Apart from this past week’s example, normal water-based sprinkler systems work wonderfully in a high-rise building. Here in Massachusetts, they are required in every building over 70 feet tall (no grandfathering, either). In 1986, there was a rip-roaring fire in the Prudential Building in Boston (unsprinklered) that was a cast-iron female dog to put out. Burnt quite a few floors, and smoked out most of the building. Had there been sprinklers, the fire would have been contained to the area of origin. In 1988 the legislation was passed requiring sprinklers in all 70+ foot tall buildings by 1997. Since then, no problems.
The engineering of highrise systems is really rather straightforward. Lower floors (around 12 and lower) are fed from normal street pressures. No big deal there. The rest of the building is divided up into pressure zones of about 12 floor each. Risers (the pipes that go up and down) feed these zones, with “express” risers feeding higher-up zones. Higher zones are allowed to take water from lower zones if you allowed for enough water in the lower zone.
These higher-up zones are where things become a bit more complex, but by no means difficult. In theory, you can pump water all the way to the top of a highrise with one or two pumps at the base of the building.
Rough and tumble sprinkler system calculation
Sprinklers are not designed for every sprinkler head to activate at the same time. You plan for an expected fire, based on the type of occupancy you’re protecting. An aerosol can warehouse would be protected differently than a warehouse storing cement block on metal shelves. The size of this expected area is based on the applicable fire code you’re using for this design (NFPA 13, 1999 ed for me).
Now that you have an expected design area, you can tell how many sprinkler heads are in this area (if the area is 2000 sq ft, and each head protects 100 sq ft, you need to plan for 20 heads to flow). You also need to know how much water is coming from each head. 0.25 gallons per minute per square foot is a decent flow for a normal office building, so we’ll use that.
The next step is to figure out how much water is flowing from each head. Since you don’t know where the fire is going to be, you plan for it to be as far away from the water supply (the riser) as possible. That way, if its closer, you know you’ll have enough water. The heads are laid out on pipes, so you can have multiple heads along one run of pipe. If the head at the end activates, water will flow past closed heads to reach that open one. If the next head opens, there will be a higher pressure there than in the first one, so more water will flow from that head. That rationalle continues all the way down the line. So if that first head is flowing 25 gpm (0.25 gpm/sqft * 100 sqft), the second head may flow 31, the third 46, and so on down the line.
A rough number, since we don’t actually have a bunch of pipe laid out in front of us (and I’m not going to design a system at 11pm), we’ll say we have 460 gallons per minute flowing through our pipes total. Thats at the top of our 110 story building (1,353 ft), since thats the toughest place to get water to. So we have to get 460 gallons per minute up through a pipe to 1,353’.
The simple calculation for getting water through a pipe somewhere is:
EP = NP + FL + EL
EP = Engine pressure (the push you need at the bottom)
NP = Nozzle pressure (what you want at the end)
FL = Friction loss (the resistance of the water to be pushed through a pipe)
EL = Elevation loss (0.433lbs/ft of elevation)
Lets say we have an 8" pipe going all the way to the top of the building (a good size for a very tall building). Nozzle pressure would be (an estimate) 141 psi at the very end of the system (where the farthest head is). Friction loss for 460gpm through 1,353’ of 8" pipe is 3 psi (thats the water resisting going through the pipe). Lastly is the elevation loss. Thats 0.433lbs per foot rise, or 586 psi.
Adding it all up:
EP = 141 + 3 + 586 = 730 psi
Do they make pumps that can do that? Sure. Even better, only pump at 365 psi and put a second pump halfway up the building. Getting water up there, no problem.
The problem arises when you don’t have that “expected fire.” No fire protection engineer in his/her right mind ever expected a 757 to slam into their building. The risers and pumps can’t push enough water up there to battle that fire. 15 floors of fire is beyond the design area by far. Plus, the risers very likely were compromised when the planes came through the building. Can’t put the fire out if there’s no water being delivered. Can you add foam to the water? Sure, but it requires a special type of head, and it doesn’t like to be used with a normal fire/plain water. Could you use a halon/FM200/CO2 system? Not really. That requires storage of the agent nearby where its intended to be used. That means every floor has to have a large bank of agent. Servicing and/or refilling these banks would be a huge undertaking. Plus, once the system discharges, thats it, there is no more agent to put the fire out with. If the fire didn’t go out, you’re not going to contain it (which is what sprinklers are designed to do anyway). Halon has to have a 5% concentration in air to work properly. Thats a lot of agent to put over a 7000 square foot floor.
High rises are a challenge to fire protection, but so far (excluding the very severe fire of last week) sprinklered high rise buildings have fared excellently under some very nasty fires. One Meridian Plaza in Philadelphia in 1991 for example. The fire burned for something like 9 or 10 hours over 8 or 9 floors. It was stopped by 9 sprinkler heads on the 27th(?) floor. What the entire Philly Fire Dept couldn’t do (including the loss of 3 firefighters) was stopped with 9 little metal devices with a water hole in the middle.
Wow…thats a long post…sorry…