For years, departments have relied on pump charts for establishing pump discharge pressures (PDP) on high-rise fires. And generally, these charts are based on 125 plus five psi per floor in elevation loss. Using these charts may work very well on many mid-rise structures, however on taller and more modern high-rises, there may be a better way.
Discard The Pump Card?
Pump charts based on five psi per floor in elevation loss are based on an average floor height of ten feet. Ten feet is multiplied by .434 psi (the pressure it takes to raise a column of water one foot), which equals 4.34 psi, round up to 5, and there you go. This works great on many structures such as low- to mid-rise hotels, office buildings and the like. Or even in older structures where all the floors are generally about ten feet in height. In taller structures, and even in newer mid-rise structures, the floors are often not universally ten feet in height. Some floors may be 12 to 15 feet, or even taller. Furthermore, the floors may vary in height in the same structure, particularly in structures with large lobbies or atriums on the first floor. Using a chart is great for figuring PDP on the fly for some mid-rise structures, however on taller buildings, or some of the newer mid-rises, they may not be as reliable.
Put In The Work
If you have very tall high-rises in your jurisdiction, you’ll have better results by doing the leg work and completing a thorough pre-incident survey than by relying on a per-floor pump card. From a pumping perspective, a good pre-incident survey will include the structure’s system pressure from the stationary pump, but you’ll also want to simply do the math. First, you’ll have to ascertain the structure’s height in feet. Oftentimes, this can be found with a simple Google search. If that doesn’t work, you can speak with the building management and they can almost certainly get you that information. Once you find the building’s height, you’ll multiply that number by .434 and that number will be the amount of pressure it takes to get water to the top of the standpipe. From there, you’ll add 100 psi. Assuming you’re using 2 ½” hose equipped with a smooth bore nozzle with a 1 1/8” tip, 100 psi will account for nozzle pressure, friction loss to the hose and loss to the pipe itself, though the condition of the pipe may present variables that we can’t account for. Scale and corrosion in the pipe may cause more friction loss, but this number is still a good place to start.
The reason for doing the math and not simply trusting the system pressure is that sometimes, particularly in older (pre-1993) structures, the building’s stationary pump may not give you the pressure you need on the upper-most floors. In these cases, even if the stationary pump is working, you may need to over-pump the system pressure, essentially taking the stationary pump out of the equation. So why use 100 psi plus elevation loss? Per NFPA 14, buildings built prior to 1993 are required to deliver a minimum of 500 gpm at 65 psi to the upper, or most remote, two standpipe outlets. Buildings built after 1993 are required to deliver at least 100 psi. Also, we cannot pump more than 50 psi over the stationary pump’s system pressure. So using 100 psi plus elevation will generally give us the pressure we need for any high rise structure without over-pressurizing an older structure.
Standing at 52 stories, the Devon Tower is Downtown Oklahoma City’s newest and tallest high-rise. Using the chart at 125 plus five psi per floor, and pumping to the roof, we would set our PDP at 360 psi. However, the actual height of the structure is 844 feet. Doing the math (844 x .434 + 100), you’ll find that you’ll actually have to pump 470 psi to get adequate pressure to the roof. Using the chart, you’ll be over 100 psi short. While the pump chart may work very well on a number of structures, it never hurts to do the math.
Standard or Series?
During your pre-incident survey, another thing you’ll have to consider is the number of pumpers you’ll need to utilize to reach the substantial pressures needed in these high-rises. If your department utilizes two-stage pumpers, you’ll most likely be able to reach very high pressures with one pumper in pressure mode. However, if your department runs single-stage pumps, you may need to set up a series operation in order to reach the system’s demand in these structures. But when should you set up a series and when can you handle it with one pumper? Traditionally, the magic number has been 150 psi. For any system pressure over 150 psi, you set up a series and you add a pumper for every 150 psi of system pressure. This number may be a little conservative.
Consider the UL Date Plate on your engine. My engine has a pump rated at 1500 gpm. As you know, this means that my pump will produce 1500 gpm from a 10-foot draft at 150 psi (100% efficiency). It is important to remember that this is 150 psi Net PDP. Net PDP is your PDP minus your intake. So, as the data plate is established from a draft, the intake is zero, so the 150 psi is Net PDP. It is also important to remember that your pump will produce 50% of its rated capacity at 250 psi Net PDP. So, on my 1500 gpm pump, I can get 750 gpm at 250 psi from a draft. For standpipe operations, we only need 500 gpm; 250 gpm each for attack and backup. So, with this in mind, you could conceivably pump a 250 psi system with a 1000 gpm pumper from a draft. The UL DATA Plate doesn’t account for pumping from a charged source like a hydrant as municipal water systems vary greatly from city to city. But keeping Net PDP in mind, imagine you’re pumping that same 250 psi system while hooked up to a 100-pound hydrant. Your Net PDP is only 150 psi, or 100% efficiency for any Class A pumper. With those numbers in mind, it may be more appropriate to set up a series operation for any system pressure over 250 psi instead of 150. Any pumper rated for at least 1000 gpm can pretty easily produce 500 gpm, especially when hooked up to a hydrant on a charged municipal system.
When pumping two rigs in series, you’ll have to figure out the PDP for each pump. For this example we’ll use a 400-psi system pressure. Logic tells you that the rig at the hydrant will pump 200 psi to the rig at the FDC, which will in turn pump the full system pressure of 400 psi. Each pump will be producing 200 psi, the FDC rig producing 200 psi Net PDP with an intake of 200 psi from the hydrant rig. This makes perfect sense until you take the hydrant into account. The point of a series pumping operation is not only to achieve high pressures but also to equalize work between the pumps. So now let’s look at this example assuming a 100-pound hydrant, giving the rig at the hydrant an intake of 100 psi. If this rig now pumps 200 psi to the rig at the FDC, the Net PDP will be 100 psi. The rig at the FDC will pump full system pressure with an intake of 200 psi, giving him a Net PDP of 200. The FDC rig is doing twice the work. To mitigate this, the hydrant rig will pump half the system pressure (200) plus half the hydrant pressure (50), for a total PDP of 250 psi and a Net PDP of 150 psi. The FDC pumper will still pump full system pressure at 400 psi but with an intake of 250 psi, his Net PDP will be 150 psi. The rigs are now doing equal work.
A Few More Things..
There are some more important things to know about your pump and hose in order for this to work. First, your hose will have to be rated to withstand these high pressures. Ideally, you’ll use 2 ½”, or even better – 3”, to make your FDC connection. Ensure that the hose you’re using for the FDC is strong enough for the pressure, keeping in mind that the max operating pressure for attack hose is test pressure minus 10 percent. Next, you’ll need to know the max intake pressure for your pump. If it’s less than the intake you could receive during a series operation in your jurisdiction, you could have it adjusted by a certified mechanic or simply cap the intake pressure relief valve. It should be found under the rig. It will be a standard 2 ½” threaded pipe that will probably be clearly marked “Do Not Cap”. Go ahead and cap that. Don’t worry. You won’t hurt the pump. Just remember to remove it once your pumping operation is over. Finally, you’ll need to know the hydrodynamic test pressure for your pump. You may need to contact the manufacturer for that information. But whatever it is, under no circumstances should you over-pump that pressure. It could result in catastrophic failure of the pump.
There is a lot of information to gather, but during an incident as complex as a working high-rise fire, there is no substitute for a good pre-incident survey. The more information you have on hand when you arrive on scene, the more smoothly your pumping operation will go. And having all the pumping figured out before you arrive, the more time you’ll have to mitigate any issues that arise during the initial phase of the operation. Good luck and stay safe, brothers and sisters.