UNIT 1

Exercise 1 Read and translate the following words.

Liquid, hydraulics, pneumatics, circuit, pipes, tubes, hoses, equipment, valves, actuators, poor circuit design, leaks, available, rigidity

Exercise 2 Match the words with their synonyms.

horse power liquidhazardous operatefluid force work dangerouspower hp

Exercise 3 Read and translate the text.

TEXT 1

Any media (liquid or gas) that flows naturally or can be forced to flow could be used to transmit energy in a fluid power system. The earliest fluid used was water hence the name hydraulics was applied to systems using liquids. In modern terminology, hydraulics implies a circuit using mineral oil. Figure 1-1 shows a basic power unit for a hydraulic system. (Note that water is making something of a comeback in the late '90s; and some fluid power systems today even operate on seawater.) The other common fluid in fluid power circuits is compressed air. As indicated in Figure 1-2, atmospheric air – compressed 7 to 10 times – is readily available and flows easily through pipes, tubes, or hoses to transmit energy to do work. Other gasses, such as nitrogen or argon, could be used but they are expensive to produce and process.

Fig. 1-1: Basic hydraulic power unit.

Of the three main methods of transmitting energy mechanical, electrical, and fluid, fluid power is least understood by industry in general. In most plants there are few persons with direct responsibility for fluid power circuit design or maintenance. Often, general mechanics maintain fluid power circuits that originally were designed by a fluid-power-distributor salesperson. In most facilities, the responsibility for fluid power systems is part of the mechanical engineers' job description.

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The problem is that mechanical engineers normally receive little if any fluid power training at college, so they are ill equipped to carry out this duty. With a modest amount of fluid power training and more than enough work to handle, the engineer often depends on a fluid power distributor's expertise. To get an order, the distributor salesperson is happy to design the circuit and often assists in installation and startup. This arrangement works reasonably well, but as other technologies advance, fluid power is being turned down on many machine functions. There is always a tendency to use the equipment most understood by those involved.

Fig. 1-2: Basic pneumatic power arrangement.

Fluid power cylinders and motors are compact and have high energy potential. They fit in small spaces and do not clutter the machine. These devices can be stalled for extended time periods, are instantly reversible, have infinitely variable speed, and often replace mechanical linkages at a much lower cost. With good circuit design, the power source, valves, and actuators will run with little maintenance for extended times. The main disadvantages are lack of understanding of the equipment and poor circuit design, which can result in overheating and leaks. Overheating occurs when the machine uses less energy than the power unit provides. (Overheating usually is easy to design out of a circuit.) Controlling leaks is a matter of using straight-thread O-ring fittings to make tubing connections or hose and SAE flange fittings with larger pipe sizes. Designing the circuit for minimal shock and cool operation also reduces leaks.

A general rule to use in choosing between hydraulics or pneumatics for cylinders is: if the specified force requires an air cylinder bore of 4 or 5 in. or larger, choose hydraulics. Most pneumatic circuits are under 3 hp because the efficiency of air compression is low. A system that requires 10 hp for hydraulics would use approximately 30 to 50 air-compressor horsepower. Air circuits are less expensive to build because a separate prime mover is not required, but operating costs are much higher and can quickly offset low component expenses. Situations where a 20-in. bore air cylinder could be economical would be if it cycled only a few times a day or was used to hold tension and never cycled. Both air and hydraulic circuits are capable of operating in hazardous areas when used with air logic controls or explosion-proof electric controls. With certain

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precautions, cylinders and motors of both types can operate in high-humidity atmospheres . . . or even under water.

When using fluid power around food or medical supplies, it is best to pipe the air exhausts outside the clean area and to use a vegetable-based fluid for hydraulic circuits.

Some applications need the rigidity of liquids so it might seem necessary to use hydraulics in these cases even with low power needs. For these systems, use a combination of air for the power source and oil as the working fluid to cut cost and still have lunge-free control with options for accurate stopping and holding as well. Air-oil tank systems, tandem cylinder systems, cylinders with integral controls, and intensifiers are a few of the available components.

Exercise 4 Decide whether the following statements are true, false or not mentioned in the text.

