An Introduction To Gate valve , Selection , Types , And Apllication

An Introduction To Gate valve , Selection , Types , And Apllication

Introduction

A gate valve is a linear motion valve used to start or stop fluid flow; however, it does not

regulate or throttle flow. The name gate is derived from the appearance of the disk in the flow

stream

Gate Valve Working 

The disk of a gate valve is completely removed from the flow stream when the valve is fully

open (figure 1). This characteristic offers virtually no resistance to flow when the valve is open. Hence there is little pressure drop across an open gate valve

FIG 1

When the valve is fully closed, a disk-to-seal ring contact surface exists for 360°, and good

sealing is provided. 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 close

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. For these reasons, gate valves are not used to regulate or

throttle flow

Gate Valve Application

Gate valves are used in almost all fluid services such as air, fuel gas, feedwater, steam, lube oil, hydrocarbon, and all most any services.
Some special gate valve is used in slurry and powder product also such as knife gate valve
The advantages to the use of a gate valve are
          - low resistance to fluid flow when completely open
          - they can work in both fluid directions
          - gate opening or closing requires the same energy of other valves
         - as the speed of closure is generally slow, using that valves does not raise physical shock to the             system.
The major disadvantages to the use of a gate valve are :

              -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

FIG 2

Gate Valves Types Of Based On Function
There are essentially 7 types of gate valves, but we can also classify them by types of disk, type of body bonnet joint or type of stem movement.
Wedge valves 

They are the standard gate valves and work by lifting a gate wedge out of the line of a fluid.

FIG 3

Knife valves 

This kind of gate valves use a sharp and beveled edge wedge to allow thicker fluids to easily flow. Usually these valves are self-cleaning and we can classify them between having either a rising or non-rising stem design.

FIG 4

Brass Valve 

They are universally considered the most durable gate valve that best fit where problem of corrosion, temperature and high pressure are serius.

FIG 5

Slide valves 

We do have to prefer slide gate valves when we need to suspend the flow of a product from discharge points.

FIG 6

Non-return valves 

They allow fluids or gas to safely flow in just one direction when pressure conditions could reverse it.
Extended body valves – These valves are use for tapping pressure vessels as they have a threaded or welded connection.

Check valves 

They are so called non-return valves because they have two gates: one for fluid to enter and other for fluid to leave.

Gate Valve Types Of Based On Disk Design 

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, or 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

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

1- Solid Wedge

The solid wedge gate valve shown in Figure 7 is the most commonly used disk because of its simplicity and strength

A valve with this type of wedge may be installed in any position and it is suitable for almost all fluids. It is practical for turbulent flow

FIG 7

2- Flexible Wedge

valve illustrated in Figure 8 is a one-piece disk with a cut around the perimeter to improve the ability to match error or change in the angle between the seats. The cut varies in size, shape, and depth. A shallow,narrow cut gives little flexibility but retains strength. A deeper and wider cut, or cast-in recess, leaves little material at the center, which allows more flexibility but compromises strength

A correct profile of the disk half in the Flexible Wedge Gate Valve flexible wedge design can give uniform deflection properties at the disk edge, so that the wedging force applied in seating will force the disk seating surface uniformly and tightly against the seat

Gate valves used in steam systems have flexible wedges. The reason for using a flexible gate is to prevent binding of the gate within the valve when the valve is in the closed position. When

steam lines are heated, they expand and cause some distortion of valve bodies. If a solid gate fits snugly between the seat of a valve in a cold steam system, when the system is heated and pipes

elongate, the seats will compress against the gate and clamp the valve shut. This problem is overcome by using a flexible gate, whose design allows the gate to flex as the valve seat compresses it

The major problem associated with flexible gates is that water tends to collect in the body neck. Under certain conditions, the admission of steam may cause the valve body neck to rupture, the bonnet to lift off, or the seat ring to collapse. Following correct warming procedures prevent these problems

FIG 8

3- Split Wedge

Split wedge gate valves, as shown in Figure 9 are of the ball and socket design. These are self-adjusting and selfaligning to both seating surfaces. The disk is free to adjust itself to the seating surface if one-half of the disk is slightly out of alignment because of foreign matter lodged between the disk half and the seat ring. This type of wedge is suitable for handling noncondensing gases and liquids at normal temperatures, particularly corrosive liquids. Freedom of movement of the disk in

the carrier prevents binding even though the valve may have been closed when hot and later contracted due to cooling. This type of valve should be installed with the stem in the vertical position

FIG 9

4- Parallel Disk

The parallel disk gate valve illustrated in Figure 10 is designed to prevent valve binding due to thermal transients. This design is used in both low and high pressure applications

The wedge surfaces between the parallel face disk halves are caused to press together under stem thrust and spread apart the disks to seal against the seats. The tapered wedges may be part of the disk halves or they may be separate elements. The lower wedge may bottom out on a rib at the valve bottom so that the stem can develop seating force. In one version, the wedge contact surfaces are curved to keep the point of contact close to the optimum

