The MetraSeal 120 Firewalker from Metraflex is a fire stop for pipes in motion. Creating a snug, fire-tight, watertight seal the unique design permits unlimited axial pipe movement, while at the same time keeping the fire stop properly positioned in the hole. The Firewalker is a fast, permanent, intumescent, mechanical seal made of interlocking links that works as a fire stop, filling the gap with an instantly verifiable thickness around any metallic, steel or copper tubing. UL listed with up to a two-hour fire protection rating, it has an intumescent formulation that forms a hard, crust-like barrier when exposed to fire. Metraflex.