Engine Company, Equipment

Measuring Water Flow for Fire Suppression

Issue 4 and Volume 21.

With water being the primary extinguishing agent for fire suppression, it’s important to be able to deliver the required flow-gallons per minute (gpm)-at the required pressure.

Water delivery systems on the fireground, a.k.a. hose evolutions, have several restrictions that can decrease the flow of the water. Things such as elevation gain or loss, hose friction loss, appliances, plumbing, and nozzle pressures can all impede water flow. Firefighters can overcome pressure restrictions within the specifications of the equipment being used mainly by increasing pump discharge pressure on the engine. Hose evolutions can also be designed to overcome some of the pressure restrictions.

Firefighters need to be well-versed in all equipment and procedures involved with their water delivery deployment. By being well-versed, I mean being educated on all that is involved with moving water for their departments. This is something they can’t accomplish at a fire without having conducted the flow tests properly first.

1 The best way to come up with water flow statistics to be used in designing hose evolutions is to conduct flow tests. Flow tests give you specifics on the gpm and pressure requirements for different evolutions and equipment such as hose nozzles and appliances. (Photos and illustrations by author
1 The best way to come up with water flow statistics to be used in designing hose evolutions is to conduct flow tests. Flow tests give you specifics on the gpm and pressure requirements for different evolutions and equipment such as hose nozzles and appliances. (Photos and illustrations by author.)

The best way to come up with water flow statistics to be used in designing hose evolutions is to conduct flow tests. Flow tests give you specifics on the gpm and pressure requirements for different evolutions and equipment such as hose nozzles and appliances. This article is going to talk about specific tests that can be done as well as equipment that is needed and how to properly use it.

My background in teaching water delivery has involved just about every flow test you can think of over the past 30 years. I, like most of you, have learned a lot from the school of hard knocks. I believe it is extremely important to contact flow-testing equipment manufacturers to learn how it is best used.

Pressure Gauge/Pressure Measuring Equipment

Pressure gauges, as they relate to the fire service, have been around for many years. There are two types of pressure gauge instruments that are used for flow testing. One is an inline pressure gauge and the other is a pitot gauge. The inline gauge is considered a wall gauge. In other words, the gauge is inserted into the housing of the inline gauge unit itself from the sidewall. The pitot gauge measures the nozzle pressure of a fire stream, which is than referenced on a chart for the correct gpm. It has a blade with a very small hole on the end that is inserted into the fire stream and sends the pressure to the gauge to come up with a reading. The pitot gauge can either be handheld or set up in a stationary fixture.

The two most common pressure gauges that are used in water delivery flow testing in the fire service are gauges that have two- and five-pound-per-square-inch (psi) increments. The five-psi-increment gauge is the most common one of the two found. Some say that the problem with the five-psi-increment gauge is that you’re guessing what the exact pressure reading is between the increments. This more than likely will not be crucial in your flow tests.

2 There are two types of pressure gauge instruments that are used for flow testing. The pitot gauge (photo 2) measures the nozzle pressure of a fire stream, which is then referenced on a chart for the correct gpm. The inline gauge (photo 3) is considered to be a wall gauge. The gauge is inserted into the housing of the inline gauge unit itself from the sidewall
2 There are two types of pressure gauge instruments that are used for flow testing. The pitot gauge (photo 2) measures the nozzle pressure of a fire stream, which is then referenced on a chart for the correct gpm. The inline gauge (photo 3) is considered to be a wall gauge. The gauge is inserted into the housing of the inline gauge unit itself from the sidewall.

I have found that when I am doing flow tests, more specifically measuring the nozzle pressure with a pitot gauge, the rise in pressure from the engine pump does not increase evenly from the discharge of the pump to the nozzle pressure. In other words, let’s say that the test requires the pump operator to increase the discharge pressure 10 psi. The nozzle pressure will probably not see a 10-psi increase. Here’s why. When the pressure is raised, so is the potential flow, which produces higher friction loss. Higher friction loss eats up some of the pressure, not allowing the nozzle pressure to increase evenly with the discharge on the pump. See “Friction Loss Effect on Flow” for a simple test that verifies this theory.

I prefer to round off the pressure reading to the next highest increment. Flow tests have proved by doing this that there is a minimum increase in gpm, if any, which does not make a difference at all. Rounding off the pressure reading on the gauge makes it simple to read on the fireground.