In other parallel disk gates, the two halves do not move apart under wedge action. Instead, the upstream pressure holds the downstream disk against the seat. A carrier ring lifts the disks, and a spring or springs hold the disks apart and seated when there is no upstream pressure

FIG 10

Another parallel gate disk design provides for sealing only one port. In these designs, the high pressure side pushes the disk open (relieving the disk) on the high pressure side, but forces the disk closed on the low pressure side. With such designs, the amount of seat leakage tends to decrease as differential pressure across the seat increases. These valves will usually have a flow direction marking which will show which side is the high pressure (relieving) side. Care should be taken to ensure that these valves are not installed backwards in the system

Some parallel disk gate valves used in high pressure systems are made with an integral bonnet vent and bypass line. A three-way valve is used to position the line to bypass in order to equalize pressure across the disks prior to opening. When the gate valve is closed, the three-way valve is positioned to vent the bonnet to one side or the other. This prevents moisture from accumulating in the bonnet. The three-way valve is positioned to the high pressure side of the gate valve when closed to ensure that flow does not bypass the isolation valve. The high pressure acts against spring compression and forces one gate off of its seat. The three-way valve vents this flow back to the pressure source

Gate Valve Types Of Based On Stem Design 

Gate valves are classified as either rising stem or nonrising stem valves.  

1- The nonrising 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 valve will almost always have a pointer-type indicator threaded onto the

upper end of the stem to indicate valve position.

The nonrising 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

FIG 11

2- The rising stem gate valve 

Rising stem gate valves are designed so that the stem is raised out of the flowpath when the

valve is open. Rising stem gate valves come in two basic designs. Some have a stem that rises

through the handwheel while others have a stem that is threaded to the bonnet

Gate Valve  Types Of Based On Seat Design

Seats for gate valves are either provided integral with the valve body or in a seat ring type of

construction. 

1- Seat ring Type

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 serviceز

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

FIG 12

2- Integral seats Type

Integral seats provide a seat of the same material of construction as the valve body 

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

new seat rings installed

FIG 13

Gate Valve Types Of Based On Body, Bonnet Connection

1- screwed bonnet

This is the simplest design available and it is used for inexpensive valves.

FIG 14

2- bolted-bonnet

This is the most popular design and used in a large number of gate valves. This requires a gasket to seal the joint between the body and bonnet.

FIG 15

3- Welded-Bonnet

This is a popular design where disassembly is not required. They are lighter in weight than their bolted-bonnet counter parts.

FIG 16

4- Pressure-Seal Bonnet

This type is used extensively for high-pressure high-temperature applications. The higher the body cavity pressure, the greater the force on the gasket in a pressure -seal valve.

FIG 17

Gate Valve Types Of Based On End Connection

 Common end finishs are socket, threaded, flanged (RF or RJ) and butt-weld ends. 

1- Screwed End Type

Screwed end is simple connection method often used for small valves (figure ). This connection is usually made by machining the valve in and out of the end into tapered or straight pipe threads, which can be connected to tapered pipe threads or the pipe line. Since this connection may have large leakage channels, sealant, sealing tape or packing can be used to plug these channels. If the material of the valve body is weldable but the expansion coefficient varies greatly, or the operating temperature varies widely, the threaded joint must be honey sealed. Thread connected valves are mainly nominal through the valve in the 50mm or less. If the size is too large, it is very difficult to install and seal the connecting parts. To facilitate installation and removal of threaded valves, pipe connections are available at appropriate locations in the piping system. Valves with nominal sizes up to 50 mm May be fitted with a sleeve, where the thread holds the two parts together.

FIG 18

2- Flanged End Type

Flanged ends are the most commonly used type of valve end connection. Flanged valve installation and disassembly are more convenient. But they are heavier and more expensive than threaded valves. Therefore it is suitable for pipe connections of all sizes and pressures. However, when the temperature exceeds 350 degrees, because the bolts, gaskets and flanges become slack, also significantly reduce the load of the bolts, the flange connection may produce leakage under great stress.

FIG 19

3- Welded End Type

Welded end connection is suitable for all kinds of pressure and temperature, and is more reliable than flange connection when used under the condition of higher load. However, soldered valves are difficult to remove and reinstall, so their use is usually limited to long-term reliable operation, or use conditions at high temperatures. Such as thermal power stations, nuclear energy projects, ethylene projects on the pipeline. Welded valves with nominal size below 50mm usually have a welding socket to carry the pipe at the flat end of the load. Due to the gap formed between the socket and the pipe, the gap may be corroded by some medium, and the vibration of the pipe may make the joint fatigue, so the use of socket welding is limited. In the larger nominal diameter, the use of conditions engraved, high temperature occasions, the valve body often used groove (butt weld ) on the welding, at the same time, the welding seam has the original requirements, must choose a skilled welder to complete the work.

