How to Check Fog Nozzles For Constant Flow and GPM

How to Check Fog Nozzles For Constant Flow and GPM

The Volunteers Corner

From watching pumping operations and listening to questions, it is evident that there are still areas of confusion regarding fog nozzles, the gallonages they deliver and the resultant friction losses.

First of all, it is necessary to know whether the particular adjustable-pattern fog nozzle you are discussing is the constant flow or the variable-flow type. The newer adjustable-pattern nozzles are often the constant-flow type. This means that at the original nozzle pressure, there is no change in the gallons per minute delivered as the pattern is changed from 90° to a solid stream.

With a variable-flow nozzle, the gallons per minute delivered will change as the pattern is changed, even though the pump pressure remains the same. The largest gallonage is delivered in the 90° pattern and the smallest in a solid stream.

You can check which type of nozzle you are using by placing a line gage behind the nozzle. If the pressure remains the same as the pattern is changed, the nozzle is the constant-flow type. With the variable-flow type, the pressure changes with the gallonage. If you start with the 90° pattern, the pressure at the nozzle will increase as you narrow the pattern and finally go into a solid stream. This also proves that less water is flowing when a solid stream is used with this type of nozzle.

The explanation is simple. The larger the gallonage flowing through a hose line, the greater the friction loss. Therefore, if you start with the largest gallonage, the friction loss will decrease as narrower patterns deliver lesser gallonages. With the pump pressure remaining the same, the excess psi will show at the nozzle, and a gage behind the tip will indicate this with a higher reading.

There are two other types of fog nozzles available. One is the constant-flow, adjustable-gallonage nozzle. This nozzle has a device built into the tip to constrict the flow of water so that you have a choice of gallons per minute which will remain constant through the various fog patterns and solid stream.

You will notice that the so-called solid stream developed by adjustable-pattern fog nozzles is not a true solid stream such as is put out by a straight tip. The water passing the circumference of the center device in the tip converges a short distance from the tip to approximate a solid stream.

The other variation in fog nozzle design is the navy type. Unlike the adjustable-pattern fog nozzle, which develops a hollow fog cone, the navy type produces a solid cone of fog. The cone pattern can not be changed. A separate orifice is used for a true solid stream. Again, the gallons per minute delivered may or may not be the same for the two positions—fog and solid stream. Using the gage test we described above will determine whether a particular navy-type fog nozzle has the same or different gallonages for fog and solid streams.

Now, how do you detennine the gpm of a fog nozzle? The easiest way is to look it up in the manufacturer’s catalog. One thing to remember is that fog nozzles are rated at 100-psi nozzle pressure. If the nozzle pressure is less, the gpm will be less. Conversely, if the nozzle pressure is more, the gpm will be more,

What if you can’t find out the rating or wish to check it for yourself? Again, use a line gage behind the nozzle. You can check the gpm at any nozzle pressure, but you will probably wish to find out what the gpm is at 100 psi. When you have this pressure at the nozzle, put the stream into a 55-gallon drum—or any other container that you can easily calibrate. With nozzles that deliver more than 50 gpm, you will need two or three drums.

With a watch, preferably a stop watch because of its more easily read second hand, fill the drum, or drums, for exactly one minute. With large flow nozzles, each drum can be emptied as soon as it is filled. By this time, the stream has been directed into an empty drum. By counting the number of drums emptied and the amount in the last drum, that may be only partially filled, you can get at least a rough idea of the number of gallons per minute the nozzle delivers at a specific nozzle pressure.

You can make a fitting for a line gage by using a pair of couplings with the bowels brazed together so that the male thread is on one end and the female swivel on the other. Bore a hole far enough from one of the ends so that you can still make up a hose coupling without interference from the gage. Tap the hole for 1/4-inch IPT. The gage can then be screwed into the brazed double coupling. Or you can use an air hose snap connector so that one half can remain permanently in the coupling and the other half can be permanently attached to the gage. Then the gage can be removed for easier storage without wearing out the few threads in the coupling hole. Just remember to snap the gage on before charging the line.

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