1 The earliest fluid used was mineral oil hence the name hydraulics was applied to systems using liquids

2 Other gasses, such as nitrogen or argon, could be used in fluid power circuit because they are cheap to produce and process

3 In most plants there are few people with direct responsibility for fluid power circuit design or maintenance

4 The distributor salesperson is happy to design the circuit and often assists in installation and startup

5 Fluid power cylinders and motors are competitive6 Most pneumatic circuits are below 3 hp7 Air circuits are less expensive to build

Exercise 5 Answer the questions.

1 What could be used to transmit energy in a fluid power system?2 What are the three main methods of transmitting energy?3 What are the main disadvantages of fluid power cylinders and

motors?4 What is a general rule to use in choosing between hydraulics or

pneumatics for cylinders?

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Exercise 6 Make up your own questions to the text.

Exercise 7 Write out key words to the text.

Exercise 8 Give a title to each paragraph of the text.

Exercise 9 Make up a short (not more than 10 sentences) written summary to the text.

UNIT 2

Exercise 1 Read and translate the following word combinations.

In a confined body, in a pressurized container, the resistance of the load, in the entire circuit, the pressurized oil, piston area, to move upward, fluid power, pressure gauge, troubleshooting hydraulic circuits, pump out let, hydraulic leverage, lever-arm length, rotary actuator, limitless force or torque, a force traversing through a distance, trapped air

Exercise 2 Match the words with their definitions.

Pressure gauge, piston, torque, actuator, work

1) one that activates, especially a device responsible for actuating a mechanical device

2) a solid cylinder or disk that fits snugly into a larger cylinder and moves under fluid pressure, as in a reciprocating engine, or displaces or compresses fluids, as in pumps and compressors

3) a device for measuring the pressure of a gas or liquid4) the measure of a force traversing through a distance5) the moment of a force; the measure of a force's

tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force and the force vector

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Exercise 3 Read and translate the text.

TEXT 2

The reason fluids can transmit energy when contained is best stated by a man from the 17th century named Blaise Pascal. Pascal's Law is one of the basic laws of fluid power. This law says: Pressure in a confined body of fluid acts equally in all directions and at right angles to the containing surfaces. Another way of saying this is: If I poke a hole in a pressurized container or line, I will get PSO. PSO stands for pressure squirting out and puncturing a pressurized liquid line will get you wet. Figure 1-3 shows how this law works in a cylinder application. Oil from a pump flows into a cylinder that is lifting a load. The resistance of the load causes pressure to build inside the cylinder until the load starts moving. While the load is in motion, pressure in the entire circuit stays nearly constant. The pressurized oil is trying to get out of the pump, pipe, and cylinder, but these mechanisms are strong enough to contain the fluid. When pressure against the piston area becomes high enough to overcome the load resistance, the oil forces the load to move upward. Understanding Pascal's Law makes it easy to see how all hydraulic and pneumatic circuits function.

Fig. 1-3: How Pascals Law affects a cylinder

Notice two important things in this example. First, the pump did not make pressure; it only produced flow. Pumps never make pressure. They only give flow. Resistance to pump flow causes pressure. This is one of the basic principles of fluid power that is of prime importance to troubleshooting hydraulic circuits. Suppose a machine with the pump running shows almost 0 psi on its pressure gauge. Does this mean the pump is bad? Without a flow meter at the pump outlet, mechanics might change the pump, because many of them think pumps make pressure. The problem with this circuit could simply be an open valve that allows all pump flow to go directly to tank. Because the pump outlet flow sees no resistance, a pressure gauge shows little or no pressure. With a flow meter

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installed, it would be obvious that the pump was all right and other causes such as an open path to tank must be found and corrected.

Fig. 1-4: Comparison of mechanical and hydraulic leverage

Another area that shows the effect of Pascal's law is a comparison of hydraulic and mechanical leverage. Figure 1-4 shows how both of these systems work. In either case, a large force is offset by a much smaller force due to the difference in lever-arm length or piston area.

