Pressure Drop Hydraulics in Revamp Projects: An Engineer’s Perspective

In revamp projects, managing pressure drop hydraulics is crucial for ensuring both process efficiency and safety, particularly in complex systems like refineries. Accurate pressure drop calculations are essential for properly sizing pipes, pumps, and other equipment to ensure fluids flow at the desired rate while maintaining system integrity. Whether performed manually or with advanced software tools, understanding hydraulics deeply is key for the process engineer tasked with optimizing the system.

However, a long-standing debate exists: Should pressure drop hydraulics be the responsibility of the process engineer or the mechanical engineer? In my experience, the process engineer is often better suited to handle this task, given their comprehensive understanding of fluid dynamics and unit operations. That said, this debate often arises due to the overlapping skillsets of both disciplines, with mechanical engineers specializing in equipment design, while process engineers focus on overall system performance.

The key considerations of pressure drop hydraulics below will allow you to explore a revamp example as a starting point. Further below it is also discussed how close collaboration between process and mechanical engineers can lead to more successful project outcomes, particularly in revamp scenarios.

Key Considerations in Pressure Drop Hydraulics

Several important factors must be considered when dealing with pressure drop hydraulics, whether in manual calculations or using software tools like KORF, Aspen HYSYS, etc:

• Fluid Properties: Understanding the physical and chemical properties of fluids—such as viscosity, density, and temperature—is crucial. In revamp projects, process modifications often change these fluid properties, affecting the hydraulics. For instance, increasing the temperature of a liquid can lower its viscosity, reducing frictional losses in pipelines.
• Piping and Equipment Layout: The design and routing of the piping, including pipe lengths, diameters, bends, fittings, and valves, all contribute to the system’s pressure drop. Updating the layout to account for new equipment or modified process parameters is a common challenge in revamp projects.
• Flow Regime: The flow regime (whether laminar or turbulent) significantly impacts pressure drop. The Reynolds number helps identify which regime is in effect, guiding the engineer in selecting the appropriate equations—whether it’s the Darcy-Weisbach equation for turbulent flow or Hagen-Poiseuille for laminar.
• System Interactions: In revamp scenarios, adding new equipment or changing process conditions can alter the pressure profile throughout the plant. Understanding these interactions is key to avoiding operational problems or inefficiencies that can arise from changes in flow resistance.
• Instrumentation and Control: Changes in control systems are common in revamp projects. Pressure control valves (PCVs) and related instrumentation may need recalibration or re-sizing to match the new system dynamics. Ensuring proper integration between hydraulic models and control systems is essential.

Manual vs. Software-Driven Hydraulic Calculations

Manual calculations, based on fundamental principles like Bernoulli’s equation or Darcy-Weisbach, are insightful for small-scale systems or preliminary estimates. However, modern refineries typically use software tools like KORF or Aspen HYSYS to simulate complex fluid systems more efficiently. The software can account for a large number of variables and system components, giving process engineers accurate insights into the pressure drop profile.

Example: Pressure Drop in a Revamp Scenario

Consider a revamp project where the goal is to increase throughput in an existing crude distillation unit (CDU). The piping network feeding the atmospheric column will need re-evaluation for pressure drop constraints. Here’s a simplified walkthrough:

• Fluid Properties and Flow Rate Adjustment: As the flow rate increases, the system will likely shift further into the turbulent regime, increasing friction losses. Calculating the Reynolds number for both pre-and post-revamp cases helps us understand how the fluid behaves under these conditions.
• Friction Factor and Pressure Drop Calculation: Using the Darcy-Weisbach equation, the pressure drop is estimated. For instance, the original system may have had a pressure drop of 5 psi, but the increased flow may result in an 8 psi pressure drop. This increase will likely affect downstream systems, such as the feed pumps.
• Equipment Re-sizing: With the increased pressure drop, the feed pumps must be re-evaluated. Ensuring that suction and discharge pressures are within acceptable limits is critical to avoid cavitation and ensure efficient operation.

