Follow these an easy steps to ensure fire hydrants can supply adequate water flow during a fire

Firefighting operations have the right to be seriously endangered if fire hydrants no ready and also able to deliver the forced water flow and pressure. Municipalities and also other authorities urge upon consistent hydrant flow testing to ensure that adequate water is available when it’s needed most—which is whereby pitot gauges come in.

You are watching: Psi to gallons per minute calculator

In theory, pitot gauges make it simple to measure up the flow of fire hydrants and convert readings in pounds per square customs (PSI) come gallons per minute (GPM), permitting inspectors to conveniently uncover diminished flow rates. However in reality, the equation and also chart have the right to be confusing—and rebab.net phones are constantly ringing v the question: exactly how do I do this right?

In this blog, we will certainly take girlfriend step-by-step with the procedure of convert a pitot gauge’s PSI analysis to GPM. Us promise the we’ll do it so straightforward even her grandmother can do that accurately—especially due to the fact that we’ve provided a calculator to help you make the conversion. Feel cost-free to jump straight to the formula, adhered to by a step-by-step process and a calculator.

In the industry for pitot gauges? Be sure to browser our an option of pitot gauge kits and also accessories, consisting of our best-selling Inspector’s selection Pitot Gauge Kit, which costs a little more but it is intended 1 percent full-range accuracy.


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Firefighting operations deserve to be seriously endangered if fire hydrants can’t deliver proper water flow.


Pitot gauge 101: here’s how they work

Fire hydrants room color-coded according to their strength of output, permitting fire departments to assess your water source capabilities quickly when they come on the scene of an emergency. Because that instance, red hydrants frequently have a rated capacity of much less than 500 GPM and light blue hydrants have actually a rated capacity of more than 1,500 GPM. Through so lot at stake, it’s an important for hydrants to execute as expected throughout a fire.

A handheld pitot gauge is the quickest, simplest method for measure straight-tip and also hydrant-flow GPM. Analyzing the data it collection can likewise reveal heavy pipe-wall deposits and also closed valves that can considerably impede a hydrant’s pressure and also flow.

Previously, we’ve readily available a comprehensive guide on just how to use a pitot gauge because that hydrant circulation testing. However here’s a quick overview of how pitot gauges work:

A pitot gauge is composed of 3 components: a blade, handle, and also pressure gauge. After it’s placed into an open up fire hydrant’s water discharge, a small tube inside the tongue directs water toward the gauge to produce a push reading. Generally, the edge of the blade is set parallel to the direction the fluid-stream movement, in ~ a distance around one-half the diameter of the opening in the facility of the stream. Put simply, that method if the orifice is 2.5 inches, the blade should be about 1.25 inches out.


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Analyzing the data collected by a pitot gauge deserve to reveal hefty pipe-wall deposits and also closed valves the can considerably impede a hydrant’s pressure and also flow. Source: City of south Salt Lake Fire Marshal Regulations


The captured PSI readings indigenous a team of hydrants in the same vicinity space then supplied to calculate the disparities in pressure between two points follow me the distribution system. Yet in bespeak to recognize the results, the PSI readings need to be converted to GPM, the typical measurement that water flow. And also that’s where it it s okay a little tricky.

PSI can’t be directly converted come GPM; these are two various units the measure. PSI procedures pressure and GPM measures flow rate.

But if various other variables are known, the water’s flow rate in GPM can be calculated with the aid of the Bernoulli equation because that incompressible fluid and also careful unit conversion. Bernoulli, a Swiss mathematician, arisen a formula that represents the relationship in between velocity and pressure follow me a streamline.

Bernoulli’s initial equation must be amendment so that it accounts because that the friction created when water flows through a hydrant. As soon as represented mathematically, the friction is dubbed the “hydrant’s coefficient” and also is determined by the lose in circulation caused by the hydrant’s internal orifice design.

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The formula: what perform all those variables mean?

After successfully completing the circulation test and recording the information, this is the formula supplied to determine the rate of discharge flow from a single fire hydrant in GPM:


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Source: Fire security Handbook


Let’s breakdown what those variables mean:

29.84 is a consistent derived from physical legislations relating to water velocity, pressure, and conversion factors. In short, this number keeps the prize in GPM. Some sources offer a slightly smaller constant of 29.83.cd = the coefficient that discharge, which represents friction loss.d = the actual within diameter the the hydrant orifice in inches.p = the pressure in PSI read at the orifice by the pitot gauge. Because this formula take away the square root of p—rather than p itself—large rises in PSI will have actually a fairly small impact on the last GPM.Q = a number offered to stand for the result, or discharge in GPM.

