Water supply basics: Hydraulics for pump operators
Reviewing the fundamentals of water and how to estimate your available supply
The legendary football coach of the Green Bay Packers, the late Vince Lombardi, is said to have uttered these words to his team in the locker room after a poorly played game the day before: “Gentlemen, we’re gonna get back to basics. This is a football.”
Call them what you will, but the “basics” or “fundamentals” are the foundation for good performance in any discipline, and being a competent motor pump operator (MPO) is no different.
In a previous article, How to calculate and overcome friction loss, we discussed methods for motor pump operators (MPOs) to use in calculating friction loss. For this article we’re going to review a couple more MPO basics and give you and your team an opportunity to brush up on these fundamentals of firefighting hydraulics. Ready?
A quick note: Different fire departments have different titles for the person who operates the water pump on fire apparatus (e.g., engineer, driver/operator, chauffeur). In the department where I served, Chesterfield (Virginia) Fire and EMS, our title was MPO, and that’s what I’ll use throughout this piece.
BASIC PROPERTIES OF WATER
Ladies and gentlemen, this is water. (We did say basics, right?) A couple facts regarding one cubic foot of water:
- Weighs 62.5 pounds
- Occupies 1,728 cubic inches of space
- Contains 7.5 gallons U.S. (one gallon of water weighs 8.35 pounds and occupies 231 cubic inches)
It’s always a good idea to keep these numbers in mind when you’re supplying hoselines pumping 200 to 400 gpm into the second floor (or higher) of a structure. Another way of saying that is that you’re pumping between 1,670 and 3,340 pounds of water into the building every minute. And it’s not long before today’s structures – those built using lightweight construction materials – cannot resist the forces of gravity and collapse.
POSITIVE-PRESSURE WATER SOURCE
Every pumping operation begins with a water supply, and after initiating fire flows from the tank water on the apparatus, that likely involves connecting to a positive-pressure water source: a fire hydrant. Many MPOs know the terms “static pressure” and “residual pressure,” but do not understand them in the context of hooking their pumper up to a hydrant, so here we go.
Static pressure is the pressure that shows up on your pump’s intake manifold pressure gauge (compound gauge) after you’ve made the hose connection, fully opened the hydrant, and have not yet begun flowing water from the pump’s discharge manifold.
Residual pressure is the pressure showing on your pump’s intake manifold pressure gauge once you start flowing water from the discharge manifold. The MPO must know how to use the difference between the two pressures (static and residual) to determine how much additional water is available and, consequently, how much additional fire flow they can provide to the incident commander.
ESTIMATING REMAINING AVAILABLE WATER FROM A HYDRANT
As an incident commander, there were always two pieces of information I wanted my MPOs to have on the tip of their tongue: How much water are you currently flowing and how much more can you give me?
You can easily have that intel for your IC by noting the amount of pressure drop on the intake manifold pressure gauge and using either the First Digit Method or the Percentage Method.
Let’s review the two methods for estimating available water supply.
First Digit Method: To use the First Digit Method, note the difference between the static pressure and the residual pressure. If that difference is less than or equal to the first number in the static pressure reading, then you can provide three times the initial fire flow. Let’s say your static pressure reading is 65 psi, your residual pressure reading is 60 psi, and you’re flowing 400 gpm.
That’s a drop of 5 psi (between static and residual pressures), which is less than the first digit of the static pressure (6). So, you can flow an additional 1,200 gpm (400 gpm x 3 = 1,200 gpm). See Table 1 below.
You try it #1: Let’s say your static pressure reading is 60 psi, your residual pressure reading is 40 psi, and you’re flowing 500 GPM. How much more water can you get from that hydrant?
Percentage Method: When using this method, the MPO determines the percentage drop in pressure on the intake gauge when a discharge is opened.
Here’s an example: The MPO records the static pressure as 70 psi and, after flowing a handline at 250 gpm, notes the residual pressure as 60 psi. Starting from the initial 70 psi static pressure reading:
- A 10% drop would be 7 psi
- A 15% drop would be 10.5 psi
- A 25% drop would be 17.5 psi
Based on the numbers and scenario (10 psi is about 15%), you would be able to flow twice the amount you’re already flowing (250 gpm x 2 = 500 gpm). See Table 2 below.
You try it #2: Your static pressure reading is 80 psi, your residual pressure reading is 65 psi, and you’re flowing 750 gpm to an elevated master stream device. How much more water can you get from that hydrant?
ATMOSPHERIC PRESSURE AND WATER
When drafting from a static water source (e.g., lake, pond, river), it’s important to know that 1 psi of atmospheric pressure will elevate a column of water 2.304 feet. So if atmospheric pressure at sea level is 14.7 psi, theoretically, the maximum lift for drafting from a static water source would be 34 feet.
Here’s the math: Atmospheric pressure (14.7 psi) multiplied by the number of feet each psi elevates water (2.304 feet) equals 33.87 (34 feet). (From a practical standpoint, lifts over 20 feet are usually not recommended, which answers the question, “Why don’t pumpers routinely carry 34 feet of hard suction hose?”)
ELEVATED HEAD PRESSURE
Fact: A one-inch by one-inch column of water 1 foot tall exerts an elevated head pressure of 0.434 psi upon its base. See Figure 1 below.
So the MPO must figure in 0.434 psi to overcome the back pressure from any elevation (e.g., fire building is higher than the pumper, supplying a standpipe system). You have firefighters working off a standpipe you’re supplying and they’re 100 feet above you and your pump. Better make sure you’re adding 43.4 psi (100 feet x 0.434 psi) to the 150 psi called for in your department’s SOGs.
You try it #3. You’re an MPO and you’ve connected your pumper to supply a standpipe system in an eight-story office building. Your fire department SOG calls for an initial flow pressure of 150 psi to the standpipe connection. How much additional pressure should the MPO add to compensate for the elevation change?
Keep up on basics
As firefighters, it can sometimes seem overwhelming as we try to keep up with the ever-increasing body of knowledge required to do our job safely, effectively and efficiently. But periodically reviewing the basics, like water supply, should always be a part of your professional development.
And here are those answers:
- Answer #1: Not 500 GPM but perhaps a smaller amount. The pressure drop of 20 psi (60-40) is greater than three times the first digit of the static pressure (3 x 6 = 18). Look back at Table 1.
- Answer #2. The pressure drop was 15 psi (80 psi - 65 psi). From your static pressure:
- Answer #3. Figuring 10 feet per floor for an office building, the MPO needs to add an additional 35 psi to the SOG’s required 150 psi for a total of 185 psi (8 floors x 10 feet/floor x 0.434 = 34.72).
Editor’s Note: What MPO tips do you have? Share in the comments below.