How a 3-Way Ball Valve Works: L-Port vs T-Port

A 3-way ball valve has three ports — typically one inlet (the common port) and two outlets — controlled by a single quarter-turn handle. Instead of installing two separate 2-way valves and a pipe tee, one 3-way valve handles the same job in a single body: it can route flow from one source to either of two destinations, combine two incoming lines into one, or in some configurations shut off flow entirely. That means fewer connections, fewer potential leak points, lower installed cost, and one handle to operate instead of two.

What the valve actually does at each handle position depends entirely on the shape of the hole machined through the ball — the bore. The two standard patterns are the L-port and the T-port, and choosing the wrong one is the most common mistake buyers make with 3-way valves. This guide explains how each pattern works, maps out the flow paths at every handle position, and walks through how to select the right valve. Apex Flow Solutions stocks high-pressure steel block-body 3-way ball valves in both L-port and T-port patterns.

In This Guide

How the Ball Bore Determines Flow

In a standard 2-way ball valve, the ball has a straight bore: rotate it 90° and flow is either open or shut. In a 3-way valve, the bore is shaped so that rotating the ball changes which ports connect to each other, not just whether flow passes.

L-Port (90° Bore)

An L-port ball has a bore shaped like the letter L — two passages meeting at a 90° angle. At any handle position, the L-port connects the common port to one of the two outlet ports, never both. Rotating the handle 90° switches flow from one outlet to the other. This makes the L-port the standard choice for diverting or selecting service: choosing between two tanks, two pumps, or two destination lines.

Many L-port valves also offer an all-ports-blocked position. Because the L-shaped bore only spans 90° of the ball, rotating the handle to an intermediate or opposite position can point both bore openings at solid body wall, giving bubble-tight shutoff of all three ports. Check the manufacturer's position chart — not every L-port valve is built with a shutoff detent, but the geometry allows it.

T-Port (T-Shaped Bore)

A T-port ball has a bore shaped like the letter T — a straight-through passage with a third branch at 90°. Depending on handle position, a T-port can connect all three ports at once, or any pair of ports (straight-through with the branch closed, or common-to-either-branch). This makes the T-port the standard choice for mixing service (combining two inlet streams into one outlet) and for systems that need straight-through flow with an occasional branch take-off.

The trade-off: because the T-shaped bore spans 180° of the ball plus a branch, there is typically no handle position that blocks all three ports. A T-port valve redirects flow; it does not provide full shutoff. If positive isolation is required, use an L-port valve or add a separate 2-way shutoff valve.

L-Port vs T-Port Comparison Table

Characteristic L-Port T-Port
Bore shape 90° elbow (L) Straight run + 90° branch (T)
Ports connected at once 2 (common + one outlet) 2 or all 3
Primary function Diverting / selecting between two lines Mixing two streams, or straight-through + branch
Can connect all three ports No Yes
Full shutoff position Often available (all ports blocked) Typically none
Straight-through flow No (flow always turns 90°) Yes
Typical uses Tank selection, pump changeover, gauge isolation Blending, bypass loops, sampling take-offs

3-Way Valve Flow Diagram by Handle Position

The table below works as a position-by-position flow diagram for a typical bottom-entry 3-way valve, where the common port is the bottom (center) port and the left and right ports are the two outlets. Always verify against the position plate on your specific valve — handle orientation conventions vary by manufacturer.

L-Port Flow Positions

Handle Position Ports Connected Result
0° Common → Left port Flow diverted to left line; right line closed
90° Common → Right port Flow diverted to right line; left line closed
180° None All ports blocked — full shutoff (where the design allows)

T-Port Flow Positions

Handle Position Ports Connected Result
0° Left ↔ Right (straight through) Straight-through flow; common (bottom) port closed
90° Common + Left + Right (all three) Mixing or full distribution — all ports open
180° Common → one side port Branch flow to one outlet; opposite port closed

Note the key difference: rotate an L-port through its full travel and at least one port is always closed; rotate a T-port and at least two ports are always open. That single fact decides most L-versus-T selections.

