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Engineer's Guide to Gate Valves and Their Configuration

作者:互联网    浏览:56504    发布时间:2015-8-14 9:34:48

Gatevalves (also known as knife valves or slide valves) are linear motion valves inwhich a flat closure element slides into the flow stream to provide shut-off.They are one of the most common valves used.

 

Gatevalves and knife valves are advantageous in applications involving slurries, astheir “gates” can cut right through the slurry. They are also used inapplications that involve viscous liquids such as heavy oils, light grease,varnish, molasses, honey, cream and other non-flammable viscous liquids. Theyare available in large sizes to better handle thick flow. However, gate valvesdo have low-pressure limitations, and are not optimal in applications thatrequire cleanliness or sanitary conditions. They are excellent for use anywherea shutoff valve is needed. When needed, they can also be used where throttlingcapabilities are desired, although this is not generally recommended as erosionof the seat and disc occurs due to the vibrations of the disk in throttlingapplications.

 

Gatevalves and knife valves are designed to minimize pressure drop across the valvein the fully opened position and stop the flow of fluid completely. Thedirection of fluid flow does not change, and the diameter through which theprocess fluid passes is essentially equal to that of the pipe. Hence, they tendto have minimal pressure drop when opened fully.


Gatevalves and knife valves advantages and disadvantges.

 

ValveFunction

Gatevalves are primarily designed for on-off services. They are best used insystems which require infrequent use of the valve. The valves are designed forfull-area flow to minimize the pressure drop and allow the passage of apipe-cleaning pig. Since most of the flow change occurs near the shutoff, therelatively high fluid velocity causes disk and seat wear and eventual leakageif the valve is used to regulate flow

 

Applications

Gatevalves are used in many industrial applications including the oil and gasindustry, pharmaceuticals, manufacturing, automotive, and marine.

 

Non-risingstem gate valves are very popular on ships, in underground applications, orwhere vertical space is limited because they don't take up extra space. Gatevalves can be used in demanding environments such as high temperature and highpressure environments. They are often seen in power plants, water treatments,mining, and offshore applications.

 

Types

Gatevalves are usually divided into two types: parallel and wedge-shaped.

 

1. The parallel gate valve uses a flat disc gate between twoparallel seats, upstream and downstream. Shut-off is obtained by thefree-floating seat or disk gate allow the upstream pressure to seal the seatand disk against any unwanted seat leakage. Some parallel gate valves aredesigned to allow the seat to be spring-energized by an elastomer that appliesconstant pressure to the disk gate seating surface. In the double-diskparallel-seat type, the valve is closed by lowering the disks from the valveneck to a height equal to that of the valve seats. Once so positioned, aninclined plane mounted between the two disks converts downward stem force intoaxial force and presses the parallel disks firmly against the valve seatssealing the two openings. These types of valve design can accommodateasymmetric or angularly misaligned valve seats. Parallel gate valves are usedin low pressure drops and low pressures, and where tight shutoff is not animportant prerequisite.

2. Knife valves are a specific type of parallel gate valve.They have a sharp edge on the bottom of the gate to shear entrained solids orseparate slurries.

3. The through-conduit gate valves have a rectangular closureelement. The closure has a circular opening equal to the full-area flowpassageway of the gate valve; as the element is lowered the opening is exposedto the flow. Raising the element shuts off the flow. In this design the seatingsurface of the gate is in contact with the gate at all times.

Wedge-shapedgate valves use two inclined seats and a slightly mismatched inclined gate,allowing for tight shut-off.

1. Disk flexibility is inherent to the split wedge design.This flexibility allows the split wedge to seal more easily and it reducesstickiness between the sealing surfaces in cases where the valve seats areangularly misaligned. The mismatched angle is also designed with some freemovement to allow the seating surfaces to match with each other as the actuatorforces them closed. It is also best used for handling non-condescending gasesand liquids at normal temperatures, particularly corrosive liquids. Pressure-energizedelastomer inserts can be installed on a solid gate to provide a tight seal.This type of disk should be installed vertically.

2. Solid wedges are the most common because of theirsimplicity and strength. They can be installed in any position, are suitablefor almost all fluids, and are practical for turbulent flow.

