Trunnion mounted or floating — where is the crossover?

Floating valves are cheaper and simpler but load the downstream seat with the full pressure force through the ball, so they run out at roughly DN200 in the lower classes and earlier at class 600. Specify trunnion mounted when the bore is larger, the class is 600 and above, actuation torque must be minimised, or the specification calls for API 6D, double block and bleed, or seat sealant injection. Pipeline and station valves are trunnion mounted almost by default.

What is the difference between DBB and DIB, and between SPE and DPE seats?

Single piston effect seats self-relieve body cavity overpressure into the line, giving inherent cavity protection. Double piston effect seats stay energised from the cavity side too, so each seat seals in both directions and the valve gives double isolation even from one side, but the cavity then needs an external relief valve. In API 6D terms a DBB valve seals with both seats and vents the cavity; DIB adds independent sealing from either end. State the required configuration, because the seat design follows from it.

Should I specify a bolted or fully welded body?

Bolted two-piece and three-piece bodies are standard where the valve is accessible and may be maintained in place, such as stations and process units. Fully welded bodies remove body joints as leak paths and are the norm for buried pipeline service, usually with stem extensions and sealant lines brought to grade. The welded valve cannot be reseated on site, so it is specified where design life without intervention is the goal. State installation, burial depth if any, and pigging requirements.

What changes for sour service?

Wetted materials must comply with NACE MR0175, which controls hardness, heat treatment and alloy selection to resist sulphide stress cracking. State the H2S partial pressure or simply that MR0175 applies, and ask for material certificates showing compliance. Elastomers and any weld overlays are affected as well as base metals, and testing documentation should be reviewed before shipment, ideally with third-party inspection for export orders.

Trunnion Mounted Ball Valve Suppliers India

About Trunnion Mounted Ball Valves

Trunnion mounted ball valves to API 6D for pipeline and high-pressure process duty, DN50 to DN1200, class 150 to 2500, with double block and bleed and fire-safe options.

A trunnion mounted ball valve fixes the ball on upper and lower trunnions so it rotates on bearings instead of floating between the seats. Spring-loaded seat rings are pushed against the ball and further energised by line pressure, which keeps operating torque low and sealing reliable at large bores and high pressure classes where floating designs run out. This is the standard pipeline ball valve, built to API 6D for transmission lines, terminals, metering and compressor stations and severe process isolation. Indian manufacturers commonly supply two-piece and three-piece bolted bodies in forged A105, LF2, F316 and duplex F51, and cast WCB and CF8M, with fully welded bodies for buried service. Typical features include double block and bleed capability, self-relieving or double piston effect seats, anti-blow-out stems, sealant injection to seats and stem, and full bore for pigging. Sour service trims comply with NACE MR0175. Ends are flanged to ASME B16.5 or butt-weld, with fire-safe design to API 607 or ISO 10497. In an enquiry state size, class, bore, body construction, seat configuration, materials, end connection, operation and the service conditions including any sour or low-temperature requirement.

Specifying Trunnion Ball Valves: Class, Seats, DBB and Body Type

What the valve is for

Trunnion mounted ball valves are the isolation standard for pipelines, terminals, metering and compressor stations and high-pressure process units — anywhere the bore is large, the class is high, or double isolation must be proven. Because the ball is carried on bearings, operating torque stays practical at sizes and pressures where a floating ball would seize against its seat.

Trunnion versus floating

Choose trunnion mounted when size exceeds roughly DN200, class reaches 600 and above, actuation torque must be minimised, or the specification calls for API 6D, double block and bleed or seat sealant injection. Below that a floating valve is usually the economic choice.

Seat configuration

The seats are spring-energised rings pressed onto the fixed ball. Single piston effect seats self-relieve body cavity overpressure into the line. Double piston effect seats seal in both directions, so the valve provides double isolation from one side alone, but the cavity then needs external relief. A common compromise is one seat of each type. State whether you require double block and bleed or double isolation and bleed in API 6D terms, because the seat choice follows directly from it.

