Trunnion Mounted Ball Valve, 2-56 Inch, ASME Class 150-2500, API 6D

A trunnion mounted ball valve is a quarter-turn isolation valve in which the ball is anchored top and bottom in bearings carried by the body, so the line pressure load is taken by the trunnion instead of by the seat, and only the seat rings move to make the seal. That single change in the force path is what lets the design work from 2 inch to 56 inch and from ASME Class 150 to Class 2500, where a floating ball valve would need a torque no actuator can supply.

J-VALVES builds trunnion mounted ball valves in Wenzhou, Zhejiang, in cast and forged bodies, two piece and three piece, full bore and reduced bore, with soft, dual and metal seats. Buyers are pipeline contractors, EPC packagers and plant operators who need a valve that seals in both directions, can be actuated, and comes with the test record to prove it. This page sets out the force path, the body styles, the seat logic, the material and torque data, and the test sequence that decides acceptance.

The Force Path: Why a Trunnion Exists

In a floating ball valve the ball is held only by the two seats. Line pressure pushes the ball downstream into the seat, and that same pressure multiplied by the seat contact area becomes the sealing force. It also becomes the friction the stem has to overcome. Because the projected seat area grows with the square of the bore, the torque of a floating valve climbs roughly with size squared and with pressure, and somewhere around 8 inch Class 600 the numbers stop being practical.

A trunnion valve breaks that link. The ball sits on a lower trunnion and an upper stem, each running in a bearing, so the pressure load goes into the body through the bearings. The seats are the parts that float: each seat ring is pushed toward the ball by a spring and by line pressure acting on a small piston area. Sealing force no longer depends on squeezing the whole ball, and operating torque falls to bearing friction, seat preload and packing drag, which is why a 24 inch Class 600 trunnion valve can still be turned by a gear operator that a person can crank.

The trade is part count. A trunnion valve has bearings, seat carriers, springs and, in most designs, a body joint with a gasket, all of which have to survive the same pressure and thermal cycle. The seat also has to be protected from over-travel, because a trunnion ball does not centre itself against the seat the way a floating ball does. Those details, not the ball, are where trunnion valves fail.

Two Piece, Three Piece and Top Entry Bodies

The body style decides how the valve is assembled, how it is repaired, and how many potential leak paths it carries. All three styles use the same trunnion principle; the difference is where the body is cut open.

Two piece is the default for pipeline service up to 24 inch. One joint means one gasket and one set of bolts to control, and the closure can be machined as a single piece with the downstream flange. Three piece costs more machining but pays back on lines that get opened: the centre section comes out with the ball and both seats still assembled, so a seat change is a bench job rather than a line job.

Top entry is the answer when the valve sits in a position nobody wants to cut out, and it is covered in its own page. Fully welded removes the joint entirely and is the only correct choice for buried transmission line valves; it is also the only one of the four that cannot be repaired, which is a maintenance decision rather than an engineering one.

Seat Designs: Single Piston and Double Piston

Two seat behaviours exist, and confusing them is the most common specification error in trunnion valves. Both look identical from outside.

An SPE seat is self-relieving by geometry: the seat piston area exposed to cavity pressure is larger than the area exposed to bore pressure, so trapped cavity pressure lifts the seat off the ball and escapes upstream. That is exactly what a liquid-filled body needs when the sun comes up. It is also why an SPE valve cannot hold a positive double block, because the same path that relieves the cavity will pass the process.

A DPE seat seals from both sides, so the cavity is a closed space between two seals. On a liquid line that space must be protected, either by a thermal relief valve on the body, by a bleed connection that is left open to a closed drain, or by a seat carrier designed to relieve at a set differential. On gas duty, DPE is the normal choice and the bleed valve is operated, not bypassed. This is the same logic that decides a DBB valve, which has its own page.

Body Joint, Bolting and Gasket

The body joint is a bolted assembly of flange, gasket and studs, and it is the one place on a trunnion valve where three different materials have to stay clamped together through a thermal cycle. Stainless flange, graphite or spiral wound gasket and alloy steel studs expand at different rates, so preload changes with temperature and the joint either relaxes or crushes the gasket.

Practice that holds up in service: a confined gasket groove so the gasket cannot extrude, spiral wound 316L with graphite filler for Class 600 and above, controlled bolt tightening in a star pattern with a calibrated torque wrench and the values recorded, and studs to ASTM A193 B7 with A194 2H nuts, or L7 and L7M for low temperature. Above Class 900 the joint usually moves to a ring type joint, which is metal-to-metal and does not depend on gasket compression for its seal.

J-VALVES records joint bolt torque by serial number and re-torques after the shell test, because the first pressurisation seats the gasket and drops preload. A valve shipped without that step weeps at the joint on site, and the leak is always blamed on the gasket material.

Body, Ball and Trim Materials

Material choice is driven by the fluid and the minimum temperature, not by the pressure class. Pressure sets wall thickness and bolting; the fluid sets the grade.

Two notes that decide service life. First, the ball coating: electroless nickel plating at 50 to 75 micrometres is standard for carbon steel balls, and hard chrome is used where the fluid is abrasive. A ground stainless ball without coating is acceptable in clean, non-abrasive duty and is common in stainless valves. Second, the stem: 17-4PH carries roughly twice the torque capacity of F316 at the same diameter, which matters on large valves where a bigger stem would mean a bigger bonnet and a bigger actuator.

