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VALVES INTRODUCTION Valves are mechanical devices designed to direct, start, stop, mix or regulate the flow, pressure or temperature of a process fluid. The common types of valves availa availabl ble e are gate gate valve valves, s, globe globe valve valves, s, butte butterfl rfly y valve valves s etc. etc. the the mater material ials s commonly used for construction are iron, steel, plastic, brass or a mixture of special alloys. According to their function valves may be classified as on-off valves, non-return valves, and control valves. The on-off valves are used to start or stop the flow through the process. Gate valves and pressure pressure relief valves are examples of onoff type of valves to mention a few. The non-return valves allow the fluid to flow in one part particu icular lar direc directio tion n only. only. The The contr control ol valve valves s are are used used to regul regulate ate flow, flow, temperature or pressure through a system.
ON-OFF VALVES:
GATE VALVES Gate valves are linear motion valves having a closure element perpendicular to the process flow that slides into the main stream to provide shut off. These are used in low-pressure systems. The problem with these valves is that they cannot handle throttling operations, are easily fouled and cannot be used in systems having high-pressure drops. It is difficult to obtain tight shut off with these valves and they take longer to open or close than any other manual valves. The different types of gate valves are parallel gate valves knife edged gate valves and through conduit gate valves.
BUTTERFLY VALVES These valves are mainly used as an on–off valve. It is mainly a rotary motion valve that uses a rotating round disk as a regulating element. There are two types of butterfly valves –concentric and eccentric butterfly valves. These valves can be directly installed in between two flanges without any special end connections owing to their very narrow face-to-face dimensions. It has a large flow coefficient and due to rotary motion of shaft the friction forces generated are far less than a linear motion valve. They have a high pressure recovery factor. These valves are used in low pressure applications. Cavitation and choked flow can occur easily with these valves when installed in an application with high pressure drop.
PLUG COCKS AND BALL VALVES For temperature below 250 C, metallic plug cocks are useful in chemical process lines. As in laboratory stopcock, a quarter turn of the stem takes the valve from fully open to fully closed, and when fully open, the channel through the plug may be as large as the inside of the pipe itself, and the pressure drop is minimal. In a ball valve the sealing element is spherical, and the problems of alignment and “freezing” of the element are less than with a plug cock. In both plug cocks and ball valves the area of contact between moving element and the seat is large, and both can therefore be used in throttling service. Ball valves find occasional application in flow control.
NON RETURN (CHECK) VALVES Non-return valves allow the fluid to flow only in the desired direction. The design is such that any flow or pressure in the opposite direction is mechanically restricted from occurring. All check valves are non return valves. Non return valves are used to prevent back flow of fluids, which could damage equipment or upset the process. Such valves are especially useful in protecting a pump in a liquid application or compressed gas applications from back flow when pump or compressor is shut down . Non return valves are also used in process systems that have varying pressure which must be kept separate. There are two types of check(non-return) valves, swing types and spring types. In the swing type, the pressure of the water forces the valve gate to 'swing' open, but once the flow stops, gravity causes the gate to fall closed , preventing a reversal of the flow. This type of valve must be mounted vertically or horizontally to work properly. In contrast, the gate in a spring check valve is spring loaded. Water pressure forces the gate open just as in the swing type, but when the flow stops, the spring, not gravity, forces the gate closed. This enables the valve to be mounted in any position and at any angle.
GLOBE VALVES A globe valve is a linear motion valve characterized by a globe style body with a long face to face dimension that accommodates smooth, rounded flow passages sufficiently long enough to ensure smooth flow through the valve without any sharp turns. These valves can be used in both gas as well as liquid applications and can handle severe conditions of temperature and pressure. The majority of the globe valves have a top entry design thus permitting a easier servicing of the internal parts and allowing the valves to remain in line when maintenance is taking place. But these valves have certain disadvantages also. They have a high cost and a large size factor and cannot be used for unclean liquids. They are mainly used for flow control and in cases involving vacuum or high temperature extremes.
