Trunnion Ball Valve

Trunnion Ball Valve Design Features

1. Gerneral Design

In the trunnion ball valve, the ball is fixed by a trunnion, and the seat rings are floating, free to move against the ball along the valve center line. The trunnion and bearings absorb the side load created by the pressure acting on the ball.

At low pressure, the seat-tight sealing is ensured by the preload of the springs acting on the seat rings. Along with the pressure increasing, medium pressure pushes the seat rings against the ball to provide additional load for tight sealing.

The ball and stem are independent of each other to minimize the effect of the side thrust generated by the pressure acting on the ball.

2. Seat Design

The standard seat design of the trunnion ball valve is a primary soft seal and a secondary metal-to-metal seal. 

The seat insert is designed as a pressed-in type, which is easy to maintain. (Fig. 1) Optional design with primary metal-to-metal seal and secondary soft seal seat design is also available upon request. (Fig. 2)

Primary soft seal, and secondary metal to metal seal (PMSS seat)

3. Double Block and Bleed 

Double block: both seats are single piston seats; when the ball is closed, each seat seals off the process medium independently from the upstream and downstream;

Bleed: the trunnion ball valve has a drain and vent valve, which allows bleeding of the trapped cavity pressure through the drain or vent valve. 

Double block and bleed trunnion mounted ball valve

4. Self Relieving Seats (for overpressure in the body cavity)

In a trunnion ball valve, both seats are single piston seats, medium pressure in the body cavity creates a resultant thrust to push the seat rings away from the ball.

The design permits the automatic release of any overpressure in the body cavity when the valve is fully open or fully closed. 

Self relieving seat trunnion ball valve

5. Double Isolation and Bleed (Optional)

DIB 1: Both seats are double piston seats; each seat seal off the fluid from the upstream and downstream. A safety valve is needed as a cavity pressure relief device to reduce overpressure build-up in the body cavity.

DIB 1 Trunnion ball valve

DIB 2: One seat is a single piston seat, and the other is a double-piston seat; the double-piston seat seals off the fluid from the upstream and downstream, and the single piston seals off the fluid from upstream.

DIB 2 trunnion ball valve

6. Low Fugitive Emission

In our trunnion ball valve, accurate machining of stem, gland, and bonnet sealing surfaces with double sealing (O-ring primary seal plus graphite gasket seal) ensures low emission, which complies with the most severe pollution-control regulations, such as API 641. 

7. AED O-Ring

When the trunnion ball valve is used in high-pressure gas applications (e.g., hydrocarbon gas service under class 600 and above), the gas may be absorbed into the molecular structure of elastomeric O-rings. If the valve is subjected to sudden decompression, the rapidly expanding gas may destroy the O-rings. Special AED O-rings or lip seals suitable for such service conditions are available upon request to avoid this possibility.

8. Fire Safe
a) External leakage prevention
In the trunnion ball valve, all the possible external leakage points between the stem and gland flange, gland flange and body, and adapter are sealed with a primary O-ring, then a secondary graphite gasket. When the fire burns out the primary O-ring seal, the secondary graphite gasket seal can still prevent the process fluid from external leakage. (Fig. 9)

b) Internal leakage prevention
For a trunnion ball valve, when the fire burns out the primary O-ring seal between the floating seat ring and adapter, also the seat insert between the seat ring and ball, the secondary graphite seal between seat ring and adapter, and seat ring & ball metal to metal contact pre-loaded by spring will minimize the internal process medium leakage. (Fig. 10,11)

Internal leakage prevention
 

9. Anti-static Device

Spring plus graphite-type anti-static devices are applied between the ball, stem, gland flange, and body to keep the electrical continuity between all the metallic components and ensure the resistance is lower than the most severe service requirement. (Fig. 8)

10. Blow-out Proof Stem

In a trunnion ball valve, the stem is made separately from the ball with an integral T-type round shoulder retained by the gland (other designs are available on request). (Fig. 8)

Anti-static device, blow-out proof stem
 

11. Emergency Sealant Injection System

6” and larger trunnion ball valve (or smaller sizes on request) is equipped with sealant injection located at the stem and seat area. The injection is integrated with a check valve to provide backup sealing. Also, a check valve is equipped at the front of the seat sealant injection to avoid blowing out in case of wrong operation. When the soft sealing materials (seat inserts and o-rings) are damaged, and leakage happens by fire or other accident, the sealant can be injected through the injection fittings. (Fig. 12)

12. Ball Seat Alignment

Mechanical stops are equipped on all trunnion ball valves to ensure the ball is never over-rotated.

Trunnion Ball Valve Drawing & Material Specifications

Trunnion Ball Valve Drawing
(Sdie entry, 3 piece, forged steel)

Material Specifications

Related Information

Valve body material selection guide

Ball valve seat material selection guide 

Trunnion ball valve VS floating ball valve

API 6D testing requirements

Ball valve torque chart