Shielding Your Assets
Case study: Engineering Tactics Against Water Hammer Risk
My Engineering Journey
While writing this article, it brought me back to the pivotal moment when I made two resolute choices: embarking on my engineering journey, and joining Pontem. Back in high school, I found myself captivated by the thrill of unraveling math and physics puzzles. There was this magical satisfaction in diving deep into complex problems and emerging victorious with solutions. It was what led me to pursue my bachelor’s degree in engineering back home in China.
After that, the wanderlust kicked in, pushing me to explore beyond my comfort zone. So, off I went to the US for a master’s in Petroleum Engineering from the University of Southern California (USC). Sure, I was so nervous about stepping into the unknown; on top of it, I had severe fear of flying while I had to take a 15+ hrs flight. But hey, facing those fears head-on was quite the ride. Little did I know by then, I’d end up living in three different countries over the next 10 years, conquered my fear of flying and embracing the adventure of life in LA, Houston, London and now Calgary…
My engineering career officially kicked off as a Flow Assurance engineer with AFS, where I got into these brilliant minds - Tommy, Andrew, Kostas, Conor, and Pete. I was so lucky to be involved in the legendary “Leviathan” project from very early phase, and contributed to the simulation works that facilitated its successful start-up. With over 20 projects under my belt from all over the globe, each presenting its own mix of challenges and connections to operations, business, and policy, I’ve found immense fulfillment in helping people in overcoming their challenges. It’s where I feel I make the most meaningful impact.
In my years working in the energy industry, I’ve realized how sound engineering advice can save millions of dollars, enhance operation efficiency while prioritizing safety. However, the challenge lies in effectively conveying these messages to decision-makers. Communication between disciplines and people with different background isn’t always a walk in the park. That’s the reason Pontem’s mission of “Bridging the Gap” resonates so deeply with me - it’s a vital step towards making a tangible difference.
Plus, there’s this curiosity burning within me - an itch to apply engineering skills beyond the traditional boundaries. My mom, an anatomy professor, once linked human blood flow to “oil and gas” flow. Now, I’m curious about tackling blood clotting from an engineering perspective. With Pontem’s broad vision and our strongly bonded team, guess where we’ll end up? Let’s dive in together, and see where it takes us!
Case study
Background
Water hammer is a pressure surge phenomenon occurs when the fluid flow inside a pipeline is abruptly halted, typically due to the instantaneous closure of a valve downstream in the flow direction. This sudden stoppage leads to a sharp increase in pressure, known as surge, immediately upstream of the closed valve. The high momentum of the flowing system encountering an abrupt change of direction results in this pressure surge. The surge pressure travels backward in the initial flow direction, forming a pressure shockwave that propagates through the entire pipeline. This pressure shockwave is anticipated to diminish as flow stops from the feeding side, system pressure then gradually reaches hydraulic equilibrium. The simplified schematic below depict a relatively straightforward water hammer event with the pressure surge behavior at each end of the flowing direction.
It is crucial to recognize that the surge pressure generated can far EXCEED normal operation pressures in the pipeline, potentially surpassing the Max Allowable Operating Pressure (MAOP) of the pipeline. Without proper front-end planning or mitigation measure, this can pose serious risks to the pipeline system.
Hence, it is vital to thoroughly evaluate water hammer during the initial phases of pipeline design and operations. This involves assessing potential flowing conditions across various ranges, providing insights on both pipeline design and operational philosophy. The goal is to ensure a well-defined strategy is implemented in advance to effectively mitigate the potential impact of water hammer. By incorporating these considerations into the planning process, operators can be prepared to prevent unforeseen pressure surges, and ensure the overall integrity of the system.
For a preliminary assessment of water hammer events in pipelines, we use Joukowsky Equation, a commonly employed tool, for a first-pass estimation on the maximum surge pressure caused by water hammer event, where the velocity instantly stops:
As we can tell from the equation, the maximum pressure surge is highly dependent on fluid type, pipeline wall material, and initial flow velocity. Tables below give sound of speed in some common fluids in oil and gas industry, as well as Young’s Modulus of common pipeline materials. Generally, the pressure surge caused by water hammer in gas-dominated system is much less of a concern than that in oil-dominated or water distribution system. And, carbon steel pipelines would be expected to experience higher pressure surge than elastic pipelines.
