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Isolation Valves - Linear Movement : International Site for Spirax Sarco

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  09/10/2014 11:42Isolation Valves - Linear Movement : International site for Spirax SarcoPage 1 sur 9http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-linear-movement.asp International site for Spirax Sarco Tel : +44 (0)1242 521361 Fax : +44 (0)1242 573342Enquiries@SpiraxSarco.comhttp://www.SpiraxSarco.com/  Isolation valves are used for diverting processmedia, facilitating maintenance, equipmentremoval and shutdown. The operation, applicationand construction of gate, globe, piston anddiaphragm valves are studied in this tutorial. Use the quick links below to take you to the mainsections of this tutorial: Contact Us The printable version of this page hasnow been replaced by The Steam and Condensate Loop Book View the complete collection of SteamEngineering Tutorials  You are here: HomeResourcesSteam Engineering TutorialsPipeline Ancillaries Isolation Valves - Linear Movement Isolation Valves - Linear Movement Isolation valves are a key component in any fluid system as they are used to stop the flow of fluid into aparticular area of the system. They are also sometimes used to manually control the flow of the fluid. TheEuropean standard EN 736-1:1995 distinguishes between isolating, regulating and control valves as follows: Isolating valve -  A valve intended for use only in the closed or fully open position. Regulating valve -  A valve intended for use in any position between closed and fully open. Control valve -  A power-operated device which changes the fluid flowrate in a process controlsystem.Isolation valves are used in a wide variety of different applications where on/off type control is required, theseinclude:Diverting process media.Flow isolation to:- Facilitate maintenance- Allow the removal of equipment- Allow the shut down of plant A multitude of different types and designs of isolation valve have been developed in order to meet this rangeof applications and the diverse operating conditions in which they are used. Valves are commonly classifiedinto two groups (see Table 12.1.1), according to the operating motion of the closure device (or obturator): Linear movement valves - The obturator moves in a straight line. Included in this category are gatevalves, globe valves, diaphragm valves and pinch valves. These valves are covered in greater depthwithin this tutorial. Rotary movement valves - The obturator rotates about an axis at right angles to the direction of flow.Ball valves and butterfly valves are the two most important rotary valves associated with steamapplications and are covered in greater depth in Tutorial 12.2, Isolation Valves - Rotary Movement. Pipeline Ancillaries Isolation Valves - Linear Movement Isolation Valves - RotaryMovementCheck ValvesStrainersSeparatorsGauges, Sight Glasses,Vacuum Breakers Related Content Globe Valves Bronze bodied globe stopvalves in sizes to suit your application. Bellows Sealed StopValves Bellows sealed designensures stem seal leaksare totally eliminated. Valve Sizing Calculator  Use the calculator to sizeyour isolation valves. The Steam andCondensate Loop Book  A comprehensive bestpractice guide to savingenergy and optimisingplant performance, thisbook covers all aspects of steam and condensatesystems. Feature   HomeAbout UsProducts & ServicesIndustries & ApplicationsTrainingResourcesContact  09/10/2014 11:42Isolation Valves - Linear Movement : International site for Spirax SarcoPage 2 sur 9http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-linear-movement.asp Table 12.1.1 Obturator motion in the basic valve types Linear movement valves Linear movement valves have been developed from the early forms of sluice gates used to control the flow of water in irrigation channels. Since then, a large number of different designs and types have been developedfor use in almost every type of flow application. Although linear movement valves are characterised bystraight-line obturator movement, the flow of the fluid may be at right angles to this movement (as in the caseof gate valves), or in the same direction, as with globe valves. The main feature of the linear movement valveis that tight shut-off may be achieved by tightening down the obturator on a threaded stem. Gate valves Gate valves are probably the most common valves in use today due to their widespread use in domesticwater systems, but it should be noted that their popularity in industry has declined in recent years. However,they are still used where an uninterrupted flow is required, because the gate fully retracts into the bonnet,creating a minimal pressure drop, when the valve is in an open position. Gate valves are specifically intendedfor use in isolation applications. A gate valve consists of four main components, the body, bonnet (or cover), gate and stem. A typical gatevalve is shown in Figure 12.1.1. Order your copy today  09/10/2014 11:42Isolation Valves - Linear Movement : International site for Spirax SarcoPage 3 sur 9http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-linear-movement.asp Fig. 12.1.1 Typical wedge gate valve The gate, which slides between the seats, is lifted in a direction at right angles to the flow until clear of theflow path. The fact that the gate fully retracts into the bonnet ensures that the pressure drop across the valveis low. Gate valves are divided into a number of different classes, depending on the design of the gate and itsseating faces. Solid wedge gate valve The gate is wedge shaped and it seats on corresponding faces in the valve body. The mechanical advantageof the activating thread, together with the wedge angle, enables adequate seating forces to be appliedagainst the fluid pressure without excessive handwheel effort. The seat can sometimes be coated with PTFEto assist a high integrity shut-off. A typical solid wedge gate valve is shown in Figure 12.1.1. Flexible wedge gate valve  Although there are several types of flexible wedge gate valves, they all make use of a flexible two-part disc,which is shaped like two wheels on a very short axle. The flexibility of the disc ensures tight seating over awide range of temperatures and pressures.The most common type of flexible wedge gate valve used in steam applications is the parallel slide valve.The two plates that constitute the gate are held against the seat by a spring, encased between them. Thefluid pressure moves the upstream disc off its seat, and the force is transferred onto the downstream disc,thereby ensuring a tight shut-off. The high degree of flexibility in the gate allows for expansion andcontraction when subjected to temperature variations, making it suitable for use in steam systems. Globe valves Globe valves constitute a major class of linear movement valves; they have become more popular than gatevalves as there is a wide variety of configurations available to suit most applications. The movement of fluidthrough the valve seat is longitudinal to the operating motion of the obturator; this means that for a valve inwhich the inlet and outlet are horizontally opposed, the fluid must follow a changing course. The mainadvantage of this arrangement is that a globe valve opens more rapidly than a gate valve as the disc onlyneeds to move a small distance from its seat to allow full flow. This is an advantage when there is frequentoperation of the valve. The disadvantage is that the fluid has to change course, increasing the resistance toflow and generating turbulence. This results in a higher pressure drop across a globe valve than a gate valve.  09/10/2014 11:42Isolation Valves - Linear Movement : International site for Spirax SarcoPage 4 sur 9http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-linear-movement.asp Fig. 12.1.2 A conventional globe valve Globe valves are less likely to leak than gate valves, which means that they can be used for higher pressureor higher volume applications, for example in steam systems, or where fluid loss can be hazardous or costly.The increased cost of globe valves over gate valves is therefore offset by the additional safety they provide,and a reduced chance of fluid loss.The pressure of the fluid acting over the area of the disc generates an axial load on the stem. This makesclosing the valve difficult, so much so, that it limits the size of a standard globe valve to DN250. On highdifferential pressure closed systems, balancing plugs can be used to overcome this effect, allowing valveswith a nominal diameter of up to 500 mm to be used (Figure 12.1.3(a)). The balancing plug contains a pre-lifting plug that acts as a pilot valve. When the valve is opened, the pre-lifting plug opens first, allowing themedium to pass through it at a controlled rate (Figure 12.1.3(b)). This reduces the differential pressure across the valve, enabling the disc to be easily liftedoff its seat (Figure 12.1.3(c)). To assist closing of the valve, isolation valves fitted with a balancing plug haveto be fitted in reverse so that the top of the plug is acted on by the upstream pressure.
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