Short answer: Choose spring return (SR) when the line must go to a known port if the air supply fails, and double acting (DA) when the valve should hold its last position. Choose L-port to divert flow between two outlets and T-port to mix or split it. A pre-assembled package is the valve with a rack-and-pinion actuator mounted on its ISO 5211 pads, plus a NAMUR solenoid valve and limit switch box if ordered, assembled and tested in the plant before shipment.
A pneumatic 3-way ball valve is a three-port ball valve with an air actuator mounted on top, so one signal from the control panel switches, diverts or blends two lines instead of someone walking to a lever. Specifying one comes down to three decisions: what the valve must do when air is lost (double acting or spring return), what the bore inside the ball must do to the flow (L-port or T-port), and whether the valve, actuator and pilot valve arrive as one tested unit or as parts to match on site.
This guide takes those decisions in that order, using our MV-3DM actuated 3-way ball valve as the worked example. If the position may not need an actuator at all, or you want the longer treatment of mounting pads and brackets, start with why the ISO 5211 mounting pad should be chosen before the actuator.
Key takeaways
- Double acting (DA) uses air for both strokes and holds its last position if the air supply drops. Spring return (SR) uses air one way and a spring the other, so on air loss it moves to the port you nominate at order time. The actuator type and the fail position are built in at the factory and cannot be changed afterwards.
- L-port connects the bottom port to either side port, so it diverts. T-port can run straight through, split to a branch or pull two inlets into one outlet. Neither bore has a fully closed position, so a three-way ball valve switches flow but does not isolate it. The bore and the actuator's stop positions decide which two flow patterns the 90° stroke gives you.
- The pilot valve follows the actuator: a 5/2 solenoid valve for double acting, a 3/2 for spring return. Ordering the wrong function is the most common mistake in pneumatic valve packages.
- On the MV-3DM, spring return runs one actuator frame larger than double acting on the same valve size, because the spring takes part of the air torque. Bray's actuator-selection bulletin states that both ends of a spring-return stroke have to be checked against the valve's torque.
- The MV-3DM ships with the actuator mounted on dual ISO 5211 pads, no bracket and no coupling, in eight sizes from 1/4″ to 2″, threaded PT, NPT or PF to the thread stated on the order. Each assembly is stroke tested, torque tested and checked for leakage before shipment.
What a pneumatic 3-way ball valve is, and what the package contains
A pneumatic 3-way ball valve is a quarter-turn ball valve with three ports and a rack-and-pinion air actuator mounted on the valve's top pad. Compressed air turns the ball through 90° between two end positions; the shape of the bore through the ball decides which ports are connected at each end. A small solenoid valve bolted directly onto the actuator usually does the switching, and a limit switch box can report the valve's position back to the control system.
On our MV-3DM the valve is the same investment-cast A351 CF8M (316) body as the manual 3-way ball valve, with CF8 (304) to order. It has PTFE (polytetrafluoroethylene) seats, a 1000 WOG rating (a cold working pressure for water, oil and gas, not a steam rating), a valve temperature range of −20°C to 180°C, and the third port on the bottom of the body. It comes in 1/4″, 3/8″, 1/2″, 3/4″, 1″, 1-1/4″, 1-1/2″ and 2″, threaded PT (tapered), NPT (the American tapered thread) or PF (parallel) to the thread stated on the order. The aluminum rack-and-pinion actuator runs on 4 to 8 bar of clean, dry air (about 60 to 116 psi), in double acting or spring return form.
The MV-3DM is sold as the valve with its actuator mounted. The solenoid valve, limit switch box and positioner are options added to that package. When the solenoid valve and limit switch box are ordered with it, the valve, the actuator, the solenoid valve and the switch box are assembled and tested together before they leave.
Double acting or spring return: decide by what must happen when air is lost
If the process can tolerate the valve staying wherever it was when the air failed, double acting is enough. If the valve must reach a defined port on air failure, whether to protect a tank, a pump or a product batch, specify spring return and write down which port it must return to. Actuator size, pilot valve and price all follow from that one answer.
