THE WATER DRAGON
APPARATUS & EQUIPMENT
The New Zealand Fire Service has developed a high-performance eductor for accessing static water supplies for rural-urban firefighting.
In New Zealand, as well as many other countries, it’s not uncommon for large, valuable homes to be constructed on “ten-acre blocks” around the fringes of major cities. Although this prime real estate is relatively close to the city, it’s still rural. As such there is no reticulated water supply to the area; rather, each home usually has its own large, staticwater supply called a “terrace tank” — a concrete, enclosed, in-ground tank, capable of holding up to 10,000 gallons, that’s fed by roof gutter drains. The tank usually doubles as a patio or terrace. The regular New Zealand rainfalls make these tanks a reliable source of domestic water, but they are very difficult to access for firefighting.
The need for drawing on this ample water supply was painfully obvious; one fire in a rural-urban residence used up the booster supply from six metropolitan pumpers and still wasn’t extinguished. The homestead was totally destroyed. The great, lumbering fire pumper, when taken away from the hydrants and water mains, is like a battleship without ammunition. We weren’t equipped for drafting the terrace tank —suction hose is heavy and bulky, and it’s rare for apparatus to carry more than 32 feet of it.
Usually the greatest barrier to practical suction—drafting—is the horizontal reach needed to get from the roadway, around the back of the house, and to the terrace tank or swimming pool, as the case may be. It is not a theoretical problem: We have the power, the pump, the primer, and the water supply. All we haven’t got is the ability to deliver the educted water back to the pump.
Eductors have been around for a long time. Usually fed from a booster or 1 Vi-inch line, they pick up static water and drive it back to the pumper through standard fire hose. They bring back enough water to run another line of the same size for “betterthan-nothing” firefighting—rarely more than 60 gpm. The problem with the small eductors used to date has been the restricted flow available because of the need to start operations from the booster tank, and the limitations to pressures around 150 psi for the water jet providing the venturi action or the driving fluid. We needed an eductor driven initially from the 400-gallon booster tank of a standard heavy New Zealand pumper using layflat hoses to bring back 400 gpm of educted water from the static supply 100 feet away for some real firefighting.
After a 14-month development period, the New Zealand Fire Service developed the “Water Dragon.” The theory behind this unit is derived from the design of “jet pumps”— called eductors, ejector pumps, and (incorrectly) venturi pumps—that share the kinetic energy of a jet of moving fluid with a body of static fluid and impart a total energy to the mixed mass of fluid, inducing it to flow where directed.
Power in a flowing fluid is proportional to flow rate and pressure. If we can’t increase the flow’, why not increase the pressure? That would come free from the motor on our fire truck parked on the roadway outside the burning house. The New Zealand Fire Service pumpers all use American multistage pumps. They all supply main line flows of 1,000 gpm through the standard 2’/t-inch outlets at 1,000 psi, and the special booster pumps simultaneously supply 500 psi to the twin 1-inch booster reels, each of which is rated to deliver 65 gpm. These booster pumps have capacities of 200 gpm, so they are only idling when working.
The thought occurred to us: Why not take water at 500 psi and feed it to an eductor? We could get more than three times the flow’ rate of that at 150 psi working pressure. This was when the research and development really started —it didn’t work quite that easily. We had to use a multiseries-staging system of jet pump elements in which the high velocity jet from a very small ⅜-inch initial drive nozzle picks up static water in a series of “laminations” through the unit.
(Photos by author.)
Pressure needed to supply water back to the pumper is reformed in a involute and then large-diameter hose delivers the eductor supply plus the drafted water back to the pump inlet.
All this theory is very well, but the crunch is, How much water do you get back? The answer, of course, depends on the flow rate and pressure you supply to the original drive nozzle and the size and length of the hose back to the pumper. Ideally you want high flow and high pressure in, with large-diameter short hose on the return. The new New Zealand layout supplies 80 gpm into the Water Dragon at 500 psi and gets back a net gain of400 gpm through parallel, 100-footlong, 3’V-»-inch hoselines. This is enough to supply three handlines. We’ve conducted experiments supplying up to 800 psi into the unit and feeding back through 5-inch HI-VOI. hose. Flows well over 500 gpm were obtained, but the 800 psi is not regarded as a practical system at the moment.
Input to the Water Dragon is through special 1 Vi-inch hose developed for snow making at ski resorts, where very high pressures are regularly pumped up mountainsides.
The complete unit can be held in one hand, has no moving parts, and can be stowed on the pumper with supply and return hose preconnected. One hundred-foot runs give the figures detailed above. Two hundredfoot runs give about 15 percent less.
The whole project has been a worthwhile exercise in the development of a unit for a defined need. Many have been in service throughout New Zealand now for six years and have proven themselves many times when it really counts: when something big is burning and there are no water mains handy *