AI’s Role in Supporting Hydraulic Calculations

Emerging technologies like Artificial Intelligence (AI) are making their mark in process design and revamp projects, and their role in pressure drop hydraulics is no exception. AI has already been implemented in several industrial settings to optimize fluid dynamics and system behavior. Let’s explore how AI can transform this process:

• Data-Driven Modeling: AI can analyze vast amounts of operational data, learning from historical performance and predicting system behavior under various conditions. For example, Chevron has implemented AI-driven models to optimize refinery operations, reducing pressure losses across complex systems by detecting patterns that humans might miss. These models provide real-time diagnostics, enabling process engineers to adjust operations more efficiently.
• Predictive Maintenance: AI can monitor pressure drops continuously and flag potential issues, such as pipe scaling, fouling, or pump degradation before they escalate. Shell has used AI in its predictive maintenance programs to detect inefficiencies in hydraulic systems, reducing downtime and maintenance costs.
• Design Optimization: AI algorithms can run millions of simulations in a fraction of the time traditional methods require, optimizing piping networks and flow paths. For example, BASF has piloted AI-based design tools to optimize process flows, leading to a 5% improvement in overall system efficiency. This capability allows engineers to identify the most efficient configurations for revamping projects, providing optimized solutions faster than manual or even simulation software can.

In the future, AI is likely to play an even bigger role in system optimization, as machine learning algorithms continue to improve, providing even more accurate insights into the complex behaviors of fluid systems.

The Debate: Should Pressure Drop Calculations Be Done by Process or Mechanical Engineers?

The debate on whether process or mechanical engineers should handle pressure drop calculations is an ongoing one. While mechanical engineers excel in equipment design—such as pump performance curves, valve selection, and material strength—process engineers possess a holistic understanding of how changes in pressure drop impact the overall operation of a plant.

In reality, successful revamp projects often depend on close collaboration between both disciplines. For example, the process engineer might model the system hydraulics, considering the unit operations and fluid dynamics, while the mechanical engineer focuses on ensuring that pumps, valves, and equipment are sized appropriately for the system's mechanical integrity. Regular cross-discipline reviews can prevent issues such as equipment under-sizing or over-stressing, which can lead to inefficiencies or mechanical failures down the line. Effective communication between these two disciplines is crucial to ensure that both fluid dynamics and equipment limitations are accounted for.

Equipment Considerations: Beyond Pump Sizing

Equipment re-sizing is often necessary during revamp projects, especially when pressure drops increase. In the earlier example, where pressure drops increased from 5 psi to 8 psi, it's not just about re-sizing pumps to handle the increased load. Engineers must also consider potential risks, such as cavitation, where vapor bubbles form in the pump due to low suction pressure, leading to mechanical damage. Increased pressure drop can also induce mechanical stress on pipelines, especially near elbows, valves, or fittings, causing wear and tear over time.

When making equipment changes, it’s critical to evaluate the system holistically—understanding the interplay between flow, pressure, and mechanical forces. For instance, if a pump is oversized to accommodate a higher flow rate, the increased velocity could cause erosion in downstream piping, leading to long-term degradation. These considerations are often missed if the pressure drop calculations are viewed in isolation, without collaboration between the process and mechanical teams.

Final Thoughts

Pressure drop hydraulics is a complex but essential aspect of revamping projects, particularly in refineries. Both process and mechanical engineers play critical roles, and effective collaboration between these disciplines is key to success. Process engineers, with their understanding of fluid dynamics, often lead the system design, while mechanical engineers ensure equipment is appropriately sized and designed to handle the revised system conditions.

As AI technology advances, its role in hydraulic calculations and system optimization will likely expand, providing engineers with powerful tools to enhance efficiency, reduce downtime, and optimize system design. In the end, a successful revamp project requires careful attention to fluid properties, piping layout, flow regimes, and equipment sizing, with a forward-looking approach that incorporates the latest technological advances.