3 There are two types of pressure gauge instruments that are used for flow testing. The pitot gauge (photo 2) measures the nozzle pressure of a fire stream, which is then referenced on a chart for the correct gpm. The inline gauge (photo 3) is considered to be a wall gauge. The gauge is inserted into the housing of the inline gauge unit itself from the sidewall
3 There are two types of pressure gauge instruments that are used for flow testing. The pitot gauge (photo 2) measures the nozzle pressure of a fire stream, which is then referenced on a chart for the correct gpm. The inline gauge (photo 3) is considered to be a wall gauge. The gauge is inserted into the housing of the inline gauge unit itself from the sidewall.

All pressure gauges should be calibrated at least annually but preferably three times a year if you are doing a lot of flow tests. When more than one gauge is being used for your flow tests, such as it would be for calculating friction loss in fire hose, it is important to make sure that the two gauges read exactly the same. A good way to test for gauge accuracy is to put the gauges together connecting one side to a pump discharge or a fire hydrant and capping off the discharge end of the gauges with a nozzle. Putting a static pressure on these gauges will show you how close the gauges are to one another.

A good practice to follow when conducting flow tests is to have one person-and one person only-read all of the gauges. I have found that if more than one person is reading gauges there is more probability of reading one incorrectly. This will involve the gauge reader doing some traveling around on the fireground, going from gauge to gauge. However, I think it’s worth the effort.

We have already talked about the difference between an inline gauge and a pitot gauge. When it comes to measuring nozzle pressures from smoothbore tips, I prefer using the pitot gauge over the inline gauge. I have found that the inline gauge connected between the hose and the inlet of the nozzle does not always give an accurate or comparable reading to the actual nozzle pressure measured at the outlet of the tip. The main reason for this is that the waterway’s diameter at the inline gauge is usually larger than the tip orifice. This issue can vary from nozzle to nozzle and could happen more at higher flows. See “Smoothbore Tip Flow Test” for the simple flow test that proves this theory.

When the flow passes through the inline gauge with the 1½-inch waterway and reduces down to the 15⁄16-inch tip, it increases in velocity, which raises the pressure. You should always use a pitot gauge when measuring a stream from a smooth bore tip. Flow testing combination nozzles is a completely different story. First of all, you cannot pitot a straight stream on a combination nozzle to record a reading that can be referenced on a conversion chart like a smoothbore nozzle. The inline gauge should be placed at the nozzle inlet to get the most accurate nozzle pressure reading as possible. Again, it may not match up to the actual nozzle pressure, but this is the best we can do. The actual flow will be determined from a flowmeter.

4 It will be extremely difficult to dial in exact pressures with this 10-psi-increment apparatus gauge. Rounding off the number makes more sense
4 It will be extremely difficult to dial in exact pressures with this 10-psi-increment apparatus gauge. Rounding off the number makes more sense.

Using the Pitot

When it comes to properly placing the handheld pitot gauge into the stream to measure the pressure, most books tell you that the blade of the pitot gauge needs to be centered in the stream, and the distance from the tip to where the blade is positioned should be equal to half the diameter of the tip itself. They also tell you to make sure that the end of the blade is not angled as it’s inserted into the stream. Make sure that it’s as parallel to the stream direction as you can get. I have found that the rule about positioning the blade does not hold true. I place the blade in the center of the stream parallel to the stream itself, but then I just rest into the blade against my thumb, which then braces against the nozzle. See for yourself. First, do the placement of the blade just like the book says and get a reading. Then place the blade as close to the end of the tip as possible and then at a farther distance from the tip than the book calls for.

Hose Friction Loss Flow Test

Now let’s use some of the techniques discussed in this article to conduct a flow test to measure the friction loss in hose. It’s up to you to decide how long the hose will be that is to be tested. The standard is 100 feet. However, when you think about it, we are generally using 50-foot sections on the fireground with the exception of large-diameter hose (LDH), which is usually 100 feet long. With this being said, you can test the hose in 50- or 100-foot sections and divide the friction loss in half.

The following are the instructions for conducting flow tests to determine friction loss in fire hose:

  1. Determine the flow and use the proper flow measuring device to develop the required flow in the hose. Smoothbore tips require a pitot gauge, and combination nozzles require a flowmeter. These devices should be set up as discussed in this article.
  2. You will need two inline gauges to measure the friction loss. Place one at the beginning of the section to be tested and one at the end of the section to be tested.
  3. There should be at least one section of straight hose going into the first gauge and exiting the second gauge. This will prevent possible turbulence in the gauge itself, which could have a negative affect on the gauge reading. With this being said, a discharge line of 200 feet will work for the flow test.
  4. Finally, simply throttle up to the required flow to be tested and read both inline gauges and subtract the lower number from the higher number. That will give you the friction loss.
Figure 1: NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants Sanctioned, Hydrant Flow Test This is the standard for testing hydrants and water mains. This system of testing can show a good hydrant, however it will not show what is available when using actual supply lines. That is reality
Figure 1: NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants Sanctioned, Hydrant Flow Test This is the standard for testing hydrants and water mains. This system of testing can show a good hydrant, however it will not show what is available when using actual supply lines. That is reality.

