3-pc Forged Trunnion Mounted Metal Seated Ball Valve

The 3-piece forged steel trunnion mounted metal seated ball valve is a quarter-turn isolation product for severe service in oil and gas, refining, power and mining where the valve must take high differential pressure, survive high temperature, abrasive or slurry media and fire risk, yet still be opened for inline service. Its distinction is that it carries the trunnion's fixed-ball stability and bidirectional sealing inside a forged three-piece body whose centre can be pulled for maintenance, while the closure is a hard-faced metal-to-metal seat rather than a polymer seat.

Compared with a soft seated trunnion valve of the same body, the metal seated version trades bubble-tight shutoff for a much wider temperature window, abrasion and fire survival, and a defined ISO 5208 Rate D leakage basis. Compared with a three-piece forged floating metal seated valve, the trunnion version seals bidirectionally and reaches larger bores and higher differential pressures. The following sections explain the working principle, the body and hard-facing options, isolation arrangements, dimensions, materials, testing and the information needed for a quotation.

Trunnion Support with Hard-Faced Metal Seats

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.

Unlike a soft seat, the sealing faces here are hard-faced: the ball and the seat rings are finished with a wear-resistant overlay (such as Stellite, tungsten carbide or chromium carbide) and lapped to line contact. Because both seats are independently loaded, the valve seals against pressure from either direction, and the metal-to-metal closure survives temperatures, abrasion and fire that would degrade a resilient polymer. Placing this mechanism inside a forged three-piece body is what gives the product its combination of high-differential-pressure capability, severe-service survival and inline serviceability.

Design Standards

Design features

1. Double sealing system of packing, and firesafe design
2. Wear-resistant and scour resistant sealing design
3. Integral anti-blowout design structure of the stem
4. Ball and seat surface are coated with super wear-resistant alloy to ensure service life and sealing reliability.
5. Low torque design
6. Unique and effective dust-proof design to ensure safe andreliable use
7. Scraper design of seat with automatic cleaning function
8. Fugitive emission test meets API 624, ISO 15848 and other specifications.

3-pc Forged Trunnion Mounted Soft Seated Ball Valve drawing

Material of Construction

Dimension & Weight

CLASS 150

CLASS 300

CLASS 600

CLASS 900

CLASS 1500

CLASS 2500

Metal-to-Metal Seat Design

The closure is formed by precision-lapped line contact between a hard-faced ball and a hard-faced seat ring. Both faces carry a wear-resistant overlay, and the seat is deliberately softer than the ball so that any wear is taken by the renewable seat rather than the ball. A spring - usually an Inconel disc spring - sits behind the seat to keep contact through thermal expansion and low-pressure conditions. Because there is no polymer in the sealing path, the valve is inherently tolerant of heat and fire: if a fire removes any auxiliary sealing, the metal-to-metal contact still closes the line, and the body gaskets and stem packing use graphite so the valve remains seal-capable at temperature.

Metal-to-metal sealing is intentionally not bubble-tight. The valve is tested and rated to ISO 5208 Rate D, which defines a small permitted leakage past the metal faces. Testing is performed per API 598, ISO 5208 and EN 12266-1. The comparison with soft seats is direct: a soft seat aims for bubble-tight closure (ISO 5208 Rate A), whereas a metal seat trades zero leakage for temperature, abrasion and fire survival.

Compliance note: seat leakage for this valve is stated as ISO 5208 Rate D. Testing follows API 598 / ISO 5208 / EN 12266-1. No "API 598 Class VI" leakage class is claimed.

Trunnion vs Floating in a Metal Seated Three-Piece Body

The two designs share the three-piece forged body and the hard-faced metal seats but solve the ball-support task differently. This comparison explains where a trunnion version is the better fit within the same severe-service body style; for the full discussion see the floating vs trunnion comparison, and for the floating alternative see the 3-piece forged floating metal seated 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 metal seats is accepted to ISO 5208 Rate D; 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 primary seal and graphite body gaskets and stem packing, the valve retains shutoff after fire exposure; the metal seat itself acts as the fire-safe secondary seal.

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.

