Dynamic Vapor Breakthrough: Avoiding Pitfalls for ERS Design

Vapor breakthrough (also known as gas blowby) can occur after an instrument fails, allowing uncontrolled flow from high-pressure to low-pressure equipment. After a loss in upstream liquid level, high-pressure vapor may enter the low-pressure system, resulting in overpressure of the low-pressure equipment.

In this newsletter, Daniel Wilkes presents a comprehensive overview of modelling dynamic vapor breakthrough. Different modelling methods for sizing vapor breakthrough scenarios are reviewed, including steady state modelling, implementing further complexities to reduce the relief requirement, dynamic modelling, and investigation of DIERS coupling equations for predicting vapor/liquid disengagement characteristics in a low-pressure system.

Vapor breakthrough scenarios should be carefully considered since they are frequently the sizing case for pressure relief system and flare (PRFS) design, as they have been known to be handled improperly. In some cases, the sizing credits were applied incorrectly. In others, the scenario went completely unnoticed in the hazard analysis.

What is Vapor Breakthrough?

Case Study: 1987 Grangemouth Hydrocracker Explosion and Fire

The 1987 Grangemouth Hydrocracker Explosion is an example of a vapor breakthrough hazard that was not adequately managed and resulted in an explosion. During recommissioning of the hydrocracker, effluent from the hydrocracker to the High Pressure (HP) Separator stopped whilst flow to the Low Pressure (LP) Separator continued. This resulted in a loss of liquid level that allowed high pressure vapor to flow from the HP system to the LP system.

Design issues:

• The pressure relief valve had been sized for fire, not for vapor breakthrough
• In the relief system design, a low-level trip on the HP separator liquid level was credited to prevent a breakthrough. However, the trip was disabled and provided no protection

What is Vapor Breakthrough?

Vapor breakthrough may occur if the liquid seal is lost between a high-pressure and low-pressure system. This loss of seal could occur due to a control valve failure or inadvertent bypass valve opening between the high-pressure and low-pressure systems. It could also occur following a loss of liquid flow to the high-pressure system.

Vapor breakthrough is a complex, dynamic scenario with many elements to consider in relief system design:

• All scenarios that could initiate a vapor breakthrough event
• The connecting pipeline between the high-pressure and low-pressure vessel
• Continued inflow and outflow to and from the high-pressure and low-pressure vessels and how it affects the liquid levels in both vessels
• The possibility of high-pressure surges in the low-pressure vessel
• The phase of flow leaving from the bottom of the high-pressure vessel
• The possibility of liquid entrainment or full liquid flow through the relief system

Steady State Relief System Sizing Methodologies

Steady state methodologies for vapor breakthrough sizing are often used for conservative relief sizing. The relief requirement is typically determined with hydraulic calculations.

Relief system engineers sometimes credit two-phase flow from the upstream system to reduce the vapor mass flowrate. However, 100% vapor flow may still break through even if liquid remains in the upstream vessel due to:

• Sloshing liquid
• High vapor velocity
• Complex internal geometries

So be cautious crediting two-phase flow.

Additional Considerations for Steady State Methods

It is not sufficient to only consider the maximum vapor flowrate into the low-pressure vessel. The resulting liquid level in the low-pressure vessel also needs to be considered. There are 3 main potential outcomes:

1. The liquid partially fills the downstream vessel:

• The downstream liquid level is low (usually below the inlet nozzle)
• Complete vapor/liquid disengagement occurs
• The relief valve discharges vapor only

2. The liquid significantly fills the downstream vessel:

• The downstream liquid level is significantly high (above the inlet nozzle, but not filled)
• Partial vapor/liquid disengagement may occur
• The relief fluid is expected to be two-phase

3. The liquid overfills the downstream vessel:

• The downstream vessel becomes liquid full
• Limited vapor/liquid disengagement may occur
• The relief fluid is expected to contain a significant amount of liquid

The resulting downstream liquid level can be calculated with a simple volume balance, however, this methodology has limitations:

• Flow may continue
• Flashing of liquid to vapor may occur

To improve the accuracy of this calculation, reduce the calculated relief requirement, and avoid design mistakes a dynamic model should be used instead:

To read the rest of this presentation, visit our website > https://bit.ly/3ITNNZw

SuperChems? Virtual Training

Master techniques for addressing relief sizing for various scenarios, relief piping system design, flare header modeling, and consequence modeling, as well as overviews of the Process Safety Office? SuperChems? v11.6 interface and its models. The course is taught virtually and provides a dynamic learning environment anywhere without sacrificing effectiveness. Learners can ask questions, talk to the instructor, and take part in group discussions.

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Exploring the Power of Flow Dynamics: Modelling Examples in SuperChems Part 1

Flow dynamics modelling is a branch of fluid mechanics that deals with the prediction and analysis of fluid flow in various systems. In this 26-minute video, watch Charles Lea and James Close demonstrate several real-world examples predicting a broad scope of process safety problems, ranging from explosion dynamics and pressure relief stability to reaction forces on process and relief piping.

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