The in-line pressure gauge is an essential and often overlooked piece of equipment when operating off of a standpipe. From the earliest days of our pump operator training we are taught to start at the nozzle and work back to the pump to calculate the discharge pressure for a given line. Lets look at a simple example:
- Smooth bore nozzle with a 1 1/8" tip = 50 pounds per square inch (psi) nozzle pressure
- 200' of 2 1/2" hose @ 5 psi friction loss per length = 20 psi to compensate for friction loss
- Line operating on the 2nd floor = 5 psi for grade change
- Discharge pressure = 75 psi
This is very easy to calculate quickly in your head because all of the factors in the equation are known. How do you know how many psi to add for the piping between the fire department connection (FDC) and the standpipe outlet on the 4th floor? The short answer to this question: you don't know. It is impossible to know the piping configuration of every standpipe and you are crazy to think that if you knew it you could calculate it quickly at the time of a fire.
The quickest, easiest, and most accurate way to provide the proper pressure at the standpipe outlet is through the use of an in-line pressure gauge. This appliance, when connected directly to the standpipe outlet, allows us to calculate discharge pressure as if the pump panel of the engine was in the stairwell. The example above with the smooth bore nozzle and 200' of 2 1/2" hose can be calculated exactly the same way, ensuring 75 psi at the standpipe outlet to provide 50 psi at the nozzle.
The in-line pressure gauge becomes even more important when we have multiple lines operating off of the same standpipe. If the fire is on the 4th floor, the first line should be connected on the 3rd. floor. The second line which will either back up the first line or proceed to the floor above (5th floor) will be connected to the standpipe outlet on the 2nd floor. If the first line was 200' the second will have to be at least 300'. The second line will also be going up three floors – from the 2nd to the 5th so we will have to add 5 psi per floor to compensate for head pressure. Smooth bore nozzle (50 psi) + 300' of 2 1/2" line (30 psi) + 3 floors elevation (15 psi) = 95 psi. The only way to be sure that each line is pumped at the proper pressure is to place the in-line pressure gauge on the standpipe outlet.
There are a few steps we need to take before attaching the in-line pressure gauge to the standpipe outlet. First open the standpipe outlet and let it run for a few seconds… yes, the floor is going to get wet and so are the stairs, its okay. This serves two purposes: first, it tells us that we have a functional standpipe that has water and second, it will hopefully flush any obstructions from the outlet. Once the outlet is flushed, turn it off and look inside. If there are any obstructions still in there remove them with the channel locks that you carry in your standpipe kit. Now, connect your in-line pressure gauge to the outlet and your hose to the gauge.
Once all of the sections of hose are connected and the line is flaked out, open the standpipe outlet valve ALL THE WAY! At this point you will get a static pressure reading since there is no water moving through the line. Once the line is opened adjust the standpipe outlet valve until you get the desired residual pressure on the in-line pressure gauge. It is extremely important to set the pressure while the line is flowing water. If necessary, the nozzle team can open the nozzle in the stairwell or the public hallway to set the pressure before moving into the fire area.
Thanks for reading and be safe! As always, feel free to add your questions, comments, or suggestions.