Bi-Directional Ball Valves Trunnion vs Floating Designs This document describes the differences between bi-directional floating ball valves and trunnion ball valves used in the oil and gas industry.

 

 

Anyone in the oil and gas industry must know how to choose the best ball valve for their project. Two of the most common types are floating ball valves and trunnion ball valves. Each has its advantages, making it essential to understand their differences.

 

What are the differences between the trunnion and floating ball valves?

 

Ball Valves are 1/4 (90°) turn valves using a spherical ball with a borehole to effect closure through the valve body.  The ball valve uses seats on each side of the ball to create a reliable seal between the ball and the valve body.  Seat-to-body sealing is commonly produced using o-rings, lip seals, or both.  This entire assembly makes ball valves an excellent choice for shutoff and control applications.

 

All ball valve designs must create contact stress between the ball surface and the seat face to affect a seal. This contact stress must exceed the pressures contained for reliable valve sealing. Different ways are available to achieve this sealing force.

 

The functional distinction between floating ball designs and trunnion ball designs is that the floating design uses the ball to create a seal on the downstream side of the valve.  It achieves this by using upstream pressure to force the ball against the downstream seat.  For the ball to achieve this force in a bi-directional valve, it must be free to move or float, creating contact stress from one side to the other depending on the direction of the pressure.

 

The trunnion ball design creates its seal on the upstream side of the valve using pressure to force the upstream seat against the ball, which is held rigidly in the body with trunnion supports.  Trunnion designs may have either internal or external trunnion supports that hold the ball ridged in the body, while the seats are forced against the ball by either spring pressure, line pressure, or both.

 

From an external appearance, the floating valve and trunnion valve designs may appear similar, except that some trunnion ball valves have an external lower trunnion support visible on the bottom of the valve body.  Another external sign of a trunnion valve may be a bleed fitting in the body cavity to relieve trapped pressure due to the trunnion valve’s Double Block and Bleed (DBB) capability, which is inherent in most trunnion valve designs.

 

 

How does the Floating Ball Valve Work?

A floating ball valve design is one in which the ball is ‘squeezed’ between two seats. The design uses the body to rigidly hold seats that ‘squeeze’ the ball in place within the ball cavity. Contrary to the designation of ‘floating ball’ in its description, there is minimal, if any, movement of the ball from one seating surface to the other during operation.  The ‘float’ pertains to the contact stress force which ‘floats’ from one side to the other, depending on which direction the upstream pressure is coming from. For low-pressure sealing, the floating design relies on this ‘squeeze’ for sufficient contact stress to make the initial seal of the ball to the downstream seat.

As pressure increases, the upstream pressure pushes the ball against the downstream seat, which enhances the contact stress on the downstream seat. This contact stress is essential for achieving a tight seal at the valve’s full working pressure. If the flow direction is reversed, the forces required for sealing shift, and the opposite seat performs the same function with the upstream pressure pushing the ball to create the sealing force from the reverse direction. The bi-directional floating ball design can seal in each direction, but not simultaneously.  Regardless of the operating direction, the body cavity remains under constant pressure; therefore, ball cavity isolation, or Double Block and Bleed (DBB), is not possible.

 

 

How does the Trunnion Ball Valve Work?

A trunnion (mounted) ball valve is one in which the ball is held securely in the body of the valve by trunnion supports, and the two moving seats are typically pushed against the ball with spring force.  For low-pressure shutoffs, the sealing force between the ball and the seats is generated by spring(s) engineered to create sufficient contact stress to affect a low-pressure seal. The trunnion design uses the seat geometry to allow upstream pressure

to create a piston force behind the seat, which exerts the proper contact stress on the ball and affects a tight shutoff up to full working pressure. As pressure increases, the seat piston force increases. The trunnion design allows the design engineer to reach the optimal forces necessary to create reliable sealing using both the seat springs and the seat piston force for the desired contact stress.  Because the sealing takes place upstream of the ball, a trunnion valve can simultaneously seal bi-directionally. This simultaneous bi-directional seal is referred to as a Double Block and Bleed (DBB) – not to be confused with a Double Isolation and Bleed (DIB) valve, which features two independent trim sets within one common valve body.

Another design consideration for trunnion ball valves is cavity relief. During operation, line pressure will become trapped inside the ball cavity when the valve is closed against pressure. Trapped pressure can also happen in a closed ball valve due to thermal expansion.  This trapped pressure will be relieved by the downstream seat lifting off the ball until the spring force overcomes the trapped pressure.  This pressure in which a seat will self-relieve is called the ‘Cavity Relief Pressure.’ Trunnion ball valves should include bleed fittings in the body cavity to safely remove any trapped pressure not relieved by the Cavity Relief. This bleed fitting also allows for the diagnosis of a leaking valve while the valve is in-line. In comparison, this body bleed fitting for valve diagnosis is not possible in the floating ball valve design.

 

 

 

 

 

Characteristics of Floating Ball Valves

• Lower Cost: Generally, more affordable than trunnion ball valves due to fewer component parts.
• Lower Cost of Repair Parts: Maintenance is more economical due to the simplicity and fewer parts involved.
• Simple Design: Easy to operate and maintain, making them user-friendly.
• Reliability Issues at Lower Pressures: Provides reliable sealing when designed correctly, but may be less dependable in lower pressure ranges if the ball and seat contact stress is not high enough.
• Inconsistent Torques: Require a more considerable actuation force, especially in larger valves, higher pressures, and when valves are closed for long periods.
• Pressure Exposure of Stem Seals: When in the closed position, the body cavity is under pressure, which exposes stem seals to pressure and prevents the design from having a double block and bleed (DBB) feature and body bleed fitting for diagnosis.

 

 

 

 

Characteristics of Trunnion Ball Valves

• Reliable sealing: Provides dependable sealing capabilities from relatively low pressure to maximum rated working pressure when designed correctly. Allows an experienced design engineer the ability to balance the tradeoff between contact stress (reliable sealing) and excessive torques.
• Cavity Pressure Relief: Seats can automatically relieve trapped pressure in the body cavity, preventing damage.
• Durability: Supports the use of Metal Seating Surfaces in abrasive and severe service applications.
• Seat-to-body sealing: Capable of supporting the use of lip seals for seat-to-body sealing, extending lifespan.
• Long life: The use of metal seating surfaces and lip seals contributes to extended lifespan in high-pressure critical service applications.
• Consistent torque: Properly designed valves achieve more consistent torque control due to regulated seat spring and piston forces.
• Higher cost: Typically, more expensive to construct and repair due to additional components.

 

 

 

 

 

Which ball valve is right for your operation?

Both trunnion and floating ball valves possess unique features designed for distinctive uses. It’s crucial to assess your specific needs against these characteristics. Most importantly, partnering with a trusted manufacturer is vital for ensuring reliability and effectiveness. The expert valve team at CORTEC can help you decide which type of ball valve will work best for your operation.

 

CORTEC stands out as a leader in the industry, specializing in innovative ball valve designs tailored to meet your needs. Our experienced engineering team utilizes state-of-the-art technology in our advanced facility to fulfill all design, testing, and manufacturing requirements. We pride ourselves on exceeding industry standards and customer expectations, ensuring you receive reliable products and exceptional support you can trust for years to come.

 

For more information, visit our valve product page or reach out to our expert CORTEC valve team today to get started.