WO1992015386A1

WO1992015386A1 - Separating apparatus

Info

Publication number: WO1992015386A1
Application number: PCT/GB1992/000409
Authority: WO
Inventors: David Andrew Webb
Original assignee: Merpro Montassa Ltd
Current assignee: Merpro Montassa Ltd
Priority date: 1991-03-07
Filing date: 1992-03-09
Publication date: 1992-09-17
Anticipated expiration: 1993-09-07
Also published as: GB9104803D0

Abstract

An apparatus for separating a mixture of gaseous and liquid phases comprising a tubular screen (3) containing a helical vane (4) which defines, with the screen, a helical passage. The passage has an inlet (1) for the mixture under pressure at one end of the screen (3) and an outlet (8) for the gaseous phase at the other end of the screen. In use, the mixture is caused to follow the helical passage and the liquid phase migrates centrifugally towards, and passes out of the passage through the screen (3).

Description

SEPARATING APPARATUS

A mixture of fluids, particularly gas, oil and water, is usually recovered at an oil- or gas-producing well head and has to be separated into its constituents prior to subsequent treatment. Initial coarse separation is conventionally achieved by transferring the mixture into a retention pressure vessel, commonly called a three phase separator, in which the mixture settles and separates into an upper gaseous phase, a middle oil phase, and a lower water phase, the various phases being withdrawn from the vessel through exits at appropriate levels. The settling and separation of the phases takes some minutes and, in order to accommodate, without interruption, the typical fluid flow from a production well, the retention vessel or vessels used must have a sufficiently large capacity corresponding to the total fluid flow during the period in which settlement and separation takes place. It follows that several disadvantages exist with three phase separators, particularly when utilised on an offshore production platform or subsea completion, as the vessels are large, heavy and very sensitive to motion such as may be experienced on a floating production or drilling facility. A further problem to be overcome by three phase separators is that a large change in the relative proportions of the constituents of the fluid mixture can occur very quickly. For example, if the percentages of the different phases is 60% oil, 30% gas, and 10% water, these can change for a short period of time up to 100% of any one of the phases, due to surging or so called "slugging". The effect is particularly severe when a slug of gas is carried to the well head. Owing to the unpredictability of this surging effect, the three phase separator must be designed with enough volume and hence retention time, to effect a separation of the most extreme levels of the phases likely to be experienced. This in turn leads to the use of pressure vessels which are much larger than would be required for steady flow and hence requires even more of the valuable deck area on an offshore production facility.

In accordance with the present invention, apparatus for separating a mixture of gaseous and liquid phases comprises a tubular screen containing a helical vane which defines, with the screen, a helical passage with an inlet for the mixture under pressure at one end of the screen and an outlet for the gaseous phase at the other end of the screen, whereby, in use, the mixture is caused to follow the helical passage and the liquid phase migrates centrifugally towards, and passes out of the passage through, the screen.

Using this apparatus, a fluid mixture containing gaseous and liquid phases can be subjected very quickly to a preliminary, moderately efficient, separation of the gaseous and liquid phases. The apparatus will usually operate upstream of two or three phase separators in which final separation will take place. However the quick preliminary separation of the gaseous and liquid phases should result in a significant reduction in the overall size, weight and cost of the separating equipment.

In one application, one or more of the apparatus may be mounted within a conventional three phase separator, a gas outlet from the apparatus leading into the upper portion of the retention pressure vessel and a liquid outlet from the apparatus leading into a lower portion of the retention vessel, so that final separation of the oil and water phases, and of any residual gas from these phases, occurs by conventional settlement.

It is preferred that the pitch of the helical vane reduces in the direction along the passage away from the mixture inlet end so that the cross sectional area of the passage becomes smaller. This will cause the fluid passing along the passage to accelerate which will assist in causing liquid droplets not immediately migrating to and through the screen, and still entrained within the residual gas flow, to migrate centrifugally to and through the screen.