 

 

 

Packaging and shipping

Provide the following to receive a quotation: process medium, operating and design temperature and pressure, nominal size and pressure class, body material (for example A105 or F316), end connection, required seat hard-facing, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, inspection and documentation requirements, and delivery destination with requested schedule. A data sheet or drawing helps confirm the severe-service specification.

Request a Quotation

Applications

High-temperature steam - superheated and saturated steam lines, boiler blowdown and drain

Thermal oil and heat-transfer-fluid loops where a soft seat would soften or degrade

Abrasive slurries and mining - catalyst lines, fly ash, sand-laden and mineral slurries

Fire-risk service - inherently fire-safe because no polymer is in the sealing path

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

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 metal seated valve is selected where high differential pressure, bidirectional isolation, frequent automated operation or severe temperature/abrasion/fire conditions make low stable torque, DBB/DIB and metal-seat survival 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 metal 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 hard-faced metal seats. The forged three-piece body lets the centre be removed for inline service while the end caps stay in the pipeline, the trunnion support holds high differential pressure with low stable torque, and the metal-to-metal seat survives high temperature, abrasion and fire that would destroy a soft seat.

How does a metal seated trunnion valve differ from a soft seated trunnion valve?

Both use the same forged three-piece trunnion body, but a metal seated valve seals by line contact between a hard-faced ball and a hard-faced seat ring, while a soft seated valve uses a resilient polymer against a lapped ball. The metal seat trades bubble-tight shutoff for service at much higher temperature (indicatively up to about 538 degC), resistance to abrasion and slurry, and inherent fire survival; its leakage is accepted to ISO 5208 Rate D rather than Rate A. Soft seats remain the choice for clean, bubble-tight, moderate-temperature duty.

How does a trunnion mounted metal seated valve differ from a floating metal seated valve?

The three-piece forged body and the hard-faced metal seats are shared, but a trunnion valve anchors the ball on fixed trunnions with spring-loaded seats moving toward it, whereas a floating valve lets line pressure push the free ball into the downstream seat. The trunnion version therefore seals bidirectionally, handles higher differential pressure and larger bores (typically NPS 2 to 24 versus NPS 1/4 to 2 for the floating type) and offers inherent DBB/DIB isolation; the floating type is simpler and suited to smaller, lower-pressure lines.

Why is the metal seat not bubble-tight, and what leakage rating applies?

Metal-to-metal sealing relies on line contact between two hard-faced surfaces, so it intentionally permits a small, defined leakage rather than zero. The seat is rated to ISO 5208 Rate D, which defines the permitted leakage past the metal faces, and is verified per API 598, ISO 5208 or EN 12266-1. That trade-off is accepted because the metal seat survives temperatures, abrasion and fire that would destroy a soft seat; it is not a defect but the operating basis of severe-service valves.

Which hard-facing materials are applied to the ball and seat?

Common hard facings are cobalt-based Stellite 6, 12 or 21, tungsten carbide (WC-Co), chromium carbide (Cr3C2-NiCr), nickel-based Ni60 and hard chrome. Tungsten carbide is chosen for severe abrasion and slurry, Stellite or chromium carbide for high-temperature steam and thermal oil, and corrosion-resistant overlays for sour or chloride media. The ball and seat are deliberately of dissimilar hardness so wear occurs on the renewable seat rather than the ball.

Is a metal seated trunnion valve fire-safe?

Yes. Because the primary closure is metal-to-metal with no polymer in the sealing path, and the body gaskets and stem packing use graphite, the valve retains shutoff after fire exposure; the metal seat itself acts as the fire-safe secondary seal. It is fire-tested per API 607, API 6FA or ISO 10497, and anti-static continuity between ball, stem and body is verified to API 608.

What size and pressure range does a forged 3-piece metal seated trunnion 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 where severe service and inline serviceability coincide; 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 information is needed to request a quotation?

Provide the process medium, operating and design temperature and pressure, nominal size and pressure class, body material (for example A105 or F316), end connection, required seat hard-facing, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, inspection and documentation needs, and delivery destination with schedule. A data sheet or drawing helps confirm the severe-service specification.

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