Now, before discussing the design of Gas Lift systems, Let's understand the Gas Lift valves. A Gas Lift system requires a source of gas and sufficient pressure to inject it at the proper place in the system. The injected gas may come from production operations or an outside source of supply, often sufficient supply and pressure is available from the high-pressure separator. If you're available operating pressure is not high enough, then a compressor will be needed. Prior to injections against typically passes through a flow control choke which controls the injection rate. Gas Lift valves located in the tubing are sized and spaced according to the overall design. The method of operation and the type of installation depend largely on the type of valves used. As you might expect there are different types of Gas Lift valves.
Simply, we should discuss three types: - The Casing pressure operated valve, - The Fluid operated valve, and - The Throttling valve. Later we should discuss a 4th type called the Pilot operated valve. They're distinguished by their sensitivity to the casing pressures including pressures needed to open and close them. The Gas Lift in history categorizes Gas Lift valves according to which pressure has the greater effect on the opening of the valve. The sensitivity is determined by the mechanical design of the valve because it is the pressure exposed to the largest area in the valve that controls the valves operation. "Remember that P=F/A .So, F=P*A. a schematic of a typical Gas Lift valve installed in a tubing string is shown here. Nitrogen is normally injected into the dome and charged to a specified pressure. The bellows serve as a flexible or responsive element. Movement of the bellows causes the valve stem to rise and fall, and the ball to open and close over the port. When the port is open, the annulus and tubing are in communication. Because the area of the bellows A (b) is much larger than the area of the port in A (p), and since the bellows is exposed to casing pressure, it is casing pressure that controls the operation of the valve. This type of valve ten is referred to as a casing pressure operated valve or, more simply, a pressure operated valve. It requires a build-up in casing pressure to open and a reduction in casing pressure to close."
Now let's look at a graph of Flow Rate (q) verses Tubing Pressure (P). It will help us understand the performance characteristics of the Throttling valves. The vertical axis is flow rate and the horizontal axis is tubing pressure. At very low tubing pressure to the left of point 1, the valve is closed. As the tubing pressure reaches point 1, the valve begins to open and gas close from the casing to the tubing. The flow rate increases as the port continues to open. Throttling occurs from point 2 point 3 at which point the point is fully opened and throttling ends. The maximum flow rate occurs at point 4 as the giving pressure increases from point 4 to point 5 the tubing and casing pressures become balanced and the flow rate drops to zero. During the reverse cycle as the tubing pressure decreases, the valve opens at point 5, throttling takes place between points 3 and 2 and the valve throttle close between points 2 and 1. There other valves referred to as Combination valves which are also available for Gas Lift operations. Information on these and other special-purpose valves are available for manufacturers. The type of valve to be used for a given installation depends on whether the well to be placed on intermittent or continuous lift. But it is not certain which type of Gas Lift operation will take place as in cases where wells performance is borderline then valves maybe selected which are suitable for both continuous and intermittent lift. Values used for continuous flow must be sensitive to tubing pressure when in the open position. As the tubing pressure decreases, the valve should begin the throttle closed so as to decrease gas throughput.as the tubing pressure increases, the valve should open so as to increase gas throughput. This proportional response to increase and decrease in tubing pressure, maintains the established flow in tubing pressure and intends to keep a constant pressure inside the tubing. The ideal valve for continuous flow Gas Lift then is the Throttling valve.
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