Training Aids Can Show Hydraulics Theory at Work
The Volunteers Corner
Hydraulics theory is the basis of both pump operation and confusion in the minds of learners. However, effective use of simple training aids can cut this confusion.
Let’s look at friction loss-the problem pump operators live with but cannot see. The use of some inline gages may not make friction loss visible, but they certainly will make the results of friction loss visible.
For a simple demonstration, hook up a six-length line of 2 1/2-inch hose to a pump outlet. At the 100-foot and 200-foot couplings, insert an inline gage. You can either buy inline gage couplings already tapped for a gage or you can braze a male and a female coupling back to back and make a ¼-inch tap for a gage.
With a straight tip on the end of the line, you are now ready to demonstrate friction loss. You should have 1, 1½ and 1 ¼ -inch tips available so you can flow the usual hand line volumes with the proper tips at the normal nozzle pressures of 45 to 50 psi.
Starting the water: When you charge the line, have a man stand at each inline gage and place a foot lightly on it so that as the hose twists under the initial pressure, the gage won’t be damaged by being slapped against the ground. Once the line is charged, the gages will remain in place and the only thing the men at the gages have to do is read the gage pressures.
Don’t look for perfection because friction losses vary according to the condition of the hose lining and the expanded diameter of the hose under pressure. However, there will be a step-down in pressure readings from the pump to the first and second inline gages and finally to the pitot gage you use at the straight tip. This provides visibility of the results of friction loss in action.
Use all three sizes of tips during this demonstration while maintaining normal nozzle pressures. The fire fighters can then see greater friction losses for larger tips. If you wish to use a fog nozzle, put an inline gage back of the nozzle. Once pressure is established, constant flow nozzles will maintain the same pressure at the gage as the fog pattern is changed. However, variable gallonage, constant flow fog tips will show different pressures at the gage as the gpm rate is changed provided the pump pressure is kept constant. This shows the need for the pump operator to change his engine pressure to meet the demands of changes in the variable gallonage fog tip.
The same demonstration can be done with 1 ½ -inch lines, but most fire departments do not have an adequate variety of straight tip sizes to make a 1 ½ -inch line your major training aid. The 1 1/2-inch nozzles and the friction losses associated with them can best be demonstrated after use of a 2 1/2-inch line has clarified understanding of friction losses.
Back pressure: Generally, we talk about back pressure and let it go at that. We can dramatically show back pressure in the classroom. Bore a hole in a 1-quart plastic frozen food container that is translucent and insert a fitting for a ⅜-inch rubber hose—8 or 10 feet long. At the other end of the hose, connect a drain cock and add to this a ¼ -inch brass elbow for a pressure gage.
Put some water in the container, raise it above the level of the gage, and fill the hose with water by opening the drain cock and letting the air escape.
Now hold the gage level with the surface of the water in the container. The gage will read zero. As you raise the container higher above the gage, the gage will reflect a rise in pressure—back pressure. You should use at least a 2-inch gage reading only to 15,psi. With a gage of this type, it is easy to see the needle move as the elevation of the container is changed.
This is easier than charging a hose line, shutting down the pump, closing the nozzle and raising and lowering the nozzle along the side of a drill tower with a length of rope.
Elevating platform use: Back pressure also can be demonstrated with an elevating platform that has a turret pipe with a gage, as most of them do. Have a pumper supply your normal nozzle pressure while the platform is at any specific height. Have the pump operator maintain this engine pressure.
Now raise and lower the platform. The turret pipe gage will show a drop in pressure as you raise the platform and an increase in pressure as you lower the platform. Again, the results of back pressure are visible in gage readings.
With an articulated boom, you also can show that when the basket is lower than the knuckle, the back pressure in the section of pipe rising above the platform is canceled by the “reverse back pressure” in the pipe descending from the knuckle to the platform.
Demonstrations such as these—and you can add many variations—show how theory really works on the fireground.