Gate Valve Design Features

This post is initiated to give some basic information regarding gate valve design features, it uses, standards, types, advantages, disadvantages etc.

What is Gate Valve?

Gate valve name itself implies as, gate called vertical disc, flat face that slides down through the valve is used to stop fluid flow. It is used primarily for on/off functions, and not used for throttling purpose, since accurate control is not possible. The pressure loss in gate valve is very less.

Gate valves are commonly multiturn valves. It consists of main components as,

• Valve Body
• Seat and Disc
• Spindle or Stem
• Gland and rotating wheel

The seat may be of fixed or removable type. In order to actuate a gate valve the disc is either lowered or lifted by means of a stem that projects outside the valve body by means of a hand wheel. The protrusion of the stem outside the valve body necessitates the containment of the fluid inside the valve and this is done by means of a gland packed with a fluid resistant barrier. Rising stem and non rising stem gate valves are available.

Repacking gland boxes can be achieved by using a feature called back-pressure seating. This effectively seals the packing retaining chamber from the pressure of the fluid flow.

Packing glands are of simple construction with a threaded gland follower and a graphited packing material for smaller valves. Other types are lantern-type or bellow seals for packing with an OSY to hold the gland follower.

Standards for Gate Valves

For users of gate valves, API 600 is the key document. It details all design and material criteria. API 600 also lists important dimensions such as stem diameter minimums, wall thickness and stuffing box size.

Small carbon steel gate valves such as the forged 150#, 300#, 600#, 800# & 1500# class valves manufactured by several companies worldwide are covered by API 602. This specification covers the same details small forged gate valves that API 600 does for larger valves. API 602 further gives dimensions for extended body valves which are used extensively in industrial facilities.

Another important gate valve specification is ASME B16.34. This document gives extensive details on valves built to ASME boiler code pressure temperature ratings. One important area in which API 600 differs from ANSI B16.34 is minimum wall thickness. API 600 requires a heavier wall for a given pressure rating than does ASME B16.34. API 602 also requires a heavier wall for 150#, 300# & 600# classes than does B16.34.

Differential pressure is the difference in pressure from one side of the wedge to the other when the valve is closed. As the valves get larger or the pressure differential increase, the input torque necessary to close the valve increases. This soon reaches a level where 1 person cannot operate the valve. A gearbox makes the valve easier to operate, though it will take longer to close or open the valve. Larger gate valves are provided with a bypass around the valve seat, which may assist in pressure equalization or warm up of a steam carrying pipe line.

There are main three varieties of gate valves

Non Rising Stem Valves: this requires a minimum amount of clearance for their installation. Usually the threads are cut in the disc itself. In a highly corrosive environment this type is preferred since the protrusion of the stem is very less. Sometimes an indicator is provided to show whether the valve is open.

Rising Stem: These are the most common type and is used when the operator needs to know whether the valve is open or closed. The threads in the stem engage with the threads in the valve bonnet which causes the stem and hand wheel to rise or lower upon being turned. These require more space when open.

Outside Screw and Yoke Valves: They have their threads on the upper end of the stem completely outside the flow of material thus protecting the working parts of the valves from the corrosive or erosive action of the fluid being handled. The threads are easy to reach for lubricating purposes. The exposed stem threads indicate on sight whether the valve is open or closed.

Quick opening Gate Valves: These are used where the lines are to closed or opened in minimum time. They are usually used in lines carrying inflammable fluids. There are two types:

• Sliding Stem valves whose disc is raised or lowered by a hand lever
• Rotating stem valves with discs fastened to a horizontal stem which is rotated by a lever arrangement. 

Pressure sealed bonnet joint valves are recommended for use in high pressure high temperature and feed water services.

Best Suited Control for Gate Valve:

Quick Opening

Recommended Uses:

• Fully open/closed, non-throttling (stop valves)
• Infrequent operation
• Minimal fluid trapping in line
• Gate valves are used in on/ off operations for steam service where a bubble tight seal is not required.
• Suitable for wide open service such as at the outlet of storage tanks, fire hydrant lines, etc.

