Category: water distribution

How do you calculate the change in water pressure as the diameter of the hose changes? – JH

When water flows through a hose, it has three main forms for its energy: kinetic energy, gravitational potential energy, and an energy associated with its pressure—which I’ll call pressure potential energy. Since energy is conserved, the water’s energy can’t change as it flows through the hose (we’ll ignore frictional forces here, although they really are pretty important in a hose). Let’s assume that the hose is horizontal, so that the water’s gravitational potential energy can’t change. When the water enters a narrowing in the hose, the water must speed up to avoid delaying the water behind it. This increase in speed is associated with an increase in kinetic energy. Since the water’s energy can’t change, the increase in kinetic energy must be accompanied by a decrease in pressure. If the water then enters a widening in the hose, it slows down, its kinetic energy drops, and its pressure rises to conserve energy! If the hose then rises upward, so that the water’s gravitational potential energy rises, the water’s pressure must drop to conserve energy. In general, one form of energy can become another but the sum of those three forms can’t change.

How do you determine the volume of water passing through a weir? – R

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 an air pump work and how does the air pocket in a Nike Air or Reebok pump shoe keep its form? — MD, Toronto, CA

A typical bicycle pump uses a piston to squeeze air that it has trapped inside a cylinder. As you push the piston into the cylinder, the trapped air molecules are packed more tightly together and their pressure rises. Moreover, because you are transferring energy to the air by doing mechanical work on it, the air’s temperature also rises. Air always accelerates toward regions of lower pressure, so this pressurized air will tend to flow through any opening that leads to lower pressure—such as the inside of an underinflated bicycle tire. A one-way valve at the base of the cylinder allows this pressurized air to flow out of the cylinder through a pipe and enter the bicycle tire. Thus each time you push down on the piston, you pressurize the air inside the cylinder and it accelerates and flows toward the lower pressure inside the bicycle tire. As you pull the piston out of the cylinder, a second one-way valve allows new air to enter the cylinder from outside so that you can repeat this process.

In a pumped air athletic shoe, squeezing a rubber bulb packs together air molecules and increases their pressure. When the pressure is high enough, a one-way valve allows this pressurized air to flow into the underinflated air pocket of the shoe. A second one-way valve allows the bulb to refill with outside air when you stop squeezing the bulb. Once the air pocket has been filled with large numbers of air molecules, these molecules exert substantial outward forces on the inner surfaces of that air pocket. The more molecules there are inside the pocket, the more often they collide with the surfaces and the more force they exert on those surfaces. These outward forces from the air molecules allow the air pocket keeps its shape.

How does a gravity powered water pump work? — JA, Hiawassee, GA

I believe that the pump you’re interested in is one that uses the energy released when water flows downhill to lift a small fraction of that water upward. While there are many possible designs for such 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. According to a reader, 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 that the water in the pipe reaches a 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 Bourdon tube pressure gauge work? – AM

A Bourdon tube pressure gauge works on much the same principle as a party favor that inflates and unrolls when you blow in its tube. The hollow Bourdon tube of the pressure gauge isn’t circular in cross-section—it’s somewhat oval. When the pressure inside the tube increases, the tube’s oval walls are distorted and the tube’s cross-section becomes slightly more circular. However, the tube is wrapped in a coil and as its walls become more circular, the tube uncoils slightly. The amount of uncoiling that occurs is almost exactly proportional to the pressure inside the Bourdon tube. As the tube uncoils, its motion activates a rack-and-pinion gear system that turns the needle on the pressure dial of the gauge. While all that you see when you look at the gauge is this needle pointing at the current pressure, you should understand that there is a small, bent tube that’s coiling and uncoiling with each change in the pressure inside that tube.

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 of 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.

Let me assume that you want to know about a turbine flow meter for gas. The most common of these 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 it that when I am in my dorm room with my window open and the door closed, there isn’t a change in temperature and no wind comes in or blows around. But if I open the door, the room becomes cold and wind is felt throughout the room?

When the wind blows into your room, it comes to a stop and experiences a rise in pressure. This is an consequence of Bernoulli’s equation, which recognizes that energy is conserved and that in a fluid, energy can exist either as kinetic energy (energy of motion), pressure energy, or gravitational potential energy. In this case, the wind’s kinetic energy becomes pressure energy as it slow down in your room. As the pressure in your room rises, it prevents more air from entering, so you have high pressure but no movement inside your room. As soon as you open the door, the high-pressure air in your room accelerates toward the relatively low-pressure air in your hall. The pressure in your room drops and the wind can get in now. Soon the wind is blowing right through your room, as though you were part of a wind tunnel. If the wind is cold, you will be too.

Why is there a relationship between speed and pressure? What is that relation? Why are they inverses of each other?

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.

Can air have gravitational potential energy?

Yes. However, you often don’t notice this because as you lower a volume of air downward, you displace a similar volume of air upward. Thus you can’t just raise or lower air to observe changes in its gravitational potential energy. You’d have less trouble if you compressed the air tightly together, perhaps turning it into a liquid, and then raised or lowered it. It’s gravitational potential energy would then be much more noticeable.

How does water move toward your mouth through a straight straw if you don’t suck on the straw?

If the straw is horizontal and the water wasn’t moving to begin with, it won’t move toward your mouth unless you suck. To make the water accelerate, it must experience net force and the two ways to achieve that net force are (1) to create a pressure imbalance on the water’s ends and (2) to have the water’s weight accelerate it. In a horizontal force with no pressure imbalance on it, there is no net force on the water and it doesn’t accelerate.