Fully Welded Trunnion Mounted Soft Seated Ball Valve

The fully welded trunnion mounted soft seated ball valve is a quarter-turn isolation product for long-distance gas and liquid pipelines, compressor and metering stations, and other buried or subsea duty where external leak risk must be minimised and maintenance access is limited. Its distinction is the body: the shell and ends are welded into a single pressure-containing envelope, so there is no bolted body joint and no body gasket, and the stem becomes the only path to atmosphere.

Compared with a bolted-body valve - a top entry, two-piece or three-piece design (top entry, 2-piece cast trunnion, 3-piece forged trunnion) - the welded body removes the joint leak paths, reduces weight and is welded directly into the line. Compared with a floating valve, the trunnion support holds the ball against line thrust and gives bidirectional sealing with low, stable torque.

How a Fully Welded Trunnion Ball Valve Works

In a trunnion mounted valve the ball carries integral upper and lower trunnion shafts that ride in bearings, so the ball is held at the centre of the cavity and cannot shift laterally under line pressure. 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 wave 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 the seat 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, which is the basis of its bidirectional capability and of the double block and bleed (DBB) or double isolation and bleed (DIB) function described below. In the fully welded version this mechanism is enclosed in a body that is closed by welding, so the assembly is built and welded under controlled conditions and then set for long service.

Design Standards

features

1. Fully-welded structure, the welding strength meets the standard requirements, and valve can fully withstand the internal and external pressure of the pipeline.
2. Forged solid ball, and carbon steel with ENP coating
3. Seat and stem are designed with grease injection valve + check valve
4. Integral forged stem and anti-blowout design
5. The self relief design to effectively prevent overpressure in the cavity, and provide more reliable sealing.
6. Low torque design to realize valve easier open and close
7. Optional buried design
8. Super thick coating to ensure the service life of the valve
9. Advanced welding equipment with mature and reliable welding process to ensure the valve service life not less than that of the pipeline
10. Optional rubber seat design

Fully Welded Body: No Bolted Joints

The defining feature is that the pressure-retaining shell is welded, not bolted. Forged pipe sections and forged end pieces are machined, assembled with the ball, trunnions, bearings and seats inside, and then joined by full-penetration welds to form a single body. There is no body flange, no body bolting and no body gasket, so the conventional body-joint leak path is eliminated and the valve is lighter than a split-body design of the same bore.

The body welds are made to qualified welding procedures (WPS/PQR) and receive full radiographic testing, so the seam quality is verified rather than assumed. Because the body cannot be opened afterwards, a fully welded valve is intended to run for its service life without internal maintenance; sealing is protected by emergency sealant injection ports and by an anti-blow-out stem, and the design is matched to buried, subsea and other low-access duty where a bolted joint would be a liability. Where internal access is needed instead, a bolted or top entry trunnion valve is the appropriate choice.

Stem Sealing: The Only Atmospheric Path

With the body welded shut, the stem is the only component that penetrates the pressure envelope to atmosphere, so all external sealing is concentrated there. The stem is sealed by a stack of O-rings or by graphite packing, backed by an anti-blow-out stem head that internal pressure cannot eject. Because there is no second barrier at a body joint, the stem seal is built with multiple sealing elements and, where required, low-emission packing to ISO 15848-1.

Emergency sealant injection ports at the stem allow a temporary seal to be restored if the primary seal is damaged in service, without shutting down the line. This is the same approach used at the seats: with a welded body, sealant injection is the on-line remedy that complements the design's long-life, low-maintenance intent. Fugitive-emission performance and the sealant system are confirmed against the purchase order.

Fully Welded Trunnion Mounted Soft Seated Ball Valve drawing

Material of Construction

Dimension & Weight

CLASS 150 Full Bore

CLASS 300 Full Bore

CLASS 400 Full Bore

CLASS 600 Full Bore

CLASS 900 Full Bore

CLASS 1500 Full Bore

CLASS 2500 Full Bore

Operating Torque

Seat

Sealing

* Other elastomeric materials are available upon request.

Flow Coefficient (Cv value) Specification

 Notes:
1.All sizes are full port.
2.Pressure ratings are per API 6D.

Method of Calculating Flow
The Flow Coefficient CV value is the flow rate of water (gallons/minute) through a fully opened valve,
with a pressure drop of 1 psi across the valve. To find the flow of liquid through the valve from the Cv,
use the following formulas.

Liquid Flow:
QL = Cv (P/G)1/2
QL = Flow rate of liquid (gal. /min.)
P = differential pressure across the valve
G = specific gravity of liquid (for water, G=1)

Gas Flow:
Qg = 61Cv (P₂ P/g)1/2
(For non-critical flow, P/P<1.0)
Qg = Flow rate of gas (CFH at STP)
P2= outlet pressure (psia) 2
g = specific gravity of gas (for air,g=1.0)

Max Operating Torque

Basic Torque vs Differential Pressure

Break away torque depends on the combination of differentialpressure,media, and cycle frequency.
It can be determined as follows:
Basic torque x safety factor x cycle frequency factor

Medium Factor

Cycle Frequency Factor

Pressure and Temperature Ratings

ASTM A105 and A350 LF2

ASTM A182 F316

Fully Welded vs Top Entry vs Two-Piece vs Three-Piece

The four common trunnion body styles differ in how the body is closed, how many leak paths they carry and whether the internals can be reached. This comparison shows where the all-welded body is the better fit; for the working principle see the floating vs trunnion comparison.