Applications:

Oil, gas, air, slurries, heavy liquids, steam, non-condensing gases, and corrosive liquids

Advantages:

• High capacity
• Tight shutoff
• Low cost
• Little resistance to flow

Disadvantages:

• Poor control
• Cavitate at low pressure drops
• Cannot be used for throttling

General Specification for Gate Valves

For 150# valves

Stem-Packing: Corrosion-inhibited die-formed flex. Graphite with anti-extrusion rings

Gaskets: API 600, API 602

For 300# valves

Trim: 13 CR, Hard face Seats, API 600 TRIM # 8

For 600# valves

Trim: 316 SS, HARDFACE SEATS, API 600 TRIM #12

For 800/ 900# valves

Trim: Full Stellite, API 602 TRIM #5

For 1500# valves

Trim: 13 CR, Hard face Seats, API 600 TRIM # 8

Gate Valve Ratings and End Connections

Gate Valve Body Material and Trim Material

Single Wedge Disc Gate Valve

This is usually a solid single wedge, which fits into tapered valve seats. They are the most commonly used because of its simplicity in construction. Best suited to overcome the misalignment and dimensional changes in the valve body due to temperature variations. Therefore they cause very less chatter due to close guiding. The tapered construction gives a large seating area which results in a wide in a wide and true contact between the disc and the corresponding faces of the valve seat.

A variation is the flexible wedge. Here the faces can have some relative movement since it is solid through the center. Another design is the split wedge.

Double Disc Gate Valves

Split wedges are used to ensure uniform seating pressure over the entire seat. They are used principally to hold light gases and fluids. The discs are forced against the valve seats by a wedging mechanism as the stem is tightened.

In double disc gate valves the seat can move relative to each other. This gives good shut off even if the seats are misaligned or have different seat angles. Seat wear is minimized since sliding contact is less.

In one design the discs are held together by a ball and socket that permit movement of the discs when they mate against tapered seats.

Parallel seating double disc designs use spreaders or wedges to force the discs against the seating surfaces.

Knife Gate Valves

Knife gate valves which is a variation of the single wedge gate valve has a very thin knife like wedge and are used in three primary applications:

• Non-abrasive slurry services such as in the pulp & paper industry
• Abrasive slurry applications as found in mining applications
• For large diameter water services as found in waste water systems.

Advantages of the Knife Gate Valve include the ability to cut through slurries, scale and surface build ups, unobstructed flow paths, small face-to-face dimensions and large line size availability. The unobstructed flow path not only provides high flow capacity (Cv), but even allows large objects, rocks and items routinely found in mining processes to safely pass through the valve. Similar to a butterfly valve, the knife gate valve has a small face-to-face dimension, greatly reducing the weight of this valve and facilitating piping design.

Sizes available range from standard cast configurations as small as 2″ to special fabricated valves exceeding 100″. Standard cast configurations have ASME 125/150 bolting patterns and are rated at 150 psi. Fabricated valves can address higher pressures and are often manufactured to match specific service requirements. With regards to bubble tight shut-off, most knife gate valves due to the uni-directional flow path and metal seats are not capable of achieving ASME Class VI ratings. However some bi-directional knife gates valves utilizing an elastomer perimeter seal do comply with ASME Class VI shut-offs.

Disadvantages of a knife gate valve are the relatively low pressure limitation, lack of cleanliness and general inability to provide bubble-tight shut-off. As mentioned above, general pressure limitations are 150 psi maximum. Relative to cleanliness, the knife gate valve contains cavities which promote contamination. Hence this valve is not recommended for high purity applications.

For abrasive and slurry applications, the knife gate valve is an excellent choice. For proper selection the Technical Sales Representative should be consulted since the application is key to developing the proper valve configuration particularly in larger line sizes. Many users are not aware of the recent design developments and capabilities.

Special knife gate valves configurations are used for handling abrasive slurries. These range from hardened components to the use of polyurethane seats and liners to combat the abrasion. The most recent development for abrasive slurries is the double seated slide gate valve which uses two opposing elastomer seats that provide an elastomer conduit through the valve. This type does best in the most difficult high solids and scaling applications.

Conduit Gate Valves

This nomenclature has been adopted for large size valves. They are mostly used in oil services and has a smooth round bore that permits passage of pigs or scrapers. The gate itself may be single gate or double gate assembly. As an additional feature most conduit valves have a base that permits easy valve support from any supporting structure.