FIG 20

Gate Valve Materials And Pressure Ratings

Gate valves are used in a wide variety of applications and may come into contact with an assortment of media. When selecting a gate valve, the material used to construct the valve is a critical decision to prevent premature valve failure or system delays. In order to select the proper valve material there are several important criteria to be considered:
 - The composition of the media in contact with all wetted (exposed) parts
 - Service temperatures
 - Operating pressures
  - Effectiveness of coating on materials
 - Material availability and cost
 - Compatibility of materials with injected media.
 - How long the valve will be exposed to the media

Carbon steel is the most commonly used material for bodies. It handles most non-corrosive liquids and gases up to 800 degrees Fahrenheit for continuous service, or to 1000 degrees Fahrenheit for occasional service.

Chrome-moly steel is used for higher temperatures and pressures than carbon steel, including such services as high pressure steam or flashing condensate which requires corrosion and errosion resistance. Chrome-moly is stronger than carbon steel and, in some cases, is as strong as stainless steel. It costs less than stainless steel, but is not as corrosion resistant.

Stainless steel is specified for high temperature services (1000 degrees Fahrenheit and up) or in corrosive applications. It is more corrosion resistant than either carbon steel or chrome-moly.

Special alloys—such as Hastelloy B and C, Monel, nickel and titanium—are also available. Consideration should be given to the types of material that have been used successfully in the past for similar applications.

Material specification is based upon the ASTM specifications listed in ANSI B16.34 for standard service valves, and ASME (American Society of Mechanical Engineers) specifications.

Body material pressure and temperature ratings according to ANSI B16.34

Actuation Method
There are several basic types of valve actuators: manual, electric, pneumatic, and hydraulic.
Manual valve actuators do not require an outside power source. 
1- Handwheel Manual Type
They use a handwheel or lever to drive a series of gears whose ratio results in a higher output torque compared to the input (manual) torque.

FIG 21

2- Gear Box Manual Type
Gearing can be applied to valves instead of the standard handwheel to make operation easier.

FIG 22

3- Motorize Type
Motorized controls may be applied to valves of any size for operation in any position or location.

FIG 23

4- Pneumatic Type
Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion.

FIG 24

5- Electro Hydraulic Type

electrohydraulic valve actuators and hydraulic valve actuators convert fluid pressure supply into linear or rotary motion.

FIG 25

Gate Valve Parts

Here you can see the main parts of the gate valve. The disk of a gate valve is also known as a wedge. To learn about each of this part read complete guide of valve parts.

FIG 26

Cv Flow Coefficients & Valve Sizing

With the flow coefficients capacities of valves at different sizes, types and manufacturers can be compared. The flow coefficients are in general determined experimentally and express the
flow capacity in imperial units - GPM (US gallons per minute) that a valve will pass for a pressure drop of 1 lb/in² (psi)

FIG 27

The flow factor - Kv - is also commonly used with capacity in SI-units.

The flow coefficient - Cv - required for a specific application can be estimated by using specific formulas for the different fluids or gases. With an estimated Cv value - the correct size of control valve can be selected from the manufacturers catalogs.
Note: Kv is the metric equivalent of Cv.

Cv = 1.16 Kv 

Kv = 0.862 Cv 

For Liquids:
For liquids the flow coefficient - Cv -is expressed with water flow capacity in gallons per minute (GPM) of 60F⁰ with pressure drop 1 psi (lb/in²).

low expressed by volume

Cv = Q (SG / dp)¹⁄² 

where :
Q = water flow (US gallons per minute)
SG = specific gravity (1 for water)
dp = pressure drop (psi)

Flow expressed by weight

Cv = w / (500 (dp SG)¹⁄² ) 

where
w = water flow (lb/hr)
SG = specific gravity (1 for water)
dp = pressure drop (psia)

.

For more you can read from 
https://mechasource.blogspot.com/2018/03/cv-flow-coefficients-valve-sizing.html

Basic Calculation of Thrust Force and Torque for Gate Valve

Seating and Un-seating Force without Double-block & Bleed Valve

FIG 28

For Disc Area:

Where :

f: Axial force to disc area against
flow direction under the pressure.
P: Pressure
D: Inside diameter of the valve bore.
tan(𝞴+𝝷):Friction coefficient.

For Stem Area:

For Gland Packing Area:

l: Total length of Gland Packing
Pg: Tightening Stress of Gland Packing
𝞵₀: Coefficient of tighten friction factor for Gland Packing

Toatal Seating Force : 

FS=F1+ F2 + F3 -WT

Toatal Un-seating Force: 

FUS=F1- F2 + F3 +WT

WT: Total weight of Stem & Disc.

Gate Valve Parts View

FIG 29