A gas boot is a tall, slim vessel that gives oil one last chance to release entrained gas before it drops into the stock tank. By flashing that gas off in a controlled vessel rather than in the open tank, the boot cuts tank vapors, reduces emissions and product loss, and keeps the stock tank from surging. This guide explains what a gas boot is, how it degasses oil, and where it sits in a tank battery.
Gas Boot in one line: A gas boot (also called a degasser or degassing boot) is a vertical, low-pressure vessel installed ahead of the stock tank that lets residual dissolved and entrained gas break out of the oil, routing that gas to a vapor-recovery or vent line so it does not flash off inside the atmospheric stock tank.
Oil leaving an upstream separator or treater is at a higher pressure than the atmospheric stock tank it is headed for. As that oil drops toward tank pressure, dissolved gas comes out of solution. If that flash happens inside the open stock tank, it shows up as tank breathing, odor, emissions, and potential overpressure of the thin tank shell. The gas boot intercepts the oil first: it operates at a low pressure just above atmospheric and gives the gas a quiet vertical space to break out cleanly.
The oil enters the boot, gas separates and rises to a top gas outlet, and degassed oil falls to the bottom and flows on to the stock tank - usually by gravity through a large downcomer, sometimes with a small back-pressure to keep the boot slightly above tank pressure. The tall vertical geometry gives the rising gas a straight, unobstructed path away from the descending liquid, which is why boots are slender and tall rather than squat.
A gas boot is not a high-pressure separator. A production separator operates at line pressure and does the bulk gas-liquid split; the boot is a low-pressure polishing step that captures the last of the gas that only breaks out once the oil approaches tank pressure. Think of the separator as the primary degas and the boot as the final degas right before storage.
The boot works hand in hand with a vapor recovery unit. Gas the boot liberates is routed - along with stock-tank vapors - to a VRU or a vapor-recovery tower and compressor, which recovers it for sale or fuel rather than flaring or venting. Capturing gas in the boot rather than the tank means it comes off at a steadier, slightly elevated pressure that is easier for a VRU to handle, improving recovery and reducing emissions.
In the tank battery the boot sits between the treater or separator liquid outlet and the stock tanks: treater - gas boot - stock tank, with the boot's gas leg tied into the vapor-recovery header. It is common on facilities that are being emissions-optimized, because moving the flash out of the open tank is one of the cheapest ways to cut tank vapor losses.
Key measured values are the boot's operating pressure, the liquid level, and the vapor-line flow to the VRU. A cloud SCADA such as Merobix reads those pressure, level, and flow tags from the site controller over Modbus, DNP3, or OPC UA. Trending boot pressure and VRU flow remotely helps confirm the boot is actually pulling gas off - a boot running at tank pressure with no vapor flow means gas is flashing in the tank instead.
It degasses oil just before it reaches the stock tank. As oil drops from separator or treater pressure toward atmospheric, dissolved gas breaks out; the gas boot gives that gas a quiet vertical space to separate and routes it to vapor recovery or vent, so the gas does not flash off inside the open stock tank as vapors and emissions.
A separator operates at line pressure and performs the primary gas-liquid split of raw well fluid. A gas boot is a low-pressure vessel that captures only the residual gas that breaks out as oil approaches atmospheric tank pressure - a final polishing degas step right before storage, not a primary separation stage.
By flashing the residual gas in a controlled vessel connected to vapor recovery instead of letting it flash inside the open, atmospheric stock tank. Gas captured in the boot can be routed to a VRU and recovered rather than vented or flared, cutting tank breathing losses and the emissions and product loss that go with them.
The vertical geometry gives rising gas an unobstructed straight path up and away from the descending liquid, maximizing clean gas-liquid separation in a small footprint at low pressure. A tall, slim boot separates the counter-flowing gas and oil far better than a short, wide vessel would at the same low operating pressure.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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