reaching the monitor, a sudden increase in flow resistance occurs, causing a sharp pressure rise that propagates as a pressure wave, generating further unbalanced loads.
The analysis considered both pressure surge-induced loads and slug loads, the latter arising from momentum changes as the water front fills the system. For initial system priming, a quasi-static assumption was employed, justified by the relatively slow water front velocity (~10 m/s) compared to the pressure wave speed in the piping (~700 m/s). This allowed for the system to be modeled as a series of stepwise filled segments, with each segment analyzed as the front advanced.
Modeling Data
The modeled system encompasses the fire water storage tank (AF801), four main pumps (AG203 A, B, D, and E), the deluge valve AHV2208, and monitor valves AHV2211, AHV2213, and AHV2215. The rest of the interconnected fire water network was assumed isolated for this study.
The pumps exhibit a range of operational characteristics, with both electric and diesel-driven units included. For transient analysis, pump properties such as efficiency, suction pressure, inertia, and start-up time were derived from vendor data and empirical relations. Sequential pump start-up is controlled by header pressure, with built-in delays to avoid rapid system pressurization.
Valve characteristics, specifically for the renewed 6″ ball valve AHV2208 and the AVK-gate monitor valves, were modeled according to Cv/Kv curves. Monitors themselves were treated as flow resistance elements, with a design pressure drop of 7 bar at 600 m³/hr. During surge events, only one monitor is expected to be open at a time.
Fluid properties were standardized to those of water, given the low concentration of foam (3%), and all downstream piping beyond AHV2208 was modeled as fiberglass (GRE), with upstream sections in steel. The system’s maximum allowable pressure was set at 18.6 barg, in accordance with ISO-14692, representing 1.33 times the design pressure.
Key surge scenarios modeled included start-up via different monitors, system stop via valve closure, and monitor switching—both simultaneous and staggered. The most critical and representative cases were emphasized in reporting.
Results and Discussion
Start-Up Scenario
The analysis determined that a controlled opening of AHV2208 to a maximum of 36° (with 90° fully open) permitted system priming within the 240-second requirement. During this phase, maximum pressures reached 12 barg (65% of allowable), and the peak unbalanced load was 8.3 kN. These figures are well within structural and operational limits, demonstrating that a gradual start-up mitigates both surge pressure and force on the piping. The hydraulic grade line analysis confirmed that the flow regime becomes increasingly stable as the water front approaches the monitor, with transient pressure spikes effectively damped by system friction and resistance.
Monitor Switching
Switching between monitors (e.g., from AHV2215 to AHV2211) with a minimum closure/opening time of 40 seconds resulted in transient surge pressures up to 13 barg (70% of allowable) and maximum unbalanced loads of 11.3 kN. The simulation revealed that the most significant flow and pressure changes occur during the initial and final phases of valve movement, corresponding to the non-linear valve characteristic curves. Despite these transient excursions, the system remained within safe operating limits.
System Shutdown
A system stop via closure of AHV2208 (with AHV2215 operational) and a minimum closure time of 40 seconds produced a minimum pressure of -0.7 barg, with peak unbalanced loads below 2 kN. Faster closure rates risked pressures dropping below vapor pressure, potentially causing vapor cavity formation and dangerous pressure oscillations. The recommended closure time thus balances operational efficiency and safety.
Entrapped Air and Cavitation Risks
The study identified the potential for significant air entrapment, particularly when priming through upstream monitors with downstream sections remaining air-filled. Entrapped air can migrate and cause secondary surges or impose unbalanced loads on the system, especially if displaced suddenly during subsequent operational events. Mitigation strategies include installing appropriately sized air release valves near the downstream monitor (AHV2215), allowing temporary discharge via sprinkler lines, or priming with all monitors open to expel air before closing unnecessary outlets.
Potential cavitation at AHV2208 during start-up was highlighted as a concern due to the high-pressure differential across the valve. The analysis could not conclusively determine if cavitation would occur under the modeled conditions, so consultation with the valve supplier was recommended for further assessment.
Conclusions
The surge analysis of foam line AFW-023024 demonstrated that, with proper operational procedures, surge pressures and unbalanced forces can be effectively managed within the system’s design limits. Critical findings include:
- A controlled start-up with AHV2208 limited to 36° opening allows system priming within required timeframes and with acceptable transient loads.
- System shutdown and monitor switching, when executed with a minimum of 40 seconds for valve actuation, prevent excessive surge pressures and mechanical stresses.
- Entrapped air remains a potential source of transient loads and must be proactively managed through air release systems and careful operational sequencing.
- The risk of cavitation at the control valve during start-up requires further validation with the equipment supplier.
Recommendations
The surge analysis of foam line AFW-023024 demonstrated that, with proper operational procedures, surge pressures and unbalanced forces can be effectively managed within the system’s design limits. Critical findings include:
- A controlled start-up with AHV2208 limited to 36° opening allows system priming within required timeframes and with acceptable transient loads.
- System shutdown and monitor switching, when executed with a minimum of 40 seconds for valve actuation, prevent excessive surge pressures and mechanical stresses.
- Entrapped air remains a potential source of transient loads and must be proactively managed through air release systems and careful operational sequencing.
- The risk of cavitation at the control valve during start-up requires further validation with the equipment supplier.