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+--- Thread: What is a Ball Valve and How Does It Work? (/showthread.php?tid=127638)
What is a Ball Valve and How Does It Work? - profilll - 01-03-2022
What is a Ball Valve and How Does It Work?
A ball valve is a shut-off valve that allows, obstructs, and controls the flow of liquids, gases, and vapors in a piping system by rotating the ball having a bore inside the valve. The ball is mounted against two seats and has a shaft that connects it to the operating and control mechanism that rotates the ball. When the cross-section of the bore is perpendicular to the area of the flow, the fluid is not permitted to pass through the valve. The fluid flows through from the valve, and the fluid flow rate depends on the area of the bore exposed to the floor.
Ball valves are a type of quarter-turn valve along with plug valves and butterfly valves. They can be operated manually or by using an actuator. The simplest operation of a floating ball valve is through the use of a wrench or a lever manually turned by an operator. Torque is applied to rotate the lever arm by 90° by either clockwise or counterclockwise to open or close the valve. If the lever arm is parallel to the pipe, it indicates that the valve is open. If the lever arm is perpendicular to the pipe, it indicates that the valve is closed.
Ball valves come in many designs and features to satisfy various industrial needs. The standards and specifications for ball valves vary depending on the industry where it is utilized.
The ball is a sphere that has a hole in its center. The hole in its center is called the bore. The bore serves as the flow opening of the fluid when the cross-section of the fluid flow path and the bore is coplanar. Otherwise, the flow is throttled. A ball valve may have a solid ball or a hollow ball. A solid ball has a constant opening diameter throughout its structure, which helps the fluid to smoothly flow at a constant velocity. A hollow ball, on the other hand, has a hollow internal structure, and the space inside it allows more fluid to pass through the valve. However, the larger space creates turbulence and high velocities. A hollow ball is more lightweight and cheap compared to a solid ball.
Shaft
The shaft connects the ball to the control mechanism that rotates the ball. The shaft has seals such as O-rings and packing rings to seal the shaft and the bonnet to avoid leakage of the fluid. The shaft may be manually operated by a lever or a handwheel or operated by an electric, pneumatic, or hydraulic actuation.
Bonnet
The bonnet is an extension of the valve housing that contains and protects the shaft and its packing. It may be welded or bolted to the body. It is also made of hard metal and it covers the opening made from connecting the shaft to the external control mechanism.
Seat
The valve seats provide sealing between the ball and its body. The upstream seat is adjacent to the inlet side of the valve. The downstream seat is found on the opposite side of the upstream seat which is adjacent to the discharge side of the valve.
A one-piece ball valve has a single-piece cast body that houses the internal components of the trunnion mounted ball valve. This eliminates the risk of leakage of the fluid from the valve. One-piece ball valves are the cheapest ball valves and always have a reduced bore. A welded one-piece ball valve is more common but cannot be dismantled for cleaning and repaired once damaged; therefore, it is only used for applications with a low possibility of particle build-up, and where sanitation is not a major concern. On the other hand, screwed one-piece ball valves can be cleaned, serviced, and repaired, but dismantling requires special tools.
The floating ball is the most common ball design in ball valves. The ball is suspended inside the valve and free to move in a lateral direction when the valve is in a closed position. It is sandwiched between two seats that support the valve and hold it in place. The ball is connected to the shaft in a slot on one end while the other end is free. When the valve is in an open position, the shaft connection to the slot at the top of the ball prevents the ball from moving laterally.
The sealing action is only dependent on fluid pressure. During an operation of a floating ball valve, the inlet pressure of the fluid forces the ball to the outlet seat which prevents the fluid from escaping from the valve body. The fluid pressure on the ball and the seats are higher when the ball valve is in the closed position.
Floating ball valves have the simplest design. They come in smaller diameters and are suitable for liquids and gases operating under low to moderate pressures. The application of floating ball valves is limited by the amount of pressure the seats can handle. At high fluid pressure, the seats can be deformed from the pressure exerted by the ball which can affect the sealing characteristics of the valve under low pressure. Furthermore, the torque to rotate the stem depends on the force required to counteract the same fluid force acting on the ball and seals.
