fire flow test

[cvg]

Consider the following quote from the city water department:

"In the fire flow test you had done on the fire hydrant... they flowed the 2 1/2" port and it showed a result of 1,140 gpm at 58 psi. Extending the graph out to the end, you were showing a fire flow of 3800 gpm at 25 psi. Knowing my experience with fire flow tests, and what I know about non looped systems, I believe the top end of the graphs for flowing higher volumes is not accurate when flowing the 2 1/2" port. The 4" port is more accurate, and therefore, I had the fire flow test repeated flowing the 4" port to flow a larger volume of water. The results of this test are much lower, and more accurately depicts what happens when the flows get higher in the dead end 12" main…the static pressure was 64 psi, and the hydrant flowed 1,718 gpm at 42 psi, or 2341 gpm at 25 psi which is 2,498 gpm at 20 psi, far short of the 3,000 gpm needed for fire flows...”

Any thoughts, comments why the 2 ½” test and the 4” test should be so different – 3,800 gpm vs. 2,341 gpm? Any recommendations on how we can proceed given the apparent inadequate water system?

 

[FireH2O]

cvg,

1) It appears the 2 tests were conducted by different people and hence different gauges were used. Correct? Bear in mind that different gauges can show different pressures. A few psi difference can result in significantly different results. Did each entity witness the other's test? Was the orifice diameter measured or assumed to be exactly 2.500000 in. and 4.00000 in? And was it really a 4" diameter outlet, as opposed to a 4-1/2" outlet. This may sound silly, but on several occasions, I have encountered people who mistakenly claimed one when it was the other.

2) Please provide the following for EACH test:
a) Type of hydrant (wet or dry barrel)
b) Actual inside diameter of orifice
c) Orifice Coefficient
d) Was a Pitot tube used to measure the pressure or a diffuser or a nozzle?
e) Pitot Pressure
f) Static Pressure
g) Residual Pressure
h) Were the static and residual pressures measured at the same hydrant or different hydrants?
i) Were the gauges recently calibrated? What is their respective accuracies (+/-1%, +/-3%, etc.)

Your description is not clear as to whether the 58 psi was a Pitot Pressure or a Residual Pressure.

 

[cvg]

I can answer a few of your questions, and am working on getting more information on the tests.

Yes, the tests were done by different people, both done by the city water department and I doubt that they witnessed each others test. I can't confirm that the actual diameter was measured in either test. Both tests were measured at the same hydrant. I know that the orifice coefficient on the second test was 0.9. The 58 psi is a residual pressure.

 

[chadl]

The type of water system can have a dramatic effect as well, in theory, a completely gravity feed type system should provide similiar results regardless of how much water is flowed during the test, however most systems utilize pumps, either in combination with elevated storage, or in a direct pumping arrangement.

If during the first test a pumping station near the area was operating, it could create a far flatter supply curve out to the limits of the pump. By flowing the larger opening, you may have passed the limits of the pump, and as a result had a larger pressure drop than what would be predicted by the first test.

When we perform flow testing, we encourge the water department to turn off all pumps not used continiously, and we try to always flow at or near our design points (these flow tests are for fire protection systems with design flows betweem 1000 gpm and 1500 gpm so this is easier to do than when trying to determine the flow rate available at 25 psi)

 

[cr1973]

3800 gpm? What size is the water main? I doubt a 6" or 8" main could actually achieve this flowrate, as the friction losses would be collosal. Most hydrant leads are 6" I believe...

 

[cvg]

CR1973 - the fireflow would be pulled from several hydrants in the vicinity with fireflow test only done at one location

 

[AegisFPE]

The extrapolated flowrate to 20 or 25psi is theoretical, as you point out cr1973, yet is where the code looks for "fireflow" in order to have a common point of reference. NFPA standards call for a 25% drop from staic to residual pressure, and a pitot reading of 10-30psi. If the static pressure drop observed was less than this or your pitot reading was higher than 30psi, you probably need to open a larger port (as you requested for the 2nd test) and/or open more hydrants in order to adequately draw-down the system.