Gas separation: apparatus – Degasifying means for liquid – With control means responsive to sensed condition
Reexamination Certificate
2000-06-22
2002-06-11
Smith, Duane S. (Department: 1724)
Gas separation: apparatus
Degasifying means for liquid
With control means responsive to sensed condition
96, 96, 96, 96, C137S187000, C137S810000, C137S811000, C137S812000, C137S813000
Reexamination Certificate
active
06402820
ABSTRACT:
BACKGROUND
This invention relates to a method and system to control the level of a liquid in a pressurised vessel or at least in a reservoir in which a pressure difference exists between the fluid above the liquid whose level is being controlled and an outflow of the liquid from the reservoir. This invention finds particular application in a fluid separation system to separate immiscible fluids of different density. The invention also relates to an improved fluidic valve.
In the petroleum extraction industry, but also elsewhere, there is frequently a requirement to separate different density immiscible fluids such as oil and water or oil and gas or all three. Indeed such mixtures may be found in large volumes and often in rapidly varying ratios of one component with respect to the other. A major problem associated with such multiphase flow is the fact that the constituent parts of the flow are extracted at a variable rate, such that in operation there is poor control over, for example, the amount of gas followed by the amount of liquid obtained from the well. This sometimes results in what is known as “slugging flow”, which can cause control problems.
Partial processing is a system where coarse separation of the various components is effected adjacent a well site, or other location near where the mixed components requiring separation first emanate. This results in much reduced transportation costs. In the petroleum extraction industry, for example, water and oil are frequently combined products of an oil well, and while the oil is to be recovered and transported to a refinery, the water is to be reused for pressurising the well. Consequently, to transport the water to a refinery and then back to the well is wasteful.
However, separation is not straightforward. As mentioned above, there are wide variations in the ratio of one component with respect to the other. Secondly, there is frequently solid matter entrained in the flow, which also need separation and isolation. Thirdly, the separation may need to be performed sub-sea, or in a remote site, where system reliability becomes of paramount importance. Gravitational separation in a vessel is possible, using a weir system for example, but maintaining the different levels of the components in the vessel is problematic when widely varying in-flow of the components occurs. Then, it is necessary to control the outflow of the components so that an appropriate interface level between the components is maintained. However, a simple weir system to maintain a level cannot function if there is a pressure difference between the less dense fluid and the outflow of the dense fluid. In this event there is the danger that the difference will simply result in forcing of the less dense fluid through the dense fluid outflow.
Pressure variation which the vessel may occur as a result of changes to the inflow rate, or alternatively, from variations to the outflow rate for one or more of the fluids in the system.
It is therefore important to maintain steady-state levels of, for example, oil, gas and water in the vessel so that separation of the fluids can be adequately controlled. Pre-separation of an oil-water steam allows the use of more compact downstream equipment. Further benefits of partial processing include the reduction of bottlenecking in the vessels and an increased yield from new and mature sites.
As mentioned above, another problem associated with production of oil and gas is sediment, which has to be removed from the fluid phase, but poses the added problem of obstructing outlets, and causing wear and stress on the component parts of systems with which is comes into contact.
Fluidic valves are known and have various design possibilities employing vortices or other properties of fluid flow to control flow from an input to an output.
It is known to employ vortex valves, which are commonly referred to as vortex amplifiers, and which comprise a vortex chamber, input and output ports, and a control port. The control port is tangential to the vortex chamber and induces a vortex in the chamber when there is flow through it. The input and output ports are generally arranged axially and/or radially with respect to the vortex chamber, one at least being on the circumference of the vortex chamber so that vortex flow in the chamber interfaces with flow into or from the circumferential port. Where a conical vortex chamber is employed, the input and output can be aligned so that resistance to flow, when there is no control port flow, is minimised.
DE-A-2431112 discloses such a valve employed to control the outflow of flood retention reservoirs. A radial main flow to an axial outflow is controlled by two tangential control ports opposing one another. The first port is supplied with flow when the level of the reservoir rises above a low level, thereby tending to reduce flow through the valve from the radial input to the axial output. The second control port is supplied with flow when the level of the reservoir rises above a high level. In this event, the flows through the control ports cancel one another's effect and the valve reverts to low resistance. Thus, as the reservoir rises from a minimum level to a maximum level, the valve starts with a low resistance because there is no flow through the control ports. The valve switches to high resistance when the first control port receives a flow as the reservoir level rises above that control port's input. Finally, the valve switches back to low resistance when the reservoir fills to its maximum level and the other control port is provided with flow as its input is flooded by the rising reservoir level.
However, a problem associated with this arrangement is that the valve is trying to maintain a fixed outflow rate despite changes in the driving hydrostatic head as set by the reservoir level. The valve is not, therefore, suited to level control where a high resistance to flow is required at a low liquid level while low resistance is required at levels above target.
Another problem with the double control vortex amplifier arrangement is that at low liquid levels the vortex chamber can very easily entrain gas and operate partly filled with gas. If the reservoir is pressurized, or suction applied to the outflow, the valve may be prone to the gas venting through one or more of the control ports and this could be highly undesirable in many chemical processing situations.
GB-A-1193089 disclosed a vortex valve having an axially arranged outlet port, two tangential control ports and subsequently no other ports such that inflow to the valve is through the control ports and outflow is through the outlet, the control ports being opposed to one another to reduce any vortex formation when flow occurs through both control ports from a common pressure source.
EP-A-0009335 discloses a T-junction modulator having a divided mainstream flow path to either side of the modulator and two control cylinders to oscillate a control flow across the modulator to inhibit mainstream flow therethrough.
It is therefore an object of the present invention to provide a system in which the level of a liquid in a pressurised chamber can be controlled so that the aforementioned problems are overcome, or at least their effects are mitigated within the design limits of the system.
It is a further object of the invention to provide a fluid separation system incorporating such a level control system.
It is moreover, an object of a different aspect of the present invention to provide a novel form of fluidic valve, suitable for use in level control and/or separation systems in accordance with the present invention or otherwise.
SUMMARY
According to the first mentioned objective, the invention therefore provides a pressure vessel containing a reservoir of fluid and having a valve controlling an outlet of the vessel and wherein there is a pressure differential across said valve beyond any hydrostatic pressure head of the reservoir fluid, the vessel comprising a system for the control of the level of reservoir fluid in the reservoir, the syst
Priestman Geoffrey H.
Tippetts John R.
Rose Robert J.
Sheldon & Mak
Smith Duane S.
The University of Sheffield
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