Notice that hydraulic leverage is not restricted to a certain distance, height, or physical location like mechanical leverage is. This is a decided advantage for many mechanisms because most designs using fluid power take less space and are not restricted by position considerations. A cylinder, rotary actuator, or fluid motor with almost limitless force or torque can directly push or rotate the machine member. These actions only require flow lines to and from the actuator and feedback devices to indicate position. The main advantage of linkage actuation is precision positioning and the ability to control without feedback.

At first look, it may appear that mechanical or hydraulic leverage is capable of saving energy. For example: 40,000 lb is held in place by 10,000 lb in Figure 1-4. However, notice that the ratio of the lever arms and the piston areas is 4:1. This means by adding extra force say to the 10,000-lb side, it lowers and the 40,000-lb side rises. When the 10,000-lb weight moves down a distance of 10 in., the 40,000-lb weight only moves up 2.5 in.

Work is the measure of a force traversing through a distance. (Work = Force X Distance.). Work usually is expressed in foot-pounds and, as the

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formula states, it is the product of force in pounds times distance in feet. When a cylinder lifts a 20,000-lb load a distance of 10 ft, the cylinder performs 200,000 ft-lb of work. This action could happen in three seconds, three minutes, or three hours without changing the amount of work.

When work is done in a certain time, it is called power. {Power = (Force X Distance) / Time.} A common measure of power is horsepower - a term taken from early days when most persons could relate to a horse's strength. This allowed the average person to evaluate to new means of power, such as the steam engine. Power is the rate of doing work. One horsepower is defined as the weight in pounds (force) a horse could lift one foot (distance) in one second (time). For the average horse this turned out to be 550 lbs. one foot in one second. Changing the time to 60 seconds (one minute), it is normally stated as 33,000 ft-lb per minute.

No consideration for compressibility is necessary in most hydraulic circuits because oil can only be compressed a very small amount. Normally, liquids are considered to be incompressible, but almost all hydraulic systems have some air trapped in them. The air bubbles are so small even persons with good eyesight cannot see them, but these bubbles allow for compressibility of approximately 0.5% per 1000 psi. Applications where this small amount of compressibility does have an adverse effect include: single-stroke air-oil intensifiers; systems that operate at very high cycle rates; servo systems that maintain close-tolerance positioning or pressures; and circuits that contain large volumes of fluid.

Another situation that makes it appear there is more compressibility than stated previously is if pipes, hoses, and cylinder tubes expand when pressurized. This requires more fluid volume to build pressure and perform the desired work. In addition, when cylinders push against a load, the machine members resisting this force may stretch, again making it necessary for more fluid to enter the cylinder before the cycle can finish.

As anyone knows, gasses are very compressible. Some applications use this feature. In most fluid power circuits, compressibility is not advantageous; in many, it is a disadvantage. This means it is best to eliminate any trapped air in a hydraulic circuit to allow faster cycle times and to make the system more rigid.

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Exercise 4 Decide whether the following statements are true or false.

1 Pascal’s Law says: Pressure in a confined body of fluid acts equally in two directions and at right angles to the containing surfaces

2 PSO stands for pressure squirting out3 When pressure against the piston area becomes high enough to

overcome the load resistance, the load forces the oil to move upward4 Pumps give pressure5 A common measure of power is horsepower6 Gasses are very compressible

Exercise 5 Make up 10 questions to the text.

Exercise 6 Write down a plan to the text.

Exercise 7 Explain Pascal’s Law and show how it works by any example.

Exercise 8 Make up a short (not more than 15 sentences) written note on any other law used in Hydraulics and Pneumatics (Charle’s Law, Boyle’s Law, etc.)

UNIT 3

Exercise 1 Read and translate the following words and word combinations.

Service life, reliable operation, fire resistance, fire hazard, fireproof, internal combustion, valves that control actuators, power losses, lubrication qualities, ample lubrication, viscosity, seal, erode, measure, cavitation, bypass, leakage, refined oil, additives, incompatible, part life, counteract, efficiency, rubber, resiliency, exceed.

Exercise 2 Give definitions to the following words. If this task causes difficulties to you, do this exercise again after you read the text.

