Liquid seals are a common piece of equipment in process flare systems, providing separation of a gas (or vapor) system into upstream and downstream sections. They are used to:
- Maintain positive header pressure
- Prevent flashback by acting as a flame arrestor
- Keep flare headers separated in staged applications
By using a liquid—often water or a glycol/water mixture—liquid seals provide reliable protection in flare operations.
However, under certain conditions, liquid seals face a critical challenge: blow-dry. When emergency relief events generate high gas velocities, the liquid inside the seal can become entrained and discharged through the flare tip. This liquid seal blow-dry can trigger operational and safety concerns.
What Is Liquid Seal Blow-Dry?
A blow-dry event occurs when the gas velocity is high enough to entrain liquid out of the drum. This can result in:
- Extinguished pilots or main flames
- Contaminated seal fluid raining down from the flare stack
- Pressure spikes in the flare header as fluid density increases in the stack
The gas flow rate at which this happens is called the blow-dry rate, defined early in the design process. For some flares, the blow-dry rate is higher than the maximum relief rate, making blow-dry impossible for that system.
However, many facilities have emergency design rates so large that preventing blow-dry entirely is impractical. In these cases, operators must account for the potential consequences of liquid entrainment.
CFD Insights: Unexpected Pressure Spikes
John Zink performed a Computational Fluid Dynamics (CFD) study to better understand blow-dry dynamics in a low-pressure flare system.
The analysis revealed that during a blow-dry event, inlet static pressure spiked to about 47 psig—much higher than expected for a low-pressure system. While this elevated pressure lasted only briefly, it still posed potential risks to upstream equipment and overall facility safety.
These results highlight the importance of evaluating liquid seal behavior during emergency relief events and considering design strategies to mitigate backpressure risks.
A Simplified Method for Pressure Estimation
Analyzing two-phase flow during blow-dry is complex and typically requires specialized software and expertise. To support operators and designers, John Zink developed a simplified analytical method to estimate the pressure spike.
- The method was calibrated using CFD results.
- It provides a practical screening tool for estimating potential backpressure.
- It allows early evaluation of system risk without full-scale modeling.
Important caveat: this simplified approach was validated against a limited number of CFD simulations, and its accuracy for other applications is unknown. It should be viewed as a screening method, not a replacement for detailed analysis.
Proven John Zink Solutions to Mitigate Blow-Dry
If blow-dry is a concern, John Zink offers engineered liquid seal designs that can avoid or mitigate liquid entrainment and reduce the magnitude of backpressure.
1. Horizontal Liquid Seal
A standalone vessel that performs the same protective functions as a vertical seal but with enhanced liquid separation. Its horizontal orientation provides inherent knockout capability, lowering the risk of liquid carryover during high relief events.
2. Quick Dump Liquid Seal – Style 1 (External Drainpipe)
This design uses an external drainpipe and equalization line. During a high relief event, a valve opens to divert liquid into a lower reservoir, removing it from the gas path. Once the event is over, the liquid is restored to re-establish the seal.
3. Quick Dump Liquid Seal – Style 2 (Internal Drainpipe)
Here, a pressurized lower chamber serves as a reservoir. At the start of a high relief event, the chamber is vented, allowing liquid to drain from the seal. Afterward, re-pressurization with nitrogen or fuel gas restores the liquid seal quickly.
Both quick dump configurations can typically drain the seal in 10–20 seconds, reducing entrainment risk. Style 2 offers the additional benefit of rapid re-pressurization, allowing faster re-establishment of the seal after flaring.
Safer Flares Through Better Design
CFD analysis has shown that liquid seal blow-dry can produce backpressures far higher than many facilities expect, up to about 47 psig in the study. While brief, these spikes can pose risks that operators should not ignore.
John Zink’s simplified analytical method provides a practical way to screen for potential blow-dry risks, though its accuracy outside the studied cases remains uncertain. For facilities where high emergency relief rates are a concern, engineered liquid seal solutions, such as horizontal designs and quick dump systems, offer proven ways to mitigate entrainment and reduce pressure rise.
Partnering with John Zink helps operators achieve safer flare operation, reduced blow-dry risk, and greater reliability across their flare systems.
