The intake/suction gauge and what it shows is critical for knowing what is occurring with your water supply. For those who use drafting as a primary means for a water supply, it helps to know if you are getting an adequate vacuum and maintaining the water flow coming in.
Here, we’ll addresses what occurs when being supplied from a pressurized water source.
In a perfect scenario, a supply line would be deployed from a fire hydrant to the pump intake of the engine spotted at the fire. The engine would then be placed in pump gear. The hydrant would be opened before water is flowing.
Once this occurs and the intake is opened, it is important to note the static pressure reading on the intake gauge. Subsequently, once fire lines are opened, the driver will need to note the drop in pressure. Remember the more gpm going out, the more the intake gauge will decrease.
Once the static pressure and the drop after water is flowing are noted, the driver needs to calculate the percentage of drop. This will determine how many more gpm they can flow. Our department called this, “limits for mains.”
Limits for mains
For example, after opening the intake, let’s say we had an initial static reading of 70 psi. We opened our gates to two individual fire lines, each with a 125-gpm nozzle. We would be flowing 250 gpm for the time being.
The pressure drop noted on the intake gauge went from 70 psi to 60 psi. What percentage of drop did we have?
To know what your limits to mains are, the following rules apply:
Therefore, starting from our initial 70 psi static pressure, 10 percent would be 7 psi, 15 percent would be 10.5 psi and 25 percent would be 17.5 psi (10 percent + 15 percent = 25 percent).
Notice we are only calculating for the upper end of the ranges. This makes our job easier, because all we need to know is what range the calculated number falls in.
Based on the above formula and scenario (10 psi is about 15 percent), we would be able to flow 2 times the amount we are already flowing — 250 gpm flowing plus an additional 500 gpm brings the total flow to 750 gpm.
Regardless of the capacity or main size, the drop in pressure according to the gauge will allow you to calculate how much more water is available. Remember, the goal is to not let the intake gauge drop below 20 psi.
You may be able to stretch it to 10 psi. However, pay extremely close attention and make sure no one else uses that same main and robs water from your supply line or that more lines are not pulled off your truck.
Have a safety buffer in place. If the intake gauge zeroes out, you will lose pressure, lose your effective streams and pump cavitation will occur.
From tank to hydrant
One question always arises from driver trainees: What if I am already pumping fire lines from the trucks tank, then I get the supply line established afterwards; how do I know what the static pressure is after water is already flowing?
The answer is to make a best estimate. You really won’t know until another fire line is taken off the truck. Then, once it is flowing, note the drop and calculate to the best of your ability.
Let’s take the above scenario and say we were already flowing 250 gpm from the truck’s tank. After the supply is established and we transitioned from the truck’s water tank to the supply line via the intake valve there’s a reading of 60 psi.
Subsequently, another two fire lines are pulled, each with a 125-gpm nozzle. The pressure drops to 50 psi. Based on what occurred, we can assume that the initial reading would have been 10 psi more than the initial 60 psi with water flowing or 70-psi static pressure.
It was always difficult to instruct new drivers in training on how to do this. I believe it was because they had to relearn percentages, and in a way, to work backwards.
We would spend all the time needed until they grasped this important concept. After a little while, it would start clicking.
Frank R. Myers is a retired Lieutenant with the City of Miami Fire Rescue, where he served 32 years. Before his retirement, he served at the training center for six years as the driver engineer instructor. He works as a consultant for PSTrax.com, a technology service that helps fire departments automate their apparatus, equipment and inventory checks.