The three phase separation consists in the tretament of a flowrate composed by three different phases. Usually one of them is in aeriform state and the other two are in liquid state. So, in general, we have:
- gas phase: as for example methane, hydrocarbon gas, natural mixed gas;
- liquid light phase: as oil;
- liquid heavy phase: as water;
The scope of the 3Ph separation is to obtain a gas flow without a big quantity of entrained liquid droplets and two separated flows of liquid light phase and liquid heavy phase.

3phase scheme

PROCESS: In general a ST inlet separator is used to calm and distribute the flow at the entrance of the vessel and, to obtain the separation of the three phases, the flows are forced to pass through some internals at the same time but in two different sections of the vessel:
- the upper section where the gas flow is treated with, for example, a rectifier, a straightner and a vane pack.
- the lower section where the liquid flow is treated with, for example, a calming baffle, a plate pack coalescer and a weir

INTERNALS: the composition of the internal set can change on the base of the characteristic of the fluids and of the required perfomances. A sample of separator is, for example, the so called "Gravity separator", where the only used internal is usually a ST inlet device and there is not separation between the light and heavy liquid phase.
In other cases, to satisfy the required performances, some special internals are required. For example:
- a double calming baffle can be required instead of a simple one;
- a wire mesh coalescer can be required instead a parallel plate pack;
- a wire mesh demister is required instead a vane pack;

SIZING: the sizing of a 3ph separator and of the its internals is a complex process needing to consider some different parameters as:
- characteristic and property of the fluids as density, viscosity, chemical composition, molecular weight etc.;
- required residence time inside the vessel;
- required dimensions of the vessel on the base of location and other needs like, for example, existing piping;
- required performances of the process as, for example, the minimun diameter of the separated liquid droplets by the vane pack or the wire mesh demister.