Gauge, piston, fire resistance, lubrication, seal, viscosity, refined oil, SUS, cavitation, bypass, incompatible, oxidation, rust, eliminate, reduce

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Exercise 3 Read and translate the text.

TEXT 3

For long service life, safety reasons, and reliable operation of hydraulic circuits, it is very important to use the correct fluid for the application. The most common fluid is based on mineral oil, but some systems require fire resistance because of their proximity to a heat source or other fire hazard. (Water is also making its return to some hydraulic systems because it is inexpensive, fireproof, and does not harm the environment.)

Transmit energy

The main purpose of the fluid in any system is to transmit energy. Electric, internal combustion, steam powered, or other prime movers drive a pump that sends oil through lines to valves that control actuators. The fluid in these lines must transmit the prime movers energy to the actuator so it can perform work. The fluid must flow easily to reduce power losses and make the circuit respond quickly.

Lubricate

In most hydraulic systems, the fluid must have good lubrication qualities. Pumps, motors, and cylinders need ample lubrication to make them efficient and extend their service life. Mineral oils with anti-wear additives work well and are available from most suppliers. Some fluids may need special considerations in component design to overcome their lack of lubricity.

Seal

Fluid thickness can be important also because one of its requirements is for sealing. Almost all pumps and many valves have metal to metal sealing fits that have minimal clearance but can leak at elevated pressures. Thin watery fluid can flow through these clearances, reducing efficiency and eroding the mating surfaces. Thicker fluids keep leakage to a minimum and efficiency high.

There are several areas that apply to specifying fluids for a hydraulic circuit. Viscosity is the measure of the fluids thickness. Hydraulic oils

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thickness is specified by a SUS or SSU designation, similar to the SAE designation used for automotive fluids. SUS stands for Saybolt Universal Seconds (or as some put it, Saybolt Seconds Universal). It is a measuring system set up by a man named Saybolt. Simply stated, the system takes a sample of fluid, heats it to 100° F, and them measures how much fluid passes through a specific orifice in a certain number of seconds.

Viscosity is most important as it applies to pumps. Most manufacturers specify viscosity limits for their pumps and it is best to stay within the limits they suggest. The prime reason for specifying a maximum viscosity is that pressure drop in the pump suction line typically is low and if the oil is too thick, the pump will be damaged due to cavitation. A pump can move fluid of any viscosity if the inlet is amply supplied. On the other end, if fluids are too thin, pump bypass wastes energy and generates extra heat. All other components in the circuit could operate on any viscosity fluid because they only use what is fed to them. However, thicker fluids waste energy because they are hard to move. Thin fluids waste energy because they allow too much bypass.

Viscosity index (or VI) is a measure of viscosity change from one temperature to another. It is common knowledge that heating any oil makes it thinner. A normal industrial hydraulic circuit runs at temperatures between 100° and 130° F. Cold starts could be as low as 40° to 50° F. Using an oil with a low VI number might start well but wind up with excessive leakage and wear or cause cavitation damage at startup and run well at temperature. Most industrial hydraulic oils run in the 90- to 105-VI range and are satisfactory for most applications.

Pour point is the lowest temperature at which a fluid still flows. It should be at least lower than the lowest temperature to which the system will be exposed so the pump can always have some lubrication. Consider installing a reservoir heater and a circulation loop on circuits that start or operate below 60° F.

Refined mineral oil does not have enough lubricating qualities to meet the needs of modern day hydraulic systems. Several lubricity additives to enhance that property are added to mineral oil as a specific manufacturers package. These additives are formulated to work together and should not be mixed with others additives because some components may be incompatible.

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Refined mineral oil also is very much affected by temperature change. In its raw state it not only has low lubricity but also would thin out noticeably with only a small increase in temperature. Viscosity modifiers enhance the oils ability to remain at a workable viscosity through a broad temperature range.

There are several causes of hydraulic oil oxidation. These include contamination, air, and heat. The interaction of these outside influences cause sludge and acids to form. Oxidation inhibitors slow or stop the fluids degradation and allow it to perform as intended.

Wear inhibitors are additives that bond with metal parts inside a hydraulic system and leave a thin film that reduces metal-to-metal contact. When these additives are working, they extend part life by reducing wear.