In a trunnion ball valve, the ball is supported by an additional shaft at its bottom which is called the trunnion. This holds the ball in its place and limits the movement of the ball to its axis. The ball can only move if the valve shaft rotates. Trunnion ball valves also feature spring-loaded seats. The inlet fluid pressure activates the springs towards the ball held by the trunnion, which creates a tight sealing.
Trunnion ball valves are available in small to large diameters, but it is more expensive than floating ball designs. They can operate efficiently in a wide range of pressures and they are ideal for high-pressure applications since the fluid pressure is also dissipated to the trunnion and the springs of the seats. Hence, they are easier to operate with a lower operating torque or a small actuator.
A vented ball valve is constructed and operates in the same way as a standard fully welded ball valve, except that the vented ball has small orifices drilled into its side. When the valve is closed, the orifice is directed to the outlet side of the valve. The drilled hole is used to vent trapped gases which causes a build-up of internal pressure inside the valve, to prevent leaking, valve failure, and explosion.
Vented ball valves are used in compressed air systems, cryogenic processing and conveying volatile liquids are also referred to as the "cryogenic valve" because of their usefulness in cryogenic processing.
A full bore has a bore diameter similar to the pipe diameter. The flow area for the fluid for full bore valves remains constant, therefore the flow resistance offered by this type is very low. Minimal frictional loss is encountered during fluid flow; hence the pressure drop is low. A high pressure drop in a piping system causes pumping more difficult. However, since the bore diameter should be equal to the pipe size, it requires a larger ball size and housing which makes it more expensive than a reduced bore.
Full bore ball valves are easier to maintain and clean. In pipelines, the pipes are maintained and inspected by an operation called pigging. A spherical or cylindrical device called pigs is allowed to flow in the pipes to detect and remove any build-up without interfering with the fluid inside the pipeline. This operation is possible with an installed full-bore ball valve.
Full bore ball valves are also used in conveying liquids with mixed solids where flow restrictions cause the build-up of particles that can eventually cause separation of the mixtures that will flow through it.
A reduced bore has a bore diameter smaller by a pipe size than the (connection) pipe diameter. The actual reduction is determined by the agreement between the manufacturer and the customer. The flow area for the fluid becomes narrower at the downstream outlet, therefore there are frictional losses that are introduced which result in a pressure drop. Since the amount of flow discharge remains constant, the velocity increases with the decrease in the flow area.
Reduced bore ball valves are more common than full bore ball valves. They are used in applications where product flow rate and turbulence are not potential concerns and particle build-up is not likely to occur. The reduced bore is less expensive than the full bore since it requires a smaller ball size and housing. Compared to other types of valves, the reduced bore ball valves have relatively smaller pressure drop.
A segmented ball valve has a V-shaped notch on its ball. A segmented 3 way ball valve has good flow rate control which depends on the ball rotation. Aside from that, it also has a good shut-off capability. The flow characteristic in a segment ball valve approaches an equal percentage flow characteristic. The flow rate in a segmented ball valve increases exponentially as the ball reaches its fully opened position.
A cavity-filled ball valve has a seat design that fills the gap between the ball and its body. This eliminates the possibility of entrapped media or particle build-up over time around the ball which can cause contamination or blocking of the fluid flow. Cavity-filled ball valves are easier to clean and maintain.
Cavity-filled ball valves are valuable in industries where sanitation is crucial, such as in food, pharmaceutical, and bioprocessing industries. They are ideal in handling solid-liquid mixtures such as slurries.
Multi-port ball valves are used in diverting, combining, splitting, or shutting off multiple fluid streams through the use of a ball with an L-shaped or T-shaped bore segmented through its middle. A flow upstream to the inlet of a multi-port valve can be split into multiple outlet streams. It can split a flow, but cannot distribute the flow to its outlet streams in pre-determined flow rates. It can also join multiple flow streams into a single stream, or simply change the direction of the fluid flow. The schematic diagram below shows possible flow configurations of an L-shaped and a T-shaped multi-port ball valve.
