Hydraulic Analysis of Fire Protection Water Supplies
Vol 21 No 5
In prior articles, we discussed the importance of the fire pump as the heart of the fire protection system. The next link in the water supply chain is the distribution system. The pump can be working just fine but the distribution system must be able to get the water to where it is needed at the volume and pressure needed. Methods to determine this volume and pressure have also been discussed in prior articles.
Water supply tests can reveal whether or not the volume and pressure available are adequate at an existing or projected site and how well the system is actually performing compared to expected performance.
If the actual performance is worse than the expected performance, tests can help identify potential reasons such as:
- a totally shut valve
- a partially shut valve.
- the presence of obstructions such as construction debris.
- the fact that a piece of pipe was not as large as was thought.
- deterioration that reduces the inside diameter of the pipe or increases the.
- interior roughness.
increased domestic or industrial use.
- a defective hydrant.
- a problem with an off-site water supply.
This series of articles will provide techniques that can be used to quickly and easily analyze water supply test results using a very simple hydraulics slide rule.
The slide rule can solve for any of the water main friction loss variables as long as the other variables are known. Comparison of actual to expected values can then be made. If the actual value differs significantly from the expected value, an investigation into the reason should be conducted. The other side of the slide rule has flow tables for the underwriters playpipe and for common hydrant outlets.
To make the most of future articles, the reader should have the slide rule available. Please send $2 for postage and handling to Industrial Fire World Magazine, P.O. Box 9161 College Station, Texas 77842 to request a slide rule or visit the IFW web site at http://www.fireworld.com/store/index.html. The slide rule is available to the first 100 readers to request one.
The following are real world industrial water supply issues that were identified through field use of these techniques:
1) A water main at a new power plant was smaller than the original design. The plant operators responsible for the fire protection system did not know this until the test was conducted. Knowing the true size of a water main is needed to properly evaluate plant fire protection. This situation was discovered because the actual friction loss in the fire loop was much higher than it should have been for size water main thought to be installed and no evidence of a partially shut valve could be found.
2) In numerous cases where a city water valve outside the plant was partially shut. This commonly happens when a valve is shut to repair a city water main. The valve is then partially open to refill the pipe. Unfortunately, the crew sometimes forgets to open the valve all the way back up. This author has found this problem at a textile mill, a metal worker, and a conference center. If a water supply test produces flows and pressures that are less than expected, the slide rule is used to measure actual friction loss versus expected friction loss and then examine individual pipe segments until the problem can be isolated.
3) At a paper mill, a "dropped" gate valve was found during a loop test. The test yielded poor results. The valve was found by comparing the actual to expected friction loss in various segments of water main. The valve was open only about 1? inches.
4) At another utility plant, a closed "left-handed" valve was found using the above techniques. "Left-handed" valves turn in the opposite direction of the standard right-handed valve, so when the maintenance staff thought they were opening the valve they were actually closing it. This was the only left-handed valve on the site.
5) At a textile warehouse, three shut division valves were found at a multiple loop facility, again by comparing actual to expected friction losses and then taking steps to isolate the problem through further testing.
6) At a semiconductor plant, a loop yielded poor results. Using the slide rule to calculate friction loss in various sections of the underground main led to the discovery of a section of pipe where its effective diameter cut in more than in half due to obstructions in the pipe.
In the next article in the series, we will introduce the slide rule and provide some basic examples of its use. Future articles will provide more advanced examples of its use.
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