In most hydraulic systems, fast and turbulent fluid flow can lead to foaming. Anti-foaming agents make the fluid less likely to form bubbles and allow those that do form to dissipate more rapidly.

Moisture in the air can condense in a hydraulic reservoir and mix with the fluid. Rust inhibitors negate the effect of this unwanted water and protect the surfaces of the systems metal components. All of these additives are necessary to extend system life and improve reliability.

Overheating the fluid can counteract the additives and decrease system efficiency. Overheating also thins the oil and reduces efficiency because of internal bypassing. Clearances in pump and valve spools let fluid pass as pressure increases, causing more heating until the fluid breaks down. External leaks through fittings and seals also increase as fluid temperatures rise. Another problem caused by overheating is a breakdown of some seal materials. Most rubber compounds are cured by controlled heat over a specific period of time. Continued heating inside the hydraulic system over long periods keeps the curing process going until the seals lose their resiliency and their ability to seal. It is best if hydraulic oil never exceeds 130° F for any extended period. Installing heat exchangers is the most common cure for overheating but designing heat out of a circuit is the better way.

Cold oil is not a problem as far as the oil is concerned but cooling does increase viscosity. When viscosity gets too high, it can cause a pump

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to cavitate and damage itself internally. Thermostatically controlled reservoir heaters easily eliminate this problem in most cases.

Exercise 4 Write out any 5 words from the text and give them written definitions.

Exercise 5 Fill in the table using any words from the text according to their parts of speech, identify their suffixes and prefixes if they have any.

NOUN VERB ADJECTIVE ADVERB

Exercise 6 Insert the necessary preposition from the list given below.

OF IN AS TO FOR FOR FOR WITH BY ON

1 The name hydraulics was applied ____________ systems using liquids.

2 In most facilities, the responsibility ______________ fluid power systems is part of the mechanical engineers' job description.

3 With a modest amount of fluid power training and more than enough work to handle, the engineer often depends ____________ a fluid power distributor's expertise.

4 Controlling leaks is a matter _________________ using straight-thread O-ring fittings to make tubing connections

5 Some applications need the rigidity of liquids so it might seem necessary to use hydraulics ______________ these cases even with low power needs.

6 PSO stands ______________ pressure squirting out and puncturing a pressurized liquid line will get you wet.

7 One horsepower is defined _____________ the weight in pounds (force) a horse could lift one foot (distance) in one second (time).

8 Mineral oils _____ anti-wear additives are available from most suppliers.

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9 Refined mineral oil also is very much affected ____ temperature change.

10 Installing heat exchangers is the most common cure _____ overheating.

Exercise 7 Make up 10 questions to the text.

Exercise 8 Write down a plan to the text.

Exercise 9 Give a title to the text.

Exercise 10 Retell the text using linking words and phrases. You can use a written plan if necessary.

UNIT 4

Exercise 1 Give definitions to the following words and word combinations. Identify their parts of speech.

Fire resistant, operate, lubricity, drastically, compatible, efficient, benefits, valve, eliminate, mixture, implement, implementation, evaporate, specify, protective.

Exercise 2 Answer the questions. If this task causes difficulties to you, do this exercise again after you read the text.

1 Is mineral oil flammable?2 What is the difference between fireproof and fire-resistant fluids?3 What are positive and negative sides of using water or mineral oil in

hydraulic circuits?4 Is it possible to use sea water in hydraulic circuits?5 What fire-resistant fluids do you know?

Exercise 3 Read the text.

TEXT 4

Fire-resistant fluids

Certain applications must operate near a heat source with elevated temperatures or even open flames or electrical heating units. Mineral oil is very flammable. It not only catches fire easily but will continue to burn even after removing the heat source. This fire hazard situation can be

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eliminated by several different choices of fluids. These fluids are not fireproof, only fire-resistant, which means they will burn if heated past a certain temperature but they will not continue to burn after removing them from the heat source.