While a straightforward application of the Joukowsky Equation provides an estimate on the maximum pressure surge at a specific location and time, particular in response to instantaneous valve closure, relying solely on it falls short in predicting the comprehensive pressure dynamics across the entire pipeline system. To address this limitation, we employ commercial hydraulics simulation software (OLGA® and AFT-Impulse®). These tools provide a thorough and reliable prediction of pressure behavior within the pipeline and associated equipment during water hammer events. The simulations meticulously consider crucial factors that influence the propagation of pressure shockwaves along the pipeline:
The frictional pressure drop induced by fluid transferring inside the pipeline
Gravitational pressure drop due to elevation and density variations throughout the pipeline system
Valve closure time, integrating the unique characteristics of the valve (CV curve)
The graph below provides an illustrative example of a pressure shockwave with varying valve closure times (speed). A more gradual closure of the valve results in noticeable reduction in the maximum pressure surge.
Packing of the system, integrating the unique characteristics of pump operation (pump curve, control logic)
During water hammer event, pipeline keeps packing (pressure building up) as the upstream continuously feeds into the pipeline against a closed (or closing) valve. The duration of the upstream response to valve closure directly influences the magnitude of the pressure surge in a water hammer event. In a pipeline system, the upstream feed from the pump can be halted either manually or automatically by triggering a high-pressure trip setpoint. The graph below illustrates that with a lower pump high-pressure trip setpoint, the automatic shutdown of the pump occurs more rapidly, resulting in a reduced pressure surge.
Pipeline expansion factor due to different elasticity of the pipeline wall material
The graph below provides an illustration on comparison of pressure surges between two different pipeline materials. It is anticipated that HDPE pipelines, with a lower Young’s modulus value than Carbon Steel pipelines, will undergo lower pressure surge.
Considering the correlation between pressure surge and key factors discussed above, various established mitigation methods exist to prevent pipeline over-pressure risks (i.e. pressure exceeding MAOP) during water hammer events. These methods encompass different perspectives, including but not limited to:
Mechanical device - Installing water hammer arrestor, pressure relief valves, pressure regulators
Pipeline design - Incorporating a comprehensive range of potential operation scenarios to address pressure surge
Operation strategy - Implementing measures such as gradual valve closure and setting pump high-pressure trip
Problem
Our client sought a solution for installing a new pipeline section to be connected into an existing water distribution system. Their primary concern was safeguarding the system against over-pressure risks in the event of water hammer. Faced with constraints imposed by the existing equipment and pipeline configuration, our focus was on mapping out viable control logics, in terms of valve closure time and pump high-pressure trip setpoint. The goal was to uphold the operational integrity of all new and existing equipment within design limitations, while achieving the desired delivery rates.
The system characterized by a significant elevation drop of over 3000 feet, with the complexity of varying pipeline MAOPs along the water delivery route. A critical aspect of operational strategy development involved identifying where the maximum surge occurs, which was not always at either end of the pipeline due to gravitational effects induced by the elevation change. We utilized Google Earth file to construct a realistic elevation profile for a precise calculation of hydraulic behavior throughout the pipeline.
Outcome
To assist our client in developing a feasible operation strategy, parametric analysis was done. This involved considering a reasonable range of pump high-pressure trip setpoints, in conjunction with various valve closure times. Hydraulics simulations were conducted with focus on the highest planned delivery rate to simulate a worst-case scenario in terms of pressure surge during water hammer events.
In order to safeguard the entire system from over-pressure during a water hammer event, the minimum valve closure time required for different pump high-pressure trip setpoints was determined. Additionally, we identified the maximum pump high-pressure trip setpoint at which over-pressure could not be mitigated by further slowing down valve closure. This analysis provided the operator with a comprehensive guideline for maintaining a safe operating window and outlined the necessary actions to protect their system from over-pressure during water hammer event.
Conclusion
This case study exemplifies how engineering simulation helps operators to establish an operation guideline to mitigate over-pressure risks in pipeline system during water hammer events. In the absence of proper mitigation measures, frequent water hammer incidents pose a substantial threat to the integrity of pipelines and equipment, resulting in environmental damage and significant financial loss due to high repair costs and increased operational downtime. A thorough upfront hydraulic analysis can provide valuable insights for infrastructure design and operation strategy, effectively mitigating risks and ultimately saving operators on both CAPEX and OPEX.