This invention relates to a three phase separator design and in the preferred embodiment provides a separator suitable for separating gas, hydrocarbon liquid and water emanating from a crude oil well. It is conventional to separate the gas, hydro¬ carbon and aqueous phases of fluid emanating from an oil well by means of a closed horizontal vessel which, in use, is approximately half filled with liquid to provide a substantial space at the top of the vessel in which liquid particles may disengage from the gaseous phase.

Such separators must be of substantial size, and, becuase of the large free space above the liquid are susceptible to the formation of waves in the liquid phases due to flow surges or movement of the vessel. According to the present invention a three phase separator comprises a first, lower, vessel which in use is substantially filled with liquid and a second, upper, vessel, mounted atop and in communication with the lower vessel, in which liquid/gas disengagement is effected, the maximum cross-sectional area of the upper vessel, measured in the horizontal plane, being smaller than the maximum cross-sectional area of the lower vessel, easured in the horizontal plane.

In the preferred embodiment of the invention the upper vessel is a substantially cylindrical vessel mounted with its axis vertical. Incoming crude oil enters the

vertical vessel horizontally through an inlet diffusing device containing baffles and/or vanes: this diffuser directs the gas and liquid generally downward into an area where the bulk of the liquid passes downwards into the lower vessel, and the gas together with small en¬ trained droplets or foam passes upwards past the inlet diffuser into the upper section of the vertical vessel. The invention will be better understood from the following description of preferred embodiments thereof, given by way of example only, reference being had to the accompanying drawings wherein:

Figure 1 illustrates schematically in cross- section a first preferred embodiment of the invention; and Figure 2 illustrates a second preferred embodi¬ ment of the invention.

The three phase separator 1 shown in the drawing comprises a lower horizontal cylindrical vessel ^" 2 mounted on legs 3- I use, the rate at which liquid is removed from the vessel is controlled by level sensors 4,5 so that the vessel is maintained substanially filled with liquid, a typical working liquid level being indicated at 6 in the drawing. Mounted atop the horizontal vessel 2 at one end thereof is an upper cylindrical vertically extending vessel 7 - The vessel 7 extends from the upper surface of the vessel 2 up¬ wardly to a gas outlet δ defined at the top end of the vertical vessel. An inlet passage 9 extends through the wall of the vertical vessel 7 and terminates in an inlet diffuser 10 which directs gas and liquid from the inlet passage 9 downwardly in the vessel 7 towards the surface 7 of the liquid in the horizontal vessel 2.

After flowing initially downwardly from the diffuser 10, gas with entrained liquid droplets or foam, flows upwardly in the vessel 7 past the inlet diffuser

10 to a separator 11 mounted in the upper portion of the vertical vessel 7 - Within this separator the gas and entrained liquid or foam are separated. The separator may be of any suitable form, and may for example be a bundle of multicyclone separators as illustrated, or may be a chevron vane separator, or may be of any other suitable form. In the preferred multicyclone separators entrained liquids and solids are subjected to a spinning action causing collapse of any foam, and throwing liquids and solids to the walls of the cyclone tubes. Liquid and solids from the cyclone tubes drain out into a collecting hopper 12 which is connected by a drain-pipe 13 either to The horizontal vessel or to a small separate storage tank. Gas, substantially free from entrained liquids and solids, passes outwardly through the outlet 8.

Optionally, an agglomerator 14 is located in The path of upwardly flowing gas before the gas enters the separator. The agglomerator may be of any suitable type, and may for example consist of wire mesh or a plurality of closely spaced plates. In passing through the agglomerator any very small dispersed droplets impinge on the surface of the agglomerator and are agglomerated into larger dropletswhich either fall down into the horizontal vessel or are entrained by the gas and taken off into the multicyclone bundle for separation. Similarly, any foam entrained in the gas will be wholly or partly collapsed by the surfaces of the agglomerator.

Within the horizontal vessel 2 the hydrocarbon and water phases separate and are removed from the vessel from respective hydrocarbon and water outlets 15 _> 16. In order to prevent water from flowing through the hydro¬ carbon outlet a weir 17 is provided near the outlet end of the vessel 2, and the water/hydrocarbon interface to the left of the weir (as viewed in the drawing) is main- tained below the level of the upper edge of the weir by

controlling the flow of water from the outlet 16. Level sensors l8,19 are provided to determine the position of the water/hydrocarbon interface. Separation of "the water from the hydrocarbon is essentially by gravity, although separation can be aided by use of a coalesing pack 20 comprising woven metallic mesh, tilted parallel plates, or opposed corrugated sheets.

Preferably stilling baffles e.g. as illustrated at 21 and 22 are provided in the horizontal vessel to minimise the disturbing effects or surging of process flow and movements occasioned by movement of the separator structure itself, such as would be incurred if the separator was mounted on board a floating production barge or the like. It is particularly preferred that a stilling baffle such as baffle 21, or other suitable means, be provided at the interface of the upper and lower vessels to inhibit movement of the liquid in that area. Movement of the liquid in the remainder of the lower vessel is inhibited because of the small space above the liquid surface aided by the use of optional stilling baffles such as baffle 22.

If desired, the horizontal vessel can be pro¬ vided with a water jet system for clearing sand and other debris from the bottom of the vessel, sand outlets 24,25 being provided for removing such material from the horizontal vessel.

In use, the space between the liquid level 6 and the top of a horizontal vessel is maintained at a minimum consistent with allowing satisfactory separation of gas bubbles from the liquid within the horizontal vessel. Gas liberated within the horizontal vessel either flows upwardly through the vertical vessel 7, or is taken via a gas balance line 26 to an upper region of the vertical vessel 7 for removal via gas outlet 8. By main- taining a minimum clearance above the liquid in the

horizontal vessel the effects of movement of the support structure and the effects of waves caused by flow surges are minimised, enabling effective separation of oil, liquid, and gas to be carried out in a lighter and more compact separator than was possible with the prior art. Referring now to Figure 2 a second embodiment of three phase separator is shown, parts corresponding to those of the first embodiment being marked with the same reference numerals. In the Figure 2 embodiment the lower vessel 2 is spherical and the upper vessel 7 is a vertical cylinder mounted centrally atop the lower vessel. A suitable stilling baffle (not shown) is located at the interface of the upper and lower vessels, and a weir (not shown) is provided within the lower vessel between the outlets 15, 16. Level sensors 28 control the rate of withdrawl of hydrocarbon and aqueous liquids to maintain the lower vessel substantially completely filled with liquid.