Slippery Water Cuts Friction Loss
—New York F. D. photo.
Tests of “slippery water” by the New York Fire Department indicate that friction losses need not continue to limit flows through 1 1/2-inch hose to the extent that they now do. Slippery water is made either by educting an additive into a hose line or by putting the additive into a booster tank.
At a public demonstration in May, two NYFD pumpers each supplied a 1 1/2-inch line with the same engine pressure. The line with the slippery water had a flow rate 50 percent higher than the plain water line and a reach that was 40 to 50 percent better, according to the department report. Also, the nozzle pressure on the slippery water line was double that of the plain water line.
Slippery water consists of an extremely dilute solution of high molecular weight, straight-chain polymer in water. The polymer is polyethylene oxide which Union Carbide, the sole producer, has trademarked as Polyox. Only 30 pounds of Polyox are enough to make “slippery” one million pounds of water (about 120,000 gallons).
Slippery water requires much less energy than ordinary water to be pumped under identical conditions (same flow rate, same hose, etc.) in turbulent flow—the chaotic flow that occurs when water flows rapidly in fire hoses. But in nonturbulent, quiescent flows, slippery water is nearly indistinguishable from ordinary water. Most of its properties, including the conventional viscosity, are nearly identical to pure water. This is not really surprising, since there are roughly 10 billion water molecules for every Polyox molecule in the solution.
The slippery effect, then, is a basic change in the nature of fluid turbulence. When one pumps water through a 1 1/2-inch hose at a rate of 100 gpm, the pressure loss per 100 feet is approximately 25 psi. Of this pressure, only 2 percent, 0.5 psi, is lost in overcoming true friction—the molecular attraction between parts of the fluid and between the fluid and the hose lining. The remaining 98 percent is lost in tbe chaos of the turbulence.
In a way not yet well understood, the long polymer molecules dampen, or reduce, the chaotic fluid motions that normally occur in this turbulent flow and hence reduce the energy dissipated.
An analogy might be a crowd of people pushing and shoving wildly to get out of a packed corridor. As long as the people expend a lot of effort jostling each other, forward progress will be slow. If we put guide rails or ropes (like those used in theaters) into the corridor and get people to walk briskly in relatively narrow lines, we may be able to reduce the chaos and get the crowd to pass more quickly. It is believed that the long polymer molecules in slippery water uncoil and align themselves in the direction of the main flow and then act something like guide ropes. Slippery water, then, is actually water with reduced chaos—reduced turbulent friction.
In fire hoses, how much of a flow rate increase can be achieved depends on the extent to which the flow with ordinary water is limited by friction. In early feasibility tests performed with fire department equipment, gains of 50 to 70 percent were achieved.
Polyox is not the only polymer with friction-reducing properties, but it is reported to be more effective in smaller quantities, is known to be nontoxic, is substantially lower in price than the alternatives, and is available in large quantities in a variety of forms. Polyox also is reported to be compatible with salt water.
Mobility and volume needed
In interior fires, especially in tenements and apartment houses, potential damage from fire spread and potential hazards to human life are great. Hose crews need speed and mobility to protect vital hallways, stairways, etc., and enough water to control and extinguish even intense fires.
For speed and mobility in a building, you need charged lines that are light and maneuverable. Since the weight of water in hose is proportional to the square of the hose diameter, light weight inescapably means small diameter. On the other hand, the smaller the hose diameter, the less water that can be pumped over a given distance with a given pressure at the pump.
Friction is disproportionately higher in smaller diameter hoses. As hose diameter decreases, the effective capacity (the volume flow rate achievable through a given length with a given pump pressure) decreases nearly as the cube of the hose diameter, while weight decreases only as the square. One can compensate for this lowered capacity by pumping through small hose at pressures higher than those used for large hose. But the pressure needed to maintain capacity becomes quite large as the hose becomes small, and for hoses that are light, far exceeds the pressure ceiling imposed by the bursting strength of the hose.
Desirable flow limits
Systems studies by chief officers in the New York Fire Department indicate that for interior fires in multiple dwellings, the flow from a hose line should be about 150 to 250 gpm. Except in unusually large and intense fires, much more water appears to be necessary. Much less gpm appears to be too risky too often.
Therefore, the main application of Polyox will be in expanding the watercarrying capabilities of small hose so that it can provide nearly as much water as 2 1/2-inch hose. New York hopes to make light, small-diameter hose the standard fire fighting line in structural fires. The advantages of a small-diameter hose are that it is highly mobile, and a charged line weighs only one-third as much as a 2 1/2-inch hose. Under almost any fixed set of circumstances—pump pressure and hose length and diameter—the adition of Polyox will result in greater rates of flow through the hose and greater pressures at the nozzle. The greater flow rates will reduce the need for or simplify relay pumping.
This permits an engine company to have a larger volume of water on the fire much more quickly. In New York Fire Department tests, a 1 1/2-inch hose has been hauled to the top of a threestory building in substantially less time than a 2 1/2-inch hose. A small hose carrying slippery water has the desired speed and mobility, yet it puts enough water on the fire.
To turn slippery water into an operation capability, the Rand Corporation, Union Carbide and the New York Fire Department are conducting a step-by-step development program.
Pumper modified for tests
The fire department has appointed a battalion chief with engineering experience for the slippery water development program. Fire Department personel and engineers and scientists contributed by Union Carbide, modified an NYFD pumper to carry the Polyox solution.
It still must be determined whether a pump is needed to inject Polyox into solution and if so, what kind of pump is best. To minimize the amount of Polyox carried on pumpers, the development team must also determine the highest concentration of premixed Polyox solution that can be used in regular operations.
The adapted pumper will be tested in regular fire operation to check out tactical fire fighting problems and to ascertain what is needed. A logistics system must be developed and tested to refill Polyox tanks in the firehouse and at major fires. Existing service groups, such as the mask service unit, may have a role in this system.
Specifications for 80 pumpers in the 1970 capital budget have been modified to provide tank space and connections for a slippery water system. These changes cost no more than a few hundred dollars on these $40,000 pumpers. The development phase of the program is expected to be complete by the time these vehicles are delivered.