Gate valve - Introduction| Standards | Operator Guidelines

Gate Valve

Gate Valve - Introduction

A Gate valve is a linear motion on-off or shut-off valve. The gate-valve comprises a disc or wedge having two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow. The pressure differential between the upstream flow direction and downstream side of the wedge causes the wedge or disc to force against the downstream seat creating more effective sealing on the downstream side of the wedge. A gate valve is not used to regulate flow as the erosion of the disc and seat may cause leakage. A gate valve in a partially open or closed position is also susceptible to vibration.

Gate valve

Where the quick and frequent operation is an essential use of quarter-turn valves such as ball valves and plug valves must be considered.

Classification of Gate Valves based on Disk or Wedge Design

Gate valves can be broadly classified based on the disk design. Gate valves are available with a variety of disks.

• Wedge Type Disks
• Solid Wedge
• Flexible Wedge
• Split Wedge
• Parallel Disks
• Conventional Parallel Disk
• Conduit Gate
• Knife Gate

With the exception of the parallel disk gate valve which has parallel seats, all other types namely, solid wedge gate valve, flexible wedge gate valve, and split wedge gate valve have inclined V-shaped seats like a wedge. The Wedge gate valve is the most commonly used valve. This valve may be installed in any position.

Solid Wedge: Solid Wedge Disks are most commonly used because of simple and less expensive design.

Flexible Wedge: A Flexible Wedge gate valve has a disk with a cut around the perimeter. A shallow cut gives less flexibility. A deeper cut provides more flexibility. Flexible Wedge gate valves are often used in steam systems. The flexible gate prevents the disc from binding into the seat when the lines are heated which could cause some distortion in the valve bodies

Split Wedge: Split Wedge gate valves are of ball and socket design. In a split wedge gate valve, the two disk halves are forced outward against the body seats by a spreader after the disk has been fully lowered into its seating position.

Wedge disk construction - Gate valve

Parts of a Gate Valve

The following sketch illustrates the various parts of a Gate Valve.

Parts of Gate valve

Seat Ring Design in Gate Valves

Seat Rings in Gate Valves can be classified into the following broad categories:

• Integral Body Seat Ring.
• Renewable Seat Rings which can be either threaded or press-fit into the body of the valve. This facilitates easy removal and replacement of seat rings.
• Non-Renewable Seat Rings which are seal welded or full welded to the body of the valve.

Advantages of Gate Valves

Some of the advantages of Gate Valves are as follows:

• Gate valves have good shut-off characteristics as the contact between the disk and seat ring extends over the entire circumference.
• The pressure drop across a gate valve is minimal as the disk offers no resistance to flow when the valve is in a fully open position.
• Gate valves are bi-directional valves and can provide shut-off in either flow directions.

Disadvantages of Gate Valves

Some of the disadvantages of Gate Valves are as follows:

• Stem travel in the gate valve requires multiple turns of the handwheel or gear operator and hence longer time duration to open or shut the valve.
• As the gate or wedge must travel across the entire cross-section of the valve bore, the overall height of the gate valve is larger compared to a ball valve which is a quarter-turn valve and does not require vertical movement of the stem.
• Gate valves are provided with renewable seats which are usually difficult to change out in-situ.
• The valve flow characteristics of the gate valve can cause a high velocity of flow to occur for a longer duration while opening or closing the valve due to the slow movement of the wedge or gate. High velocities can cause erosion of the seat and the disc thereby reducing the design life of the valve. This condition is also referred to as the “wire drawing” effect.
• Gate valves are susceptible to the solid wedge or disc getting stuck between the seats due to differential contraction between the valve seats and the wedge during shutdown of a high-temperature piping system. The piping system needs to be reheated for the disc to travel smoothly again. This makes the gate valve unsuitable for use in piping systems subject to high fluctuations in temperature. However, this problem can be overcome by using a gate valve with a flexible wedges.
• Solid wedge gate valves are sensitive to thermal expansion loads or bending moments which can cause the wedge or disc to lock between the valve seats or result in improper seating of wedge between the seats and leakage across the valve seats.
• Gate valves being metal seated do not provide bubble-tight shut-off and should not be used in applications where small leakage across the seats are not permissible.

Industry Codes and Standards

• API 600 - Steel Gate Valves, Flanged, and Butt-welding Ends, Bolted and pressure seal bonnets
• API 602 - Compact Steel Gate Valves, Flanged, Threaded, Welding and Extended-body ends
• BS 5352 - Specification for Steel Wedge Gate, Globe and Check 50mm and smaller
• MSS SP 80 - Bronze Gate, globe, check, and angle valves
• API 598 - Valve Inspection and Testing

Solid wedge gate valves 40mm and below are generally specified to BS 5352.
Solid wedge gate valves 2-inch and above are generally specified to BS-1414 or API-600 as appropriate.

Compact Solid wedge gate valves 4-inch and below are specified to API-602.
Split wedge gate valves 1-inch and above are specified to API-600.
Conduit valves 2-inch and larger are specified to API-6D.
Double disc gate valves 1-inch and larger are specified to API-6D or API-600 as appropriate.

Operators for Gate Valves

Most operators use the following guidelines to select the type of operators for Gate valves

Operator	Size	Rating
Handwheel	8-inch and below	Class 150 and 300
Handwheel	4-inch and below	Class 600 and 900
Handwheel	3-inch and below	Class 1500 and 2500
Gear Operator	10-inch and above	Class 150 and 300
Gear Operator	6-inch and above	Class 600 and 900
Gear Operator	4-inch and above	Class 1500 and 2500