Generally, the fire-resistant fluids do not have the same specifications as mineral oil-based fluids. Pumps often must be down rated because the fluids lubricity or specific gravity is different and would shorten the pumps service life drastically at elevated pressures or high rotary speeds. Some fire-resistant fluids are not compatible with standard seal materials so seals must be changed. Always check with the pump manufacturer and fluid supplier before using or changing to a fire-resistant fluid.

Water

Originally, hydraulic circuits used water to transmit energy (hence the word hydraulics). The main problem with water-filled circuits was either low-pressure operation or very expensive pumps and valves to operate with this low viscosity fluid above 500 to 600 psi. When huge oil deposits were discovered, mineral oil replaced water because of its additional benefits. Water made a brief comeback during an oil shortage crisis but quickly succumbed when oil flowed freely again.

In the late 90s, water again made inroads into oil-hydraulic systems. Several companies have developed reliable pumps and valves for water that operate at 1500 to 2000 psi. There are still limitations (such as freezing) to using water, but in certain applications it has many benefits. One big advantage is that there are fewer environmental problems during operation or in disposing of the fluid. Price also is a factor because water costs so little and is readily available almost anywhere.

Some suppliers are making equipment that operates on seawater to eliminate possible contamination of the earths potable water sources. These systems operate at elevated pressures without performance loss.

High water-content fluids

Some types of manufacturing still use water as a base and add some soluble oil for lubrication. This type of fluid is known as high water-content fluid (or HWCF). The common mixture is 95% water and 5%

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soluble oil. This mixture takes care of most of the lubricity problems but does not address low viscosity concerns. Therefore, systems using HWCF still need expensive pumps and valves to make them efficient and extend their life.

Rolling mills and other applications with molten metals are one area where HWCF is prevalent. Often the soluble oil is the same compound used for coolant in the metal-rolling process. This eliminates concerns about cross-contamination of fluids and the problems it can cause.

Water-in-oil emulsions

Some systems use around 40% water for fire resistance and 60% oil for lubrication and viscosity considerations. Again, these are not common fluids because they require special oil and continuous maintenance to keep them mixed well and their ratio within limits. Most manufacturers do not want the problems associated with water-in-oil emulsions so their use is very limited.

Water glycol

A very common fire-resistant fluid is water glycol. This fluid uses water for fire resistance and a product like ethylene glycol (permanent anti-freeze) for lubricity, along with thickeners to enhance viscosity. Ethylene glycol will burn, but the energy it takes to vaporize the water present quickly quells the fire once it leaves the heat source. This means a fire would not spread to other parts of the plant. Always remember fire-resistant not fireproof.

Water glycol fluids are heavier than mineral oil and do not have its lubricating qualities, so most pump manufacturers specify reduced rpm and lower operating pressures for water glycol. In addition, the water in this fluid can evaporate, especially at elevated temperatures, so it must be tested regularly for the correct mixture.

Cost is also a consideration. Water glycol is more expensive than oil and requires most of the same considerations when disposing of it.

Always check with the pump manufacturer before specifying water glycol fluid to see what changes are necessary to run the pump with this fluid. Seal compatibility is usually not a problem, but always check each

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manufacturers specifications before implementing this fluid. In addition, it is imperative to completely flush a system of any other fluids before refilling with water glycol.

Synthetics

The other main fire-resistant fluids are synthetic types. They are made from mineral oil, but have been processed and contain additives to obtain a much higher flash point. It takes more heat to start them burning but there is not enough volatile materials in them to sustain burning. These fluids may catch fire from a pot of hot metal but quickly self-extinguish after leaving the heat source.

Synthetic fluids retain most of the qualities of the mineral oil from which they are derived, so most hydraulic components specify no operating restrictions. However, most of these fluids are not compatible with common seal materials so seal specification changes are usually necessary. Special consideration must be given to handling of synthetics because they can cause skin irritation and other health hazards. Also most synthetic fluids require protective epoxy paint for all components in contact with them.

Of all the fluids discussed, synthetics are the most expensive. They can cost up to five times more than mineral oil.

No matter which fluid is chosen, design the circuit to work in a reasonable temperature range; install good filters and maintain them; and check the fluids regularly to see if they are within specification limits.

