“I love the sound of breaking glass.” Nick Lowe
When I was a kid, the song, “I Love the Sound of Breaking Glass,” was on the radio. I didn’t get it. I had memories of the sound of breaking glass, none of them good: The sound of an errant baseball going through the neighbor’s picture window. The sound of my mom’s favorite glass vase shattering against the wall after being knocked over during an out-of-control pillow fight. The sound of a waitress dropping a tray loaded with dishes. What was there for me to love?
Nothing. And there still isn’t.
Explosions and Flying Glass
When most people think of explosion effects, they first think of blast waves. Then they think of flying fragments. Glass, being one of the weakest elements in a building, breaks long before walls fail, floors collapse, and roofs cave in. When a blast wave breaks glass, it can break it into lethal shards that it then launches at high velocities for quite some distance. With explosions in urban settings, glass fragments cause most of the injuries.
The hazard of flying glass following an explosion is so prevalent that the EPA chose 1 psi overpressure as the endpoint for explosion scenarios in the Risk Management Planning rule (40 CFR 68). Why 1 psi? Not because 1 psi is the pressure that bursts eardrums, liquifies internal organs, or break bones. No, the EPA chose 1 psi as the overpressure endpoint because 1 psi is the overpressure at which glass shatters.
The EPA’s Guidance for Offsite Consequence Analysis states that “an overpressure of 1 psi may cause partial demolition of houses, which can result in serious injuries to people, and shattering of glass windows, which may cause skin laceration from flying glass.” “Skin laceration” sounds pretty tame; just a fancy way of saying “scratches,” right? But annealed glass, the glass typically used in construction, fractures into fragments that are both dagger-shaped and razor sharp. The injuries can be considerably worse than scratches.
What Overpressure Should You Consider in Facility Siting Studies?
Process facilities are not urban settings. Yet there are windows: in control rooms, in laboratories, even in the fancy administration building that sits at the entrance to the plant. When the blast calculations are done and you know what the blast wave will look like, how will you know if the windows need to be replaced or removed? And if the windows are to be replaced, replaced with what?
The EPA uses an endpoint of 1 psi overpressure in its RMP rule. However, there are several other sources with guidance. FEMA, for instance, suggests that broken windows occur at incident overpressures in the range of 0.15 to 0.22 psi. There is a difference, however, between cracking and fragmenting, between fragments and flying shards.
NOAA lists glass responses at several different overpressures:
0.15 psig Typical pressure for glass failure
0.5 -1 psig Windows usually shattered; some window frame damage
1 – 8 psig Range for slight to serious laceration injuries from flying glass
As NOAA points out, “Unlike toxic levels of concern , no well-defined guidelines or standards exist to evaluate the overpressure hazard. So, ALOHA uses default overpressure values (in pounds per square inch, psi) that are based on a review of several widely accepted sources on overpressure and explosions.” For glazing, ALOHA assumes that 1 psi overpressure shatters glass.
Whether glazing needs to be replaced depends on the anticipated overpressure. If the anticipated overpressure is less than 0.15 psi, glass doesn’t need to be replaced. Up to 0.5 psi, glazing might crack during an incident, but to replace it preemptively because it might crack seems silly. Above 1 psi, there is potential for injuries, even fatal injuries. This is glazing that should be replaced.
The range between 0.5 psi overpressure and 1 psi overpressure is where the tough questions are. So, there is one more factor to consider. Size.
The bigger the window, the lower the overpressure needed to break. British Gas published some work many years ago showing that both area and thickness mattered. The pressure to shatter glass is proportional to the thickness of the glass, while the pressure to shatter glass is inversely proportional to the square of the area. In rough terms, windows smaller than 1 sq.ft. take an awful lot of pressure to break. As they get bigger, windows require much less pressure to break, with 15~20 sq.ft. being the size at which bigger doesn’t matter that much anymore. Since a 15~20 sq.ft. window is pretty ordinary, this probably explains why most tables published these days simply give a single overpressure value for glass.
Replace It? With What?
Most window glass is annealed glass, also known as float glass, plate glass, or sheet glass. The published tables all assume that the glass being considered is ordinary annealed glass. If it is to be replaced, one option is to replace it with thicker panes. As the folks at British Gas confirmed, the pressure to shatter glass is proportional to the thickness of the glass.
One option is to replace annealed glass with thermally tempered glass (TTG), also known as safety glass. This is the kind of glass found in automobiles. Building codes require TTG in construction glass that the public will physically touch, such as glass doors. TTG is four to five times stronger than annealed glass, and when it breaks, it fractures into small cubes.
Another glazing option is not glass at all. Polycarbonate glazing is very strong and very blast resistant. It is comparatively expensive, though, and unlike glass, is combustible and suffers from abrasion and environmental degradation, especially from aromatic hydrocarbons. So, polycarbonate glazing is not a good option for the control room windows at a styrene plant.
Georgian Wired Glass
There was a time when the safety glazing of choice was wire-reinforced glass, also known as Georgian Wired Glass. Instinctively, people just accepted that wired glass was more secure. Putting wire in glass was like putting re-bar in concrete. It had to make it stronger, right?
Photo credit: Pixabay
As it turns out, the wire actually weakens the glass. Wire makes annealed glass more susceptible to fracture and the embedded wire mesh does not prevent pieces of glass from flying free. Worse, if the blast pressure throws someone into the wired glass, the wire can ensnare them, compounding their injuries.
A better option for reinforced glazing is laminated glass. Laminated glass is stronger than monolithic glass of the same thickness, and when the glass does break, the polymer interlayer tends to hold the fragments together.
Consider the Glass
As you work on your facility siting studies, don’t forget to consider the glazing. Long before the walls collapse, the glass will break. The charts and tables that various organizations have published are based on single-pane annealed glass. It is clear that at overpressures above 1 psi, windows can become dangerous, even fatal, hazards.
Unfortunately, the first thought that many have upon recognizing the hazard of their glass windows is to replace them with wire-reinforced glass. Don’t. This is typically worse than leaving the annealed glass panes in place.
Fortunately, there are options for replacing ordinary glazing. TTG, polycarbonate, and laminated glass can all serve to reduce the hazard posed by windows during an explosion. If this not enough, consider smaller windows.
Just don’t give up on your windows. You don’t have to brick them in and you don’t have to learn to love the sound of breaking glass. You have other options.