Designing overpressure protection means balancing safety against capital cost, and conservative relief load estimates often tip that balance toward oversized equipment. In a recent AspenTech webinar, Emerson’s Erika Belmont, AspenTech Product Manager, and Katherine Cobb, AspenTech Product Marketing Director, showed how dynamic simulation and flare network analysis work together to size safety systems on predicted process behavior rather than assumptions.

Why It Matters

Flare systems must handle many possible emergencies, and each scenario drives decisions on valve sizes, header diameters, knockout drum capacity, and materials of construction. When relief loads are estimated without accounting for how the process actually responds during an upset, safety equipment tends to be overdesigned, which raises capital expenditures (CAPEX). Accurate, scenario-based loads let engineers keep the plant safe while spending only what the design requires. They also help teams confirm whether an existing flare network can absorb new loads from debottlenecking, brownfield modifications, and plant expansion projects.

Key Takeaways

• Aspen HYSYS Dynamics predicts relief loads from transient behavior during upsets, helping avoid overdesign of safety equipment.
• The safety analysis environment in Aspen Plus and Aspen HYSYS sizes pressure safety valves (PSVs) across scenarios such as fire, tube rupture, control valve failure, and blocked outlet.
• Aspen Flare System Analyzer designs, rates, and debottlenecks flare networks and flags issues such as choked flow, slug flow, and Mach number violations.
• Valve sizes and relief scenarios import directly from the simulators into the flare model, keeping the safety workflow connected.
• Customers have used dynamic simulation to reduce peak flare loads by 20% and save $500,000 on a new flare design.

A Connected Toolkit for Overpressure Protection

Katherine explained that overpressure protection analysis involves several steps: calculating relief loads, sizing PSVs, and then designing and rating the flare system. AspenTech tools cover each step. Dynamic modeling in Aspen HYSYS Dynamics or Aspen Plus Dynamics calculates relief loads. The safety analysis environment, built into the Aspen Plus and Aspen HYSYS simulators, sizes relief devices across multiple scenarios and documents the results. Blowdown technology in Aspen HYSYS models rapid emergency depressurization and helps pinpoint low-temperature areas where more expensive materials of construction are required. Aspen Flare System Analyzer completes the chain.

Engineers can convert steady-state Aspen HYSYS models to dynamic models entirely within Aspen HYSYS. Erika noted that dynamic simulation supports a site-wide evaluation of possible scenarios and helps identify potential safety risks so teams can take preventive measures.

Modeling a Blocked Outlet Scenario

Erika’s demonstration began with a dynamic model of a light hydrocarbon feed, mostly propane, passing through a knockout drum, pump, and preheater into a column that separates propane from the other components. Running in dynamics required sized equipment, pump curves, heat exchanger volumes, pressure specifications on boundary streams, and controllers in place.

To simulate a blocked outlet, she used the event scheduler to fail the propane vapor outlet valve shut, then switched several controllers to manual so they froze at their current operating points. Freezing those controllers kept them from compensating, which represents the worst case. When the relief valve opened, she captured the temperature, pressure, flow, and composition at peak conditions and used them in the safety analysis environment to size the valve, selecting an orifice from the American Petroleum Institute (API) 526 standard.

Asked why not build the blocked outlet case directly in steady state, Erika said that works if you already know the relieving conditions. Dynamics gives a more accurate prediction based on the actual system and can reveal when assumed conditions are wrong.

Rating the Flare Network

Erika then imported the sized valves into an existing flare model in Aspen Flare System Analyzer. Valves with matching names mapped automatically, and auto-map rules linked the simulator’s relief scenarios to flare scenarios such as local and total power loss. Mapping individual relief scenarios to global flare scenarios helps teams complete flare analysis faster and with fewer mistakes. Data can also come from simulator streams, blowdown and depressuring analyses, or a text file.

In rating mode, the software checks an existing network for pressure drops, velocities, temperatures, noise, and sound power level, then highlights problems in red with supporting messages. In the demo, it warned of choked flow, slug flow, and a Mach number violation in the stack pipe. Resizing the stack cleared the warning. Design mode sizes new headers to defined criteria, and debottlenecking mode resizes problem sections, which is useful for revamps. The tool models single and multiphase flare and vent systems in convergent, divergent, and looped configurations. Since version 14, it also offers more detailed emissions predictions, radiation assessment at various distances, and Aspen Properties methods.

During the Q&A, Erika confirmed that heat transfer in pipes can be enabled model-wide or for individual pipes. The tool can help size the stack pipe but does not perform rigorous flare tip design. For rigorous depressurization studies, she recommended blowdown over the legacy depressuring analysis, which remains a good first approach.

Results in the Field

Katherine shared two customer examples. The first customer used Aspen HYSYS Dynamics to evaluate an offshore flare system for available capacity. The analysis showed a 20% reduction in peak flow rate to the flare header, and the system could handle an extra 20% flow from expansion projects. The second customer estimated peak flare load with dynamic simulation in Aspen HYSYS during a refinery reconfiguration, saving $500,000 on a new flare design and reducing the sterile zone area.

Learn More

Watch the on-demand webinar, Optimize Process Safety with Dynamic Modeling and Accurate Flare Design, to see Erika’s full demonstration and Q&A. To explore the tools further, visit the