Distillation tray hydraulics: the vertical distance between the top of the bubble-cap slots and the quiescent liquid level when the static liquid is just about to flow over the overflow weir is called what?

Introduction / Context:
Understanding the terminology of tray hydraulics is essential for designing and troubleshooting distillation columns. One key geometric parameter is the liquid level relative to the bubble-cap slots and the overflow weir under static (no vapor) conditions. This affects start-up behavior and the degree of slot coverage at incipient flow.

Given Data / Assumptions:

• Bubble-cap tray with an overflow weir and downcomer.
• “Static” refers to conditions without vapor flow, just before liquid begins to spill over the weir.
• Interest is in the specific distance from slot tops to the liquid surface at that moment.

Concept / Approach:
The term for the distance between slot tops and the quiescent liquid level as the liquid is about to flow over the weir is known as the static submergence. It indicates how much of the cap slot is covered at rest, influencing vapor–liquid contact upon vapor introduction and the potential for weeping at low loads.

Step-by-Step Solution:

Verification / Alternative check:
Tray hydraulics glossaries and design texts define static submergence exactly in this manner for bubble-cap trays.

Why Other Options Are Wrong:

• Downcomer liquid seal: refers to the height sealing the downcomer, not slot coverage.
• Skirt clearance: mechanical clearance of cap skirt, unrelated to level definition.
• Foam height: pertains to froth during operation, not static condition.
• Weir crest height: fixed mechanical weir height, not the static coverage distance.

Common Pitfalls:
Confusing static submergence with operating froth height; mixing terms used for sieve/valve trays versus bubble-cap trays.

Final Answer:
Static submergence