#138 – Basic Hydraulic Calculation Tips

By Jacob Raines

We know that learning higher math concepts can make you feel a little queasy.

Not knowing the math basics needed for hydraulic calculations is intimidating and lack of confidence on top of that can keep you from advancing to the next level in your career.

If you are not confident in math as a subject, then you can build upon some basic math concepts to master a more complex concept: HYDRAULIC CALCULATIONS!

When studying for a test, you never really figure out how well prepared you are until test day comes around. While I was good at math in high school, I was taught a harsh lesson during my first geometry test: I had memorized all the right equations, but I had no idea how to solve the actual problems.

My issue with geometry was that I knew how to plug in the numbers, but I did not know what the numbers really meant. Many individuals who take the NICET exams encounter a similar issue when it comes to the concept of hydraulic calculations. So here are a few tips in hopes that you don’t end up like me in geometry class when you take the NICET exams.

First, let’s answer the question:

What are Hydraulic Calculations?

Hydraulic calculations are used to determine the pressure and output of water (flow) for a given sprinkler system. It all starts with a fundamental equation found in NFPA 13, which is used throughout the calculation process:

K = Q ÷ √P

This equation shows the relationship between flow rate, pressure, and the orifice [link to: https://en.wikipedia.org/wiki/Orifice] size of the sprinkler. There are three variables that are used for this equation, so let’s break those down.

Q = Flow Rate (gpm). The flow rate is the amount of water that flows through the sprinkler head and is measured in gallons per minute (gpm).

P = Pressure at the sprinkler head (psi). This one is simple; it is basically the pressure at which the water flows from the sprinkler, measured in pounds per square inch (psi). A sprinkler with a lower pressure will yield a slower stream of water, where a sprinkler with a higher pressure will produce a much faster stream.

K = K-Factor of the sprinkler head. The K-Factor depends on the orifice diameter of the sprinkler. A sprinkler with a low K-Factor will restrict the flow of water, whereas a sprinkler with a higher K-factor will allow more water to pass through the sprinkler head. Large k-factors yield larger flows and small k-factors yield smaller flows.

If pressure is held constant and k increases, it results in a larger flow rate.

Now that we have the definitions out of the way, lets look closer at the basic hydraulic formula itself.

The basic hydraulic formula can be re-arranged in three different ways as follows:

Solving for K-Factor K = Q ÷ √P
Solving for pressure (psi) Q = K × √P
Solving for flow rate (gpm) P = (Q ÷ K)2

Now that we have all the pieces, lets put this puzzle together with a practice problem!

What is the K-Factor of a sprinkler head that is flowing at 18 psi at a flow rate of 28 gpm?

The question is asking for the K-Factor, so we know to use: K = Q ÷ √P
Plug everything in, and our formula will look like: K = 28 gpm ÷ √18 psi
That leaves us with: K = 28 gpm ÷ 4.24 psi
Finally, after we divide, we get: K = 6.6

Hydraulic calculations become much more digestible when we know what is really going on behind the equation. When faced with more advanced concepts that will show up on higher level NICET exams, knowing the basics of the problem becomes very important.

This mindset of understanding the particulars of an industry concept do not just apply to mathematical equations, but everything in the fire protection industry. Developing an understanding for the topics that show up on the NICET exams will not only help you pass the test, but it will make you a better worker in this life safety industry.

To build on this concept and further prepare for your NICET Level I exam, check out our resources!

To learn more about hydraulic calculations, see these online courses:

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Intermediate / Level II: Hydraulic Calculations L2

Advanced / Level III: Hydraulic Calculations L3