Trunnion Ball Valve - How They Work

Figure 1: Free floating ball valve (left) and a trunnion mounted ball valve (right)

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Trunnion ball valves are quarter-turn valves used to control the flow of media in a pipeline. Unlike the more common "free floating" ball valve design, trunnion ball valves have a unique configuration. In a trunnion ball valve, the ball is secured by connecting it to the stem on one side and a shaft called the trunnion on the other, ensuring the ball remains firmly in place. In contrast, a floating ball valve connects the ball only to the stem, allowing it to "float." This distinction is illustrated in Figure 1, highlighted by a red box.

Trunnion ball valves have many different advantages over free floating valves, they are suitable for low, and high-pressure applications, and offer lower operational costs then free floating ball valves.

Trunnion ball valve advantages

High pressure applications

For high pressure applications, a trunnion ball valve is the ideal ball valve design type. In the floating ball valves the ball has some freedom of movement. During use, the ball floats slightly downstream against the seat ring and creates a sealing only on one side of the valve. In addition, the torque on the stem is also increased.

This is the point where the trunnion design offers a solution. In the trunnion valve design, the ball is not floating as it is connected to the trunnion. This is advantageous since it will maintain a proper seal on both sides and there is no added torque to the stem. Therefore, for higher pressure applications a trunnion ball valve will provide better sealing and require less torque to turn.

Figure 2: A cross section of a trunnion ball valve, clearly showing the trunnion (A), and spring mounted seats (B)

Spring-loaded seats

Trunnion mounted ball valves are fitted with spring loaded seats. In a floating ball valve the pressure forces the ball against the seats, which creates a seal. Unfortunately the ball valve will need high pressure to create an effective seal, which would render them ineffective in low pressure systems.

Trunnion mounted ball valves are effective in both low and high pressure situations. In low or no-pressure situations the spring-loaded seats will create a seal, while also working for high pressure applications.

Lower operational costs

A trunnion mounted ball valve provides lower operational costs due to the fact that any additional pressure on the valve is absorbed by the trunnion and the stem. This leads to a lower operating torque, and subsequently eliminates the need for a large actuator.

To learn more about making the right selection of ball valve for your application, read our ball valve selection technical article!

Trunnion mounted ball valve weight

A trunnion ball valve is designed by API608 or API 6D and complies with several other industry standards. API 6D gives basic design, dimensions, and testing standards for the trunnion valve. The API 6D standard also gives the standard weights of trunnion ball valves corresponding to different bore sizes and body dimensions.

Trunnion double block and bleed (dbb) ball valves

Double block and bleed (DBB) is the process of shutting a pipe section on both sides of the valve rather than just one. Therefore a trunnion ball valve can block both the upstream and downstream sides of the working area and then vent (bleed) any pressure that remains in the piping and valve. A double block and bleed (DBB) is often used for stricter isolation of process fluid from other equipment. It usually consists of two trunnion ball valves that act as block valves and a bleed valve (usually ball or needle valve) to a safe location, consistent with local environmental regulations.

A DBB valve is a side-entry ball valve designed for downstream, midstream, and upstream gas and oil applications with either a welded or bolted body. The valve has two seats, each providing a seal against pressure separately. One seat seals the fluid from upstream, and the other seat seals the fluid from downstream. For a DBB trunnion mounted ball valve, when the valve is in the closed position, and the fluid comes from upstream, it pushes the seat against the ball to release the sealing.

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When the pipeline is under pressure, the trunnion ball valve is closed. The fluid remains trapped within the ball, which creates a resultant pressure in the ball. The pressure within the ball rises when the temperature in the ball becomes higher. It is necessary to vent/bleed the pressure when the pressure in the ball becomes high enough. For a trunnion ball valve with a bleed function, the pressure in the ball pushes the seat back as the spring behind the seat compresses, and there is vent/bleeding between the sealing surfaces. According to API 6D specifications, the venting/bleeding is required at a pressure of less than 1.33 times the design pressure rating for the ball valves. However, several critical projects need to vent at a pressure less than 1.1 times the originally designed pressure.

Learn more about ball valves in our extensive ball valve guide.

Figure 3: Double block and bleed valve

&#;&#;Floating ball&#;&#; and &#;&#;trunnion ball&#;&#; are concepts which are used generally. But what are the exact difference between these two designs and when to use which one?

The most important difference between these two design is the construction of the ball and the way in which it is assembled inside the valve body. A trunnion ball is attached and centred inside the valve body through both a top shaft -the valve stem- and a bottom shaft &#; the trunnion. A floating ball is attached to the valve body only through the valve stem. As a result, the floating ball &#;&#;floats&#;&#; in the valve seats.

In a floating ball design the ball is pushed against the downstream seat by the in-line pressure, resulting in tightness. When operated from closed to open position, the ball is to be rotated against both the in-line pressure (&#;p) and the friction of the seats. In other words: the torque needed to operate the valve is created by both in-line pressure and the nature of the valve seats. The amount of torque required increases significantly when operating pressure (&#;p) and/or valve size increase, and/or whenever the nature of the seat is made more robust. The latter applies in case of a metal seated valve design.

In a trunnion design, the ball is inserted in a central bottom shaft which is called the trunnion. The ball is fixed between the stem and the trunnion, which inclines that the ball is not floating but fixed and centred. The inline pressure presses the seats against the ball, causing the tightness. This inclines that during operation, the ball does not have to be rotated against the in-line pressure (&#;p) and the valve seats, but that is solely needs to be rotated against the pressure of the seats.

As a result, the required torque of a trunnion mounted ball valve is generally lower than the torque required of a comparable floating ball valve. For example: a DN200 metal-seated floating ball valve would require a significantly larger actuator than a DN200 comparable trunnion valve, leading to significantly lower costs of the overall package. Also, in general the trunnion seat design offers higher stability which makes it more suitable for extreme conditions and especially varying pressure levels.

So, the trunnion-mounted ball valve is more suitable for high pressure applications and bigger dimensions compared to the floating ball. Another advantage of the trunnion design vis-à-vis the floating design is the fact that a trunnion generally is included with a drain or bleed connection, making it suitable to function as a dual safe device. Furthermore, it functions as an relief valve automatically whenever the pressure in the central cavity is higher than the spring force of the seats. When this happens, the seat springs relieve automatically in order to drain the excess pressure back into the main line. Because of these reasons, the trunnion is commonly used in offshore- & oil & gas applications, where extreme conditions pose the standard.

Off course, a large disadvantage of the trunnion compared to the floating design is associated with its costs; which are significantly bigger. Because of these costs, trunnions are used solely when they have to be used.

Our specialist happily assist you in advising the right ball valve design for your application.

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