Hydrant Flow Tests
Hydrant flow tests are important to determine the flow rate and pressure in any location throughout a water distribution system. To ensure that fire hydrants are capable of providing water at an acceptable pressure and flow rate for public health and firefighting operations, hydrants must be tested regularly.
Hydrant flow tests are conducted to provide information to design a water-based suppression system and to determine if the water supply will meet firefighting requirements.
To minimize time and monetary cost, it is important to ensure that flow tests are conducted properly to reduce the chances of having to conduct multiple tests. For example, if tests conducted incorrectly may determine that a fire pump is not necessary when it actually is, and end up wasting time and money while harming reputations. To avoid this, it is best to complete hydrant flow tests as correctly as possible.
Introduction to the NFPA 291
Fire hydrant flow tests can vary greatly across the industry and the nation. To ensure that flow tests are correctly performed, we recommend following the recommended practice detailed by the The National Fire Protection Association (NFPA) in NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants.
Notes About the NFPA 291
Current editions of NFPA 291 recommend group testing, where you flow more than one hydrant. Although this may be problematic and not entirely necessary, using one hydrant for static and residual pressures while using a different hydrant for flow is the best practice.
Chapter 4 of NFPA 291, 2016 edition, recommends having a static, or test, hydrant and one or more residual, or flow, hydrants. To determine how many flow hydrants are required, it is recommended to flow enough water to provide at least a 25% drop in residual pressure when compared to the static pressure.
It is generally accepted that a 25% drop is not required to design a fire suppression system. Since the hydrant flow tests are also used to determine whether the water supply meets firefighting requirements, it is best practice to flow a similar amount of water as the most demanding flow, regardless of pressure. It is impractical in some jurisdictions to get a 25% drop in pressure, even when required fire flows are easily achievable. Ultimately, the results of the flow test should provide enough information to accurately plot the static and residual points on a water supply graph to demonstrate water demand against water supply.
It may be best to show the water supply curve at 20 psi because the International Fire Code Appendix B and the NFPA 1 have required fire flows at 20 psi.
Choosing a Hydrant to Test
When choosing which hydrants to test, it is important to consider traffic issues and any potential damage to the area surrounding the flow hydrant due to the water flow. After determining which hydrants will be used for the flow test, a time should be selected when there is “ordinary” water demand. For residential areas, this time might be between 6:00-9:00am and 4:00-8:00pm, when most people are at home using water. For industrial areas, this time might be between 7:00am and 6:00pm, as this is when most facilities will be operating.
It is better to use 2 ½ inch hydrant outlets as opposed to the pumper outlets, which is anything larger than the 2 ½ outlet, because the 2 ½ inch outlets are completely filled across the entire cross section of the outlet. On the other hand, pumper outlets will have voids, which can result in inaccurate pressure readings. If using the 2 ½ inch outlets is impractical or if they cannot produce the necessary flow, the pumper outlets can be used, but the resulting flow should be modified to account for the voids in the water flow.
How to Conduct the Flow Test with a Pitot Tube
Ideal Pressure Readings
When conducting the test, it is important to completely open all hydrants to reduce the potential for damage to the hydrant and preserve the accuracy of the test. For the greatest accuracy when conducting a reading using a handheld pitot tube, NFPA 291 recommends keeping the pitot tube pressure readings between 10 and 30 psi at full flow, or when the hydrant is completely open.
This is due to the problems that arise when pressure is outside of that range. If pressures are below 10 psi, the flow is not enough to completely fill the cross section of the open outlet and can result in inaccurate readings. If the pressure rises above 30 psi, it can be difficult to align and maintain the position of the pitot tube for long enough to take an accurate reading. Furthermore, pressures above 30 psi may bend or break pitot tube blades.
That being said, the 10 to 30 psi range is a recommendation. If it is impractical to open multiple hydrants at once, it is best to keep pressures as close to 30 psi as possible while opening as many hydrants at full flow as feasible.
Angling the Pitot Tube
When using a handheld pitot tube, make sure to hold the blade perpendicular (at a right angle, or 90 degrees) to the outlet, so that the orifice of the pitot tube blade is approximately half the distance downstream of the outlet (1 ¼ inches for a 2 ½ inch outlet) and in the center of the flow for the most accurate reading.
