1. Gate Valves

2. Structure
A gate valve has a movable "gate" that is raised and lowered by turning a hand wheel at the top to turn a screw which raises or lowers the gate to control flow.
Can be controlled by a electric, pneumatic or hydraulic actuator.

3. Operating Method
The closure element of a gate valve is a replaceable disk.
To open the valve, the disk is completely removed from the stream and offers virtually no resistance. Therefore, there is little pressure drop across the open gate valve.
A fully closed gate valve provides good sealing due to the 360° disk-to-seal ring contact surface. Proper mating of a disk to the seal ring ensures there is very little or no leakage across the disk when the gate valve is closed.

4. Applications
Another factor to be considered for applications is the type of bonnets on the valve which provide leak proof closure.
Screw-in bonnets: this is the simplest, that offers a durable, pressure-tight seal.
Union bonnets: Suitable for applications which require frequent inspection and cleaning. Also provides added strength to the body
Bolted bonnets: Used for larger valves and higher pressure applications.
Selection of the right gate valve for applications depends on the many factors that have to be taken into consideration:
The process fluid
Gas
Corrosive or dirty
The operating temperature & pressure
Working environment
Duty and shut off performance

5. Applications Oil or Gas Subsea & surface Process Isolation
Production manifolds surface & subsea
High Integrity Pipeline Protection systems
Pipeline end termination
Emergency shut down
Double block and bleed
Subsea & surface
Severe service
Abrasive flow
High temperatures & Pressures
Corrosive environments
Deep water
Hot tapping

6. Advantages Available in large sizes
Can be used with slurries and viscous liquids
Used as a shut off valve
Easy to maintain and disassemble
Inexpensive
Inherently fire-safe (when used with a metal sheet)
Bidirectional

7. Disadvantages Low pressure limitations Slow open and close time
Erosion of the seat and disk can occur
Poor throttling characteristics
Difficult to repair
Should not be used in sanitary applications

8. Flow Route
Often used when a straight-line flow of fluid and minimum restric­tion is desired.
The gate faces can form a wedge shape or they can be parallel.
Gate valves are primarily used to permit or prevent the flow of liquids, but typical gate valves shouldn't be used for regulating flow, unless they are specifically designed for that purpose.
Often used in the petroleum industry because of the ability to cut through fluids.
On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percentage of opening. Meaning that flow rate does not change evenly with stem travel.
Most of the flow change occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage if used to regulate flow.
They designed to be fully opened or closed. When fully open, there is no obstruction in the flow path, resulting in very low friction loss. Partially open gates tend to vibrate from the fluid flow.

9. http://www.youtube.com/watch?v=BdIpriOKiPk&feature=player_embedded -