Positive Pressure Fire Labels
Evolution and Explanation of Positive Pressure
The 1997 revision of the Uniform Building Code (UBC) includes a requirement for Fire Door Assemblies to be tested in a manner which more closely duplicates observations made in room fire experiments and which also corresponds to the test levels already specified in most international jurisdictions.
When a fire starts in a closed room, heat generated causes gases to rapidly build and expand. This creates a "positive" pressure in the room and this pressure attempts to force smoke and gas out of the area. The hot gases also tend to rise.
Once the fire is fully developed, the pressure differential between the inside of the room and outside reaches an equilibrium where negative pressure in the room exists in the lower part of the room and positive pressure exists above that. The area where these two meet is generally called the "Neutral Pressure Plane".
In the United States, this neutral plane is considered to be located at 40 inches above the floor for door assemblies and is consistent with observations made during room testing. In fact, door tests require that the test chamber be neutral in pressure as the test begins and that the neutral plane be elevated in the test chamber in the first few minutes of the test to be at 40 inches above the door within four minutes after the test in initiated. This simulates the behavior of a fire beginning in a room and becoming fully involved in four minutes.
The UBC test standard that applies is UBC 7-2-1997, Part I (UL-10C).
Positive Pressure and Wood Doors
The positive pressure inside the test room creates a technical challenge for wood doors in particular. The pressure tends to drive hot gases out of the room between the door and its frame, requiring in most designs special treatment on stiles and the top rail.
It is possible to seal the door to frame gap with an intumescent material, a compound which expands under heat to fill the gap and prevent hot gases from causing the door to fail at its edges. A number of manufacturers offer this material.
A&L Shielding's Experience in Positive Pressure Labeling
Pairs of lead lined doors require a lead lined astragal to shield against radiation leakage at the meeting stiles. This area is especially vulnerable in fire. A&L Shielding Inc. designs have successfully passed a 20 minute test with a wood lead lined astragal by incorporating intumescent material in the stile of one of the doors. In a fire, the intumescent expands at the meeting stiles just as it does between the hinge stile and top rail and the metal frame. This system is used by A&L Shielding Inc. for all three types of 20 minute pair labels: Neutral Pressure as well as Positive Pressure Categories A and B. It is also possible to use a listed metal edge and lead lined astragal, and some customers prefer this approach. In this case, intumescent material must be applied to the metal edge of one leaf for Positive Pressure Category A and B doors only.)
For 45 minute and 60 minute pairs, a lead lined metal edge and astragal must be used.
This type of labeled door, if properly designed, constructed, and tested can be listed for use with almost all types of listed door hardware. See the ITS-Warnock Hersey listings for hardware exclusions by manufacturer.
Door weight is a special consideration for lead lined doors and the use of pivot type hinges is common. Pivot hinges transfer some of the door weight to the floor, reducing stress on the frame and its attachment to the building. A&L Shielding has 20 minute, 45 minute and 60 minute PP listing in both Category A and B for pivots, earned through specific testing. Continuous hinges are also made for this high door weight and are listed for Positive Pressure installations.