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+Jason Hugo

#46 - How to Use a Pitot Gauge for Hydrant Flow Testing

Posted by on 5/20/2016
Fire hydrants must supply adequate water during an emergency, and the Pitot Gauge is a popular and effective tool for testing water flow. To prepare for the worst, municipalities and other authorities require regular hydrant flow testing. Read on as we describe what a Pitot gauge is, why hydrant flow testing is necessary, who conducts these tests, and how to use a Pitot gauge during a flow test.

The handheld Pitot gauge remains the fastest and easiest method for measuring both straight tip and hydrant flow GPM (gallons per minute) to uncover reduced flow rates. Heavy pipe-wall deposits and closed valves are often uncovered when analyzing data collected by a hand-held Pitot gauge.

How does a Pitot gauge work?

A Pitot gauge consists of three components: a blade, handle, and gauge. Once placed within an open fire hydrant’s water discharge, a narrow tube inside the blade directs water towards the gauge to enable a pressure reading. The captured PSI readings can be used to calculate the disparities in pressure between two points along a distribution system. But wait, don’t we want GPM?!

Components of a Pitot Gauge

The desirable unit of measure we seek from hydrant flow testing is a hydrant’s GPM (gallon per minute). To convert from PSI to GPM, we refer to Bernoulli’s principle. A Swiss mathematician, Bernoulli developed a formula to represent the relationship between velocity and pressure along a streamline:
Bernoulli’s equation, however, doesn’t account for friction produced when water flows through a pipe system. This friction represented mathematically is called the hydrant’s coefficient and it represents a pipe's wall roughness or abnormalities. The pipe's coefficient is required to convert to GPM. The Moody diagram or Colebrook equation are both resources that can help in estimating your pipe's coefficient. Once the coefficient is identified, you're ready to convert to GPM. Watch this video to see an example:

You can also see a theoretical discharge table we provide for our Pitot gauges here.

Who performs Fire Hydrant Flow Tests with Pitot Gauges?

City officials and professional contractors perform hydrant flow tests with Pitot gauges. The guidelines and regulations they follow are commonly drafted by the National Fire Protection Association (NFPA), which provides guidelines for hydrant flow testing procedures. It is recommended that a city’s entire distribution system be tested every five years. Besides peace of mind for emergencies, the tests enable municipalities to color code their fire hydrants. City officials use GPM readings to color code fire hydrants according to their strength of output; this allows fire departments to assess their water resource capabilities quickly when arriving on the scene of an emergency.

How to Use a Pitot Gauge in 9 Steps

Provided below are 9 high-level steps for performing a hydrant flow test with a Pitot gauge. It is by no means a comprehensive step-by-step guide. Consider consulting your local fire department or inspector if you want detailed, local information.

Step 1: Determine the location of your test by selecting a group of hydrants in the same vicinity. Mark one hydrant as ‘residual’, which signifies that is the test hydrant. Both static pressure (when flow hydrants are closed) and residual pressure (when flow hydrants are open) are to be measured. Per NFPA 291:
  • “This hydrant is chosen so it will be located between the hydrant to be flowed and the large mains that constitute the immediate sources of water supply in the area.”
  • “The number of hydrants to be used in any test depends upon the strength of the distribution system in the vicinity of the test location.”
Step 2: Flush residual hydrant to remove any sediment and fasten nozzle cap with gauge on the hydrant’s outlet.

Step 3: Slowly release the main valve until air is vented and take a static pressure reading.

Step 4: Measure the inside diameter of the outlet nozzle or hydrant outlet flow occurs. Again, hydrants are usually 4”.

Step 5: Field personnel should slowly open each fire hydrant, one at a time. This avoids pressure surges.

Step 6: Readings are taken using the Pitot gauge at each hydrant once the residual pressure reading on the outlet cap has stabilized.
  • Residual pressure and Pitot gauge readings must be taken simultaneously
  • For accurate Pitot gauge readings, the Pitot tube should be held downstream and in the center of the nozzle
Step 7: Record both the residual and Pitot gauge readings

Step 8: Slowly close each fire hydrant

Step 9: Combine the PSI readings from the residual hydrant’s static and residual pressure, with the coefficient determined by measuring the inside diameter of the hydrants outlet nozzle, and enter both into Bernoulli’s principle to formulate your GPM value

Voila! Now you know how to conduct a hydrant flow test with a Pitot gauge. Check out this short clip showing a Pitot gauge in action:

How to Buy a Pitot Gauge

When you’re in the market for a new Pitot gauge, keep the following tips in mind. Maintaining positive grip is a must, especially considering the Pitot's operating circumstances. Make sure the Pitot gauge is light but firm in your grip. Look for a rotating pitot gauge on a quick disconnect fitting for easier reading and faster draining of the system. Also, it’s handy to have a notched blade for Playpipe or hose nozzle readings in addition to the straight blade for hydrant and pump testing.

If you're ready for the best, most accurate Pitot gauge, look no further than the Inspector's Choice Pitot Gauge Kit.

Best Pitot Gauge Kit: Inspector's Choice

From the ground up, this Pitot gauge was built to exceed your expectations. The Sure-Grip handle and lightweight construction (only 14-ounces!) fits comfortably in your hand test after test. Improve your accuracy with the Ashcroft 1009 Duralife Gauge. N.I.S.T.-certification guarantees 1% full-range accuracy. The quick disconnect brass fitting enables 360-degree gauge rotation and easy system draining.

The Inspector's Choice Pitot Kit is ready for any job. Measure discharge from hydrants, nozzles, and Playpipes using the included straight and notched ultra-thin blades. Carry up to two gauges in the foam lined, lockable carrying case that comes with an extra roll of Thread Seal tape.

Buy the Inspector's Choice Pitot gauge from QRFS.com and choose from eight gauge ranges up to 600 PSI FREE. You'll have your Pitot gauge kit fast since most orders ship the same day. Ready to check out the Inspector's Choice Pitot? Click the link to get started.

This blog originally posted on May 20, 2016 by Jason Hugo and Cameron Sharp of Quick Response Fire Supply. Like what you read? Check out the rest at QRFS.com/blog

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