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1. A flow stop valve positionable in a downhole tubular, the flow stop valve comprising:
a housing; a valve selectively permitting fluid flow through the flow stop valve, the valve comprising a first valve element and a second valve element movably located within the housing, wherein the first valve element is movable with respect to the second valve element; and a first biasing element acting on the valve, wherein the valve is actuated between an open position and a closed position in response to a pressure difference acting on the valve, the pressure difference comprising a difference between a fluid pressure outside of the downhole tubular and a fluid pressure inside of the downhole tubular, or a difference between a fluid pressure at a first, uphole end of the housing and a fluid pressure at a second, downhole end of the housing, or both, wherein the valve is in a closed position when the pressure difference is less than a threshold value and the valve is an open position when the pressure difference is greater than the threshold value, wherein the first biasing element is preloaded by movement of the first and the second valve elements together in response to an increase in the pressure difference in response to lowering the flow stop valve downhole, and wherein the second valve element comprises a port and a passage, the port being in fluidic communication with the passage such that a flow from the first end of the housing to the port is selectively blocked by movement of the second valve element or first valve element, or both, wherein when the valve is in the open position a flow path exists from the first end of the housing, through the port and the passage of the second valve element to the second end of the housing, and wherein, when the valve is in the closed position, the first valve element covers the port of the second valve element, such that the first valve element blocks the flow from the first end of the housing from proceeding to the port.
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