Did you know standpipe systems are designed to deliver a prescribed amount of water at a prescribed pressure to the top most outlet? Unless you are a fire inspector or you are really into engine company operations you may not have put much thought into the design of a standpipe system. At first glance a standpipe system may appear to just be a pipe with some valves and fire hose threads that we can supply water to with a fire engine. If you are having trouble sleeping one night, try reading NFPA 14, it has all of the information on design and installation of standpipe systems. For firefighting purposes we need to be concerned with pressure (PSI), gallons per minute (GPM), and what hose/nozzle combination we are using.
There is great debate in the fire service over the proper hose/nozzle combination to use when operating from a standpipe system. That debate is driven by arguments over GPM, PSI, obstructions, etc. The easiest part of that debate to put to rest is the GPM argument. In my research and experience doing fire inspections I have never found a standpipe system that was designed for fire department use with a design flow less than 250 GPM at the top most or most remote outlet. We all know a 2 1/2" hand line flows approximately 250 GPM, therefore a properly maintained standpipe system can provide the appropriate flow.
The next issue is pressure (PSI). We all know that smooth bore nozzles operate best at 50 psi and there are a multitude of combination nozzles that operate at various pressures. Standpipe systems design standards were written based upon firefighters using 2 1/2" hose with smooth bore nozzles. These standards were first written in the 1960's and the modern versions are still based upon 2 1/2" hose and smooth bore nozzles. NFPA 14 required 65 psi residual pressure from the top most standpipe outlet up until 1993. From the 1993 edition of NFPA 14 until present 100 psi residual pressure is required from the top most or most remote outlet. The 65 psi requirement was intended to ensure a productive fire flow when 3 lengths of 2 1/2" hose with a smooth bore nozzle was attached to the top standpipe outlet. 50 psi nozzle pressure + 5 psi friction loss per length of hose = 65 psi outlet pressure. Operating in a building with a standpipe system designed before 1993 is our "worst case" scenario, so we will use the 65 psi requirement for further discussion.
Having the proper hose/nozzle combination plays a pivotal role in standpipe operations being a success or failure. New fire hose is not the same as older fire hose. Newer hose creates much less friction loss than older hose which further enables fire departments to employ 1 3/4" hose for standpipe operations. Regardless of which type of hose your department has you need to know its friction loss per length. The easiest way to determine this is to hook 100 feet of hose up to the engine and place an in-line pressure gauge behind the nozzle. Flow the line so you have the proper nozzle pressure at the in-line gauge and calculate the difference between the gauge on the pump panel and the in-line pressure gauge.
Based on the above information about standpipe design we can rule out 100 psi combination nozzles for standpipe operations. Even with the best fire hose with low friction loss and a system designed to have 100 psi residual pressure at the top most outlet we can reasonably conclude that we will not have 100 psi at the nozzle. This is true with 2 1/2" line and will only be worse with smaller line as we know friction loss increases exponentially as hose size decreases.
1 3/4" hose can create friction loss as high as 18.5 psi per 50 foot length, or 20 psi per length for firefighter friendly math. To dismiss 1 3/4" hose as a realistic standpipe line lets look at a smooth bore nozzle with 1 length of hose. 50 psi nozzle pressure + 20 psi friction loss for 1 length of 1 3/4" line = 70 psi outlet pressure. We know that older systems are only required to supply 65 psi at the top most outlet. We should never connect less than 3 lengths of hose to a standpipe outlet. We need at least 1 length to go from the floor below to the fire floor, one length for the hallway, and one length for the fire apartment. For arguments sake lets be extremely generous and assume a fancy new piece of 1 3/4" line only creates 10 psi of friction loss per length, half of what the older hose produced. With 50 psi needed to operate the nozzle + 30 psi friction loss = 80 psi needed at the standpipe outlet. You can easily see how these issues are further compounded if using a nozzle that requires more than 50 psi.
It is no secret that I like smooth bore nozzles. They are the simplest and NEARLY fireman proof. Check out our Nozzles: Truths From The Street article for my thoughts on smooth bore versus combination nozzles. There are some very nice low pressure fog nozzles on the market that will work well on a well maintained standpipe. I still prefer the smooth bore nozzle because as hard as we try to ensure systems are well maintained, we really don't know how well they are maintained regardless of what the paperwork says. When speaking of nozzle obstructions, I am far more concerned with items placed in the fire department connection (FDC) than items placed in the standpipe outlet. We all (I hope) flush any standpipe outlet before hooking up to it. Items placed in the FDC are not so easy to see or remove and can take some time to make it through the piping and into your nozzle. Those items can be dealt with much easier when using a smooth bore nozzle as described in the Nozzles: Truths From The Street article.
The other type of obstruction when dealing with standpipe systems is pipe scale. Pipe scale occurs when rust and corrosion adhere to the inner surface of the pipes. This also serves to increase friction loss in the pipe and can further reduce outlet pressure. The fitting it the picture was removed from a sprinkler system that was found to have some pipe scale issues. Pipe scale will not be found unless an internal pipe inspection is conducted. Pipe scale will have no effect on the operability of a smooth bore nozzle but can have a devastating impact on combination nozzles by clogging the tip.
Feel free to add your comments, questions, or suggestions. I will expand on this topic soon with some information about in-line pressure gauges for standpipe operations.