Gate Valve

What is a Gate Valve

A gate valve is a linear motion valve used to start or stop fluid flow. The name gate is derived from the appearance of the disk in the flow stream. 

Gate valve Diagram

The disk of a gate valve is completely removed from the flow stream when the valve is fully open.When the valve is fully closed, a disk-to-seal ring contact surface exists for 360 degree.With the proper mating of a disk to the seal ring, very little or no leakage occurs across the disk when the gate valve is closed.

A gate valve can be used for a wide variety of fluids and provides a tight seal when closed.  

Major disadvantages  of a gate valve

• It is not suitable for throttling applications.
• It is prone to vibration in the partially open  state. 
• It is more subject to seat and disk wear than a globe valve.
• Repairs, such as lapping and grinding, are generally more difficult to accomplish.

Disk Design of Gate Valve

Gate valves are available with a variety of disks. Classification of gate valves is usually made by the type disk used: 

• solid wedge
• flexible wedge
• split wedge
• parallel disk

Solid wedges, flexible wedges, and split wedges are used in valves having inclined seats. Parallel disks are used in valves having parallel seats.

Gate valve

Regardless of the style of wedge or disk used, the disk is usually replaceable. In services where solids or high velocity may cause rapid erosion of the seat or disk, these components should have a high surface hardness and should have replacement seats as well as disks. If the seats are not replaceable, seat damage requires removal of the valve from the line for refacing of the seat, or refacing of the seat in place. Valves being used in corrosion service should normally be specified with replaceable seats.

Gate Valve Stem Design

Gate valves are classified as either rising stem or non rising stem valves. For the non rising stem gate valve, the stem is threaded on the lower end into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. 

The non rising stem configuration places the stem threads within the boundary established by the valve packing out of contact with the environment. This configuration assures that the stem merely rotates in the packing without much danger of carrying dirt into the packing from outside to inside.

Rising stem gate valves are designed so that the stem is raised out of the flow path when the valve is open. Rising stem gate valves come in two basic designs. Some valves have a stem that rises through the hand wheel while others have a stem that is threaded to the bonnet.

Gate Valve Seat Design

Seats for gate valves are either provided integral with the valve body or in a seat ring type of construction. Seat ring construction provides seats which are either threaded into position or are pressed into position and seal welded to the valve body. The latter form of construction is recommended for higher temperature service.

Integral seats provide a seat of the same material of construction as the valve body while the pressed-in or threaded-in seats permit variation. Rings with hard facings may be supplied for the application where they are required.

Small, forged steel, gate valves may have hard faced seats pressed into the body.In some series, this type of valve in sizes from 1/2 to 2 inches is rated for 2500 psig steam service. In large gate valves, disks are often of the solid wedge type with seat rings threaded in, welded in, or pressed in. Screwed in seat rings are considered replaceable since they may be removed and replaced.

Butterfly Valve Basics

What is a Butterfly Valve

A butterfly valve, is a quarter turn ,rotary motion valve that is used to stop, regulate, and start fluid (air,gas,water,steam) flow in pipes. Butterfly valves are easily and quickly operated because of 90^o rotation of disk from a fully closed to fully opened position.That,s why butterfly valves are also called quick turn valves.

Butterfly Valve How It works

Butterfly valves are built on the principle of a pipe damper.These are also called butterfly damper valves.The flow control element in butterfly valve is a disk of approximately the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis. 

When the disk lies parallel to the piping run, the valve is fully opened. When the disk of this rotary valve approaches the perpendicular position, the valve is closed.In butterfly valves intermediate positions can also be attained for throttling purposes.Intermediate position in butterfly valves are usually achieved by handle-locking devices.

Larger butterfly valves are actuated by hand wheels connected to the stem through gears that provide mechanical advantage at the expense of speed.Butterfly valves may be motor operated or motorized ,air operated or pneumatically actuated also.

Advantages of Butterfly Valves

Butterfly valves have many advantages over gate, globe, plug, and ball valves, especially for large industrial applications of valve.

• Butterfly valves present savings in weight, space, and cost.
• The maintenance costs of butterfly valves are usually low because there are a minimal number of moving parts .
• Also in butterfly valves there are no narrow spaces or pockets where fluid can be trapped.
• Butterfly valves are well-suited for the handling of large fluid flows of liquids or gases that are at relatively low pressures .
• Butterfly valves are also good for the handling of slurries or liquids which have large amounts of suspended solids.

Disadvantages of Butterfly Valves

• Butterfly valves  have limited throttling range .
• Butterfly valves  have although ability to handle slurries but it is limited.
• Butterfly valves  require special seal for tight shuttoff.
• Butterfly valves require high torque required for control.

Types of Butterfly Valves

• The most common and less expensive or standard type is the wafer type butterfly valve  that fits between two pipeline flanges also called flanged butterfly valves.This type is also called general butterfly valve.
• Another type, the lug wafer design, or lug style butterfly valve,is held in place between two pipe  flanges by bolts that join the two flanges and pass through holes in the valve's outer casing.

Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded end construction.

Mechanical assembly of Disk ,Seat and Stem in Butterfly Valve

Butterfly valves stop the flow of fluid by the valve disk that rests on a seat that is on the inside diameter periphery of the valve body. Many butterfly valves have an elastomeric seat against which the disk seals. Other butterfly valves have a seal ring arrangement that uses a clamp-ring and backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings.Metal disk on metal seat (in metal seated butterfly valves) has failed to provide tight seal in butterfly valves.

The stem and disk for a butterfly valve are separate pieces. The disk is bored to receive the stem. Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned. In the first method, the disk is bored through and secured to the stem with bolts or pins. The alternate method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-shaped stem. This method allows the disk to "float" and seek its center in the seat.

Sealing (Packing or O-ring) in butterfly valves

O-ring seals or conventional stuffing box packing are used in butterfly valves to prevent the leakage or accomplish the valve sealing.If a stuffing box or external O-ring is employed, the fluid passing through the valve will come into contact with the valve stem.