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The Hydrant Flow Test Part I

Proper procedures for water flow tests can be found in NFPA 291 "Recommended Practice for Fire Flow Testing and Marking of Hydrants"

I can't begin to emphasize the importance of making sure you have up to date and accurate flow test data before you begin the layout of any project.

All sprinkler system design and layout begins with the flow test and there is one thing you need to remember.

Garbage In = Garbage Out

During your career you are going to make plenty of mistakes running into beams you didn't see, measuring a room and missing a foot or hitting some ductwork you didn't see on the mechanical drawings. These types of mistakes you will want to avoid to the best of your ability and while they can be costly they are seldom total disasters.

Designing a sprinkler system to erroneous flow test information, especially if the actual results are lower then what you designed to, has the very real potential of leading to a total disaster. On the right (or wrong) project this can be a six figure mistake.

Getting a water supply is usually the first thing you do in designing a system and, in my opinion, the most important part of the project. Don't minimize its importance.

First thing you will need is a pitot set of gauges that may look something similar to this:

To conduct a flow test you are going to need a good set of gages with a annual "Calibration Certificate" issued by a recognized test facility.

A calibration certificate looks like this:

Make your test accurate and insist on calibrated gauges.

You are also going to want a test report form and N^1.85 graph paper which you can find in pdf format under Graphs, Forms and Tables below.

Download Graphs and Worksheets Here
Graphs, Forms and Tables Click Here

So now that we have everything we need let's go do a flow test.

We're going to conduct a test of a 8 city watermain where we are contracted to install a sprinkler fire sprinkler system.

It's going to be a grocery store and while not cast in stone yet I am willing to bet a 6" or even a 4" run in *80' of 6" line from SRC to the building) will do nicely.

Whenever possible you want your test hydrant to be located downstream the property as shown in the sketch. Our test hydrant will be Hydrant A which is where we will obtain a static and residual pressure while Hydrant B will be our flowing hydrant.

If possible we want to use the nearest downstream hydrant, relative to the project site to be the hydrant used to obtain our static and residual hydrant.

It is important to note the elevations of the hydrant outlets, especially the test hydrant, relative to the finished floor of the project you are protecting which, in this case, we determined the finished floor of the project was going to be 2'-0" higher than the gauge. We don't have to get super accurate on the elevation shots and if the difference is just a foot or two most designers will ignore the elevation difference especially if the hydrant butt (that's the outlet) is higher then the site finished floor. As far as the flowing hydrant B's elevation it isn't critical but not a bad idea to note it.

But before we turn the flowing hydrant on we obtained our "static pressure" or what the pressure in the line is without any water moving. As the photo shows the static pressure was 54 psi.

After obtaining our static pressure we'll move down to our flowing hydrant to obtain a pitot reading that can be converted to obtain a gpm flow rate. During this flow test we obtained a pitot reading of 38 psi.

4.5 Test Procedure.
4.5.1 In a typical test, the 200-psi (14-bar) gauge is attached to one of the 2-in. (6.4-cm) outlets of the residual hydrant using the special cap.
4.5.4 A reading (static pressure) is taken when the needle comes to rest.
4.5.10 After the readings have been taken, hydrants should be shut down slowly, one at a time, to prevent undue surges in the system.
4.6 Pitot Readings.
4.6.1 When measuring discharge from open hydrant butts, it is always preferable from the standpoint of accuracy to use 2-in. (6.4-cm) outlets rather than pumper outlets.
4.6.2 In practically all cases, the 2-in. (6.4-cm) outlets are filled across the entire cross section during flow, while in the case of the larger outlets there is very frequently a void near the bottom.
4.6.3 When measuring the pitot pressure of a stream of practically uniform velocity, the orifice in the pitot tube is held downstream approximately one-half the diameter of the hydrant outlet or nozzle opening, and in the center of the stream.
4.6.4 The center line of the orifice should be at right angles to the plane of the face of the hydrant outlet.
4.6.5 The air chamber on the pitot tube should be kept elevated.
4.6.6 Pitot readings of less than 10 psi (0.7 bar) and more than 30 psi (2.0 bar) should be avoided, if possible.

Avoiding pitot readings above 30 psi is very difficult to do and most readings, from single hydrants, are going to be in the 30's and 40's psi range.

One more reading and we're done. The reading we are looking for is the residual pressure or what pressure is available with the downstream or flowing hydrant fully open. In this example the residual pressure is 47 psi.

We have everything we need which is:
Static Pressure: 54 psi
Residual Pressure: 47 psi
Pitot pressure: 38 psi
Flowing orifice size: 2 1/2"

As for the rate of discharge it is obtained by the formula:

which is the formula used to determine the theoretical discharge from a perfect circular orifice.

With a pitot pressure of 37 psi our theoretical discharge is 1,134 gpm.

But no orifice is perfect and we need to multiply the theoretical discharge of 1,134 gpm by a coefficient of discharge which, for most hydrants, is 0.90.

Assigning a coefficient of discharge can be tricky. Some insurance companies, most notably Factor Mutual, will only accept a discharge coefficient of 0.80 and older style hydrants, where the outlets protrude into the barrel or are square (found on real old hydrants... older than you and I put together) a discharge coefficient of 0.90 is used.

Using a coefficient of 0.90 we determine the actual rate of flow to be 1,020 gpm.

Everything we need is:
Static Pressure: 54 psi
Residual Pressure: 47 psi
Pitot pressure: 38 psi
Flowing orifice size: 2 1/2"
Actual rate of flow: 1,020 gpm

Don't like digging out the calculator to solve the formula? I made it easy... in the packet of graphs, forms and tables you'll find a blank flow test report which, when filled out, should look like this:

Now we will transfer our information onto a full size sheet of N^1.85 graph paper.

Points "A" and "B" are good while "C" and "D" are bad. It doesn't matter where but our design point must be below the water supply line.

This isn't a bad water supply to work with, pipe sizes will be moderate along with the price.

With a density of .20 over 1,500 sq. ft. we can expect overhead sprinklers to need approximately 320 to 350 gpm with the total requirement, which includes the required 250 gpm hose stream demand, of between 570 to 600 gpm. If demand turns out to be 600 gpm we are good to go as long as we keep our pressure requirement below 45 or 46 psi.

Below are four additional flow tests were we will analyze the results.

Fire Hydrant Flow Tests
Graphs, Forms and Tables Click Here
A Flow Test, Interpretations and Analysis Lesson #1 Click Here
A Flow Test, Interpretations and Analysis Lesson #2 Click Here
A Flow Test, Interpretations and Analysis Lesson #3 Click Here
A Flow Test, Interpretations and Analysis Lesson #4 Click Here

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