3-pc Forged Trunnion Mounted Soft Seated Ball Valve

3-piece forged steel trunnion mounted soft seated ball valve is a quarter-turn isolation product for oil and gas, chemical, power and skid-mounted service where the valve must both take high differential pressure and be serviced without a full line shutdown. Its distinction is that it carries the trunnion's fixed-ball stability inside a forged three-piece body: the ball is held on top and bottom trunnions, the soft seats are spring-loaded toward it, and the two end caps remain in the pipeline while the central body is unbolted for maintenance.

Compared with a floating 3-piece valve, the trunnion version seals bidirectionally and holds larger bores and higher differential pressures. Compared with a two-piece cast trunnion valve, the forged three-piece body favours inline serviceability and is most effective in compact, high-differential-pressure duty rather than the largest pipeline bores. The following sections explain the working principle, the body and seat options, isolation arrangements, dimensions, materials, testing and the information needed for a quotation.

Trunnion Mounting in a Three-Piece Forged Body

In a trunnion mounted valve the ball carries integral upper and lower trunnion shafts that ride in bearings housed in the body, so the ball is held at the centre of the cavity and cannot shift laterally under line pressure. The line thrust is taken through the trunnions and bearings into the body, not through the seats. The two seats are annular assemblies backed by disc or coil springs; at low pressure the springs provide the initial sealing force, and as line pressure rises it adds a self-energising force behind each seat, pressing it against the stationary ball.

Because the seats carry only the contact stress needed to seal, seat loading stays roughly constant and operating torque does not climb with pressure. With both seats independently loaded, the valve seals against pressure from either direction - the basis of its bidirectional capability and of the double block and bleed (DBB) or double isolation and bleed (DIB) function described below. Placing this mechanism inside a forged three-piece body is what gives the product its combination of high-differential-pressure capability and inline serviceability.

Design Standards

Design features

1. Double sealing system of packing, and firesafe design
2. Stem with emergency grease injection
3. Integral anti-blowout design structure of the stem
4.Floating preload seat ring design to ensure good sealing under Iow pressure
5. Double block and bleed function (DBB)
6. Double isolation and bleed function (DIB)
7. Anti-static function
8. Low torque design
9. Top flange self-limited design

3-pc Forged Trunnion Mounted Soft Seated Ball Valve drawing

Material List

Dimension & Weight

CLASS 150 Full Bore

CLASS 150 Reduced Bore

CLASS 300 Full Bore

CLASS 300 Reduced Bore

CLASS 600 Full Bore

CLASS 600 Reduced Bore

CLASS 900 Full Bore

CLASS 900 Reduced Bore

CLASS 1500 Full Bore

CLASS 1500 Reduced Bore

CLASS 2500 Full Bore

Trunnion vs Floating in a Three-Piece Body

The two designs share the three-piece forged body but solve the sealing task differently. This comparison explains where a trunnion version is the better fit within the same body style; for the full discussion see the floating vs trunnion comparison, and for the cast two-piece trunnion alternative see the 2-piece cast trunnion page.

Testing, Fire-Safe and Safety Features

Each valve is subjected to pressure testing per the governing specification: shell test at 1.5 times the rated pressure and seat test at 1.1 times the rated pressure. Seat leakage for soft seats is accepted to ISO 5208 Rate A; testing is performed per API 598, ISO 5208, EN 12266-1 and API 6D as applicable.

Fire-safe design per API 607 / API 6FA / ISO 10497, with a metal-to-metal secondary seal and graphite fire seals so the valve retains shutoff after soft seats are exposed to fire.

Anti-static device per API 608: a spring or contact path keeps the ball, stem and body electrically continuous; the resistance is verified not to exceed 10 ohms.

Blow-out-proof stem with a shouldered or integrally retained head that internal pressure cannot eject.

Emergency sealant injection ports on the stem and seats (optional per API 6D) allow temporary sealing if a seat is damaged in service.

Cavity self-relief on SPE arrangements prevents over-pressurisation of the trapped body cavity.

ISO 5211 actuator mounting pad for gear, pneumatic, electric or hydraulic actuation; ISO 15848 low-emission packing available where specified.

Material traceability is documented by a Material Test Certificate to EN 10204 (3.1 as standard, 3.2 with third-party involvement on request), covering chemistry, mechanical properties, heat number and, where applicable, hardness and NACE compliance. Third-party inspection (for example SGS, BV, TUV) is supported when required by the contract.