Double acting (DA): air in both directions, no defined fail position
A double acting actuator has two air chambers. Air on one side turns the valve one way, air on the other side turns it back. Remove the air and nothing pushes the piston in either direction, so the valve holds its last position. That is useful when a line must not move unexpectedly, but it also means the valve has no defined fail position: it sits at whichever end it reached last, or part way if air was lost mid-stroke.
Because both strokes are air driven, the actuator's output torque is the same in both directions and constant along the stroke. That is why a double acting unit can be smaller than a spring return unit on the same valve.
Spring return (SR): air one way, spring the other, fail to the port you nominate
A spring return actuator uses air for one stroke only. A set of springs drives the return stroke, so when the air supply fails the springs move the valve to a preset position. On a two-way valve that preset is open or closed. A three-way ball valve has no fully closed position, so the preset is a port: on air loss the ball returns to the side port you nominated when you ordered. Our MV-3DM and the Tri-Clamp MV-3M-TC are both built this way, with the spring set to fall back to the port you name at order time.
That port has to be on the inquiry, not in a later email. The actuator type and the fail position are set at the factory and cannot be changed afterwards: a spring return unit ordered to fail to port A cannot be flipped to port B on site, and a double acting unit cannot be converted into a spring return one.
Why spring return needs a larger actuator
In a spring return actuator the air stroke has to turn the valve and compress the springs at the same time, so part of the air torque is spent on the springs. The spring stroke then has only the stored spring force to turn the valve, and that force falls as the springs extend. Bray's actuator selection bulletin for ball valves states the rule: for a spring return unit, the torque at the end of the air stroke and the torque at the end of the spring stroke must both exceed what the valve needs. In its worked example, the same valve that a double acting actuator handles needs a larger spring return frame with three or four springs.
On the MV-3DM, spring return runs one actuator frame larger than double acting on the same valve size, because the spring takes part of the air torque. Torque ratings for the valve and actuator are available on request; send the line pressure and medium with the inquiry and the package is sized in the plant.
The pilot valve follows the actuator
A double acting actuator has two air ports and both have to be fed, so it takes a 5/2 solenoid valve: one side is pressurised while the other vents. A spring return actuator takes air on one port only, so a 3/2 is enough. Our NAMUR solenoid valve for pneumatic actuators is offered in both functions, with an IP67 coil in 24V DC, 110V AC or 220V AC and a manual override for commissioning. Getting the function wrong is the most common ordering mistake in pneumatic valve packages, which is a good reason to order the solenoid with the actuator rather than separately.
| Question | Double acting (DA) | Spring return (SR) |
|---|---|---|
| How is the valve driven? | Air for both strokes | Air one way, springs the other |
| What happens on air loss? | Holds its last position; no defined fail position | Springs return the ball to the port nominated at order time |
| Fail position | None to set | Set to the nominated port in the plant |
| Actuator frame on the same MV-3DM size | Standard frame | One frame larger, because the spring takes part of the air torque |
| Solenoid valve function | 5/2 | 3/2 |
| Air supply | 4–8 bar (60–116 psi) | 4–8 bar (60–116 psi) |
| Pick it when | The valve should stay where it is if air fails | The line must reach a known port if air fails |
L-port or T-port: the bore sets what "switching" means
The two balls look identical from outside. Inside, an L-port bore connects the bottom port to one side port at a time, so the valve diverts: tank 1 or tank 2, pump A or pump B, product or return loop. A T-port bore can connect all three ports or any two, so it can run straight through, split one inlet to a branch, or pull two inlets into one outlet for mixing. Pumps & Systems describes the same split in its overview of three-way valves: the L-port diverts flow to one direction at a time, and the T-port is the pattern mostly used for mixing.
One consequence is easy to miss on a drawing: neither bore has a fully closed position. The T-port always leaves a path open, and the L-port always connects the bottom port to one side or the other, so neither can serve as the isolation valve for the line. A three-way ball valve switches flow; it does not shut it off. If a branch must be positively shut, add a two-way valve downstream, whatever actuator sits on top of the three-way.
Neither pattern is a default on our valves. T-port and L-port are both stock on the manual MV-3M and both are offered on the actuated MV-3DM; you name the bore when you ask for a price. If you can sketch the flow path, send it with the inquiry and we will confirm the porting.
