ANSI Two Piece Carbon Steel Trunnion Mounted Ball Valve

Overview of the J-VALVES 8 inch ANSI Class 300 WCB two piece trunnion ball valve

Eight inch at Class 300 is the size and class where the large bore trunnion valve stops being a special item and becomes a stock product. It is the size of a main header, a pump discharge, a cooling water ring main or a plant air distribution line, and there are more of them in a typical process plant than any other combination of bore and class.

The body on this page is cast, two pieces, and carbon steel. Each of those three words is a decision rather than a description. Cast rather than forged because at DN 200 a forging would be a very large and expensive billet; two pieces rather than three because one joint at full pressure is better than two; carbon steel because the duties that use this size are water, steam and gas rather than anything that attacks steel.

The valve is eight inch, DN 200, Class 300, which API 6D clause 7.2 records as PN 50. The body is ASTM A216 WCB, the ball is WCB with a nickel plating or a stainless ball, the seat is reinforced PTFE, PPL or metal, the ends are raised face or ring joint flanges to ASME B16.5, and the operator is a gear.

Design and Construction

A two piece body is a main body and one end cap, closed on a single ring of studs with a single gasket between them. At eight inch that is one joint at full line pressure instead of the two that a three piece body would have, and the joint is a flat face with a spiral wound gasket rather than a ring groove, which means it can be made up with ordinary workshop practice.

The maintenance access is one sided by design. The cap comes off and exposes one seat and the ball, and the ball can be lifted out through that opening. The second seat stays in the main body where it was set at assembly, and it is reached by taking the ball out first. For a valve that is expected to have its seats changed once or twice in a plant life, that is enough access.

The ball is trunnion mounted, which at this size is not optional. The ball runs on an upper and a lower shaft in bearings, and the pressure load on the ball goes into those bearings rather than into the seats. Without that, the force needed to turn a DN 200 ball against two loaded seats would be beyond any lever and most gearboxes.

Both seats are spring loaded and both are pressure energized. The spring gives the valve its initial seal at low pressure, and above a few bar the line pressure takes over and loads the upstream seat towards the ball. The downstream seat is loaded by the cavity pressure that gets past the upstream seat, or, depending on the seat pattern, by line pressure from behind. Either way the result is a valve that seals in both directions and can be installed in either orientation.

The stem passes through the body and is retained from the inside by a shoulder, so the pressure cannot eject it. Above the shoulder is the packing box and a gland, and above that a mounting flange for the operator. That flange is the part that gets used years later when a manual valve is converted to actuated, and it is worth checking at the order that it is machined to a standard pattern rather than to the works' own.

Material Options

A216 WCB is the cast carbon steel that most large valve bodies are made from, and at Class 300 it is doing a job that a forging could also do, at roughly twice the cost and with a much longer lead time. The casting route also allows the body to be shaped around the trunnion pockets and the seat bores instead of machined out of solid.

The ball material is the option that gets bought most often and for the least clear reason. A nickel plated carbon steel ball is fine in water, steam and clean gas, and it is the part that suffers first when the medium carries fines. A stainless ball costs more and does not fix an abrasive medium, because the abrasive wears the seat rather than the ball.

The seat is the temperature limit, as it is on every soft seated valve. Reinforced PTFE is the standard and it is what makes the valve tight at ambient; PPL takes the valve higher up the range in the same body. Above the point where the insert will hold, the answer is a metal seat and a different leakage acceptance, and that is a different valve rather than an option on this one.

Technical Specifications

Class 300 in group 1.1 of ASME B16.34 is 740 psi at 38 C, and the figure is worth knowing because it is roughly half of what most people assume when they hear the class number. It is also the figure the shell test is calculated from, at 1.5 times, rather than the line pressure the valve will ever see.

The port is the other number that has to be settled before the order and is often left to the works. A full port DN 200 valve passes a pig and has a lower pressure drop; a reduced port valve is lighter, cheaper and needs a smaller gearbox. On a cooling water header the difference does not matter, and on a line that will be pigged it decides whether the valve can stay in the line.

Standards and Compliance

At eight inch Class 300 both ball valve product standards apply, and this is one of the last points on the ladder where that is true. API 608 covers DN 15 to DN 600 in Class 150 to Class 600, and API 6D covers pipeline valves over a wider range. A valve at this size and class sits inside both, so a data sheet that names one of them is incomplete rather than wrong.

Casting quality is the item that is least often specified and most often argued about. MSS SP-55 is the visual acceptance standard for steel castings, and it classifies discontinuities rather than banning them. A buyer who wants radiography on the body has to ask for it on the order, because it is not part of the default inspection on a Class 300 casting.

Manufacturing Process

The body and cap are cast, then normalized and tempered, then shot blasted and inspected before any machining starts. The machining is done with the flange face, the bolt circle, the joint face and the seat pocket all referenced to one centreline per part, because that reference is what makes the two halves close with the seat pockets in line.

