In the engineering domain, accurately simulating transient events in pipeline systems is critical, particularly for essential safety mechanisms such as firewater systems. These systems, typically dry until needed in emergencies, must be capable of flooding rapidly and efficiently when activated. However, the sudden filling of a dry pipeline system introduces significant momentum changes, which can lead to substantial forces acting on the system’s components. These forces, if not properly anticipated, can cause system failures during critical moments.
Recent developments in simulation techniques have sought to address these challenges, offering more reliable and efficient ways to predict system behavior under such conditions. This article delves into the innovative modeling approach discussed at the recent DRG Conference, focusing on a method that builds upon the Method of Characteristics (MoC) to simulate firewater system behavior and improve engineering safety.
The Challenge of Firewater Systems
Firewater systems are essential for industrial safety, particularly in facilities such as oil refineries, chemical plants, and large storage areas. In an emergency, these systems need to activate immediately, flooding the dry pipelines with water to suppress fires. The process, however, is not as straightforward as one might assume. As water rushes through the dry pipes, it encounters air pockets, valves, and nozzles, creating significant momentum changes. The forces generated by these interactions can be so great that they damage the system, rendering it inoperative at the exact moment it is needed.
Many existing models for simulating pipeline flooding focus on single-pipe systems, which limits their applicability to the complex, heavily branched geometries often seen in real-world firewater systems. To ensure operational reliability, engineers require a simulation method that can handle these complexities while maintaining computational efficiency.
Method of Characteristics: The Foundation
The Method of Characteristics (MoC) has long served as a workhorse for simulating transient events in pipeline systems. This mathematical approach, which is algebraic in nature, is well-suited for modeling fluid flow in pipelines.
While MoC itself is robust, it does not inherently account for the movement of gas volumes within the system, such as the air displaced by incoming water in a dry pipeline. Recognizing this gap, researchers sought to develop an additional module that could integrate seamlessly with MoC.
This module would specifically address the behavior of gas volumes, enabling the simulation of large, branched systems with multiple air pockets. The goal was to achieve a balance between computational efficiency and acceptable accuracy, with an error margin of 10–20% deemed sufficient for engineering purposes.
Developing the New Model
Researchers at DRG began their work by adapting an existing elastic column model, modifying it to suit the unique challenges posed by firewater systems. Their approach relied on several key assumptions:
- Fast Flooding:Â The flooding process was assumed to occur quickly enough that the interface between liquid and gas could be considered perpendicular to the pipe axis at all times. This eliminated the need to account for mixed flow conditions, simplifying the model.
- Single Gas Volume Pressure:Â Within each gas volume, a uniform pressure was assumed, ignoring the flow of gas within the volume.
- No Air Bubble Entrapment:Â It was assumed that no air bubbles were entrained in the liquid phase as it moved through the system. While this assumption is not entirely accurate, it was acceptable for the initial stages of model development.
With these assumptions in place, the model was designed to track the movement of gas volumes through the pipeline system using MoC. Each volume, represented as a series of segments within the pipes, was delineated by positive and negative interfaces. The model calculated how these volumes moved and changed in size, accounting for factors such as gas expansion/compression and interactions with system inlets and outlets.
Handling Complex Geometries
One of the most challenging aspects of simulating firewater systems lies in their complex, branched geometries. At junctions within the system, gas volumes can split into multiple parts or merge back into a single volume, depending on the flow conditions. In this case two primary scenarios were identified:
- Water Approaching a Gas-Filled Junction:Â In this case, the incoming water column causes a single gas volume to split into two distinct parts. Each part can then travel independently, potentially at different pressures.
- Gas Entering a Water-Filled Junction:Â This situation is simpler to handle, as no new volumes are created. Instead, new segments are added to the existing volume as the gas moves into the branches.
To minimize computational effort, the model grouped all inlets and outlets connected to the same ambient pressure into a single entity. This approach reduced the complexity of the calculations while maintaining accuracy.
Computational Efficiency and Validation
One of the model’s primary advantages is its computational efficiency. Compared to traditional MoC simulations, the overhead introduced by the new model is modest:
- For a simple pipeline system, the runtime increases by only 22%.
- For a heavily branched firewater system with hundreds of outlets (e.g., sprinklers on an oil tank), the runtime increases by 34%.
These results demonstrate the model’s scalability and suitability for real-world applications.
By contrast, using computational fluid dynamics (CFD) for such simulations would require significantly more computational resources.
The model was validated using experimental data from the literature, including studies by Zhou et al. (2011, 2018) and Hou et al. (2014):
- Zhou et al. (2011): The model accurately predicted the speed of the liquid-gas interface in a pipeline with an entrapped air pocket.
- Zhou et al. (2018): The model captured the cushioning effect of air pockets during rapid air expulsion through an orifice.
- Hou et al. (2014): The model reproduced the front velocity of the liquid column during rapid filling of a large-scale pipeline system.
While the model showed deviations in certain scenarios (e.g., small air pockets), these discrepancies were attributed to simplifying assumptions, such as neglecting mixed flow conditions.
Current Model Limitations
While the new model represents a significant advancement, it is not without limitations. One notable drawback is its inability to simulate air bubble entrapment within the liquid phase. This omission can lead to conservative results, as entrapped air bubbles effectively lower the wave speed of the liquid, reducing pressure surges. Addressing this limitation is a priority for future development.
The team at DRG is also exploring the potential of their model to improve cavitation simulations. Unlike traditional cavitation models, which are implicit in nature, the new approach could explicitly track vapor cavities as they move through the system. This could lead to more accurate predictions of cavitation behavior, with applications beyond firewater systems.
Concluding Remarks
The innovative modeling approach presented throughout this article represents a significant step forward in the simulation of firewater systems. By building upon the Method of Characteristics and introducing a module to track gas volumes, the researchers at DRG have created a tool that is both robust and efficient. Already in use by commercial companies, the model has proven its ability to handle complex geometries and provide reliable predictions.
As our team continues to refine the model, incorporating air bubble entrapment and improving cavitation simulations, its applicability will only grow. In an industry where safety is paramount, advancements such as these play a crucial role in ensuring that critical systems perform as expected when lives depend on them.