Brass is an alloy of copper and zinc that can be distinguished by its dull yellowish to reddish color, depending on the amount of zinc. It is the most common material for ball valves. Brass is a tough, strong, and durable metal that can withstand high temperatures and pressures. The copper in brass alloy has antimicrobial properties that inhibit the growth and reproduction of microbes on its surface. Brass has good chemical, corrosion, and biofouling resistance. It is inert to most acids, alkalis, and bases, except for solutions with high chlorine content. Chlorine can cause dezincification, a reaction where chloride ions strip away zinc from the alloy, causing a porous structure. Dezincification can drastically reduce the strength of the material.
Brass ball valves are not difficult to fabricate because of their malleability, and they are also easy to cast and weld. They are lighter and cheaper than steel ball valves. They are also easy to assemble in the piping system. Common applications of brass ball valves are in food, chemical, and oil and gas processing, and in conveying gaseous fluids. It is also safe to use in the delivery of potable drinking water.
Stainless steel is a type of steel that contains higher chromium content and some amounts of nickel. The chromium content of stainless steel makes it acquire a superior corrosion resistance. Stainless steel is known for its excellent strength, toughness, and durability. It also retains its strength in high temperatures and pressures.
Stainless steel ball valves are commonly constructed in 304 and 316 Stainless Steel grades. The 304 Stainless Steel has 18% chromium and 8% nickel, while 316 Stainless Steel has 18% chromium and 10% nickel and trace amounts of molybdenum. The combination of nickel and molybdenum makes the 316 Stainless Steel resistant to chlorides.
There are applications where the use of a stainless-steel ball valve is an excellent choice. They are used in swimming pools to handle chlorinated water. In harsh industrial environments such as desalination and petroleum refining plants, they offer better resistance to corrosive chemicals under high temperatures and pressures. In breweries, stainless steel pipes and valves are used to handle wort, a reactive liquid that is extracted during the mashing process.
PVC is a tough, rigid and durable plastic material. Compared to brass and stainless-steel alloys, they generally have lower strength but they are cheaper. They are resistant to corrosion and most acids, bases, and salt solutions. However, they are not resistant to aromatic compounds and hydrocarbons. PVC ball valves have a lower temperature and pressure rating, for up to 150 psi and 140°F, respectively. Application of PVC ball valves includes plumbing, irrigation, and water distribution systems.
Chlorinated PVC (CPVC) is a type of PVC that has been chlorinated by a free radical reaction initiated by UV light. The chlorination of PVC results in higher temperature resistance. CPVC ball valves can handle higher temperatures of up to 200°F.
PP is a tough, durable, lightweight, and flexible thermoplastic made from propylene monomer. It offers good resistance to most acids and bases, but has selective compatibility with organic substances and solvents. Its chemical resistance decreases with increasing temperatures. The maximum operating temperature of PP is 82°C. PP ball valves are suitable for regulating the flow of fluids with a wide range of viscosity. They are used as materials for ball valves in the manufacturing of sugar, fertilizers, chemicals, paper, and others.
PVDF is a high purity, durable, thermoplastic fluoropolymer with high molecular weight. It is synthesized from gaseous vinylidene fluoride monomer by a free-radical polymerization process. It is also resistant to abrasion. This material has excellent chemical resistance that makes it suitable for handling reactive liquids and gaseous substances such as sulfuric acid, hydrochloric acid, nitric acid, hydrocarbons, fuels, and solvents that are aromatic, aliphatic, and halogenated in nature. It is used as a material for ball valves in industries such as water purification, wastewater treatment, and in food and pharmaceutical processing because of its good biofouling properties that is resistant to the growth of microbial films. It also remains unaffected by exposure to sunlight and UV. The maximum operating temperature of PVDF is around 115°C.