How a 90° actuator maps onto the bore
A standard quarter-turn actuator gives two end positions, 90° apart. With an L-port ball those two positions are "bottom port to side A" and "bottom port to side B", which is the changeover most diverting duties need. With a T-port ball, 90° moves between two neighbouring flow patterns; which two depends on the drilling and on the actuator's stop positions. Two T-port patterns that sit 180° apart on the valve's flow diagram cannot both be reached by a 90° stroke, so confirm the pair you need against the manufacturer's flow diagram for that valve before you order, and say on the inquiry which two patterns the valve has to give you.
The two decisions combine like this:
| Bore | What the two actuator positions do | Double acting on air loss | Spring return on air loss |
|---|---|---|---|
| L-port (diverting) | Bottom port to side A, or bottom port to side B | Stays on whichever side it last selected | Returns to the side port nominated at order time |
| T-port (mixing or distributing) | Two neighbouring patterns, set by the drilling and the actuator stops; confirm against the flow diagram | Stays on the last pattern; a path is always open | Returns to the pattern nominated at order time; a path is always open |
What a pre-assembled package includes, and why we ship it that way
A pre-assembled pneumatic 3-way ball valve package from us is the MV-3DM valve with the rack-and-pinion actuator already mounted and, if ordered, the solenoid valve and limit switch box fitted on the actuator. Those parts are assembled and tested together before they leave: each assembly is stroke tested and torque tested, and each unit is checked for leakage before shipment, so there is nothing to match up on site. Torque values are available on request. The unit arrives ready to pipe.
ISO 5211 and NAMUR: what the two interfaces fix
The MV-3DM body machines dual ISO 5211 bolt patterns, so the actuator sits straight on the valve with no bracket and no coupling. ISO 5211 is the standard for part-turn actuator attachments: it fixes the flange dimensions between actuator and valve, the dimensions of the drive component, and reference torques for each interface size, and its summary states that the attachment of an intermediate support to the valve is outside its scope. As our bracket and coupler page explains, the height above the stem and the stem's drive shape still have to be matched, which is why two parts that both claim ISO 5211 can need a bracket to meet. On the MV-3DM that matching is done once, in the plant, and the actuator bolts on directly. A bracket and coupler is the answer when a line runs hot and the actuator has to stand off from the stem, or when the valve and the actuator come from different sources.
One detail worth asking about if you are matching an actuator from another source: ISO 5211 flanges carry an F code (F03, F04, F05 and so on) with metric bolting, and in the U.S. an actuator may be built to the MSS FA flange instead (MSS is the Manufacturers Standardization Society). The Valve Manufacturers Association's magazine explains in its article on actuator and gearbox flange standards that the two share a bolt circle at the same size but differ in bolt thread and in the centering spigot, so the flange standard has to be stated when parts are bought separately. On an assembled package the match is made in the plant.
The solenoid valve and the limit switch box both mount through the NAMUR interface. The NAMUR NE 19 recommendation, which references VDI/VDE 3845, describes a standard interface for mounting a solenoid valve on a pneumatic actuator without piping between them. Every pneumatic actuator we make carries that interface. Our 4V310-08B solenoid valve sits flat on it with no fittings, and the LSB-01 limit switch box mounts the same way, with two SPDT mechanical switches (single-pole, double-throw contacts) that trip at the open and closed end positions and report back to the plant's PLC or DCS (its programmable logic controller or distributed control system). On a three-way valve that signal confirms which port is live, which is what an interlock on a tank changeover or a sampling loop needs.
How to specify a pneumatic 3-way ball valve
Eight lines on the inquiry cover the decisions above. Neither the bore nor the actuator with its fail port can be changed after delivery, so those two lines deserve a second look before the order is placed. In order:
- Size and quantity: 1/4″ to 2″ on the MV-3DM.
- Body grade: CF8M (316), or CF8 (304) to order.
- Thread standard: PT, NPT or PF, to the thread stated on the order. Use the same standard as the rest of the line.
- Bore: L-port or T-port, with a sketch of the flow path if you have one; for a T-port, name the two flow patterns the valve has to give you.
- Actuator: double acting or spring return. For spring return, name the port the valve must return to on air loss.
- Air supply available at the valve, so the package can be sized within the 4 to 8 bar range, and how often the valve cycles.