A good operating temperature range is between 70° and 130° F with the optimum being around 110° F. A rule of thumb would be: warm enough to feel hot to the touch but cool enough to hold tightly for an extended period. Overheating hydraulic fluids is second only to contamination when it comes to reasons for fluid failure.

Exercise 4 Fill in the table using the words given below according to their parts of speech, identify their suffixes and prefixes if they have any.

Application, source, flame, flammable, remove, generally, shorten, rotary, supplier, originally, hydraulic, expensive, viscosity, deposits, brief,

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shortage, contamination, potable, soluble, extend, prevalent, considerations, require, obtain, special, maintain, thumb, tightly.

NOUN VERB ADJECTIVE ADVERB

Exercise 5 Write out from the text all names of fire-resistant fluids.

Exercise 6 Decide whether the following statements are true, false or not mentioned in the text.

1 Mineral oil will continue to burn after removing the heat source2 Fire hazard situation can be eliminated by fireproof fluids3 High water-content fluid contains no oil4 Water glycol fluids have better lubricating qualities than mineral oil

does5 Water glycol fluids can cause heart problems

Exercise 7 Give a title to each paragraph of the text.

Exercise 8 Make a short summary of the text in your native language.

UNIT 5

Exercise 1 Organize a group discussion using the following questions.

1 Why is continuous filtration of any hydraulic system necessary?2 Which filters are the best? Why?3 How and where should you store fluids?

Exercise 2 Read text 5A to find out the answers to the questions in exercise 1.

TEXT 5 A

Continuous filtration of any hydraulic system is necessary for long component life. Fluids seldom wear out but they can become so

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contaminated that the parts they drive can fail. (The filter section of this book offers some good recommendations on keeping fluid clean.)

Even with the best of care, any hydraulic fluid should be checked at least twice a year. Systems located in dirty atmospheres may need to be checked more often to see if a pattern exists that requires special consideration. Pay close attention to the sampling process and packaging procedures recommended by the test facility that will process the sample. Expect a report on the level of contamination plus an analysis of the additive contents, water content, ferrous and non-ferrous material amounts, and any other problem areas the test facility finds. Use this information to know when to change fluids and to check for abnormal part wear problems.

Fig. 2-1. Filter cart (used to transfer hydraulic fluids) and its circuit schematic diagram.

New oil or other fluids from the supplier are not necessarily clean. The fluids are shipped in drums or by bulk, and there is no way of knowing how clean these containers are. Some suppliers offer filtered oil with a guaranteed contamination level at added cost. Otherwise, about the lowest level of contamination from most manufacturers is 25 microns.

Anytime a system needs new fluid, it is best to use a transfer unit, Figure 2-1, with a 10-micron or finer filter in the loop. Another way of filtering new or refill fluid is with a filter permanently attached to the reservoir, Figure 2-2. In this arrangement, the breather or other possible fill points should be made inaccessible.

Fig. 2-2 Hydraulic power unit and circuit diagram of its filter arrangement

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The filter cart shown in Figure 2-1 can also be used to filter any hydraulic unit in the plant. Instead of this filter unit sitting idle except when filling systems, set it up at a machines power unit for a timed run. Place the suction hose in one end of the reservoir and the return hose in the opposite end. This adds a continual filtration loop to any machine even when the machines main pump is shut off. Run the cart until the fluid is clean and then move is to another power unit. Repeating this process on a regular schedule can assist the hydraulic units filters and add extra life to the fluid and the hydraulic components. This process may also show a pattern on machines that have a contamination problem.

Hydraulic fluids should be stored in a clean dry atmosphere. Keep all containers closed tightly and reinstall covers on any partially used drums.

Never mix fluids in any hydraulic system. Make sure all containers are clearly marked and segregated so fluids will not be mixed with one another. Mixing fluids can result in damage to components and some combinations are very difficult to clean up. Be especially careful when mineral oils and synthetic or water-glycol fluids are used in different parts of the same plant.

Fluids are the lifeblood of any hydraulic system and should be given the utmost care.

Exercise 3 Make oral speech on the topic “Fluids in hydraulic circuits”. Remember to use linking words and phrases.

