Simple Fire Flow Tests Are Easy to Conduct

Simple Fire Flow Tests Are Easy to Conduct


The Volunteers Corner

Fire flow tests can show whether a water system will provide sufficient flow to handle areas with different levels of fire protential, and if the flow is deficient, the test results provide solid arguments for improving the water system.

A true flow test takes a fair amount of work both getting the basic readings in the field and doing the mathematical computations to obtain the results. However, a few easily taken gage readings of flowing hydrants can provide a reasonably good idea of the fire flow in a specific area. By using the table printed below, gage readings are translated directly into gallon-perminute flows.

The equipment needed is minimal. With one cap gage, you are in business. You can test one hydrant flow at a time. If you wish to test two or three hydrants, then you need two or three cap gages, one for each hydrant. However, in the flow ranges in which this type of testing is most likely to be done, hydrant flows are under 1000 gpm and opening the second hydrant will only confirm the low fire flow available from that part of the water system.

Making a cap gage: The easiest—and most expensive—thing to do is to buy a cap gage. You can save a little money by making one yourself. Generally, you can find an old cap that was used on a 2 1/2-inch hose gate on a pumper. (A hydrant cap also can be used, but its design makes it less desirable.) Drill a hole in the center of the cap to take a 1/4-inch iron pipe thread tap. After threading this hole, screw a 1 1/2-inch brass nipple into the hole. Using all ¼-inch brass plumbing fittings, add a union, a close nipple, a tee, and finally a petcock. To the branch of the tee, connect a close nipple, a stopcock and finally a gage.

The union allows the position of the gage to be adjusted after screwing the cap to the hydrant.

After the cap has been placed on a 2 1/2-inch hydrant outlet, the position of the gage can be adjusted for the best visibility by releasing and then tightening the union. If you don’t care how you have to crane your neck to read the gage, tap two 1/4-inch holes in a 2 1/2-inch hose cap and fit a gage into one hole and a petcock into the other. The gage needle may shake a little more because of the lack of a stopcock.

All the fittings can be bought at plumbing supply houses at relatively little cost. You may even have some used fittings on hand, which will cut expenses.

But don t go cheap on the gage. As long as the fittings are in good condition, they can be used. The gage is different because its quality will be reflected in the readings. Go all out and buy a test-quality gage and during its years of use, have it tested from time to time to check on its continued accuracy.

Testing procedure: Now you are ready to test a hydrant for potential flow. Remove both 2 1/2-inch caps from the hydrant and put the gage cap on one of the outlets. Then open the hydrant all the way. As water first flows, open the petcock to get air out of the piping and close it after a solid stream flows. Reduce any gage needle vibration by partially closing the stopcock. Read the gage and write the pressure on paper for later reference to the table below to determine the gpm. Shut the hydrant, replace the caps and go on to the next hydrant to be tested.

In some areas, you may wish to flow two or three hydrants simultaneously. Use walkie-talkies to coordinate the opening and closing of hydrants so that you don’t flow water any longer than necessary. We mention this because flows from open hydrant outlets can cause inconvenience to passing vehicles and pedestrians, as well as damage the ground—grassy or bare—in the immediate area of the hydrant.

Protection of ground: One way to avoid damage to the ground is to place a 2-foot-square piece of plywood so that the hydrant stream falls on it. The open end of a burlap bag also can be tied over the hydrant outlet so that the lower part of the bag limits the splash.

If you have to use a 4 or 4 1/2-inch hydrant outlet because the hydrant does not have two 2 1/2-inch outlets, the discharge gallonages in the table below should be multiplied by 2.3 for 4-inch outlets and 2.9 for 4 1/2-inch outlets.

The following table gives the discharge volume of hydrants with one 2 1/2-inch outlet flowing at various residual pressures indicated by the cap gage:

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