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Process Operation Technology

Oil, gas and petrochemical process industries


    GATE VALVES

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    Talib Al_Munawri
    Director-General
    Director-General

    Posts : 337
    Join date : 2009-07-03
    Age : 30
    Location : Sultanate of Oman

    GATE VALVES

    Post by Talib Al_Munawri on Thu Sep 17, 2009 9:36 pm

    GATE VALVES


    There are two main types of gate valves used in the process system:

    Arrow Rising stem gate valve.
    Arrow Non-rising stem gate valve

    The valve disc in a gate valve is a wedge shaped disc or gate.
    It is attached to the end of a threaded stem. The gate fits into
    a wedged shape seat in the valve body to stop flow through the valve.

    Turning the handwheel raises and lowers the valve disc. When the
    valve is fully open the gate is positioned above the passage in
    the valve body. This allows full flow in both directions through
    the ports. There is little or no restriction and no pressure drop
    across the valve.

    When the gate valve is fully closed the gate fills the passage and
    it stops the flow through the valve completely. Gate valves are
    only used in the fully open or fully closed position to start and
    stop flow. You must turn the handwheel to many times to move the
    valve to the fully open or fully closed position.


    RISING STEM GATE VALVE




    Large gate valves normally have rising stems. When the handwheel
    is turned the stem rises out of the valve bonnet through the handwheel.
    The stem shows the position of the valve disc and shows if the valve
    is fully open or fully closed. In small gate valves the wheel and
    stem may rise together.


    The threaded part of the stem is above the valve body. It is not
    in contact with the fluid flowing through the valve. This type
    of valve has less corrosion problems. The rising stem gate valve
    requires more space and more head room.


    NON RISING STEM GATE VALVE





    Some gate valves have a non rising stem. The stem does not come
    out of the valve body. The valve disc moves up and down the stem.
    With this type of gate valve there is more chance of corrosion as
    the valve stem is in contact with the liquid flowing through the
    valve. This type of valve is used where there is not enough space
    above the valve for the stem to rise out of the body.


    Gate valves provide a good seal against flow. For this reason they
    are normally used as isolation valves in high pressure hydrocarbon
    pipelines. The gate valve is the most commonly used valves in the
    petroleum industry.


    If the gate valve is partially closed there may be vibration.
    This can cause noise and possible damage to the valve disc and
    the seating surfaces. For this reason gate valves should only
    be used for throttling (restricting flow) in an emergency.


    Three main types of gate designs for gate valves are:


    Arrow Solid Wedge Gate.
    Arrow Split Gate (parallel disc).
    Arrow Flexible Gate






    SOLID WEDGE GATE


    The solid wedge valve disc was the first type that was used in gate
    valves. It does not allow for expansion or contraction of the valve
    body and stem material with large temperature changes. This type of
    wedge is not used in high temperature processes where there is thermal expansion
    and contraction of the valve body


    SPLIT GATE


    The split gate valve disc is split up the centre. This allows the
    element to expand or contract. Therefore, the disc has a better
    fit in the valve seat over a wide range of pressures and temperatures.
    The wedge can adapt itself to small amounts of distortion caused by seat wear
    or piping strain.


    FLEXIBLE GATE


    Like the split gate valve disc the flexible gate valve disc is
    split up the centre. This allows the element to expand and contract
    and to give a very tight seal in the valve seat over a wide range
    of pressures and temperatures. This type of wedge is used in high
    temperature processes


      Current date/time is Sun Oct 22, 2017 2:20 am