In order to contain the liquid phase passing through the screen, the screen is preferably surrounded by a housing. It is expected that the apparatus will be used with the axis of the tubular screen substantially upright, with the mixture inlet at the bottom, in which case the housing will have a gas outlet adjacent to its upper end and one or more liquid outlets in its side wall. The liquid outlet(s) may be provided by a vertical slot, which may diverge, i.e. increase in width, in the upward direction, or by vertically spaced ports, to present a continuous or stepwise increase in cross-sectional area to increasing levels of liquid phase in the housing. In this way, as the volume of liquid passing through the screen varies, the amount at any time within the space between the screen and housing will substantially fill the space up to a corresponding level so that little or no liquid will pass through the screen into a partially gas-filled space and be remixed with the gas. This contrasts with an arrangement with an outlet at the bottom of the housing capable of accommodating a maximum liquid flow, since at any flow less than the maximum the liquid will cascade down through a partially gas-filled space outside the screen. The screen may be a metal screen of a kind having, for example, circumferentially extending, closely spaced slits of the order of 800 to 1200, preferably 1000, microns wide.

Although the invention is useful for the pre- separation of gaseous and liquid constituents of production fluids, it is equally useful in conjunction with smaller test separators to enable accurate and reliable information to be obtained speedily during tests for monitoring an existing field or assessing the viability of a new field.

An example of separating apparatus constructed in accordance with present invention, is illustrated diagrammatically in the accompanying drawing. As shown, the apparatus has an inlet port 1 which can be bolted and sealed to a flange coupling on a feed pipe or bolted and sealed inside at the bottom of a conventional retention pressure vessel of a three phase separator. Sealed to the inlet port is a tubular section 2 which has a tubular screen 3 sealed to it at its upper end. Contained inside the tubular sections 2,3 is a helical vane 4 of upwardly decreasing pitch. The tubular sections 2, 3 and vane 4 thus define a helical passage 6 of decreasing cross-section.

Also sealed to the tubular section 2 is a housing 7 which terminates at its upper end in a gas outlet 8 and has, in its side wall, at different levels , a plurality of outlets 9, which lead to a common liquid outlet 10. In use the liquid/gaseous mixture is introduced under pressure through the inlet port 1. It is then forced up along the helical passage between the vane 4 and screen 3. Centrifugal forces cause the liquid to migrate towards and through the screen 3. The decreasing pitch of the vane 4 causes the mixture to accelerate as it rises thus increasing the centrifugal force on the mixture and improving the separation. The liquid passing through the screen 3 enters the housing 7 where it is discharged through the outlets 9 and common outlet 10. This arrangement ensures that the volume of liquid in the housing 7 varies with the volume of mixture being separated, so that liquid passing through the screen 3 does not cascade down through a gas filled space.

The gas, being lighter than liquid, continues to follow the helical path and emerges from the top of the vane 4 and passes out through gas outlet 8.

Claims

1. An apparatus for separating a mixture of gaseous and liquid phases comprising a tubular screen (3) containing a helical vane (4) which defines, with the screen, a helical passage with an inlet (1) for the mixture under pressure at one end of the screen, and an outlet (8) for the gaseous phase at the other end of the screen whereby, in use, the mixture is caused to follow the helical passage and the liquid phase migrates centrifugally towards, and passes out of the passage through the screen.

2. An apparatus according to claim l, wherein the pitch of the helical vane (4) reduces in the direction along the passage away from the mixture inlet so that the cross sectional area of the passage becomes smaller.

3. An apparatus according to claim 1 or claim 2, wherein the screen (3) is surrounded by a housing, the housing containing a gas outlet (8) and a liquid outlet (10) .

4. An apparatus according to claim 3, wherein the axis of the tubular screen (3) is substantially vertical and the housing contains a plurality of vertically spaced liquid outlet ports (9) .

5. An apparatus according to claim 3, wherein the axis of the tubular screen (3) is substantially vertical and the housing (7) contains a vertical slot to act as the liquid outlet.

6. An apparatus according to any one of the preceding claims, wherein the metal screen (3) contains closely spaced slits of the order 800 to 1200 microns wide.

7. A three phase separator within which is mounted at least one apparatus according to any one of the preceding claims.