Introduction
Pumps installed in fire engines either hoses out water from the fire tanker or suck water from a source such as fire hydrants, lakes, or pools. Different mechanisms control the pumping of water from a water source to the scene of the fire. Usually, a 6 inch suction pipe is used to extract water out of the water tank or an external water source. The water is then discharged out through exit lines or hoses with a certain force to eventually quench the fire.
The Impeller Pump
The main source that pumps water is the impeller water pump. This is a propeller type device that has curved blades which are driven by diesel. When water enters the pump, it instantly hits the inner part of the pump and is forcefully slung outwards through the centrifugal force caused by the spinning impeller. The water pressure, volume of water entering the pump, etc. are controlled by a set of levers and switches present in the control panel of the fire truck. During the fire fighting operation, the valve connecting the pump and the tank is opened through a switch which ensures water flow into the pump.
Many types of hoses are used to control different categories of fire. These hoses are also called lines that can draw out different quantities of water depending on the hose length, radius, and water pressure. Simple cubic calculations provide answers to questions how much water is pumped into a particular fire. For instance, a hose is, usually, cylindrical in shape with a certain diameter. If the radius of this hose is given as r cm, its length l cm, then the volume of water it can carry is given by the formula V= π × r2 ×l cm3. Using the conventionally used conversion factor of 1000cm3 being 1 litre, we can calculate the volume of water stored in hose.
Flow Rate and Nozzle Pressure
The flow of the liquid through the hose, or the water pressure of the water coming out of the hose is determined by Bernoulli’s Principle, which is nothing but an extension of the equation of continuity. According to the equation of continuity, the volume of liquid entering one end of a tube must be the same as the other end. Consider water enters one end of the pipe of area A1, travels a distance Δx1 and exits the hose whose area is A2 by travelling Δx2. Then, for the volume remain same at both ends, A1 × Δx1= A2 × Δx2=V where V is the volume of water travelling across. If A2 is very small, then in order to maintain the constancy in volume, more water is pushed through at much faster rates which consequently increases the water force. Efficient fire trucks can pump up to 1000 gallons per minute and more.
Ladder Movement
Another important feature of a fire truck is the ladder that can keep elongating to reach higher floors of multi-storey buildings. The ladder on these trucks are raised and lowered by hydraulic pistons, which makes use of Pascal’s principle that states that the pressure throughout any closed system remains constant. A hydraulic piston consists of a large fluid-filled cylinder with a number of cylindrical pistons in it. When pressure is exerted to one of the pistons, it causes the other piston, on which the ladder is attached to expand and rise up and causes the ladder to extend. If the pressure is released, the piston moves down and the ladder retracts. To move the ladder left and right, a hydraulic motor is used, which employs the same principle as that of a hydraulic piston.
Friction Losses
Friction losses result from the resistance water experiences due to the walls of the hose as it moves through the pipe. Many factors contribute to these losses. As the flow rate (gallons per minute, litres per minute, etc.) increases, the friction losses increase accordingly. The larger the amount of water in contact with the walls of the hose, the more is the friction losses, which is why these losses increase with flow rate. Analogous to electrical resistivity which increases with increasing conductor lengths, friction losses of water increases as the length of the hose increases. This is due to the larger distance that needs to be travelled before exiting the hose. These losses are greater when the hose is curled in rather than when it is straight. Friction losses depend strongly on the fabric of the hose, its lining and age. Again, analogous to electrical resistance, friction losses increase four times when the flow rate is doubled and increases 32 times when the radius of the hose is halved. To overcome this loss due to friction, the pump pressure must be increased so that the necessary amount of water is pumped into the fire.
Pump Design
Designing an engine with suitable nozzle pressures requires some fundamental mathematical calculations. If water head is the height of the water column due to the inherent pressure, the pressure is positive if the water column was at a height and the hose is downhill so that gravity acts in the direction of water. This condition is known as head gain. This pressure is negative if the hose faces upwards and water has to be pumped against gravity. This is loss gain. Engine and nozzle pressure can be calculated using the simple equation: Nozzle Pressure= Pump Pressure ±Head Gain or Loss Gain-Friction Losses
Since head gain is positive, its value should be added to the pump pressure and in case of loss gain, or negative pressure, then it should be subtracted. Friction losses should always be subtracted from the pump pressure since it acts in a direction opposite to the normal flow of water through the hose and opposes the pressure exerted by the water column.
Physics of Water Pumps
Physics plays an important role in determining the different parameters that govern the flow of water through a fire hose. It is important to understand the difference between the pump and the source of water when extracting water into the tank. When water is drafted, the air present in the hose is completely evacuated, creating a vacuum in the pump chamber. This causes the atmospheric pressure or the weight of the air on the water surface to force water up the suction pipe into the pump just like sipping water through a straw. There is a critical height up to which the engine can draw and lift water. For instance, at sea level, the atmosphere experts a pressure of 14.7 pounds per square inch or 175 N/m or 0.068 atm. At this pressure, the height up to which water can be pumped is about 34 feet, or about 10 meters.
Thus, while determining different parameters and components of a fire engine water pump, numerous considerations need to be made. In each case, simple laws of physics and simple mathematical calculations provide solutions to most of the requirements. However, interestingly, it is evident that physics governs most, if not all, the different aspects of a fire engine water pump.
References
Bonsor, Kevin. "How Fire Engines Work." HowStuffWorks. HowStuffWorks.com, n.d. Web. 20Nov.2014.http://science.howstuffworks.com/transport/engines-equipment/fire-engine1.htm
"Firefighter Math." Firefighter Math. United Stated Department of Agriculture, n.d. Web. 20 Nov.2014. http://www.firefightermath.org/index.php?option=com_content&view=article&id=171&Itemid=41.
Hydraulic Training Axial Piston Units." Hydraulic-Training Axial Piston Units (1998): 1-28. Insane Hydraulics. Engineering Mannsmann Rexroth. Web. 20 Nov. 2014. <http://www.insanehydraulics.com/library/files/Hydraulic-Trainings-for-Axial-Piston-Units.pdf>