Body construction

Two-piece and three-piece bolted bodies are standard for stations and plants where maintenance access exists. Fully welded bodies eliminate body joints for buried pipeline service and are supplied with stem extensions and sealant lines extended to grade.

Bore

Pipeline valves are full bore to pass inspection and cleaning pigs; reduced bore saves cost where pigging will never be required.

Materials

Forged A105, LF2 for low temperature, F316 and duplex F51, or cast WCB and CF8M, with electroless nickel plated or overlaid balls and trim. Sour service requires NACE MR0175 compliant materials with controlled hardness, stated explicitly in the enquiry.

Auxiliary features

Sealant injection fittings on seats and stem give an emergency sealing route, cavity vent and drain connections allow block-and-bleed proving, and anti-static and anti-blow-out stem features are standard. Gear operators are usual from about DN150 upward, with mounting pads for actuators where automation is planned.

Standards

API 6D governs pipeline ball valves including testing and the DBB and DIB definitions; ASME B16.34 covers pressure-temperature ratings; ends are flanged to ASME B16.5 or butt-weld; fire-safe certification is to API 607 or ISO 10497; supplementary seat and shell testing to API 598 where the purchaser specifies it.

What to state in the enquiry

Size, class, full or reduced bore, bolted or welded body, seat configuration and DBB or DIB requirement, body and trim materials, sour service and NACE MR0175 if applicable, end connections, operator or actuation, service medium and temperature range, quantity, and documentation needs such as material certificates and third-party witnessed testing. Indian forges and valve makers supply this class of valve for export against exactly this information.

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How Trunnion Ball Valves Work

Fixed ball, moving seats

In a trunnion mounted valve the ball is held by a top trunnion integral with or coupled to the stem and a bottom trunnion post, both running in bearings. The ball cannot move downstream, so sealing is done the other way round from a floating valve: each seat ring is free to slide axially in its pocket and is pushed onto the ball by springs, with line pressure acting on the exposed piston area of the ring to add force in proportion to pressure. Sealing therefore starts at zero pressure, from spring load alone, and strengthens with pressure without ever transferring the full ball load onto a polymer seat.

Why torque stays low

The pressure force on the ball is carried by the trunnion bearings, not the seats, so friction at the seat face stays roughly constant with pressure. This is what makes large high-class valves operable with gearboxes and modest actuators, and it is the fundamental reason the design scales where floating valves cannot.

Piston effect

Whether a seat relieves or holds cavity pressure is set by the relative sealing diameters on the ring. In a single piston effect seat, pressure trapped in the body cavity acts on a net area that pushes the seat away from the ball, so cavity overpressure vents into the bore automatically. In a double piston effect seat the net area works the other way and cavity pressure presses the seat harder onto the ball, so the seat seals from both directions but the cavity must be protected by an external relief valve.

Double block and bleed

With both seats sealing against their respective ends, the body cavity between them can be vented through a bleed connection. A dry bleed proves both seats are holding, which is why DBB trunnion valves are used for positive isolation before maintenance and at custody transfer points, replacing two valves and a spool.

Sealing hierarchy

The primary seal is usually a polymer insert — nylon, PEEK or filled PTFE — retained in the metal seat ring; behind it the metal ring itself gives a secondary seal, and sealant injection points allow a viscous compound to be pumped to the seat face as an emergency measure if the insert is damaged. The stem is sealed by O-rings or graphite packing with its own injection point on larger valves.

Fire behaviour

Fire-safe designs retain a metal-to-metal seat path and graphite secondary seals so that when polymer inserts burn away the valve still limits leakage to the rates defined in API 607 and ISO 10497.

Failure modes

Debris packing the seat pockets can stop the rings following the ball, wax and hydrates can fill the cavity and jam the mechanism, bearing wear from side load shows up as rising torque, and in double piston effect valves a blocked or missing cavity relief can overpressure the body on thermal expansion. Most are prevented by choosing the right seat configuration, keeping the cavity drained and vented per the operating philosophy, and stating the service honestly, including sand, wax and H2S, in the enquiry.

Trunnion Mounted Ball Valves