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Thảo luận trong 'Các vấn đề về thủy lực & ứng dụng' bắt đầu bởi khoadongluc, 29/5/10.

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    Valves are devices designed to maintain, restrict, or meter the flow of product through pipes, tubing or entire pipeline systems. They function by allowing flow while in their open position, and restricting flow when closed. Based on the mechanism that is used to restrict flow, the valves can be mainly classified as Gate valves, Ball valves, Butterfly valves, Check valves, Globe valves, Plug valves, Control valves, Pressure relief valves etc.


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    The valves for cross country pipeline systems are designed as per API 6D. Generally for high pressure pipelines, class 600 valves are used and for low pressure lines and internal piping class 300 & class 150 valves are used. Valves are of Hand Operated type (HOV) and Motor Operated type (MOV). In MOVs, actuators are mounted on valves that in response to a signal automatically move the valve to the desired position using an outside power source. The major classifications of valve actuators include electric / electronic, pneumatic, and hydraulic / electro-hydraulic. Manual valve actuators are manipulated directly by the user.The moving parts in a valve generally are the disk, the valve stem and the handwheel.The stem connects the handwheel and the disk. It is threaded and fits into the threads in the valve bonnet. The location of the valve disk in relation to the valve seat allows or restricts flow. The valves are designed based on products to be pumped, temperature & pressure of the products and the service (shut-off, throttling, pressure / flow control) for which the valve is required.


    GATE VALVES

    Gate valves are linear motion valves in which a flat closure element slides into the flow stream to provide shut-off. Gate valves are designed to minimize pressure drop across the valve in the fully opened position and stop the flow of fluid completely. The direction of fluid flow does not change, and the diameter through which the process fluid passes is essentially equal to that of the pipe. Hence, they tend to have minimal pressure drop when opened fully.

    Gate valves are excellent for use anywhere a shutoff valve is needed. They can also be used where throttling capabilities are desired, although this is not generally recommended as erosion of the seat and disc occurs due to the vibrations of the disk in throttling applications.

    Gate valves are usually divided into two types: parallel and wedge-shaped. The parallel gate valve uses a flat disc gate between two parallel seats, upstream and downstream. In the double-disk parallel-seat type, the valve is closed by lowering the disks from the valve neck to a height equal to that of the valve seats. Once so positioned, an inclined plane mounted between the two disks coverts downward stem force into axial force and presses the parallel disks firmly against the valve seats sealing the two openings. These types of valve design can accommodate asymmetric or angularly misaligned valve seats. Wedge-shaped gate valves use two inclined seats and a slightly mismatched inclined gate
    allowing for tight shut-off. Disk flexibility is inherent to the split wedge design. This flexibility allows the split wedge to seal more easily and it reduces stickiness between the sealing surfaces in cases where the valve seats are angularly misaligned.

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    GLOBE VALVES
    Globe valves are linear motion valves with rounded bodies, from which the name “Globe” is derived. They are widely used to regulate fluid flow in both on/off and throttling service. The direction of fluid flow through the valve changes several times, which increases the pressure drop across the valve. In most cases, globe valves are installed with the stem vertical and the higher-pressure fluid stream connected to the pipe side above the disk, which helps to maintain a tight seal when the valve is fully closed.

    Globe valves provide precise throttling and control and have high-pressure limits. Conversely, they offer a low coefficient of flow and are not good selections in applications that require cleanliness or sterility. When globe valves are open, the fluid flows through the space between the edge of the disk and the seat. These valves are commonly used as a fully open or fully closed on/off valves, but they may be used for throttling as long as fine adjustments are not required. Globe valves are available in three main body types i) Angle design ii) Y-design and iii) Multi-piece design. Angle valves are designed so that the inlet and outlet are perpendicular, for transferring flow from vertical to horizontal. Y-design valves derive linear action from the incline between the axis of the inlet and outlet ports. The bodies of multi-piece design valves are bolted together. The inlet and outlet are not of single piece construction. Given below is an example of an Angle Globe valve.


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    PLUG VALVES
    Plug valves are one of the most widely used valves for both on/off and throttling services for low pressure service. The body is comprised of three main parts: body, cover and plug. The plug is a cylindrical, tapered or generally cone-shaped device that can be raised or lowered within the seat to maintain, restrict or completely shut off flow. The valve is opened by rotation, with the plug itself being the only element that is capable of
    movement.

    Plug valves are high capacity valves that can be used for directional flow control. They can safely and efficiently handle gas and liquid fuel and extreme temperature flow. They can also be used to regulate the flow of liquids containing suspended solids (slurries).

    When opening the valve, the hand wheel turned to make the plug lifted and separated with the sealed surface. Then the handle is turned by 90º to align the channel of plug with the channel of valve body. When closing the valve, the hand wheel is turned by 90º to align the channel of plug with valve body and then the hand wheel is turned to descend the plug.


