Gate Valves

Gate valves (also known as knife valves or slide valves) are linear motion valves in which a flat closure element slides into the flow stream to provide shut-off. Gate valves and knife valves are designed to minimize pressure drop across the valve in the fully opened position and stop the flow of fluid completely. The direction of fluid flow does not change, and the diameter through which the process fluid passes is essentially equal to that of the pipe. Hence, they tend to have minimal pressure drop when opened fully. 

Gate valves and knife valves are advantageous in applications involving slurries, as their “gates” can cut right through the slurry. They are also used in applications that involve viscous liquids such as heavy oils, light grease, varnish, molasses, honey, cream and other non-flammable viscous liquids. They are available in large sizes to better handle thick flow. However, gate valves do have low-pressure limitations, and are not optimal in applications that require cleanliness or sanitary conditions. They are excellent for use anywhere a shutoff valve is needed. They can also be used where throttling capabilities are desired, although this is not generally recommended as erosion of the seat and disc occurs due to the vibrations of the disk in throttling applications.

Gate valves are usually divided into two types: parallel and wedge-shaped. The parallel gate valve uses a flat disc gate between two parallel seats, upstream and downstream. Knife valves are of this type, but with a sharp edge on the bottom of the gate to shear entrained solids or separate slurries. 

In the double-disk parallel-seat type, the valve is closed by lowering the disks from the valve neck to a height equal to that of the valve seats. Once so positioned, an inclined plane mounted between the two disks coverts downward stem force into axial force and presses the parallel disks firmly against the valve seats sealing the two openings. These types of valve design can accommodate asymmetric or angularly misaligned valve seats. 

Wedge-shaped gate valves and knife valves use two inclined seats and a slightly mismatched inclined gate allowing for tight shut-off. Disk flexibility is inherent to the split wedge design. This flexibility allows the split wedge to seal more easily and it reduces stickiness between the sealing surfaces in cases where the valve seats are angularly misaligned.

A Gate Valve, or Sluice Valve, as it is sometimes known, is a valve that 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. The gate faces can form a wedge shape or they can be parallel. Gate valves are sometimes used for regulating flow, but many are not suited for that purpose, having been 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 nonrising stem. Rising stems provide a visual indication of valve position. Nonrising stems are used where vertical space is limited or underground.

Bonnets provide leakproof 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 15 MPa (2250 psi). The unique feature about the pressure seal bonnet is that the body - bonnet joints seals 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 normally have flanged ends which are drilled according to pipeline compatible flange dimensional standards. Cast iron, cast carbon steel, gun metal, stainless steel, alloy steels, and forged steels are different materials from which gate valves are constructed.