Hydraulic Analysis of Fire Protection Water Supplies
In Part 2 of this series we introduced the hydraulics slide rule and worked some basic problems. You may wish to reread Part 2 before proceeding. You will need the slide rule to work the problems on your own.
In Part 2, Problem 3 posed the following: 600 gpm are flowing through 700 feet of six-inch unlined cast iron pipe. The pipe is 30 years old. What is the expected friction loss? The answer was 22 psi.
To set up the problem for this article, solve the following: 1000 gpm is flowing through 700 feet of eight-inch plastic pipe that was installed 15 years ago. What is the expected friction loss? Using the same techniques, the calculator shows an answer of approximately 4.2 psi (Figure 1) Note that the C factor for plastic pipe is 150 from the back of the calculator. It does not vary with age.?
Suppose that during a test a friction loss of 18 psi was measured instead.
The reason for this unexpected result should be investigated. The first thing to do is to check known valves to be sure they are wide open. If they are, it is time to put on your detective's hat.
The next step is to see if you can figure out where the problem is occurring. This is done by adding as many gauges along the length of pipe as possible. There should be at least two hydrants along the 700 feet and there are probably some sprinkler risers as well. The lower sprinkler riser gauge (Figure 2) will read the pressure in the main at the point of attachment if the riser lead into the main.1
Potential Cause 1: If the pressure drop is uniform along the length of pipe being investigated, the problem is probably with the main itself. The main could be smaller than what was thought. Or, another pipe type that has tuberculated could have been installed instead of the plastic pipe. Everyone has his or her own technique but one method of investigation is to first check to see if a smaller pipe was installed2.
Checking the friction loss for a six-inch" plastic pipe (Figure 3) shows a friction loss of approximately 17 psi so it is a definite possibility that a 6" plastic pipe was installed instead.??
Checking to see if an eight-inch unlined cast iron pipe could have been installed instead, a friction loss of approximately 9 psi is obtained based on a C factor of 100 for a 15 year old pipe. This is unlikely to be the cause of the higher than expected result.
Hopefully you can find as-built diagrams to verify what was installed but sometimes it is necessary to dig up the pipe to know for sure. Assuming that you confirm that this is a 6" pipe rather than an 8" pipe, it is now necessary to determine if the 6" pipe is acceptable for sprinkler and hose stream flows. It may be necessary to replace the pipe.????????????????????????????
Potential Cause 2: If the pressure drop is sudden between two points along the pipe then the problem is probably a localized problem in the main. Likely causes are:
- a partially "dropped" gate on a known valve
- a partially shut valve that you don't know is there (like a buried or paved over curb box valve)
- an obstruction such as construction debris in the main.
Sometimes, a camera or a backhoe is needed for further investigation. But before that is done, it is worth a search for older diagrams that may reveal valves that were left off revised diagrams. It may be possible to interview senior or even retired maintenance personnel who may know of the existence of other valves.
To properly develop pre-emergency plans, knowledge of the water supply is essential. This article has shown how to determine what is going on in the circumstance presented above. In part 4, we'll conclude with another problem that was identified with these techniques.
Questions can be directed to the author at John_Frank@swissre.com or +1 770-569-7082
- Technically the pressure in the main will differ based on the elevation of the main with respect to the gauge. In practice, all pressures are read above ground at hydrants or risers so the elevation effects are normally cancelled out. If the ground elevation changes then the effects must be accounted for. There cannot be any check valves between the riser gauge and the main being investigated if a riser gauge is to be used accurately.
- The author would normally check the C factor first but since the C factor does not change with plastic pipe, it was decided to check for a smaller pipe first in this case.
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