A gate valve, also known as a sluice valve, 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. 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 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.
Disc Design
Solid and Flexible Wedge
Stem
Gate valves are characterized as having either a rising or a non-rising stem. Rising stems provide a visual indication of valve position. Non-rising stems are used where vertical space is limited or underground.
Bonnet
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 15 MPa (2250 psi). The unique feature about the pressure seal bonnet is that the body bonnet joints seals improve 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.
Body-Bonnet Gasket
Carbon steel or stainless steel + flexible graphite combined gasket is used for Class150 gate valve; stainless steel + flexible graphite spiral wound gasket for Class 300 gate valve; stainless steel + flexible graphite spiral wound gasket for Class 600 gate valve, and ring type joint gasket is optional for Class 600 gate valve; ring type joint gasket is used for Class 900 gate valve; pressurized seal design is used for Class1500 to Class 500 gate valve.
Operation
Hand wheel or gearbox operator are usually applied, if required, chain wheel is available. Top flange dimension according to ISO 5211 is available for valves to be actuated.
Belleville Spring Loaded Packing
If required, Belleville spring loaded packing can be applied to enhance the durability and reliability of the packing seal.
Packing Seal
Molded flexible graphite is normally used for packing material. PTFE or combined packing material can be also adopted if required. The internal surface of the stuffing box, of which area contacts the packing, is fine finished (Ra 3.2μm). The stem surface, contacting the packing, should be rolled and pressed after being precisely machined, so as to reach to high finish and compactness (Ra 0.8μm) and to ensure the reliable tightness of the stem area.
Back Seat
All FBV gate valves have the back seating design. In most cases, the carbon steel gate valve is fitted with a renewable back seat while for stainless steel gate valves, the back seat is machined directly in the bonnet or is machined after welding. When the gate valve is at fully open position, the sealing of the back seat is very reliable. However, according to API 600, the packing shall not be added or changed under line pressure.
Seat
For carbon steel gate valve, the seat material is usually forged steel. The sealing surface of the seat is overlay welded with hard alloy specified by customers. Renewable threaded-in seat ring is used for gate valves NPS≤10, and welded-on seat is optional and can be used if required. Welded-on seat is used for carbon steel gate valves NPS ≤10. For Stainless steel gate valve, the seat is integral or hard alloy welding integrally to the body. Threaded-in seat ring or welded-on seat is also optional for stainless steel gate valve.
Stem
The stem is of one-piece wrought material, welding fabricated is not permitted for FBV valves. The connection of the stem to the disc is “T” type. The strength of the connecting area is higher than that of the T threaded part of the stem. The strength test of the area conforms to API 591.
Stem Nut
Usually, the stem nut is made of copper alloy or ASTM A439 D2. For large sized gate valves (NPS 10 for Class 150, NPS 8 for Class 300, NPS 6 for Class 600, NPS 5 for Class 900), rolling bearing is fitted underside of the stem nut in order to minimize the open and close torque of the gate valve.
|
