Trunnion Ball Valve

Trunnion Ball Valve Design Features

1. Gerneral Design

In the trunnion mouned ball valve, the ball is fixed by a trunnion shaft (sizes 4" and smaller) or retaining plate (sizes 6" and above); and the seat rings are floating, free to move against the ball along the valve center line. The trunnion shaft / retaining plate, together with 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, the process 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. Ball Seat Alignment

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

3. 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 O-rings may be destroyed by the rapidly expanding gas. Special AED O-rings or Lip seals suitable for such service conditions are available on request to avoid this possibility.

4. Low Fugitive Emission 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 is in compliance with the most severe pollution-control regulations such as API 641. The test certifications are available on request.

5. Seat Design

Trunnion ball valves are with standard seat design: primary soft seal, and secondary metal to metal seal. Seat insert is designed as pressed-in type which is easy for maintenance.(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)

6. Seat Types

a) Standard: Single Piston Effect Seats (Self Relieving Seats)
In a trunnion mounted ball valve, medium pressure, both upstream and downstream, creates a resultant thrust to the seat rings against the ball to assure tight sealing; Medium pressure acting in the body cavity creates a resultant thrust to push the seat rings away from the ball.

The single piston design permits the automatic release of any over pressure in the body cavity when the valve is in the fully open or fully closed position. (Fig. 3,4)

Double Block and Bleed Trunnion Ball Valve
When the ball is in the closed position, each seat seals off the process medium independently at the same time between the up/down stream and body cavity; it allows bleeding of the trapped cavity pressure (DBB) through drain or vent valve. The double block and bleed function makes it possible to flush the valve under pressure and verify that the seats are sealing properly.(Fig. 7)

Single piston effect seat (self-relieving seat)
 

Trunnion ball valve with double block and bleed
 

b) Option 1: Double Piston Effect Seats (Double Isolation and Bleed Ball Valve)
In trunnion ball valves, medium pressure, both upstream and downstream as well as in the body cavity, creates a resultant thrust that pushes the seat rings against the ball. Valves with double piston effect seat rings require a cavity pressure relief device to reduce the build-up of over pressure in the body cavity. (Fig. 5,6)

c) Option 2: Combination Seats (Single Piston Effect Seats*Double Piston Effect Seats)
In trunnion ball valves, combination seats design is available on request. That is a standard seat design used for upstream side and a double piston effect seats design used for downstream. The advantage is it can reach double piston effect seats design function without cavity pressure relief device to saves the cost, meanwhile only need a little care to install valve per flow direction arrow. (Fig. 3,6)
 

7. Fire Safe

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

b) Internal leakage prevention
For a trunnion ball valve, when fire burned out the primary O-ring seal between the floating seat ring and adapter, also the seat insert between 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
 

8. Anti-static Device

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

9. Blow-out Proof Stem

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

Anti-static device, blow-out proof stem
 

10. Emergency Sealant Injection System

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

Trunnion Ball Valve Drawing & Material Specifications

Trunnion Ball Valve Drawing

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