Vanaların sınıflandırılması ve imalat proseslerinin incelenmesi

Abstract

Basınç taşıyan bir eleman olan vanalar, içinden geçen akışkanın akışını kontrol ederler. Vanalar kullanım yerlerine bağlı olarak bir veya bir çok fonksiyonu yerine getirmek durumundadırlar. Bu nedenle vana kullanıcıları, vana seçimi yaparken bir çok faktörü göz önünde bulundurarak, ihtiyaçlarına uygun bir vana tipini ve malzemesini belirlemektedirler. Vanalar bir çok özelliklerine bağlı olarak sınıflandırılabilir. Örneğin kumanda milinin hare ketine göre kapama elemanının tipine göre, gövde şekline göre vs. Çeşitli uluslararası normlarla, vanaların ve vana parçalarının temel boyutları, dizayn tipleri, malzemeleri, kalite kontrol esasları belirlenmiştir. Vanalarda diğer önemli bir husus olan sızdırmazlığın sağlanması için çeşitli sızdırmazlık eleman ve sistemleri geliştirilmiştir. Aynı şekilde vanaların müsade edilebilen sızdırma miktarları ilgili normlarla belirlenmiştir. Her geçen daha yüksek basınçlar, daha geniş çalışma sıcaklık, alanları, vana dizaynını sürekli bir gelişim içinde bulunmasını gerekli kılmıştır. Ayrıca malzeme konusunda yapılan her bir yenilik, vana imalatınada yansımıştır. Son yıllarda termoplastik vanalar büyük bir oran da kullanılmaya başlamış ve çok özel ihtiyaçları yerine getirmek amacı ile komposit malzeme kullanımı da vanacı- lıkta yer almıştır. Çeşitli tezgah üreticileri vana parça imalatı için özel amaçlı tezgahları piyasaya sürmektedirler.

