Ball Valve - How They Work
dnfsdd845 > 11-24-2021, 05:46 AM
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 floating 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 trunnion 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.
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.
For more information on a flanged connection, read our electric actuator ball valve connection types article.
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.
To understand the working principle of a ball valve, it is important to know the 5 main 1 piece carbon steel 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.
The valve may have two, three or even four ports (2-way, 3-way or 4-way). The vast majority of ball valves are 2-way and manually operated with a lever. The lever is in line with pipe when the valve is opened. In closed position, the handle is perpendicular to the pipe. The ball valve flow direction is simply from the input to the output for a 2-way valve. Manually operated ball valves can be quickly closed and therefore there is a risk of water hammer with fast-flowing media. Some ball valves are fitted with a transmission. The 3-way valves have an L-shaped or T-shaped bore, which affect the circuit function (flow direction). This can be seen in Figure 3. As a result, various circuit functions can be achieved such as distributing or mixing flows.
The handle is connected to the valve stem (Figure 2 number 1) and is capable of turning the valve from the open or closed position (90 degrees). If installed correctly, the valve will be open when the handle is parallel to pipe and closed when the handle is perpendicular to the pipe. Taking note of the handle direction is important to visually know if the valve is open or closed. There are additional 1 piece ball valve handle types, like lockable handles or ball valve handwheels. These operate as their names suggest. If you have a bigger ball valve or need additional torque to open or close the valve, a ball valve handle extension may be required. If your handle breaks, is miss-placed or you are converting an automatic ball valve to a manual one, you can buy ball valve handle replacements.
Instead of a manual handle operation to turn the valve on or off, some valves can be fitted with an electric or pneumatic actuator as seen in Figure 4. They connect directly to the valve stem (Figure 2 number 1) and are capable of turning it a quarter turn. The most common flange connection between the valve and actuator is the ISO 5211 standard. Figure 5 shows an example of an ISO 5211 top ready to be connected to an actuator. By using an actuator, you can control your ball valve remotely or through a controller so that it can be used as an automatic shut off. A spring actuated ball valve, also called spring loaded, use a spring to open/close the valve in a power-off scenario and an actuator to then hold it in the open/close position. These are used for energy conservation applications or for fail-safe reasons.
Brass has the largest market share (Figure 6). Brass is an alloy of copper and zinc and has good mechanical properties. Brass valves are used for (drinking) water, gas, oil, air and many other media. Chloride solutions (e.g. seawater) or demineralized water may cause dezincification. Dezincification is a form of corrosion where which zinc is removed from the alloy. This creates a porous structure with a greatly decreased mechanical strength. A brass housing is ideal for an air ball valve. View our manual 2-way brass ball valves or 3-way brass ball valves.
Stainless steel is used for corrosive media and aggressive environments (Figure 7). They are therefore often used in seawater, swimming pools, osmosis installations, with high temperatures, and many chemicals. Most stainless steel is austenitic. Type 304 and 316 are the most common, 316 has the best corrosion resistance. 304 is sometimes referred to as 18/8 because of 18% chromium and 8% nickel. 316 has 18% chromium and 10% nickel (18/10). Stainless steel valves usually require a higher operating torque than for example brass or PVC valves. This must be taken into account when a stainless steel valve is operated by an electric or pneumatic actuator. View our manual 2-way stainless steel ball valves or 3-way stainless steel ball valves.
PVC often has a lower price (except for ISO-top valves) and are widely used in irrigation, water supply and drainage or corrosive media (Figure 8). PVC stands for polyvinyl chloride. PVC is resistant to the most of the salt solutions, acids, bases, and organic solvents. PVC not suitable for temperatures higher than 60 °C, and is also not resistant to aromatic and chlorinated hydrocarbons. PVC is not as strong as brass or stainless steel, therefore PVC ball valves have lower pressure rating. A more in-depth article about PVC ball valves can be read here.
Most valve seats are made of PTFE (Teflon). PTFE stands for PolyTetraFluorEthylene. This material has a very good chemical resistance and a high melting point (~327°C). Besides that, the friction coefficient is extremely low. A small disadvantage of PTFE is that the material shows creep, which can cause a deterioration of the sealing over time. Besides that, PTFE has a rather high thermal expansion coefficient. A solution for this problem is to use a spring in order to apply a constant pressure on the Teflon seal, like for example a cup spring. Other popular sealing materials are enforced PTFE and Polyamide (Nylon). The harder the material of the valve seat is, the more difficult it is to maintain proper sealing. For some application in which soft materials are not possible to use, for example with very high temperatures, metal or ceramic valve seats are used.
Approvals
For certain applications, approvals are desired or required. Drinking water and gas are the most common. Choosing a certified 2 piece floating ball valve, assures that the product meets important safety requirements.
Drinking water
These ball valves are suitable for drinking water applications and have a WRAS, KIWA or DVGW approval. If being used with a water holding tank, they often work in connection with a float switch to monitor water level.
Ball valves are widely used for large size piping in the subsea manifolds. In addition, small bore ball valves in size ½″ and ¾″ can be used in subsea distribution units on the piping system to be used for delivering chemicals and hydraulic oil. This chapter reviews the application, design features, and design standards of the subsea ball valves. The concept of double piston effect seat design as well as double isolation and bleed (DIB) type 1 and 2 is explained in this chapter. Different components of a typical ball valve with the possible materials have been reviewed in this chapter. Different quality control methods and approaches are briefly explained. In addition, special ball valves such as three port or three way ball valves are also explained in this chapter.
A ball valve consists of a valve body in which a large sphere with a central hole equal to the inside diameter of the pipe is mounted. As the ball is rotated, in the fully open position the valve provides the through conduit or full bore required for unrestricted flow of the fluid and scrapers or pigs. Compared with a gate valve, a ball valve has very little resistance to flow in the fully open position. When fully open, the L/D ratio for a ball valve is approximately 3.0. The ball valve, like the gate valve, is generally used in the fully open or fully closed positions. A typical ball valve is shown in Figure 12.10.