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Valves comprise a chamber having a piston disposed therein. One side of the piston defines a hydrostatic chamber in fluid communication with an outside environment, such as wellbore annulus, through a port. Operatively associated with the piston on the other side is a sleeve in sliding engagement wi

Valves comprise a chamber having a piston disposed therein. One side of the piston defines a hydrostatic chamber in fluid communication with an outside environment, such as wellbore annulus, through a port. Operatively associated with the piston on the other side is a sleeve in sliding engagement with an inner mandrel. The inner mandrel comprises a port that is initially blocked by the sleeve. Upon an increase in pressure within the annulus, the piston is moved causing the port in the sleeve to align with the port in the inner mandrel thereby allowing fluid to flow from the annulus into the bore of the inner mandrel. As a result, fluid can be circulated through the valve, or pressure within the annulus can be reduced. A return member is operatively associated with the piston to urge the piston toward the closed position after pressure within the wellbore annulus is reduced.

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1. A valve comprising: an inner mandrel, a piston housing, a top sub, a gage ring, an outer housing, a lower screen, and a guide,wherein the inner mandrel is secured to the piston housing and the piston housing is secured to the top sub to provide a string bore disposed through the inner mandrel, th

1. A valve comprising: an inner mandrel, a piston housing, a top sub, a gage ring, an outer housing, a lower screen, and a guide,wherein the inner mandrel is secured to the piston housing and the piston housing is secured to the top sub to provide a string bore disposed through the inner mandrel, the piston housing and the top sub, and the outer housing is secured to the piston housing to provide an annulus between an inner wall surface of the outer housing and an outer wall surface of the inner mandrel,wherein the gage ring is secured to a lower end of the top sub to provide a piston port in fluid communication with a piston housing bore and an environment disposed outside of the outer housing, the piston housing bore having a piston disposed therein,wherein the piston is operatively associated with a sleeve, the sleeve being in sliding engagement with the outer wall surface of the inner mandrel, the sleeve comprising a sleeve port,wherein a lower end of the outer housing is secured to an upper end of a first screen and a lower end of the first screen is secured to the guide, the guide being fixed to the outer wall surface of the inner mandrel. 2. The valve of claim 1, wherein the piston is operatively associated with the sleeve through an upper coupling and a lower coupling, the upper coupling and lower coupling being fixed to each other through a return member housing, the return member housing having a return member disposed therein. 3. The valve of claim 2, wherein the return member comprises a spring operatively associated with the upper coupling and a detent disposed on the outer wall surface of the inner mandrel. 4. The valve of claim 3, wherein the upper coupling is connected to a piston mandrel and the piston mandrel is connected to the piston. 5. The valve of claim 1, wherein the piston housing comprises upper and lower piston stops to restrict the movement of the piston within the piston housing. 6. The valve of claim 1, wherein the guide comprises at least one groove disposed in an outer wall surface of the guide. 7. The valve of claim 6, wherein the guide comprises three grooves disposed longitudinally in the outer wall surface of the guide. 8. A method of circulating fluid between a wellbore annulus and a valve, the method comprising the steps of: (a) providing a wellbore having a wellbore annulus;(b) disposing a valve within the wellbore, the valve comprising an outer mandrel comprising an inner wall surface defining an outer mandrel bore, an outer wall surface, and an outer mandrel port disposed in the outer wall surface of the outer mandrel and in fluid communication with the outer mandrel bore,an inner mandrel disposed within the outer mandrel bore, the inner mandrel comprising an inner wall surface defining an inner mandrel bore, an outer wall surface, and an inner mandrel port disposed in the outer wall surface of the inner mandrel and in fluid communication with the inner mandrel bore, the outer mandrel being fixed to the inner mandrel at a first end thereby providing an annulus between the outer wall surface of the inner mandrel and the inner wall surface of the outer mandrel, anda sleeve disposed within the annulus and in sliding engagement with the inner wall surface of the outer mandrel and the outer wall surface of the inner mandrel, the sleeve comprising a sleeve port, wherein the sleeve moves within the annulus due to an increase in pressure within the wellbore annulus until the sleeve port is at least partially aligned with the port of the inner mandrel, andwherein the outer mandrel port is in fluid communication with the wellbore annulus; and(c) moving the sleeve away from the outer mandrel port due to an increase in pressure within the wellbore annulus causing the sleeve port to at least partially align with the inner mandrel port, thereby permitting fluid to flow from the wellbore annulus into the inner mandrel bore. 9. The method of claim 8, wherein during step (c), a completion fluid is pumped down the wellbore annulus and a drilling fluid flows from the wellbore annulus into the inner mandrel bore to replace at least a portion of the drilling fluid in the wellbore annulus with at least a portion of the completion fluid. 10. The method of claim 9, further comprising the steps of: (d) reducing the pressure of the completion fluid being pumped down the wellbore annulus causing the sleeve to move toward the outer mandrel port causing the sleeve port to move out of alignment with the inner mandrel port;(e) setting a first wellbore barrier above the valve to provide an isolated wellbore annulus disposed below the first wellbore barrier; and(f) moving the sleeve away from the outer mandrel port due to an increase in completion fluid pressure within the isolated wellbore annulus causing the sleeve port to at least partially align with the inner mandrel port, thereby permitting at least a portion of the completion fluid to flow from the isolated wellbore annulus into the inner mandrel bore causing a reduction in pressure within the isolated wellbore annulus. 11. The method of claim 10, further comprising the step of: (g) after sufficient pressure has been reduced within the isolated wellbore annulus, moving the sleeve toward the outer mandrel port causing the sleeve port to move out of alignment with the inner mandrel port.