SOLENOID VALVE: VALVE: Solenoid valves are best suited for small, short-stroke on-off operations requiring very high speed of response. These valves can open or close in 8 to 12 milliseconds. However, they are limited to pressure drops below 20.7 bars although when pivoted with pilot levers or double seats, they can handle higher pressure drops. A solenoid valve contains a valve body, a magnetic core attached to the stem and disc, and a solenoid coil. A small spring assists the release and initial closing of the valve. The valve is electrically energized to open. When an electrical signal is input to a solenoid valve (magnetic changeover valve), the drawing force of the solenoid moves the spool, changing the direction of flow. Because the electrical signal is switched at the valve, remote control and automatic control are simple. Stronger springs are used to overcome the friction of the packing when it is required. Reversing the valve plug causes reverse action (open when de-energized). These valves are quite expensive.
FAIL SAFE SYSTEMS There are two readily available fail-safe schemes for control valves. The choice must be based on detailed knowledge of the valve application in the overall process or system. Two generalizations are that in a heating application, the valve should fail close and in cooling applications, it should fail open. There are applications where either failure mode is equally safe, then considerations of standardization may be used. Fail safe involves the selection of actions of actuator and inner valve. Both actuator and inner valve usually offer a choice of increasing air pressure to push the stem down or up which may open or close the inner valve. The process application of the valve must be investigated to determine whether on instrument air failure it would be better to have the valve go fully open or fully closed.
Air fail to close valve
Bellow Seal Valve Bellow seal valves are a special type of globe valves which are used for fine control of flow. The main difference between the ordinary globe valves and the bellow seal valve is that in the globe valve there is gland packing along the stem but in the bellow seal valve, a bellow is used to prevent leakage instead of the packing. Bellow seal valves are totally leak-proof. They are used to handle corrosive liquids. This type of valve has been used in the butadiene extraction unit.
Stem
Bellow seal valve (globe)
MATERIAL OF CONSTRUCTION OF VALVES The selection of the valve body material is usually based on pressure, temperature, corrosive properties and erosive properties of the flow media. Also the choice of the materials depends on economic factors. Majority of control valves involve non corrosive fluids at reasonable temperatures and pressure. Therefore cast iron and cast carbon steel are most commonly used valve body materials.
MATERIAL 1. Cast Carbon steel
ATSM DESIGNATION ASTM A216 grade –WCB
PROPERTY
2. Cast chromemoly steel
ASTM 217 gradeWC9
3. Cast chromemoly steel
ASTM A217 grade-C5
Have corrosion and creep resistance. Can withstand temp to 566 0C 1.5 times as costly as WCB Popular steel alloy can withstand temp up to 5930C. Resistant to erosion and creep at high temperature.
4. Cast type 304 stainless steel
ASTM A351 grade-CF8
Suitable for use above 5380C and below –101 0C.
5. Cast type 316 stainless steel
ASTM A351 grade CF8
Greater resistance to corrosion, pitting and oxidizing fluids. Slightly expensive than type 304 but provides greater
Can withstand higher temp (>4130C) and pressure than cast iron. Very expensive
USE Control of air, saturated or superheated steam, noncorrosive liquid and gases. For very high temperature application. Used on highpressure steam, oils, gases, petroleum vapors, seawater and other mild corrosives. Used to handle oxidizing or very corrosive fluids. Used to handle oxidizing or very corrosive
6. Cast iron
ASTM A126
7. Cast Cast bron bronz ze
ASTM ASTM B61 B61 and and B62
resistance. Inexpensive, non-ductile material.
fluids Used for valve bodies controlling steam, water and corrosive fluids.
Good resistance to corrosion and suitable for cryogenic temperatures satisfactory materials for steam, air, water, oil, noncorrosive gas and some dilute acid service.
Most frequently used for valve trim parts.
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