Finding the number to plug in for “d” and also “p” room easy. To determine d, simply measure the within diameter of the hydrant’s outlet or outlet nozzle where flow occurs; to discover p, document the pitot gauge reading.

The discharge coefficient, or “c,” varies through the form of outlet on the hydrant. Different manufacturers have different internal hydrant orifice designs, and also friction is developed as water is compelled to go about bends or spicy corners.

In most modern-day hydrants, the change between the vertical barrel that the hydrant and also the horizontal outlet is smooth and also rounded. In the diagram below, take into consideration the difference in between the transitions (which look choose wings on either side of the center rectangle) in the an initial two outlet drawings. The rounded style has a coefficient the 0.9, definition there is only a 10 percent decrease in water flow because of friction loss.


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The discharge coefficient varies through the type of outlet ~ above the hydrant. Source: Fire Engineering


When the shift is square and sharp as in the second drawing, the hydrant has actually a coefficient the 0.8. Once it is square and also projects right into the barrel of the hydrant favor the illustration on the right, the coefficient is 0.7. Through poking her fingers inside the outlet, you deserve to determine if it’s smooth and rounded or square and sharp. Compare her findings come the 3 general species of hydrant outlets in the diagram above to recognize the coefficient of discharge.

Practice solving the equation you yourself or through our practice hydrant discharge calculator

Now that you recognize where the numbers come from, let’s practice solving the equation. While testing a single fire hydrant, this is what we found:

The flow reading top top the pitot gauge is 62 psi.The outlet is smooth and also rounded, meaning it has a coefficient of 0.9.

So, what’s the complete GPM available at normal operating pressure?

Step 1: multiply the constant 29.84 time the coefficient the discharge (cd). 29.84 x 0.9 = 26.856

Step 2: Square the measure diameter. 2.55 x 2.55 = 6.5025

Step 3: main point those 2 numbers: 26.856 x 6.5025 = 174.6

Step 4: discover the square source of 62 psi, i m sorry is 7.8740

Step 5: multiply the final two number to fix for Q, the discharge in GPM. 174.6 x 7.8740 = 1,375 GPM

Now that you’ve gained a understand on how PSI, discharge coefficients, and diameter impact a hydrant’s flow rates, try our handy GPM calculator because that fire hydrants:

Watch this video clip for one more step-by-step example of just how to usage the pitot gauge’s PSI reading to calculation the rate of water flowing from the hydrant in GPM:

Theoretical discharge tables make converting PSI to GPM also easier

Theoretical discharge tables, prefer the one we provide with our selection of pitot gauges, make the conversion even simpler. Simply find the pitot pressure reading on the left and also slide your finger over to the equivalent orifice dimension to disclose the GPM.

But it’s important to save in mind the the table is based upon using a coefficient discharge of 1 in the formula, i m sorry is the theoretical discharge native a perfect one orifice.

In the real world, no orifice is perfect. When much more accurate outcomes are required, NFPA 291 explains that the coefficient appropriate for your certain hydrant outlet must be applied to the numbers on the table.

In other words, if her pitot push is 28 psi and your orifice dimension is 4, her theoretical discharge follow to the table is 2,526 GPM. If the coefficient of your hydrant outlet is 0.9, multiply 2,526 x 0.9 find the actual rate of flow: 2,273.4 GPM.

Below is a theoretical discharge table indigenous NFPA 291; it additionally uses a perfect (impossible) coefficient discharge that 1. Click on the image to view the full-size version:


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Click here or ~ above the image over for a full-size version. Source: NFPA 291: Recommended practice for Fire circulation Testing and Marking that Hydrants


Pitot gauges aid inspectors ensure the fire hydrants room ready and able to fight fires

Pitot gauges are the fastest and also easiest method to measure up the rate of discharge from fire hydrants – as lengthy as you know the proper steps to converting press readings in pounds every square customs to gallons per minute. Adhering to these straightforward instructions help inspectors provide peace that mind the fire hydrants will execute as expected during a fire.


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The Inspector’s selection Pitot Gauge Kit.


In the industry for pitot gauges? Be certain to browse our choice of pitot gauge kits and also accessories, consisting of our best-selling Inspector’s choice Pitot Gauge Kit, which prices a little more but delivers 1 percent full-range accuracy.

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