Size, Class and Torque

Torque is the number that sizes the actuator, the gearbox and the mounting flange, and it is the number buyers most often take from a competitor's catalogue instead of from the valve they are buying. The figures below are indicative break-to-open values for clean, lubricated service at the rated pressure, at ambient temperature; they must be confirmed by test on the actual valve.

Three multipliers must be applied on top of the table. Temperature: a valve at 300°C needs about 1.3 to 1.6 times the ambient torque because the seat material softens and the clearances tighten. Medium: dry gas and abrasive slurry push it up again. Seat type: a metal seated valve at the same size and class typically needs 1.5 to 2.5 times the soft seated figure, because the seal is metal-to-metal and the seat has to be forced into contact.

Actuator sizing then adds a safety factor. The common rule for pneumatic actuation is 1.5 times the maximum break-to-open torque, with the air supply at its minimum stated pressure rather than the plant nominal, and the valve at its maximum differential pressure. Sizing on nominal air and ambient torque is the reason actuators stall on the first cold morning.

Anti-Static, Fire Safe and Stem Retention

Three details separate a valve that passes a specification review from one that does not, and none of them are visible from the outside.

Anti-static: API 6D requires a conductive path from ball to stem to body, with resistance measured at 10 ohms or less. A valve with PTFE seats and a PTFE stem bearing insulates the ball, and a static charge built up by dry gas flow will eventually discharge across the seat and damage it. The path is normally a stainless spring or pin pressed against the stem and the ball, and it must be tested after assembly, not assumed.

Fire safe: the requirement is that the valve still limits leakage after a fire destroys the soft seat. The design answer is a secondary metal seat, usually a graphite or stainless ring that only touches the ball once the primary seat has burned away, plus a graphite body gasket and graphite packing. Qualification is by test to API 607 or API 6FA, and the certificate should name the size, class and seat configuration tested; a certificate for a different seat design proves nothing.

Stem retention: API 6D and ASME B16.34 both require the stem to be restrained against blow-out. The usual design is a shoulder on the stem larger than the bore of the bonnet, so pressure cannot eject it even with the gland removed. On a trunnion valve the upper stem also carries the ball, so the retention feature is doing two jobs.

Testing and Documentation

A trunnion ball valve is accepted or rejected on its test record. The sequence below is the one that is actually run, and each step exists because a different failure mode shows up there.

Documentation that should travel with the valve: EN 10204 3.1 material certificate naming the actual heats, Charpy impact results where impact tested steel is used, shell and seat test records by serial number, torque record, NACE MR0175 compliance statement for sour service, and a dimensional drawing. On a project with a third party inspector, the inspection and test plan is agreed before machining, not after.

Selection Matrix

Read the four rows below before writing a datasheet line. Most wrong valves are ordered because one of these was left undecided and the supplier filled the gap with the cheapest option.

Trunnion Mounted Ball Valve FAQ

What is the difference between a trunnion mounted ball valve and a floating ball valve?

In a floating ball valve the ball is supported only by the two seats, so line pressure pushes the ball into the downstream seat and the whole pressure load becomes sealing force and friction. In a trunnion mounted valve the ball is held by a top stem and a bottom trunnion in bearings, the pressure load goes into the body, and only the seat rings move. Torque stays low as size grows, which is why trunnion valves run from 2 to 56 inch while floating valves stop around 8 inch.

At what size does a ball valve have to be trunnion mounted?

There is no standard cutoff, but the practical line is around 6 to 8 inch at Class 300 and above, and around 8 to 10 inch at Class 150. Below that a floating valve is cheaper and perfectly reliable. Above it the floating torque becomes too high for a manual gear and too high for a standard actuator, and the seat loading distorts the seat ring. High pressure at small size also pushes the decision: a 4 inch Class 2500 valve is normally trunnion mounted.

What does double piston effect mean, and when is it required?

A double piston effect seat seals against pressure arriving from either the bore or the body cavity, so the cavity becomes a closed space between two seals. It is required wherever positive double isolation is specified: double block and bleed, DIB-1, metering runs and wellhead isolation. The consequence is that the cavity must have its own pressure relief, because trapped liquid that expands on warm-up has nowhere to go and will crack the body or push the seat out.

Is a trunnion mounted ball valve bidirectional?

Yes. Both seats seal independently, so the valve holds pressure from either end, and the upstream seat is the one that does the sealing in normal flow. That is also why a trunnion valve is tested on each seat separately and in both directions, and why the test record should show four results rather than one.

What are the size, pressure and temperature limits?

J-VALVES builds trunnion mounted ball valves from 2 to 56 inch, in ASME Class 150, 300, 600, 900, 1500 and 2500, with flanged RF or RTJ, butt weld and socket weld ends. Soft seated valves run from -29°C to 180°C with RPTFE and to 250°C with PEEK; metal seated valves run to 425°C and beyond with the right body grade. Below -46°C the design changes to a cryogenic build with an extended stem.

What do you need to quote a trunnion mounted ball valve?

Size, ASME class, body style, end connection and face-to-face dimension, body and trim material, seat type (SPE or DPE, soft or metal), full or reduced bore, design temperature and fluid, actuation, test standard, third party inspection, coating, and quantity. Sour service, fire safe testing and NACE MR0175 should be stated at enquiry. A line class sheet or datasheet shortens the review considerably.