Flowmeter

Most, if not all, flowmeters are designed to be accurate within plus or minus five percent. Let’s take a look at an example to see how much it can vary.

With 185 gpm being the target flow, a five percent reading lower than 185 gpm is 176 gpm. Five percent higher than 185 gpm is 194 gpm. When you’re conducting flow tests on a combination nozzle, the flowmeter is the only instrument that can be used to achieve the flow. Just understand that the number may not be 100 percent accurate. The best way to ensure that the flowmeter is as accurate as possible is to calibrate for the flow you are trying to reach using a smoothbore tip and a handheld pitot gauge. In other words, pitot the stream and recalibrate the flowmeter to match the numbers you get from the pitot gauge. You should do this every time you conduct a flow test.

 

5 Check the gauges for accuracy with a static pressure
5 Check the gauges for accuracy with a static pressure.

Most flowmeter manufacturers recommend that there be at least a three- to five-foot straight piece of hose going into the flowmeter tube and exiting the flowmeter tube. If the flowmeter tube is hooked to the end of the discharge or into the front of the nozzle, there could be turbulence created within the flowmeter tube itself, which could cause an inaccurate reading.

Hydrant Flow Tests

I once talked to a chief about using LDH and the benefits it has in getting the most out of a hydrant system. His department used 2½- and three-inch for its supply lines. He told me that they did not need LDH because they had an excellent municipal water system with an average pressure of 90 psi-I assume he meant static pressure-which in his thinking indicated a hot hydrant. What he didn’t realize is that no matter how good the hydrant system is, if you can’t get the water out of the ground through the proper supply line, it’s not going to do a lot of good. The small-diameter hose that his department uses is actually restricting the capabilities of the hydrant system.

6 The pitot gauge blade is one inch from a one-inch tip. The book says that it should be placed at ½ inch. The reading did not change.
6 The pitot gauge blade is one inch from a one-inch tip. The book says that it should be placed at ½ inch. The reading did not change.

The National Fire Protection Association (NFPA) 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, sanctioned hydrant flow test is illustrated in Figure 1. The test has two hydrants next to each other on the same main. One hydrant is the test hydrant. The static and residual readings are taken from it with no water flowing. The second hydrant is the flow hydrant. A pitoted reading is taken from the hydrant port. Three sets of numbers are recorded. They are the static and residual pressures from the test hydrant and the flow pressure from the flow hydrant. These numbers are placed on a water flow chart, and a graph is made that will reflect the available water from the main and the available water from the hydrant.

7 A short section of 2½- or three-inch hose is used before and after the flowmeter tube. This helps alleviate turbulence that might occur if the tube is connected to a discharge or the end of the handline at the nozzle
7 A short section of 2½- or three-inch hose is used before and after the flowmeter tube. This helps alleviate turbulence that might occur if the tube is connected to a discharge or the end of the handline at the nozzle.

The reason we do flow tests on hydrants is to give us a good idea of what we might get from the hydrant itself. The same flow test will give us an estimate on the available water in the water main at the point of the hydrant we are using for the flow test. I think these tests are needed for preplanning as well as color coding the hydrant to signify the rated flow. However, the reality is that we must be able to get the water out of the ground and into the pumping apparatus to do us any good. I believe that suppression should add an additional flow test to the NFPA standard test. Make it realistic and connect an engine to the hydrant using the supply line that is actually used on a fire to get the water delivery capabilities. I guarantee that the real-world flow will be less than the flow test of the hydrant itself. Make this a big water flow test. Try to max out the supply line evolution for master stream operations. This could involve a relay pump operation, dual supply lines from one hydrant, or dual supply lines from two hydrants. Again, make it realistic according to your department’s standard operating procedures.

Creating the proper fire stream is crucial to a successful outcome at an incident. Proper flow tests will ensure that this can be accomplished. Take the time and do it right.

PAUL SHAPIRO is director of Fire Flow Technology. He is a nationally recognized instructor on large-flow water delivery. He is also a retired engineer from the Las Vegas (NV) Fire Department. He has authored numerous articles for fire trade magazines. He has been in the fire service since 1981, is author of Layin’ the Big Lines, and produced the first in a series of videos on large-flow water delivery.