The seat pockets are the critical dimension on the cap and the body, and they are checked against each other rather than each against a drawing. If the two pockets are not coaxial within tolerance the seats will sit at an angle to the ball, the contact band will be a crescent rather than a ring, and the valve will pass its shell test and weep on its seat test.

The ball is machined and then ground, coated if the specification calls for a coating, and finally checked for sphericity. On a DN 200 ball the grinding is done on a machine that holds the ball between centres, and the result is a sphere that the seat insert can conform to rather than a surface that has been polished to look right.

Quality Control and Testing

The heat number is cast into the body and repeated on the certificate, and on a carbon steel casting the identity check is as much about excluding a mixed part as about confirming WCB. A WCC body in a WCB order is not a defect in service, but it is a documentation defect, and it is the kind of thing that stops a project file being signed off.

The shell test on a two piece body is a test of the cap joint as much as of the casting. The joint is the only place where the body is not one piece of metal, and a gasket that has been fitted dry, or a joint face that was not cleaned, will show up at 1.5 times as a weep that no amount of stud tensioning will stop.

Applications and Industry Service

This valve is bought for the utility side of a plant: cooling water, plant air, nitrogen, steam distribution, fire water and fuel gas. In all of those duties the requirement is a tight shutoff that can be restored by changing a seat, and a body that will outlast the piping it is bolted into.

The eight inch size also appears in hydrocarbon service where the line is large and the pressure is low, such as tank farm rundown, flare headers and vent lines. In those duties the class is chosen for the flange rather than for the pressure, because the piping standard fixes the flange and the valve follows it.

It is the wrong valve in three situations, and each one is discovered late. The first is a line that will be pigged, where a reduced port body stops the pig and a full port is required. The second is a medium that attacks carbon steel, where the body grade has to change. The third is a duty with a high cycle count, where the seat insert becomes a wear part and the maintenance interval becomes the design constraint.

Installation, Operation and Maintenance

The valve is heavy enough at this size that it should be supported independently of the pipe, particularly on a vertical run where the flange bolts would otherwise carry the weight. Support the body, not the gearbox, and leave the support adjustable so the flange faces can be brought parallel before the joint is tensioned.

Bring the mating flanges up parallel before tensioning, and use the gasket and the studs that belong to the eight inch Class 300 flange rather than hardware sized to the bore. Studs are not a winch: a flange drawn up out of square bends the cap rather than seating it, and the joint weeps at test with the gasket blamed for an error that belongs to the pipework.

For maintenance the line is isolated, vented and drained through the cavity connection, the cap studs are released in a pattern, and the cap comes away with one seat. Both seats should be replaced as a pair even when only one is leaking, because they have seen the same number of cycles. The gasket goes back new, and the studs are torqued to the drawing figure in a cross pattern.

How to Specify and Order

The data sheet is short: bore, class, body grade, port, insert and operator. Two more belong on the order at this size and are often left to the works: whether the body is required to be radiographed, and whether the top flange is to a standard actuator pattern.

The line answers four things the valve cannot: what flows, how hot, how hard it pushes, and whether a pig will ever run through it. The medium decides the body and ball grade, the temperature decides the seat insert, the pressure confirms the class, and the pigging question decides between full and reduced port.

Documents to name in the order: the EN 10204 3.1 certificate with the heat number matched to the plate, the casting inspection record, the shell and seat test record with a result per seat, the fire certificate if one is required with its scope read, and the general arrangement drawing carrying the face to face dimension.

FAQ

Q: Why is this body cast rather than forged?

A: Because at DN 200 a forging would be a very large billet at roughly twice the cost and with a much longer lead time, and the casting route allows the body to be shaped around the trunnion pockets and seat bores instead of machined out of solid. Above about DN 200 the casting is the only sensible route at this class.

Q: What does Class 300 actually hold?

A: 740 psi at 38 C in group 1.1 of ASME B16.34. The number in the name is an index rather than a pressure, and the index doubles from 150 to 300 to 600, so 300 is exactly twice 150 in cold rating rather than 300 psi. At temperature the figure falls, which is why the group matters.

Q: How many pressure joints does a two piece body have?

A: One, between the main body and the end cap, and it sees full line pressure. A three piece body would have two. That single joint is the reason this style is cheaper to make and it is also the joint that is proved first at the shell test.

Q: Does the body need radiography?

A: Not by default. A Class 300 WCB casting is accepted visually and dimensionally to MSS SP-55, and radiography is a purchaser option that has to be written on the order. A casting that has not been examined can still pass a hydrostatic test with internal porosity that has not yet reached the surface.

Q: Can the valve be converted to an actuator later?

A: Yes, if the top flange is machined to a standard actuator pattern rather than to the works own. That is worth confirming at the order rather than at the point where an actuator arrives and does not fit, because reworking a top flange on a valve already in the line is a shutdown.