A tundish is a vessel or fitting designed to control the flow of liquid, but the term means very different things depending on whether you’re talking about steelmaking or plumbing. In steel manufacturing, a tundish is a large intermediate container that holds molten metal between the main melting vessel and the casting mold. In plumbing and heating systems, a tundish is a small funnel-shaped fitting that creates an air gap in overflow pipes to prevent contamination and provide a visible warning when something goes wrong.
The Steelmaking Tundish
In continuous casting, which is how most steel is produced today, molten steel follows a path from the furnace into a large transport vessel called a ladle, then into the tundish, and finally into a water-cooled mold where it solidifies into shape. The tundish sits physically between the ladle and the mold, acting as a reservoir that keeps the process running smoothly. When one ladle empties and a new one takes its place, the tundish holds enough molten steel to keep feeding the mold without interruption. This continuity is critical because any pause in casting can ruin the product or damage equipment.
A typical steelmaking tundish is a wide, shallow, refractory-lined vessel. It can feed multiple molds at once (four-strand designs are common), with separate outlets at the bottom directing steel into each mold. The refractory lining, often magnesia-based, must withstand direct contact with steel at roughly 1,500°C or higher. Over time, the molten steel corrodes the lining, particularly attacking silica-rich phases in the refractory material. Basic cover fluxes applied to the steel’s surface inside the tundish also contribute to this wear.
How a Tundish Cleans Molten Steel
What started as a simple holding tank has evolved into a key refining stage. Modern tundishes are engineered to remove non-metallic inclusions, which are tiny particles of oxides, sulfides, or other impurities that weaken finished steel. The primary removal method is flotation: inclusion particles rise to the surface of the molten steel, where they’re absorbed by a layer of protective slag on top.
To encourage this, engineers install internal structures like dams, baffles, and turbulence controllers inside the tundish. These redirect the flow of molten steel, forcing it to travel a longer path and spend more time inside the vessel. The longer the steel stays in the tundish, the more time inclusions have to float to the surface. Smaller inclusions can also collide with each other, merging into larger particles that rise more easily. Research has shown that inclusion particle size significantly affects removal rates, with larger particles separating out much more readily.
In advanced designs using channel induction heating, electromagnetic forces provide an additional benefit. As molten steel flows through heated channels, some inclusion particles are pushed toward and adsorbed onto the channel walls. The heated steel also creates density differences that promote upward flow patterns in the discharge area, further helping inclusions reach the surface.
Keeping the Temperature Right
Molten steel loses heat as it sits in the tundish, and temperature consistency matters enormously for casting quality. If steel arrives at the mold too hot or too cold, the final product suffers. Two main heating technologies address this problem: plasma heating and electromagnetic induction heating.
Induction heating has become the preferred option. It works by generating an electromagnetic field that induces an electrical current directly within the molten steel as it passes through a channel. The steel’s own electrical resistance converts this current into heat (Joule heating), warming the metal from the inside. Because the heat is generated within the steel itself rather than applied from outside, efficiency exceeds 90%, and temperature can be controlled within ±2 to 3°C. The system runs quietly, introduces no chemical contamination, and requires relatively little maintenance.
Plasma heating, by comparison, operates at about 65% efficiency with temperature accuracy of only ±5°C. It increases the nitrogen content of the steel, generates significant noise and electromagnetic radiation, and requires frequent replacement of cathode materials. For most operations, induction heating is the more practical choice.
The Plumbing Tundish
In domestic heating and hot water systems, a tundish serves a completely different purpose. It’s a small, open funnel installed in the discharge pipe of a pressure or temperature relief valve, typically on unvented hot water cylinders, boilers, or water heaters. The key feature is the air gap: the pipe from the relief valve terminates above the tundish’s open funnel, and a second pipe carries any discharged water away from below. That physical gap between the two pipes prevents any possibility of contaminated drainage water being siphoned back into the clean water supply.
This air gap is a legal requirement in many jurisdictions because without it, a drop in water pressure could create suction that pulls sewage or drain water backward through the overflow pipe and into the potable water system. The tundish eliminates that risk entirely by breaking the connection between the two pipe sections.
Using a Tundish as a Warning Signal
The open design of a plumbing tundish means you can both see and hear water flowing through it. This turns the tundish into a diagnostic tool. If water is dripping or flowing from the tundish on your unvented cylinder, something in the system needs attention.
A constant drip or significant water flow typically points to one of three problems. Excessive system pressure, often caused by a faulty expansion vessel, incorrect pressure settings, or a malfunctioning pressure reducing valve, can force water through the relief valve and out through the tundish. A failing temperature and pressure relief valve (the safety device designed to open if conditions inside the cylinder exceed safe limits) can also leak water through the tundish even when pressure and temperature are normal. In either case, visible water at the tundish is your early warning that the system is venting when it shouldn’t be, and the underlying cause needs to be identified and fixed.
In installations where the relief valve’s discharge pipe terminates in an out-of-sight location, such as an external wall or hidden drain, the tundish is especially valuable. Without it, a continuously discharging relief valve could go unnoticed for weeks, wasting water and potentially masking a dangerous pressure buildup.