A gate valve is a valve that operates using a round/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. The gate faces can form a wedge shape or they can be parallel. Typical gate valves should never be used for regulating flow, unless they are specifically designed for that purpose. On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening. This means that the flow rate does not change evenly with stem travel. Also, a partially open gate disk tends to vibrate from the fluid flow. 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. 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 characterised as having either a rising or a non-rising stem. Rising stems provide a visual indication of valve position because the stem is attached to the gate such that the gate and stem rise and lower together as the valve is operated. Non-rising stem valves may have a pointer threaded onto the upper end of the stem to indicate valve position, since the gate travels up or down the stem on the threads without raising or lowering the stem. Non-rising stems are used underground or where vertical space is limited. Bonnets provide leak proof closure for the valve body. Gate valves may have a screw-in, union, or bolted bonnet. The screw-in bonnet is the simplest, offering a durable, pressure-tight seal. The union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength. The bolted bonnet is used for larger valves and higher pressure applications.
Another type of bonnet construction in a gate valve is a pressure seal bonnet. This construction is adopted for valves for high pressure service, typically in excess of 15 MPa (2250 psi). A unqiue benefit of a pressure seal bonnet design is that the seal improves as the internal pressure in the valve increases, compared to other constructions where the increase in internal pressure tends to create leaks in the body-bonnet joint.
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, gun metal, stainless steel, alloy steels, and forged steels.
Contact - The Valve Pipeline
If a valve comes back from service, always cycle (open and close) the valve a few times outside before putting it back in stock or trying to do any repairs- it is possible for pressure to get stuck in the cavity of the valve.