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    CHECK VALVES
    Check valves are mechanical valves that permit gases and liquids to flow in only one direction, preventing process flow from reversing. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while back flow forces the valve closed. Generally check valves contain a ball that sits freely above the seat, which has only one through hole. The ball has a slightly larger diameter than that of the through hole. When the pressure behind the seat exceeds that above the ball, liquid is allowed to flow through the valve. But once the pressure above the ball exceeds the pressure below the seat, the ball returns to rest in the seat, forming a seal that prevents backflow.

    Check valves can be single disc swing type, double disc swing type, lift-check type, silent type, ball-check type and cone-check type. Single disc swing valves are designed with the closure element attached to the top of the cap. The closure element can be pushed aside by the flow, but swings back into the close position upon flow reversal. These valves are not good choices in services with frequent flow reversal, as the disc is often pounded into the seat and wears quickly. These valves may be installed vertically or horizontally, although when mounted vertically a lever and counter-balance should be applied to reduce the valves tendency to stay open in situations where flow velocity changes slowly.

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    Double disc or wafer check valves consist of two half-circle disks hinged together that fold together upon positive flow and retract to a full-circle to close against reverse flow. The valve is inserted between two flanges. These are most commonly used liquid services.
    Lift-check valves have a disc that is guided. Spring-loaded types can operate in either vertical or horizontal mounting. The style that lacks the spring must be mounted in such a way as to allow gravity to close disc. Composition disc valves are available for gritty
    liquid services.

    Silent or centre guide valves are similar to lift check valves, with a center guide extending from inlet to outlet ports. The valve stopper is spring and bushing actuated to
    keep the movement "quiet.” Ball-check valves use a free-floating or spring loaded ball resting in a seat ring as the closure element. Upon reverse flow, the ball is forced back into its seat preventing backflow. These valves are good for most services. They can even handle fluids that produce gummy deposits. Since the disc is free to rotate wear is distributed evenly and this all helps to keep the valve seats clean. Cone check valves use a free-floating or spring loaded cone resting in the seat ring as the closure element. Upon reverse flow, the cone is forced back into its seat preventing backflow.


    BUTTERFLY VALVES

    Butterfly valves are quick opening valves that consist of a metal circular disc or vane with its pivot axes at right angles to the direction of flow in the pipe, which when rotated on a shaft, seals against seats in the valve body. They are normally used as throttling valves to control flow. Butterfly valves control the flow of gas or liquid by means of a disk, which turns on a diametrical axis inside a pipe or by two semicircular plates hinged on a common spindle, which permits flow in only one direction. These valves offer a rotary stem movement of 90º or less, in a compact design. Unlike ball valves, butterfly valves do not have any pockets in which fluids may become trapped when the valve is closed. Butterfly valves are available in several body styles and seal types. The flange type or lug style can be held between flanges of any type. Wafer valves are installed between two flanges using bolts or nuts and studs, while lug style valves have metal inserts installed in the valve's bolt holes and the valve is installed between two flanges using a separate set of bolts for each flange. Butterfly valves may be used in a wide variety of chemical services, they are available with small dimensions allowing for use in areas where space is limited, and they allow a high coefficient of flow. However, the design of butterfly valve creates the disadvantage of not being easy to clean, and therefore should be avoided in situations that call for regular cleaning.

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    BALL VALVES

    Ball valves are quarter-turn, straight through flow valves that have a round closure element with matching rounded seats that permit uniform sealing stress. Ball valves are used in situations where tight shut-off is required. The type of seat can vary with the valve pressure rating and materials of construction. Some valve seats are composed of
    single molded forms, while other seats with higher-pressure ratings often incorporate a trunnion design where each face of the ball is separately sealed. These valves are able to transfer gases, liquids and liquids with suspended solids.

    In general, ball valves offer many advantages when contrasted with other valve types. They provide superior ease of operation and can maintain and regulate high volume, high pressure and high temperature flow. Most ball valves offer rugged construction providing for a long service life, and a comparably low cost. Additionally, the design of the
    regulating element allows the valve to function without the complications of side loads, typical of butterfly or globe valves, and the valve design permits inspection and repair of seats and seals without removing the valves’ body from the line.

    Ball valves are available in a variety of body styles, including one-piece, two-piece, three-piece and flanged body construction. Each of these ball valve forms offers specific advantages depending upon the requirements of the given application. Common ball valve materials include brass, bronze, copper, cast iron, ductile iron, stainless and other
    steel types, metal alloys.[​IMG]



    CONTROL VALVES


    Control valves are power-operated devices used to modify fluid flow or pressure rate in a pipeline system. Control valves types include globe, gate, butterfly, ball and plug. Important specifications to consider for power-operated control valves include diameter, working pressure, and operating temperature. Metal material choices for valve body
    include brass, bronze, copper, cast iron, ductile iron, Monel, stainless steel and steel. Valve actuation methods for control valves include electric, pneumatic, hydraulic, and manual. Seat features include metal-to-metal, o-ring or soft seat, Class IV or V, and Class VI.
     

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