Author:Site Editor Publish Time: 2018-10-04 Origin:Site
A gate valve, also known as a sluice valve, is a valve which opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is desired.
If divided by the gate's faces, it can be separated by parallel gate valve and wedge gate valve. Gate valves are used to permit or prevent the flow of liquids, and are typically for on/off control rather than flow regulation.Gate valve is difference from ball valve and butterfly valve. Because of their ability to cut through liquids, gate valves are often used in the petroleum industry. On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening. This means that flow rate does not change evenly with stroke of stem. Most of the flow change occurs near shut off with a relatively high fluid velocity causing gate and seat wear and eventual leakage if used to regulate flow. Also, a partially open gate tends to vibrate from the fluid flow. Thus, typical gate valves are designed to be fully opened or closed.When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.
Gate valves are actuated by a threaded stem which connects the actuator (e.g. handwheel or motor) to the gate. They are characterised as having either a rising or a non rising stem, depending on which end of the stem is threaded. Rising stems are fixed to the gate and rise and lower together as the valve is operated, providing a visual indication of valve position. The actuator takes the form of a nut which is rotated around the threaded stem to move it. Non rising stem valves are fixed to, and rotate with, the actuator, and are threaded into the gate. They may have a pointer threaded onto the upper end of the stem to indicate valve position, since the gate's motion is concealed inside the valve. Non rising stems are used underground or where vertical space is limited.
Gate valves may have flanged ends which are drilled according to pipeline compatible flange dimensional standards. Gate valves are typically constructed from cast iron, ductile iron, cast carbon steel, gunmetal, stainless steel, alloy steels, and forged steels.
All-metal gate valves are typically used in ultra-high vacuum chambers to isolate regions of the chamber.
Bonnets provide leak proof closure for the valve body. Gate valves may have a screw-in, union, or bolted bonnet. Screw-in bonnet is the simplest, offering a durable, pressure-tight seal. Union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength. Bolted bonnet is used for larger valves and higher pressure applications.
Another type of bonnet construction in a gate valve is pressure seal bonnet. This construction is adopted for valves for high pressure service, typically in excess of 2250 psi (15 MPa). The unique feature of the pressure seal bonnet is that the bonnet ends in a downward-facing cup which fits inside the body of the valve. As the internal pressure in the valve increases, the sides of the cup are forced outward. improving the body-bonnet seal. Other constructions where the seal is provided by external clamping pressure tend to create leaks in the body-bonnet joint.