Drilling mud gas buster also called a mud gas separator or poor boy degasser. It captures and separates large volume of free gas within the drilling fluid. If there is a “KICK” situation, this vessel separates the mud and the gas by allowing it to flow over baffle plates. –wiki
The purpose of a gas buster is to remove gas mixed with the drilling fluid before the drilling fluid goes over the shale shaker. A gas buster works well in fluid with large bubbles of free gas. (Often the gas is starting to break free in the flowline.) A problem with the basic gas buster is that the heavier gases will not rise and be dissipated in the air but settle around the rig.
An old-fashioned but effective gas buster is made from a piece of 9- or
11-inch (228.6- or 279.4-mm) casing (Figure 1). An inlet, tangent to the
side but tilted up about 5, is welded into a 6-ft (2-m) length of casing
about one third of the distance from the bottom. The mud entering the
inlet spins, and the centrifugal force allows the gas to go to the center
and out while the mud goes to the sides and down. A pipe on the top
carries the gas away, and the bottom of the casing is open to the shale
tank on the shaker.
The tangential intake is used on many land rigs where the gas buster
is installed in the possum belly (back tank) of the shaker. The tangential intake balances the force of the drilling fluid and expanding gas so that the gas-buster tank does not need heavy bracing.
Another version of the pipe gas buster uses the same casing size as the
standard gas buster but directs the mud and gas mixture onto a blast
plate or ‘‘baffle,’’ which breaks up the flow pattern and separates gas and
drilling fluid. This system is unbalanced, and the pipe needs to be restrained. In heavy mud and with higher than normal initial gel strength,
the baffle system may cause some entrainment of the gas, which appears
as gas cutting.
The offshore version of the gas buster uses an 11-inch casing up to
20 feet tall ahead of the shale shaker. The offshore gas buster is closed
and usually has a U tube on the line to the shaker to build backpressure
and force gas to the discharge line.