- Accessories: solenoid valve (the function follows the actuator; state the coil voltage), limit switch box, positioner.
- Line conditions and documents: medium, pressure and temperature, and whether you want the mill test report for the body material, which is available on request.
Common mistakes on 3-way actuated valve orders
Five specification mistakes are worth checking before the order goes out. All five are decided on the inquiry, and neither the bore nor the actuator can be corrected after delivery.
Ordering the wrong solenoid function. A 5/2 on a spring return actuator, or a 3/2 on a double acting one, will not run the valve correctly. The function follows the actuator and nothing else; if you are unsure which actuator you have, send the model and we will tell you.
Specifying spring return without a fail port. Spring return is bought so the valve reaches a known port on air loss. An order that says "spring return" without naming the port can arrive set to the wrong side. Write the port on the inquiry.
Expecting the three-way valve to isolate. Neither the L-port nor the T-port bore has a fully closed position. If a branch has to be shut tight, use a two-way valve for that duty.
Sizing the actuator from a two-way torque table. Bray's bulletin cited above publishes a separate torque table for multiport valves, and at the same size its values are higher than in its two-way table. On our packages the actuator is sized to the three-way valve in the plant; if you are pairing parts yourself, ask for the three-way torque rather than reading the two-way figure.
Buying a three-way valve that cannot be automated later. Our manual MV-3M carries an ISO-style top pad, so an actuator can be added. Some three-way valves, including our manual Tri-Clamp MV-3N, have no pad at all; the ISO 5211 article linked above covers that case. If a position may ever need to switch on a signal, order the pneumatic build from the start.
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FAQ
How do L-port and T-port behave on a pneumatic 3-way ball valve when the air fails?
What happens on air loss is set by the actuator; the bore only shapes the result. With a double acting actuator, either bore holds its last position: an L-port stays on the side it last selected, a T-port stays on its last flow pattern. With a spring return actuator, the springs return the ball to the port or pattern nominated at order time. Neither bore has a fully closed position, so on air loss the valve moves to its fail position but does not isolate the line.
When is the fail position of a spring return 3-way valve set?
At the factory, from the order. The actuator type and the fail position are built in when the package is assembled and cannot be changed afterwards. A spring return actuator set to return to port A cannot be reversed to port B on site, and a double acting unit cannot be converted to spring return. State the fail port on the inquiry; if the process changes later, treat it as a new actuator specification rather than an adjustment.
Does a pneumatic 3-way ball valve need a 5/2 or a 3/2 solenoid valve?
Match it to the actuator. A double acting actuator takes a 5/2 solenoid valve, because its two air ports both need to be fed. A spring return actuator takes a 3/2, because only one port takes air and the spring handles the return stroke. Our 4V310-08B NAMUR solenoid comes in either function with a 24V DC, 110V AC or 220V AC coil; ordered with the valve, the right function is fitted and tested in the plant.
Does a 3-way ball valve need a separate shut-off valve?
Yes, if the line needs positive shut-off. Neither bore has a fully closed position: a T-port ball always leaves a path open, and an L-port ball always connects the bottom port to one side or the other. Add a two-way valve downstream of the three-way for that duty, and keep the three-way valve for the switching, mixing or splitting work it is built for.
What is included in a pre-assembled pneumatic 3-way ball valve package?
The MV-3DM valve in investment-cast CF8M with PTFE seats, the rack-and-pinion actuator in double acting or spring return form mounted directly on the valve's dual ISO 5211 pads, and, if ordered, a NAMUR solenoid valve and a limit switch box; a positioner can also be added. The valve, actuator, solenoid valve and switch box are assembled and tested together before they leave: each assembly is stroke tested and torque tested, and each unit is checked for leakage before shipment. A mill test report for the body material is available on request.
Summary
Send the size, body grade, thread standard, bore, actuator type with its fail port, air supply and cycle rate, accessories, and line conditions, and the MV-3DM can be quoted as one line: valve, actuator and pilot valve delivered as one tested unit. Air loss behaviour sets the actuator, double acting to hold position or spring return to reach the nominated port. Flow duty sets the bore, L-port to divert or T-port to mix or split, with a two-way valve added downstream wherever a branch must be shut tight, because neither bore fully closes.