Static head pressure
The weight of a fluid in a container exerts pressure on the containing vessel's sides and bottom. This is called static head pressure. It is caused by earth's gravitational pull. A good example of head pressure is a community water system. Figure 1-7 shows a water tower with a topmost water level of 80 feet. A cubic inch of water weighs 0.0361 pounds. Therefore a one square-inch column of water will exert a force of 0.0361 psi for every inch of elevation. This works out to .433 psi per foot of elevation. For the water tower in Figure 1-7, the pressure at the base would be: 80 ft X 0.433 psi/ft = 34.6 psi. This pressure is always available, even when no pumps are running. Of course, if the water level drops, static head pressure also will drop.
The specific gravity of hydraulic oil is approximately 0.9, so multiplying water's 0.433 psi per foot by 0.9 shows oil exerts 0.39 psi per foot of elevation. Usually this fraction is rounded to 0.4 for simplicity. If the water tower were filled to 80 ft with oil, it would exert a pressure of 32 psi at ground level. Other fluids would develop a higher or lower static pressure according to their specific gravities.
This pressure is only realized at ground level at the tower. Outlets at other levels would be higher or lower according to their distance below the fluid surface.
Tanks seen on most water towers simply store volume. Pressure does not drop rapidly or require frequent pump starts to maintain the fluid level. The size or shape of the tank does not affect pressure at the base. Pressure at the base of a straight 80-ft pipe would be the same, but useful volume before pressure drop would change drastically. Always remember: it is not the physical size of a body of fluid that determines pressure but how deep it is.
Head pressure can have an adverse effect on a hydraulic system because many pumps are installed above the fluid level. This means the pump must first create enough vacuum to raise the fluid and then create even higher vacuum to accelerate and move it. Therefore there is a limit to how far a pump can be located above the oil level. Most pumps specify a maximum suction pressure of 3 psi. At 4- to 5-psi suction pressure, pumps start to cavitate . . . causing internal damage. At 6- to 7-psi vacuum, cavitation damage is severe and noise levels increase noticeably. (The effects of cavitation are covered fully in Chapter 8, Fluid power pumps and accessory items.) Axial- or in-line-piston pumps are especially vulnerable to high inlet vacuum damage and should be set up below the fluid level to produce a positive head pressure.
Many modern hydraulic systems place the pump next to the reservoir so the fluid level is always above the pump inlet. With this type of installation the pump always has oil at startup and has a positive head pressure at its inlet. A better arrangement puts the tank above the pump to take advantage of even greater head pressure. Everything possible should be done to keep pressure drop low in the pump inlet line because the highest possible pressure drop allowable is one atmosphere (14.7 psi at sea level).
The earth's atmosphere the air we breathe exerts a force of 14.7 psi at sea level on an average day. This pressure covers the whole earth's surface, but at elevations higher than sea level, it is reduced by approximately 0.5 psi per 1000 feet. This pressure of earth's atmosphere is the source of the power of vacuum. The highest possible vacuum reading at any location is the weight of the air above it at that time. A reading of maximum vacuum available is given during the local weather forecast as the barometer reading. Divide the barometer reading by two to get the approximate atmospheric pressure in psi. This force could be directly measured if it were possible to isolate a one square-inch column of air one atmosphere tall at a sea level location. Because this is not possible, the method used to measure vacuum is demonstrated in Figure 1-8.
Submerge a clear tube with one closed end in a container of mercury and allow it to fill completely. (The tube must be more than 30-in. long for this example to work when mercury is the liquid.) After the mercury displaces all the air in the tube, carefully raise the tube's closed end, keeping the open end submerged so the mercury can't run out and be replaced by air. When the tube is positioned vertically, the liquid mercury level will lower to give the atmospheric pressure reading in inches of mercury (29.92-in. Hg at sea level). The mercury level will fluctuate from this point as high and low-pressure weather systems move past. If the tube had been 100-in. tall, the mercury level would still have dropped to whatever the atmospheric pressure was at its location. The reason the mercury does not all flow out is that atmospheric pressure holds it in.
This barometer could have been built using another liquid but the tube would have to be longer because most other liquids have a much lower specific gravity than mercury's 13.546. Water, with a specific gravity of 1.0, would require a closed-end tube at least 33.8 ft long, while oil, with a specific gravity of approximately 0.9, would have to be even longer.
Vacuum pumps can be similar in design to air compressors. There are reciprocating-piston, diaphragm, rotary-screw, and lobed-rotor designs. (See air compressor types in Chapter 8, Fluid power pumps and accessory items.) Imagine hooking the inlet of an air compressor to a receiver tank and leaving the outlet open to atmosphere. As the pump runs, it evacuates air from the receiver and causes a negative pressure in it.
Vacuum pumps are an added expense and normally are only found in facilities that use a constant supply of negative pressure to operate machines or make products.
Vacuum generators that use plant compressed air as a power source are also available. These components have no moving parts but use plant air flowing through a venturi to produce a small supply of negative pressure. Figure 1-9 shows a simplified cutaway view of a venturi-type vacuum generator. The device consists of body A with compressed-air inlet B that passes air flow through venturi nozzle C. The air exhausts at a higher velocity to atmosphere through orifice D. As air at increasing velocity flows past opening E near the venturi nozzle, it creates a negative pressure and draws in atmospheric air through port F. Port F can connect to any external device that needs a vacuum source. A vacuum gauge at port F shows negative pressure when compressed air is supplied to port B.
Vacuum generators are inexpensive, but can be costly to operate. For every 4 cfm of air supply required to power them, they use approximately one compressor horsepower. For this reason, venturi-type vacuum generators usually are installed with a control valve to turn them on only when needed.
Vacuum is limited to one atmosphere maximum at any location, and standard vacuum pumps only reach about 85% (approximately 12 psi) of this on average. As a result, vacuum is not powerful enough to do much work unless it acts on a large area.
Many industrial vacuum applications have to do with handling parts. Large-area suction cups can lift a large heavy part with ease, as illustrated in Figure 1-10. When the lift rises, negative pressure (vacuum) inside the suction cups causes atmospheric pressure on the opposite side of the part to push it up.
Industries such as glass and wood manufacturing use vacuum to hold work pieces during machining or other operations, as shown in Figure 1-11. The pieces are held firmly in place as the negative pressure under them causes atmospheric pressure to push against them. A resilient seal laid in a groove in the fixture keeps atmospheric air from entering the cavity beneath the part. This groove can be cut to match the contour of the part. In machining operations, the seals can isolate interior cutouts, allowing them to be removed while firmly holding the rest of the piece.
Heated plastic sheet can be vacuum-formed to make some products at a much lower cost than other types of plastic forming, as suggested in Figure 1-12. Forming heated plastic sheet in a cavity or over a shape is quick and positive. When atmospheric pressure tries to fill the negative-pressure area under the softened sheet, the sheet is forced into the desired shape. Large parts such as pickup-truck bed liners are formed by this method.