This table is a design summary. The actual acceptance basis is the purchase order, the material specification and the applicable standard edition. The configurations are not interchangeable without re-qualification.

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.

Welded body integrity: body seams made to qualified WPS/PQR and verified by full radiographic testing.

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 6D: a spring or contact path keeps the ball, stem and body electrically continuous; the resistance is verified not to exceed 10 ohms.

Anti-blow-out 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.

Extended stem and external coating for buried service, with ISO 5211 actuator mounting and 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, welding records 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: nominal size and pressure class, body and end material (for example A105, A350 LF2, A694 F52 or F316), the pipe grade the ends must match, seat material, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, actuator type, whether the valves are for buried service with a stem extension and external coating, inspection and documentation requirements, and delivery destination with requested schedule. A data sheet or drawing helps confirm the specification.

Submit Your Valve Specification

Applications

Long-distance natural gas and liquid transmission pipelines - mainline block valves

Buried and underground lines, including urban gas distribution, installed without a valve pit

Subsea and offshore pipelines; subsea variants per API 6DSS where specified

Compressor, metering and pressure-reducing stations - isolation duty

District heating and cooling networks

Tank farms and terminals - manifold and loading isolation

Suitability depends on design pressure, diameter, medium composition, temperature, corrosion control and the project specification. A fully welded trunnion valve is selected where the lowest external leak risk, welded-line continuity and long-life, low-maintenance service are decisive, and where in-line internal repair is not required; where internal access is needed, a bolted or top entry trunnion valve is used instead.

FAQ

What is a fully welded trunnion mounted soft seated ball valve?

It is a quarter-turn isolation valve whose body sections are joined by welding rather than bolts, so the body carries no bolted joint and no body gasket. The ball is held on fixed top and bottom trunnions and sealed by spring-loaded soft seats, which can provide double block and bleed (DBB) isolation. Because the body is welded shut, the stem is the only path to atmosphere, and the design is selected for buried, subsea and other long-life service.

How does a fully welded body differ from a top entry or other bolted-body valve?

A top entry, two-piece or three-piece valve is closed by one or more bolted body joints, each carrying a gasket. A fully welded body replaces those joints with welded seams, so there is no body gasket and no bolted body-joint leak path. The trade-off is internal access: a bolted-body valve can be opened for seat or ball service, while a fully welded valve is closed for its service life and internal parts are reached only by cutting the valve out of the line.

How is the stem sealed, and what happens if the stem seal leaks?

The stem is sealed by a stack of O-rings or graphite packing, with an anti-blow-out stem that cannot be ejected by internal pressure. Because the stem is the only path to atmosphere, the design adds multiple sealing barriers, optional ISO 15848-1 low-emission packing, and emergency sealant injection ports at the stem. If the primary seal leaks in service, sealant can be injected to restore a temporary seal without shutting down the line.

Can a fully welded ball valve be serviced or repaired in the line?

Routine seat or ball maintenance is not possible in the line, because the welded body cannot be opened. The design instead targets long-life, low-maintenance operation, and provides emergency sealant injection at the stem and seats to restore sealing temporarily if a seat is damaged. A permanent repair is made by cutting the valve out of the pipeline, so the design is matched to services where that is acceptable, such as buried and subsea lines.

What size and pressure range does a fully welded trunnion ball valve cover?

Typical supply is NPS 2 to 56 (DN50 to DN1400) and Class 150 to 2500 (PN10 to PN420), in full bore (piggable) or reduced bore. Larger sizes and higher pressure classes are evaluated per inquiry. The all-welded body is economical in large-diameter transmission service, where it is welded directly into the line.

What body and trim materials are used, and can it be welded directly to the pipeline?

The body and ends are typically forged carbon steel to ASTM A105 or ASTM A350 LF2 for low temperature, ASTM A694 F52 or F65 to match high-strength line pipe for direct welding, or ASTM A182 F316 stainless for corrosive service. The ball and trunnion are commonly A105 with ENP or hard facing, or F316, and the stem is 17-4PH or Inconel 718. Welded ends follow ASME B16.25 and are matched to the pipe grade per API 5L so the valve can be welded straight into the line; body welds receive full radiographic testing.

What soft seat materials are used 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); a metal-seat backup, where fitted for fire or elevated temperature, is evaluated to ISO 5208 Rate D.

What information is needed to request a quotation?

Provide the nominal size and pressure class, body and end material (for example A105, A350 LF2, A694 F52 or F316), the pipe grade the ends must match, seat material, bore type (full or reduced), applicable standard edition, DBB or DIB requirement with SPE/DPE arrangement, actuator type, whether the valves are for buried service with a stem extension and external coating, inspection and documentation needs, and delivery destination with schedule. A data sheet or drawing helps confirm the specification.

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