Attività

Ball Valve – How They Work

A ball valve is a shut off

valve that controls the flow of a liquid or gas by means of a rotary ball having a bore. By rotating the ball a quarter turn

(90 degrees) around its axis, the medium can flow through or is blocked. They are characterized by a long service life and

provide a reliable sealing over the life span, even when the valve is not in use for a long time. As a result, they are more

popular as a shut off valve then for example the gate valve. For a complete comparison, read our gate valve vs ball valve

article. Moreover, they are more resistant against contaminated media than most other types of valves. In special versions,

ball valves are also used as a control valve. This application is less common due to the relatively limited accuracy of

controlling the flow rate in comparison with other types of control valves. However, the valve also offers some advantages

here. For example, it still ensures a reliable sealing, even in the case of dirty media. Figure 1 shows a sectional view of a

ball valve.

Standard (threaded)

Standard ball valves consist of the housing, seats, ball and lever for ball rotation. They include valves with two, three

and four ports which can be female or male threaded or a combination of those. Threaded valves are most common and come in

many varieties: with approvals for specific media or applications, mini ball valves, angled ball valves, ISO-top ball valves,

with an integrated strainer or a bleed point and the list goes on. They have a wide range of options and a large operating

range for pressure and temperature.

For more information on a threaded connection, read our ball valve connection types article.

Hydraulic

Hydraulic ball valves are specially designed for hydraulic and heating systems due to their high operating pressure

rating and hydraulic and heating oil resistance. These valves are made of either steel or stainless steel. Besides these

materials, the seats also make hydraulic valves suitable for high operating pressure. The seats of these valves are made of

polyoxymethylene (POM), which is suitable for high pressure and low temperature applications. The maximum operating pressure

of hydraulic ball valves goes above 500 bar while the maximum temperature goes up to 80°C.

Ball valves are used for both on/off and throttling service. Ball valves are similar to plug valves but use a ball-shaped

seating element (Figure 4.56). They are quick-opening and require only a quarter-turn to open or close. They require manual

or power operators in large sizes and at high operating pressures to overcome the operating torque. They are equipped with

soft seats that conform readily to the surface of the ball and have a metal-to-meal secondary seal. If the valve is left

partially open for an extended period under a high pressure drop across the ball, the soft seat may become damaged and may

lock the ball in position. Ball valves are best suited for stopping and starting flow but may be used for moderate

throttling. Compared with other valves with similar ratings, ball valves are relatively small and light.

Flanged

Flanged ball valves are characterized by their connection type. The ports are connected to a piping system via flanges

that are usually designed in accordance with a certain standard. These valves provide a high flow rate since they typically

have a full-bore design. When choosing a flanged ball valve, besides the pressure rating, you also have to check the flange

compression class which indicates the highest pressure this connection type can withstand. These ball valves are designed

with two, three or four ports, they can be approved for specific media, have an ISO-top and everything else a standard

quarter turn valve could have. They are typically made out of stainless steel, steel, or cast iron.

Vented

Vented ball valves look almost the same as the standard 2-way ball valves when it comes to their design. The main

difference is that the outlet port vents to the environment in closed position. This is achieved by a small hole that is

drilled in the ball and in the valve body. When the valve closes, the holes line up with the outlet port and release the

pressure. This is especially useful in compressed air systems where depressurization provides a safer working environment.

Intuitively these valves look like 2-way ball valves while in fact they are 3/2-way due to the small borehole for venting.

Ball valves are not recommended for FO applications. Generally, it is possible to reduce the opening time of the fail

open actuated valve by installing a quick exhaust valve on the control panel to release the instrument air from the pneumatic

actuator in the fail mode quickly. However, a ball valve’s seat and disk are in contact during the opening and closing,

which can jeopardize FO. In addition, moving the relatively large and heavy ball requires a higher stem torque, a larger

actuator, and perhaps a longer opening time. The ball valve manufacturer was asked about the possibility of using a soft seat

ball valve for this application. The manufacturer believed that FO of the soft seat ball valve in 2 s could cause damage to

the soft seat because of the very quick contact with the ball. On the other hand, the manufacturer stated that a 2-s opening

time can be achieved with a metal seat ball valve. But a metal seat has the disadvantage of possible leakage, unlike a soft

seat, and it is a more costly solution than butterfly and axial control valves due to the valve and the large mounted

actuator.

Unlike FO applications, a ball valve is a good choice as a blowdown valve with less opening time than an FO valve.

Fig. 12.25 shows a blowdown ball valve to release the overpressured fluid from the equipment in an emergency mode. The

blowdown ball valve is an 18″ Class 2500 in a 6MO body and a metallic Inconel 625 seat, which may need 18 s for opening.

Blowdown or FO valves on flare lines usually see low operating temperatures because of the released gas pressure drop. Gas

pressure drop reduces the operating temperature to ? 46°C or even lower, so the minimum design temperature is typically

below ? 100°C. The low temperature application makes it impractical to use 22Cr duplex with a minimum design temperature of

? 46°C for the valve, so 6MO or Inconel 625 are the correct choices of materials. An extended bonnet is used for the valve

to keep the packing away from the relatively cold service, similar to cryogenic valves.

Ball valve working principle

To understand the working principle of a ball valve, it is important to know the 5 main ball valve parts and 2 different

operation types. The 5 main components can be seen in the ball valve diagram in Figure 2. The valve stem (1) is connected to

the ball (4) and is either manually operated or automatically operated (electrically or pneumatically). The ball is supported

and sealed by the ball valve seat (5) and their are o-rings (2) around the valve stem. All are inside the valve housing (3).

The ball has a bore through it, as seen in the sectional view in Figure 1. When the valve stem is turned a quarter-turn the

bore is either open to the flow allowing media to flow through or closed to prevent media flow. The valve's circuit

function, housing assembly, ball design, and operation types all impact the ball valve's operation are are discussed

below.Circuit function