Other Considerations
If multiple outlets and hydrants are used to achieve the desired flow, a single pitot reading at each hydrant can be taken and these values added together to determine the total flow at the residual pressure.
Conducting Flow Tests Without Pitot Tubes
As there are multiple products on the market that make finding flow pressure easier and more accurate, handheld pitot tubes are used less often. If you are utilizing one of these products, make sure to verify the flows at different pressure readings, as manufacturers may have different methods of determining the flow for products that are not covered in NFPA 291. These other products also have recommended pressure-operating ranges, so it is important to check all information before conducting a flow test.
The NFPA 291 does have an option for getting flow reading without a pitot tube or other flow pressure reading device in Section 4.9. This section states that a hydrant cap can be used on one 2 ½ inch outlet while opening and flowing the other 2 ½ inch outlet, because the reading should be approximately the same. However, this should not be considered an equivalent option in place of a pitot tube and is only to be done if a pitot tube or other flow pressure reading device is unavailable.
Gauge Accuracy
When conducting a hydrant flow test, it is best to have a selection of gauges with various pressure reading ranges, as gauges tend to be more accurate in the middle of their calibrated range. Furthermore, the range should match the flow that is being measured, as a gauge with a maximum pressure of 200 psi may have difficulty accurately reflecting a pressure between 10 and 30 psi. Conversely, a gauge with a range between 10 and 30 psi would not be able to measure a pressure that was much higher than 30 psi.
Gauge accuracy is also a percentage of the maximum reading of the gauge, so a greater range is not always better. If there is a selection of gauges to choose from, then they can be changed to keep the reading within the optimal range.
Although air-filled pressure gauges are usable, it is recommended to use digital or liquid-filled analog gauges, which can reduce the vibrations in the needle and make the reading easier to read.
To obtain the most accurate reading from residual and flow gauges, it is best to let the hydrant flow for a little while to stabilize the flow before taking a reading. Once the flow is stable, note the high and low readings of the pressure range on the gauges to determine an average. The average residual and flow pressures are the values that should be used as results, as opposed to using the minimum or maximum value obtained.
Opening and Closing Hydrants
It is important to properly open and close hydrants before and after conducting flow tests. It is particularly important when conducting a flow test with a dry barrel hydrant or in areas subject to freezing.
If a dry barrel hydrant is not completely opened, it can flow an excessive amount of water out of the weep hole and cause soil erosion around the hydrant base. Erosion over time can cause the hydrant to sink and may potentially damage the underground piping.
Furthermore, if a dry hydrant is not properly closed, water can be trapped in the barrel or cause water to continually flow from the weep hole. Water trapped within the dry barrel in areas subject to freezing can cause internal damage to the hydrant.
Proper Procedure to Open and Close Hydrants
To ensure that hydrants are properly opened and closed, follow these procedures when opening and closing the hydrant. When opening the hydrant, count the number of complete turns to verify that are made until the operating nut can no longer turn, then turn it back a quarter of a turn. When closing the hydrant, count the number of complete turns to verify that it was the same number of turns as used to open the hydrant. Once again, the hydrant should be backed off a quarter of a turn.
Before reinstalling the final hydrant cap, verify that the water has been completely drained out of the dry barrel by feeling for negative pressure at the outlet. To do so, place a hand over the open 2 ½ inch hydrant outlet for a few seconds to feel for suction. The first few times, there will be an audible pressure release when the hand is removed. Continue to do this until there is no more suction. Then, the caps may be safely replaced.
Other Considerations
Some other factors to take into account that can greatly affect your test results are whether there are any booster pumps on the water supply, whether there are water storage tanks, and the elevation of the hydrants. These effects have a higher residual pressure than static pressure, but not all water supplies have a linear relationship of flow to pressure.
These measures are important because for some water supplies, when the flow demand increases, additional water can be provided to an area through multiple pumps or valves, causing complex geometries to the pressure and flow relationship at any given point in the system. Although there is no current guidance in the NFPA 291 about this effect, it may eventually be added.
Note: Because NFPA 291 is a recommended practice, all recommendations do not have to be met exactly unless the local jurisdiction has specifically adopted NFPA 291.
Sources:
https://www.aspe.org/sites/default/files/webfm/ArchivedIssues/2011/201112/FocusonFireProtection.pdf
https://sprinklerage.com/conducting-flow-tests/
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