Introduction
Honeywell is installing a new membrane test system at its Penang plant in Malaysia, which will operate in batch mode on weekends for testing. NRC Engineering (NRCE) is handling the initial Front End Engineering Design (FEED) for the plant. As part of the process, they’ve assigned iFluids Engineering in Chennai to conduct a detailed flarenet analysis.
This analysis will focus on the plant’s Pressure Relief Valves (PSVs), as well as the new flare header and flare tip. The main objective is to verify that the proposed design, particularly the size of the main flare header, follows safety regulations set by API 521. The analysis will also check if the PSV tailpipes and flare header sizes are adequate for safe gas discharge during high-pressure events.
Scope of Work
- Check the 14-inch flare header size: Ensure it’s large enough to handle the gas flow safely.
- Mach number limits: Confirm that gas flow speeds stay below safety thresholds:
- PSV tailpipes: Mach number < 0.75 (75% of the speed of sound).
- Flare header: Mach number < 0.5 (50% of the speed of sound).
- Determine proper sizes: Adjust the size of the PSV tailpipes and flare header if needed.
- Identify critical scenarios: Pinpoint the worst-case situations for gas discharge and size the system accordingly.
- Develop a FlareNet model: Use Aspen Flare System Analyzer to simulate the flare system, including all major components like PSVs, control valves, tailpipes, flare headers, and the flare stack.
- Ensure compliance with API 521: Verify that the entire system meets the required safety standards and make recommendations if changes are needed.
Understanding Flare System and API 521:
- Flare System: A flare system is designed to safely release and burn off excess gases during emergency or maintenance conditions in a plant. The system includes the tailpipe, which connects the source of the flare to the flare header, and the flare header, which gathers gases from various sources and directs them to the flare stack.
- API 521: This is a standard published by the American Petroleum Institute (API) that outlines the design criteria for pressure-relieving and depressuring systems, including flare systems. It focuses on ensuring safe pressure relief while preventing overpressure scenarios.
Flare Load Summary
The flare load summary provides the total gas flows that must be handled by the flare system, including:
- Maximum expected flows during relief scenarios.
- Flare gas composition (affecting density, speed of sound, and pressure drop).
- Temperature and pressure conditions for the relief gas
Developing the FlareNet Model:
Using Aspen Flare System Analyzer
- The software simulates the flow of gases through the flare system, based on the provided flare load summary.
- It models the piping network, considering factors like pipe lengths, diameters
- Key variables analyzed include Mach number, pressure drop, and velocities.
Sizing Criteria:
When sizing flare headers and tailpipes, important factors from API 521 include:
- Mach Number Limit: The Mach number is the ratio of the gas velocity to the speed of sound in the gas. API 521 limits the Mach number to typically 0.7 or lower (to avoid sonic choking and minimize noise and vibration issues).
Proposing New Tailpipe and Header Sizes:
- Tailpipe Sizing: The size is adjusted so that the Mach number does not exceed the API-recommended limit (usually ≤ 0.7) under the worst-case relief scenario. If the initial pipe size results in a higher Mach number, a larger diameter tailpipe will be proposed to reduce gas velocity.
Final Report:
The technical report include:
- Simulation Results: Showing the calculated Mach numbers, for the initial and proposed piping sizes.
- Recommendations: Any additional system improvements required to achieve compliance, such as changes to pipe layouts or additional equipment installation.