Common Applications

  • Tank selection: draw from or fill either of two tanks with one L-port valve instead of two shutoff valves and a tee.
  • Pump diverting: switch a pump's discharge between two destinations, or swap between a duty and standby pump on the suction side.
  • Gauge and instrument isolation: route a pressure gauge to either of two process points, or block it off entirely for service using an L-port with a shutoff position.
  • Hydraulic flow switching: steel block-body 3-way valves rated to 5,000+ PSI direct hydraulic flow between circuits — common on mobile equipment, power units, and test stands.
  • Bypass loops: a T-port valve lets flow run straight through a filter, heat exchanger, or meter, then diverts it around the device for maintenance without stopping the system.

Selecting a 3-Way Valve

1. Port pattern. Start here. Need to select between two destinations or shut everything off? L-port. Need to mix two streams, run straight through with a branch option, or keep flow moving in every position? T-port.

2. Connection type. General industrial plumbing uses tapered NPT threads, which seal on the threads themselves (with sealant). High-pressure hydraulic systems increasingly specify SAE O-ring boss (ORB) straight-thread ports, which seal on an O-ring against a machined face and resist vibration loosening and leakage far better at hydraulic pressures.

3. Pressure rating. Match the valve's rated working pressure to system pressure including spikes. Brass 3-way valves commonly run 400–600 PSI WOG; steel block-body hydraulic valves run 5,000–7,500 PSI. Remember that ratings derate with temperature.

4. Seal material. Buna-N (nitrile) handles petroleum oils, water, and air up to roughly 200°F at low cost. Viton (FKM) extends service to about 400°F and resists aggressive chemicals and fuels. Seats are typically PTFE or reinforced PTFE in either case.

5. Size. Size by required flow (Cv), not just pipe size. Note that flow through an L-port always makes a 90° turn inside the ball, so its Cv is lower than a straight-bore 2-way valve of the same size; undersizing shows up as pressure drop.

Standards & References

  • ASME B16.34 — Valves: Flanged, Threaded, and Welding End. Establishes pressure-temperature ratings and material requirements for industrial valves.
  • SAE J1926 — Connections for General Use and Fluid Power: Ports with ISO 725 straight threads and O-ring sealing. Governs SAE O-ring boss ports on hydraulic 3-way valves.
  • ASME B1.20.1 — Pipe Threads, General Purpose (Inch). Defines NPT taper thread dimensions used on threaded valve ends.

Frequently Asked Questions

What is a 3-way ball valve used for?

A 3-way ball valve routes flow among three ports with one quarter-turn handle. It is used to divert flow from one source to either of two destinations, combine two inlet streams into one outlet, select between two tanks or pumps, isolate instruments, and switch hydraulic circuits — replacing two 2-way valves and a tee with a single valve body.

What is the difference between L-port and T-port?

An L-port ball has a 90° elbow-shaped bore that connects the common port to only one outlet at a time, making it ideal for diverting and selecting, often with a full-shutoff position. A T-port ball has a T-shaped bore that can connect all three ports or any pair, making it ideal for mixing and straight-through-plus-branch flow, but it typically cannot shut off all ports.

Can a 3-way ball valve shut off flow completely?

Many L-port valves can: rotating the ball so both bore openings face solid body wall blocks all three ports. T-port valves generally cannot — at least two ports remain connected in every handle position. If full isolation matters, confirm the manufacturer's position chart shows an all-ports-blocked position, or add a separate shutoff valve.

Which way does flow go in a 3-way valve?

Flow direction depends on the bore pattern and handle position, not the valve body. On a typical bottom-common L-port valve, flow enters the common port and exits the left or right port depending on handle position. On a T-port, flow can pass straight through the side ports, split to all three, or run common-to-branch. Always reference the position plate on the valve.

Do 3-way ball valves work for mixing?

Yes — choose a T-port pattern. In the all-ports-open position, a T-port valve combines two incoming streams into a single outlet, which is how it is used for blending water temperatures, mixing chemical feeds, or merging return lines. An L-port cannot mix, since it never connects more than two ports at once.

Shop Related Products

Ready to put the right port pattern to work? Browse our 3-Way Ball Valves in L-port and T-port configurations, or explore the full lineup of Brass Ball Valves, Stainless Steel Ball Valves, and All Ball Valves. Every valve ships with full specifications, and free shipping applies to orders over $500.