 

 

 

 

Provide the following to receive a quotation: nominal size and pressure class, body material (for example A105 or F316), end connection (flanged, socket weld or butt weld), seat material, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, actuator type, inspection and documentation requirements, and delivery destination with requested schedule. A data sheet or drawing helps confirm the specification.

Request a Quotation

Packaging and shipping

Applications

Oil and gas gathering, station and tie-in lines where compact high-differential-pressure isolation is needed

Refinery and petrochemical units - process gas, product and utility isolation

Skid-mounted and modular process packages welded into the assembly

Sampling, instrument and utility lines opened for periodic service

Low-temperature systems (A350 LF2) and sour (H2S) environments per NACE MR0175 / ISO 15156 (confirmed per conditions)

Suitability depends on design pressure, diameter, medium composition, temperature, corrosion control and the project specification. A trunnion mounted valve is selected where high differential pressure, bidirectional isolation or frequent automated operation makes low, stable torque and DBB/DIB decisive; for the largest pipeline bores a two-piece or top-entry trunnion is usually preferred.

FAQ

What is a 3-piece forged trunnion mounted soft seated ball valve?

It is a quarter-turn isolation valve with a body machined from solid forgings and built as two end caps plus a central section, in which a ball is fixed by top and bottom trunnions and sealed by spring-loaded soft seats. The forged three-piece body lets the centre be removed for inline service while the end caps stay in the pipeline, and the trunnion support lets the valve hold high differential pressure with low, stable operating torque.

How does a 3-piece trunnion valve differ from a 3-piece floating ball valve?

Both use a three-piece forged body, but a trunnion valve anchors the ball on fixed trunnions while spring-loaded seats move toward it, whereas a floating valve leaves the ball free so line pressure pushes it into the downstream seat. The trunnion version seals bidirectionally and handles higher differential pressure and larger bores than a floating 3-piece valve of the same body style.

Can a 3-piece trunnion valve be serviced inline without removing it from the line?

Yes. The two end caps remain bolted to the pipeline while the central body, which carries the ball, trunnions and seats, is unbolted and lifted out for seat, seal or ball service. The pipe run stays intact, so the valve can be cleaned, inspected and rebuilt without cutting welds, draining the system or disturbing alignment.

What size and pressure range does a forged 3-piece trunnion ball valve cover?

Typical supply is NPS 2 to 24 (DN50 to DN600) and Class 150 to 2500 (PN16 to PN420), in full or reduced bore, with flanged, socket-weld or butt-weld ends. The three-piece forged design is most effective in compact, high-differential-pressure service; larger bores and very high classes are evaluated per inquiry, and a two-piece or top-entry trunnion is often preferred above NPS 24.

What forged body materials are used, and can the valve be supplied for sour or low-temperature service?

Standard forged bodies are ASTM A105 or A105N carbon steel, ASTM A350 LF2 for low temperature, ASTM A182 F304 or F316 stainless, and ASTM A182 F11 or F22 alloy steel for elevated temperature. Sour-service valves with controlled chemistry and hardness can be supplied to NACE MR0175 / ISO 15156, and low-temperature valves to A350 LF2, subject to the confirmed service conditions.

What soft seat materials are available and what leakage rating applies?

Common soft seats are PTFE (about -29 to +200 degC), RPTFE reinforced with glass or carbon (about -29 to +230 degC), PEEK (about -20 to +260 degC), Nylon (about -20 to +120 degC) and Devlon nylon composite (about -20 to +220 degC). The usable window also depends on pressure and medium. Soft seats are tested for seat leakage to ISO 5208 Rate A (bubble-tight); metal-seat backups, where fitted, are evaluated to ISO 5208 Rate D.

What seat arrangements are available - DBB, DIB, SPE, DPE?

Because both seats are independently loaded, the valve provides double block and bleed (DBB) or double isolation and bleed (DIB). DIB-1 uses double-piston-effect seats on both sides (DPE x DPE); DIB-2 uses a single-piston-effect upstream seat and a double-piston-effect downstream seat (SPE x DPE). SPE seats self-relieve cavity pressure to the low-pressure side, while DPE seats seal against pressure from either direction. The required arrangement follows the project isolation philosophy.

What information is needed to request a quotation?

Provide the nominal size and pressure class, body material (for example A105 or F316), end connection (flanged, socket weld or butt weld), seat material, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, actuator type, inspection and documentation needs, and delivery destination with schedule. A data sheet or drawing helps confirm the specification.

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