The process industries, including chemical and petroc heraical manufacturing, oil refining, pulp and paper, pharmaceuticals, foods, and many other industries that directly or indirectly provide energy, medicines, nutrition, have one thing ain common, they all process liquids and they all use valves to do so. Values are used to permit or restrict the flow of a fluid, modify its rate, blend or distribute a fluid, iso late equipment, fill tanks or serve as safety devices in the event of a fire or other emergency. Today, industry's requirements continue to force an evalution in valve design. Higher pressure, in a wider range of temperatures; exotic, hazardous, and expensive fluids; automation and simplicity are all contributing to tomorrow's valve design. The vaves most commonly used throughout industry may be categorized as linear or rotary. Linear Valves Rotary Valves Gate Ball Globe Plug Diaphragm General-purpose butterfly Pinch High-performance butterfly What is a valve? A valve is a pressure-containing mechanical device used to shut off or otherwise modify the flow of a fluid that passes through it. The action of a valve is caused by moving the closure element (ball, plug, gate disc), which is attached to a stem located outside of the body. A valve has a body to contain the fluid pressure, a closure element toshut off or modify the flow through the body, and finally a stem to change the control position of the closure element. vxi The basic valve func tions can be classified as follows: 1- On-off service; this is basically a flow/no flow service, and this function is the most common use for valves. 2- Flow control; valves in this service are used to modify the flow of a fluid by changing the flow rate or creating a pressure drop to meet the process requirements. 3- Divert flow; the flow of a fluid may be diverted or distributed with the use of multiported valves. 4- Two-stage shutoff; batch (quantity) control systems require two-stage control. This may be achieved with the use of two vaves arranged in parallel by pass system. 5- Blending; a common application is for temperature control on a heating and cooling system in commer cial buldings where hot and cold water are blended. When making a valve selection, we should know which performances we desire for our system to select the best valve for the job. Some important questions to ask: What is the function of the valve? On, off only? Control? Frequent or infrequent operation? How will it be operated: manually, pneumatically, or electrically? What is the process medium? is it corrosive or abrasive? What is its volume and velocity? Is the valve being considered available in all materials? What is the process pressure and temperature? What is the pipe size? How fast must it operate? Should the valve be able to be maintained in line? How much should it cost? Is there any restriction about weight? A valve user should be cautious of low cost; it may mean cheap construction. Materials specification is an important step in valve selection, becouse material of a valve is effective on valve life, performance and cost. Material compatibility with a given process fluid and Vlll the physical ability to withstand wear, pressure drop and erasive fluids are basic considerations in material selec tion. We should know that a valve can achieve no more than its material will allow. "With each new, devolopment in materials or their properties the ability^ of valves to handle more severe applications is improved. Material selection is often quite simple for nonagg- ressive fluids. But even water must be carefully analyzed before a material is selected. Each combination of body, trim, and seal materials may provide desirable performance on one hand but undesirable performance on the other hand. Valve bodies and closure elements can be constructed of about any material that may be cast, forged, molded or welded. This includes iron, steel, aluminum, bronze, nickel, titanium, and many other metals and alloys. They can also be made from a variety of plastics and even ceramics. In some cases a corrosion-resistant metal may be too costly for a particular service. Of ten. a plastic or elastomer lining may be available. The strength of an iron or steel body in combination with the uniaue properties of the liner material is sometimes an excellent cost-saving alternative. Bronze is widely used in gate valves and fittings for temperatures up to 230°C. It is an alloy of copper, tin, lead and zinc. Cast iron is reqularly made in three grades- cast iron, ferrosteel-and high tensile iron. These metals are recommended for temperatures up to 230 C. Steel is recommended for high pressures and temperatures and services where working conditions, either internal or external, are too severe for iron and bronze. Stainless steel is usually used for corrosive, adhesive and aggresive fluids because of its chemical resistance. Most valve manufactures publish their material selec tion guides. When selecting a valve material, many variab les enter into the question. His also important to remember that not all valves are available with all materials. Economics generally becomes an important factor in valve selection. One of the criteria in making the proper valve selection is to ensure that it has a lifelong valve. To do so it is important to consider the initial purchase price of the valve, any anticipated maintenance, and the expected valve life for a given application. Depending on the frequency of required maintenance, service cost can became an important factor in the lifet ime cost of ownership of a given valve. IX Let's take a look at thevalve types. In general, the re- are two main classifications for valves. 1) Linear 2) Rotary-shaft valves. 1) Linear-Shaft Valves: A linear-shaft valve provides shut off or control by raising or lowering the closure member into or out of the flow path. Valves considered linear-shaft valves are gate, globe, pinch and diaphragm valves. A major feature of the linear shaft valve is that tighter shutoff, within the limits of the materials and design, can be achieved by tightening down on the threaded stem. Gate valve is commanly used as a stop valve to turn on or shutt off flow. It gets name from the gatelike disc which operates at a right angle to the flow path. The flow through a globe valve doesn't follow a linear path which causes increased resistance to flow and a consi derable pressure drop. Because of the seating arrangement, globe valves are the most suitable for throttling flow. The valve gets its name from the globular shape of the body. A pinch valve is constructed of a flexible tube section that provides on-off or throttling control by pinching off the flow. The pinching action is provided by a pinch bar that is forced against the tube by threading a linear shaft. A Diaphragm valve controls fluid like a pinch valve does. The shaft of a pinch or diaphragm valve is completely isolated from the fluid. Therefore the design eliminates stem leakage. 2) Rotary-Shaft Valves: In general, the advantages of rotary valves are that they are simple, lightweight, easy to automate, and easy to maintain and they come in a variety of materials and connections. They are also available in a wide range of sizes. Plug, ball and butterfly valves are the major types of rotary valves widely used in industrial plants for flow control in fluid-handling systems. Quarter-turn valves move from fully open to fully closed position with a 90 rotation of the closure member. x Plug valves are the oldest type of valves in use. A rotary cylindrical or tapered plug with an opening block or allows the flow of a fluid. When the plug is rotated, it permits flow through the openning of plug. a quarter turn in either direction completely blocks the flow path. Plug valves, like other rotary shaft valves, require minimum installation space and they are simple to operate. They have little internal disturbance to flow, therefore their pressure drop across the valve is low. Ball valves are basically the adaptation of plug valves. Instead ' of a plug, it has a ball with a hole through one axis which connects the inlet and outlet ports in the body. In addition to quick, quarter- turn on-off operation, ball valves are easy to maintain, require no lubrication, and give tight sea ling with low operation tork. They are recommeded for on-off service, minimum resistance to flow, and moderate temperature service where low maintenance is required. They have also several construction designs. Butterfly valves are built on the principle of a pipe damper i The flow control element is a disc, which has approximately the same diameter as the inside diameter of the adjoining pipecand rotates on a horizontal or vertical axis. When disc is parallel to flow, the valve is shut. Throtting can be achieved by rotating the disc to an intermediate position and locking 1Sb handle in this-positiaa.- Butterfly ^valves in larger sizes have weight, space, and inital cos.ta-dvantages over gate, globe plug and ball valves. Maintenance costs are low because of the minimum number of moving parts. They have different kinds of body designs. As an example, let us observe the production line of a piston valve produced by Klinger-Yakacxk Valve Casting and Manufacturing Company. The important dimensions which effect valve operation and the manufacturing operations of each part are given. The cutting conditions and some special tools which are used in the production process are also presented. The information is provided about mashine prog ramming of a CNC lathe.