Read the text below to find answers to the given questions.

Text 12 B

Water Distribution

How does standard water pump work?

The water pumps in most cars are centrifugal pumps. These pumps work by spinning water around in a circle inside cylindrical pump housing. The pump makes the water spin by pushing it with an impeller. The blades of this impeller project outward from an axle like the arms of turnstile and, as the impeller spins, the water spins with it. As the water spins, the pressure near the outer edge of the pump housing becomes much higher than near the center of the impeller. There are many ways to understand this rise in pressure, and here is one of them:

You can view the water between the impeller blades as an object travelling in a circle. Objects don’t naturally travel in a circle — they need an inward force to cause them to accelerate inward as they spin. Without such an inward force, an object will travel in a straight line and won't complete the circle. In a centrifugal pump, that inward force is provided by high-pressure water near the outer edge of the pump housing.

The water at the edge of the pump pushes inward on the water between the impeller blades and makes it possible for that water to travel in a circle. The water pressure at the edge of the turning impeller rises until it’s able to keep water circling with the impeller blades.

Why do faster moving fluids have lower pressure?

Actually, faster moving fluids don’t necessarily have lower pressure. For example, a bottle of compressed air in the back of a pickup truck is still high-pressure air, even though it’s moving fast. The real issue here is that when fluid speeds up in passing through stationary obstacles, its pressure drops. For example, when air rushes into the open but stationary mouth of a vacuum cleaner, that air experiences not only a rise in speed, it also experiences a drop in pressure.

Similarly, when water rushes out of the nozzle of a hose, its speed increases and its pressure drops. This is simply conservation of energy: as the fluid gains kinetic energy, it must lose pressure energy. However, if there are sources of energy around — fans, pumps, or moving surfaces — then these exchanges of pressure for speed may no longer be present.

How can the volume of water passing through a weir be determined?

If the speed of the water were uniform as it passes through the opening, you could measure that speed and multiply it by the cross-section of the weir to obtain the volume of water passing through the weir each second. However, since the flow is faster near the center of the flow, it’s difficult to calculate the volume flowing each second. Your best bet is probably to divide the opening into a number of regions and then to measure the water’s velocity at the center of each region. Multiply each velocity by the cross-sectional area of that region and then sum up all the products to obtain the overall volume flow per second.

How does gravity powered water pump work?

While there are many possible designs for a pump, the classic version used a phenomenon called «water hammer» to lift water upward. In this technique, a column of water is allowed to accelerate downhill through a pipe until it’s flowing at a good speed through the pipe.

The pump then closes a valve at the lower end of the pipe, so that the water has to stop abruptly. Since water accelerates in response to imbalances in pressure, the stopping process involves an enormous pressure surge at the lower end of the moving water column. A one-way valve at the lower end of the pipe opens during this pressure surge and allows a small fraction of the water to escape from the pipe.

The escaping water rises upward through a second pipe for delivery to a home or business. The escaping water actually enters a head tank that is normally filled with air and thus compresses that air.

The compressed air is then used to push water through the pump’s outlet and provide the pumping action. This pumping scheme is apparently called a «hydraulic ram». The only trick to operating such a pump is opening and closing the valve at the lower end of the first pipe.

This valve must open long enough for the water in the pipe to reach good speed and then it must close very suddenly to provide the pressure surge that lifts the small amount of water upward for delivery.

How does a turbine flow meter work?

There are many different types of flow meters; some specialized to handling gases and others to handling liquids. In each case, a true flow meter transfers gas from its inlet to its outlet one unit of volume at a time and it measures how many of those volumes it transfers.

There are also some flow rate meters that measure how quickly a gas or liquid is flowing. These devices normally use turbines to measure the speed of the passing fluid and measurements from these flow rate meters can be integrated over time to determine how much gas or liquid has passed through them. However, because flow rate meters don’t measure each volume of gas directly, they aren’t as accurate as true flow meters.

The most common of a turbine flow meter is a device that’s half filled with liquid. The «turbine» is actually a set of blades that spin in a vertical plane and spend half their times immersed in the liquid. When one of the turning blades emerges from the liquid, the empty space that appears beneath it is allowed to fill with the gas being measured.

This gas flows in from the meter’s inlet. Soon another blade begins to emerge from the liquid and a volume of gas is then trapped between the first blade and the second blade. Once the blades have turned almost half a turn, the first one begins to submerge again in the liquid.

The gas that was trapped between it and the next blade is then squeezed out from between those blades by the liquid and flows out the meter’s outlet. A geared arrangement measures how many turns the blades make and therefore how many volumes of gas have been transferred from the meter’s inlet to its outlet.

Why is there a relationship between speed and pressure? What is that relation? Why are speed and pressure inversely related?

When a fluid is flowing smoothly and steadily through a stationary environment, its energy is conserved. As long as it doesn’t lose much energy to frictional effects, you can count on its total energy remaining essentially constant as it flows downstream.

Since it only has three forms for its energy: gravitational potential energy, pressure potential energy, and kinetic energy, you can expect that a decrease in one of these forms of energy will be accompanied by an increase in one of the other forms. That’s when speed and pressure are inversely related. When the fluid slows down, its kinetic energy drops so its pressure potential energy (and its pressure) must rise.

TEXT AND VOCABULARY EXERCISES

23. Find in the text the words or phrases which mean the same as:

§ центробежный насос	§ импеллер, крыльчатка
§ лопасть, лопатка	§ высокое и низкое давление
§ объем воды	§ водослив
§ гидравлический удар	§ точные водомеры
§ осаждение	§ равновесие, баланс
§ взаимосвязь между скоростью и давлением

Дата добавления: 2015-10-21