By Larry Davis
Published Friday, March 20, 2009
| From the April 2007 Issue of FireRescue
When a high rate of flow (gpm) is required for a water-on-wheels operation, a dumpsite can be established with several portable tanks to feed the attack engine. Tank connectors, inchworms and jet siphons are the three methods for transferring water between tanks to ensure the tank from which the pumper is drafting never runs dry.
So, of the three water transfer methods, which is best for transferring water between porta-tanks? Answer: The one that provides water to the attack engine at the delivery rate required by the incident. And, generally, the jet siphon accomplishes this best.
When I first got started in rural water supply operations training more than 30 years ago, apparatus and equipment manufacturers hadn't discovered the "rural firefighting market." As a result, most of the hardware needed for rural water transport operations had to be built by rural departments themselves. This led to a wide variety of apparatus and equipment designs and ideas. Water transfer methods started this way and, as a result, they varied greatly from department to department.
In the mid-1970s I began conducting rural firefighting seminars, first in Pennsylvania and then around the country. As I started to work with homemade water transfer devices, it became obvious that some of the designs just couldn't deliver the gpm needed. It also became obvious that if flow was to be improved, some method of measuring the flow from a device under various conditions would be needed.
The Inchworm vs. the Jet Siphon
As with anything a department uses, whether homemade or purchased, personnel don't always like to admit that a piece of equipment isn't performing as well as they'd hoped. As a result, they may fiercely resist peformance measurements.
An early opportunity to test the efficacy of water transfer devices arose for me during a rural water class I taught at a regional fire school at what is now the California University of Pennsylvania. One department attending the class brought two 4" inchworms, while another department brought a jet siphon. Lacking any real method of accurately measuring the flow from each, we decided to set up a test as shown in Figure 3.
The actual test was really a comparison of the flow capability of a 4" inchworm with that of a jet siphon in a length of 6" suction hose. The siphon was set up to transfer water from the red tank to the black tank, while the inchworm was intended to allow water to flow from the black tank to the red tank. This simple test would provide a comparison of the flow rates from the two devices.
The test began with both porta-tanks full. The engine drafted and discharged about 800 gpm. The result of this simple comparative test is shown in Figure 3. The water level in the red tank shows that the inchworm can't flow enough gpm to keep pace with the jet siphon. Although this test showed that a jet siphon could outperform a 4" inchworm, its flow rate was still not quantifiable because we had no method of weighing the tanks.
Because of the variables involved, it became clear that the only real way to get accurate information about the gpm flow from water transfer devices was to conduct weight tests using porta-tanks and permanently installed truck scales.
Durham Inchworm Tests
The first flow tests I conducted on inchworms occurred in Durham, N.H. The test setup is shown in Figure 4. An empty porta-tank was placed on the truck scales while a full porta-tank was placed adjacent to the empty tank, but off of the scales. We ensured the supply tank was totally full so the inchworm would have the benefit of the height of the water and the increase in pressure this would cause.
The empty porta-tank was weighed to determine the baseline weight. The filled inchworm was then placed between the two tanks and allowed to flow for 1 minute, at which time it was removed to halt the flow. The porta-tank was then reweighed for its test weight. The baseline weight was subtracted from the test weight to determine the net weight. This sum was divided by 8.34 lbs. to determine our flow rate in gpm.
This test is shown in Figures 5 and 6. The results of this test proved most interesting. The flow from the 4" inchworm in the 1 minute period was 192 gallons. This result showed that at least five 4" inchworms would be required to transfer enough flow to keep up with a pumper drafting 1,000 gpm from a porta-tank.
Bethany Inchworm & Jet Siphon Tests
My next opportunity to conduct similar tests came during a rural water supply course hosted by the Bethany (Conn.) Volunteer Fire Department (BVFD). The first test involved an inchworm and was set up in the same manner as the Durham test (Figure 7). The BVFD's results showed a flow of 132 gallons of water in the 1 minute, or 132 gpm.
The BVFD had made a jet siphon using a ?" pipe and 6" suction hose. The siphon was supplied by a 50' section of 1 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ«" hose with a pump discharge pressure of 150 psi.
The BVFD tested this jet siphon, shown in Figures 8 and 9. As in the inchworm tests, an empty porta-tank was placed on the truck scales and weighed to determine its baseline weight. The first step in the test was to determine the flow from the jet siphon nozzle alone. This is shown in Figure 10. The 1 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ«" siphon supply line was charged to 150 psi and allowed to flow into the empty porta-tank for 1 minute. The tank was then reweighed with the baseline weight subtracted from the test weight.
Under this set of circumstances, the jet siphon nozzle alone flowed 175 gpm. The next test was to measure the flow of the Venturi created when the jet siphon was immersed in a full porta-tank and reweigh the porta-tank on the scales to obtain a baseline weight. Then the jet siphon was placed into a full porta-tank, pressurized and flowed for a period of 1 minute. At that point, the tank was reweighed to obtain the test weight. The baseline weight was subtracted from the test weight and the difference divided by 8.34 lbs. to determine the total gallons transferred in 1 minute (gpm).
In this case, the total weight transferred in 1 minute equaled 1,031 gallons, for a flow of 1,031 gpm. However, based on the earlier test of the siphon nozzle, we knew the siphon nozzle alone flowed 175 gpm (175 gallons in 1 minute). So the net gain from the jet siphon's Venturi action was 1,031 gpm less 175 gpm, or 856 gpm. Looking at it another way, the 175 gpm used to create the Venturi action at the jet siphon increased its total flow by 480 percent.
This information, as well as that from other tests I have had the opportunity to conduct, has provided some accurate baseline data on how these devices perform. Now that you know how these tests can be performed, you can measure the performance of your water transfer devices and then experiment with what you can do to improve performance.
Several years ago, a colleague spent several days in one of the largest collegiate engineering libraries in the United States looking for information on how to design the perfect jet siphon. After going through every pertinent text, he found there was no book that contained the data on how to build the perfect rural siphon. Better ways might yet be out there. Commercially available water transfer devices may not perform any better than your homemade device. The only way to find out for sure is to test it.
Next month we'll look at some of the designs rural fire departments have come up with for water transfer devices.
'Til next time, stay safe.
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