3. Flexible wedge gate valves are commonly used in steamsystems. The disk is one piece with a cut around the perimeter to improve theability to match error or change the angle between the seats. Different sizes,shapes, and depths are available. For example, a shallow, narrow cut giveslittle flexibility but retains strength. A deeper, wider cut leaves littlematerial at the center and weakens the disk, but increases flexibility. Flexiblewedges prevent binding the gate within the valve when the valve is in theclosed position. The design allows the gate to flex as the valve seat iscompressed by heat from the steam in the system. The disadvantage is that watertends to collect in the body neck.

 

Flow

Asa gate valve is opened, the flow path is enlarged in a highly nonlinear mannerwith respect to the percent opening. The flow rate does not change evenly withthe stem travel and the disk in a partially opened gate valve will vibrate fromthe fluid flow. Since this vibration can cause the seat and disk to wear andcause leakage, gate valves should only be used in the fully open or fullyclosed position. Very little friction loss occurs when the valve is in thefully open position.

 

Methodof Control

Theclosure element of a gate valve is a replaceable disk. To open the valve, thedisk is completely removed from the stream and offers virtually no resistance.Therefore, there is little pressure drop across the open gate valve. A fullyclosed gate valve provides good sealing due to the 360° disk-to-seal ringcontact surface. Proper mating of a disk to the seal ring ensures there is verylittle or no leakage across the disk when the gate valve is closed.

 

Media

Gatevalves can be used for liquid and gas services. They are especially designedfor slurries with entrained solids, granules, and powders. They can also beused for cryogenic and vacuum services.

 

GateValve Components

Thereare three main parts of a gate valve: body, bonnet, and trim.

 

Body

 

Thebody of a gate valve holds all of the operational parts of the valve. It isconnected to the system with one of the mounting options below. The mountingoption should be selected based on the current system mounting features and thetype and size of the media.

 


 

Bonnet

Thebonnet of a gate valve contains the moving parts and is attached to the valvebody. The bonnet can be removed from the body in order to allow for maintenanceand replacing parts.

 

Trim

Thetrim of a gate valve contains the functioning pieces of the valve: the stem,the gate, the disc or wedge, and the seat rings.

 

1. Stem- The stem of a gate valve is either a rising stem ora nonrising stem. The stem is responsible for the proper positioning of thedisk. Nonrising stems will almost always have a pointer-type indicator mountedonto the upper end of the stem to indicate valve position. This configurationprotects the threads from carrying dirt into the packing because the stemthreads are held within the boundary of the valve packing. Rising stems riseout of the flow path when the valve is opened. They can either have a stem thatrises through the handwheel or have a stem that is threaded to the bonnet.

2. Seat- The seat of a gate valve is either integral withthe valve body or in a seat ring type of configuration. The seat ringconstruction is either threaded on to the body or pressed into position and theseal welded to the valve body. Pressed and welded is recommended for highertemperature applications. Press-in or threaded-in seats permit variation in theseat material verses the material of the body of the valve.

Itis important to consider the size of the valve and system when selecting a gatevalve. Gate valves are linear motion valves and therefore are taller than othermanual valves. This is especially true if the valve uses a rising stem. Most ofthe time gate valves should be installed horizontally (perpendicular to theground) with the manual actuator in the top position. This allows for easyaccess for maintenance and replacing parts. Smaller gate valves can beinstalled in vertical lines, but gravity tends to pull the valve out ofalignment.

 

Materialof Valve Construction

Gatevalves are used in a wide variety of applications and may come into contactwith an assortment of media. When selecting a gate valve, the material used toconstruct the valve is a critical decision to prevent premature valve failureor system delays. In order to select the proper valve material there areseveral important criteria to be considered:

 

The composition of themedia in contact with all wetted (exposed) parts

Service temperatures

Operating pressures

Effectiveness ofcoating on materials

Material availabilityand cost

Compatibility ofmaterials with injected media.

How long the valvewill be exposed to the media

 

Thereare organizations dedicated to developing and maintaining standards for valvesand materials in particular environments. Gate valves are available in manydifferent materials. Valves can be specified by the National Association forCorrosion Engineers (NACE) and the American Petroleum Institute (API) for theirability to handle strong and corrosive media.

 

Actuator

Valveactuators open and close the valve in response to a signal or manualmanipulation. Most gate valves have manual actuators, such as a handwheel,because they are commonly used in applications where the valve does not need tobe opened or closed often or quickly. Since gate valves are not used inthrottling applications, the actuator is responsible for fully opening andfully closing the valve.