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Control valves are imperative elements in any system where fluid flow must be monitored and manipulated

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Control valves are imperative elements in any system where fluid flow must be monitored and manipulated Powered By Docstoc
					Control valves are imperative elements in any system where fluid flow must be monitored and
manipulated. Selection of the proper valve involves a thorough knowledge of the process for which it will
be used. Involved in selecting the proper valve is not only which type of valve to use, but the material of
which it is made and the size it must be to perform its designated task.

The basic valve is used to permit or restrain the flow of fluid and/or adjust the pressure in a system. A
complete control valve is made of the valve itself, an actuator, and, if necessary, a valve control device.
The actuator is what provides the required force to cause the closing part of the valve to move. Valve
control devices keep the valves in the proper operating conditions; they can ensure appropriate position,
interpret signals, and manipulate responses.

When implementing a valve into a process, one must consider the possible adverse occurrences in the
system. This can include noise due to the movement of the valve, which can ultimately produce shock
waves and damage the construction of the system. Cavitation and flashing, which involve the rapid
expansion and collapse of vapor bubbles inside the pipe, can also damage the system and may corrode
the valve material and reduce the fluid flow.

There are four general types of valves.

    1.   Electronic, or electrical valves. The movement of the ball or flap that controls flow is controlled
         electronically through circuits or digitally. These types of valves have very precise control but can
         also be very expensive.
    2.   Non-Return valves. These valves allow flow in only one direction, and are common in various
         industries. When pressure in the opposite direction is applied, the valve closes.
    3.   Electromechanical valves. These valves have electro magnets controlling whether the valve is open
         or closed. These valves can only be fully open or fully closed.
    4.   Mechanical Valves. These valves use mechanical energy in the process of opening and closing the
         actual valve. Larger valves can be opened and closed using mechanical processes such as levers
         and pulleys, whereas smaller mechanical valves can be opened or closed via a turning wheel or
         pulling a level by hand.

A butterfly valve is a valve with a handle in the shape of butterfly like wings, it shuts off with a 1/4 turn of
the valve handle. The mechanism inside is a metal ball with a hole through it, turn it one way and it opens
water, turn it 1/4 of a turn or 90 degrees and the holes are then blocked off by the sides of the valve
body. Very good valve and shuts off water immediately with no leaks, no maintenance. Highly
recommended.

A gate valve is a valve with either an octagnogle shaped handle or round handle. It shuts off the same way
as a normal tap however, a gate valve has no washer of any kind, what shuts the water off is the
mechanism inside the valve which is engineered in such a way that the metal disk inside slides up and
down inside the valve body with a very fine tolerance shutting off the water. However, gate valves have
been known not to shut off water properly either because the tolerance inside is not small enough which
allows water to flow through or when the valve gets old it starts to form deposits on the sliding disk which
prevents it from closing properly. This valve cannot be serviced. Not recommended at all.

Some general features that one can take into consideration when selecting a valve are the following:

    1.   Pressure rating
       2.   Size and flow capacity
       3.   Desired flow condition
       4.   Temperature limits
       5.   Shutoff response to leakage
       6.   Equipments and pipes connected
       7.   Material compatibility and durability
       8.   There is a vast abundance of valve types available for implementation into systems. The valves
            most commonly used in processes are ball valves, butterfly valves, globe valves, and plug valves. A
            summary of these four valve types and their relevant applications is in the table below.


Valve Type Application                                 Other information
Ball            Flow is on or off                      Easy to clean
Butterfly       Good flow control at high capacities Economical
Globe           Good flow control                      Difficult to clean
Plug            Extreme on/off situations              More rugged, costly than ball valve


       Following is a detailed description of the four main valve types.

       Ball Valves

       A ball valve is a valve with a spherical disc, the part of the valve which controls the flow through it. The
       sphere has a hole, or port, through the middle so that when the port is in line with both ends of the
       valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and
       flow is blocked. There are four types of ball valves.

       A full port ball valve has an over sized ball so that the hole in the ball is the same size as the pipeline
       resulting in lower friction loss. Flow is unrestricted, but the valve is larger. This is not required for
       general industrial applications as all types of valves used in industry like gate valves, plug valves,
       butterfly valves, etc have restrictions across the flow and does not permit full flow. This leads to
       excessive costs for full bore ball valves and is generally an unnecessary cost.

       In reduced port ball valves, flow through the valve is one pipe size smaller than the valve's pipe size
       resulting in flow area becoming lesser than pipe. But the flow discharge remains constant as it is a
       multiplier factor of flow discharge (Q) is equal to area of flow (A) into velocity (V). A1V1 = A2V2; the
       velocity increases with reduced area of flow and decreases with increased area of flow.

       A V port ball valve has either a 'v' shaped ball or a 'v' shaped seat. This allows the orifice to be opened
       and closed in a more controlled manner with a closer to linear flow characteristic. When the valve is in
       the closed position and opening is commenced the small end of the 'v' is opened first allowing stable
       flow control during this stage. This type of design requires a generally more robust construction due
       to higher velocities of the fluids, which would quickly damage a standard valve.

       A trunnion ball valve has a mechanical means of anchoring the ball at the top and the bottom, this
       design is usually applied on larger and higher pressure valves (say, above 10 cm and 40 bars).
    Ball valves are good for on/off situations. A common use for a ball valve is the emergency shut off for
    a sink.

    Butterfly Valves

    Butterfly valves consist of a disc attached to a shaft with bearings used to facilitate rotation. The
    characteristics of the flow can be controlled by changing the design of the disk being used. For
    example, there are designs that can be used in order to reduce the noise caused by a fluid as it flows
    through. Butterfly valves are good for situations with straight flow and where a small pressure drop is
    desired. There are also high performance butterfly valves. They have the added benefit of reduced
    torque issues, tight shutoff, and excellent throttling. It is necessary to consider the torque that will act
    on the valve. It will have water moving on both sides and when being used to throttle the flow
    through the valve it becomes a big factor. These valves are good in situations with high desired
    pressure drops.They are desirable due to their small size, which makes them a low cost control
    instrument.



Events that may cause valve failure

Cause: Tuberculation/Debris may have built up in the seat area, particularly in the double disc valve.
Corrective Action: Create flow through the valve (open a nearby downstream hydrant) then exercise the
valve to loosen/remove the debris.
Corrective Action: The valve may be larger than shown on the plans. Continue to operate based on the
next size valve.
Cause: The valve may have a bevel or spur gear to help during operation.
Corrective Action: Check the plans to see if it a gear was installed on the valve. The typical gear ratio is 4
or 4.5 to 1. Multiply the standard number of turns by the gear ratio.
Cause: Excessive torque may have been applied to the operating nut at some point and damaged the
stem or stem nut.
Corrective Action: Expose the valve and inspect the stem and stem nut. Replace if necessary.


The lever operation and the steering wheel operation are done by the hand.
1) Open and close the valve by turning handle smoothly.
(Turn clockwise to close and counterclockwise to open.)
2) In case of lever type (40-200 mm{1 1/2”-8”}), the direction of handle is same as the disc as shown in Fig.
6-1.
・ For the full-shut (Close) position, the handle is perpendicular to the piping axis direction.
・ For the full-opened position, the handle is parallel to the piping axis direction.
3) In case of gear type (40-400 mm {1 1/2”-16”}), the indicator shows the position of the disc on the top of
gear box.
(Fig.6-2)
・ For the full-shut (close) position, the indication shows Shut (S).
・ For the full-opened position, the indication shows Open (O).

				
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Description: health and safety hm pipeline safety construction safety