HomeFire FightersWater line capacity testing: do it right!

fire hydrant Line capacity testingWhat most water system operators call hydrant flow or fire flow testing is giving way to a more accurate name for the same process: Line capacity testing.

“That’s because this is what you’re doing: The hydrant is like a vessel for getting the water out. For years we’ve focused more on what hydrants can do than what the main can do,” said Randy Lusk, regional manager for M.E. Simpson Co. Inc. and a certified EPA line capacity testing instructor.

Lusk gave a presentation on the procedure at a regional conference of the American Public Works Association in Chicago this spring, in which he outlined six parts to the process of successfully measuring pressure and flow data. The first, he said, is to remember that what’s really being tested during the process: The water main.

“The hydrant is just the vessel to get the water out of the main … you need to get a drop in the system to see if you’re still getting safe drinking and fire water.”

The single hydrant testing method is not recognized by the American Water Works Association, the National Fire Protection Agency or International Organization for Standardization or ISO, as a valid test, he pointed out.

As a review, Lusk shared step-by-step instructions of the process for measuring static/psi, and pitot pressures while performing such a test.

“Plan to test from the large main out and keep large mains flowing into smaller mains whenever possible. You also have to work from the source out.

“Also, decide which will be the residual hydrant and which will be the flow hydrant. Dead ends are a great spot to put the flow hydrant,” he added.

“A 10 percent drop in pressure — static to residual — is the goal. If you can’t get it, switch to a different flow device — maybe from a pollard to a hose monster — or you may have to move the flow hydrant closer.

“On the flow hydrant you’ll need to either use a 2.5-inch pollard or a 4.5-inch hose monster, depending on the size of main you’re working with. The rule of thumb is to usually use a HM on 10-inch and larger mains. If you can’t get the 10 percent drop, then switch from the pollard to a hose monster even on smaller mains. Remember: Always open slowly and close slowly, flow from the source out and remember that the residual hydrant has to be between the source and the flow hydrant.”

Notify, then prepare

To ensure safety during the flow, he said, check areas — a lot of times the process will land debris in nearby landscaping.

“Be respectful, throw tarps and don’t blow on garages. If damage seems unavoidable, relocate the flow hydrant.”

Invest in gate valves, he recommends, which will give you less hammers and main breaking. And watch for traffic, use cones or vehicles to block or to deflect “car washing” and watch for children.

Equipment checklist

  • 2.5 inch static caps
  • gauges
  • hydrant wrenches
  • diffusers (2.5 inch)
  • diffuser pitot tubes
  • gate valve
  • valve key
  • metal detector to find valve
  • spare gauges (tested daily)
  • safety cones


Lusk places a high priority on the care of equipment used in the testing process.

“Inspect it daily. If you’re more than two pounds off on a test gauge, throw it away.

“Treat it like you own it. Gate valves are expensive. Get them greased.”

Steps for testing flow hydrants

The flow hydrant person has it easier than the residual runner. He just opens and closes the gate valve and records data. This job is just as important as being the residual runner, he added — it’s just easier on the back. “We recommend rotating jobs each day to save wear and tear on the workers.”

  • Remove all caps and grease them.
  • Replace pumper port.
  • Install pressure gauge.
  • Install gate valve on remaining nozzle.
  • Install diffuser only on gate valve.
  • Open the gate valve fully to flush, and then close and wait for partner.
  • Close bleeder valve on diffuser to keep debris out of gauge.
  • Wait for partner to get “static reading.” Then, once the residual runner tells you, open the gate valve slowly. Take the pitot reading, then take it again to be accurate. Close the gate valve slowly to relive pressure on the flow hydrant. One flow hydrant is usually good for three to five tests as long as you are getting your 10 percent drop. Make sure the hydrant drains and seats properly.

Steps for residual hydrants

  • Remove all caps off of ports.
  • Apply grease to threads of nozzles.
  • Replace pumper port.
  • Replace cap not being utilized for test.
  • Open hydrant slowly to clean out debris.
  • Close hydrant slowly. It’s not a race.
  • Install static cap (used to measure static and residual pressure).
  • Charge hydrant, open fully so you know you’re consistent every time.
  • Take static reading.
  • Signal partner that you’re ready for flow hydrant to be opened. WARNING: If the pressure drops below 20 psi, STOP. Watch your gauge.
  • Take residual reading using the 10 percent rule.
  • Shut the hydrant slowly.
  • Tell your partner the static and residual readings.
  • Remove equipment, replace cap.
  • Listen to hydrant with approved listening device to ensure nothing is leaking.

Example of data gathered

Ps (Static) = 140 psi; Pr (residual) = 125 psi; Pp (Pitot pressure) = 120; Cd (coefficient) (0.9); D (diameter) (2.5)

Equation No. 1 for the data:

Qr = 29.83 • Cd • D2 • M(Pp)

Qr = Actual Flow (gpm); Cd = Coefficient (found on the diffuser); D = Diameter (opening in inches); Pp = Pitot Pressure (from flow hydrant)

Results for equation No. 1:

Qr = 29.83 • Cd • D2 • M(Pp)
Qr = 29.83 • 0.9 • (2.5)2 • M120
Qr = 29.83 • 0.9 • 6.25 • 10.95
Qr = 1837 gpm

Equation No. 2 for the data (you need QR in this equation, your results from the first):

Qf = Qr • {(Ps – 20)/(Ps – Pr)}^0.54

Qf = Fire flow in gpm at 20 psi; Qr = Actual flow in gpm from previous equation; Ps = Static; Pr = Residual;

Qf = 1837 x {(140-20)/(140-125)}^0.54

Equation No. 2 results to get the flow at 20 PSI:

Qf = 1837 • {(120/15) ^ 0.54}
Qf = 1837 • (8^0.54)
Qf = 1837 • 3.07375
Qf = 5647 gpm flow at 20 psi

The data collected is used to keep a community safe and its residents protected in case of fire, as well as for water system modeling, water system hydraulics and color coding a system based on the NFPA flowchart. It’s also recorded by the ISO for the purpose of calculating insurance rates, by the fire department, and 911 dispatch and paging services.

“Fire-flow testing is aimed not only at best management practices, but to help identify hydrant issues, closed valve issues due to main breaks, service work shutdowns, etc., and to keep the flow testing history up to date,” Lusk said.

ISO testing is recommended every five years or whenever hydraulic conditions change. The NFPA states that inspections, maintenance and flushing should be conducted annually, and that fire flow testing should be done every five years or whenever hydraulic conditions change. The AWWA’s best management practices dictate that it’s a good idea to conduct fire flow testing whenever hydraulic conditions change.

The Municipal


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