Exercise 4 Read text 5B and present the information given in the text schematically.

TEXT 5 B

PUMPS

Pumps can be classified into two groups which generally describe how energy is transmitted to the fluid: dynamic and displacement. The two subclassifications of displacement pumps are reciprocating and rotary. Since these are seldom used in water conveyance systems as the principal pumps, they will not be discussed.

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The most common type of dynamic pump is the cen t r i f uga l pump. This classification is subdivided according to head and discharge characteristics into: ax ia l -flow (low head and high discharge), mi xed -f l ow (moderate head and moderate discharge), and rad i a l - f l ow (high head and low discharge). These classifications are more accurately defined by the specific speed of the machine (to be defined later), which is a function of head, discharge, and rotational speed. The axial, mixed, and radial classifications can be further subdivided into single- or multiple-stage; single- or double-suction (mixed flow only); open or closed-impeller; self-priming or nonpriming; fixed- or variable-pitch; and fixed- or variable-speed. Pumps can also be classified according to their installation or physical orientation: wet-pit, dry-pit, and horizontal or vertical.

Exercise 5 Translate text 5B in written form.

Exercise 6 Retell the text according to the scheme from exercise 4.

Exercise 7 Fill in the sentences with necessary prepositions from the list given below.

BY INTO OUT OF WITH WITH TO TO TO TO TO AS

BETWEEN BEFORE INTO IN IN OF OF OF OF ABOUT

1 Pumps can be classified _____________ two groups.

2 Pumps can also be classified according _____________ their installation or physical orientation.

3 Often, general mechanics maintain fluid power circuits that originally were designed _________ a fluid-power-distributor salesperson.

4 The main disadvantages are lack _____________ understanding of the equipment and poor circuit design, which can result _____________ overheating and leaks.

5 A general rule to use in choosing _____________ hydraulics or pneumatics for cylinders is: if the specified force requires an air cylinder bore of 4 or 5 in. or larger, choose hydraulics.

6 Pascal's Law is one _____________ the basic laws of fluid power.22

7 Oil from a pump flows ___________ a cylinder that is lifting a load.

8 The pressurized oil is trying to get _____________ the pump, pipe, and cylinder, but these mechanisms are strong enough to contain the fluid.

9 In either case, a large force is offset by a much smaller force due _____________ the difference in lever-arm length or piston area.

10 _____________ addition, when cylinders push against a load, the machine members resisting this force may stretch.

11 Viscosity is most important as it applies _____________ pumps.

12 In most hydraulic systems, fast and turbulent fluid flow can lead _____________ foaming.

13 Moisture in the air can condense in a hydraulic reservoir and mix _____________ the fluid.

14 This type of fluid is known ____________ high water-content fluid

15 This eliminates concerns _____________ cross-contamination of fluids and the problems it can cause.

16 Most manufacturers do not want the problems associated _____________ water-in-oil emulsions so their use is very limited.

17 Water glycol is more expensive than oil and requires most of the same considerations when disposing _____________ it.

18 Always check each manufacturers specifications _____________ implementing water glycol fluid.

19 Special consideration must be given ______ handling of synthetics.

20 High water-content fluid takes care _____________ most of the lubricity problems but does not address low viscosity concerns.

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FlowFig. 3.1 Typical system head curve.

UNIT 6

Exercise 1 Organize a group discussion using the following questions.

1 What should be taken into consideration while selecting a pump?2 What is total dynamic head?3 What is net positive suction head

Exercise 2 Read text 6 to find out the answers to the questions in exercise 1.

TEXT 6

PUMP HYDRAULICS

Selecting a pump for a particular service requires matching the system requirements and pump capabilities. The process (of analyzing the head-discharge requirements of a system) consists of applying the energy equation and evaluating the pumping head required to overcome the elevation difference (static lift) and the friction plus minor losses. For a pump supplying water between two reservoirs, the pump head required to produce a given discharge can be expressed as

(3.1a)or

(3.1b)

in which the constant

Figure 3.1 is a graphical representation of Eq. 3.1 showing the general shape of a system head curve. The curve shown is for a system having a relatively large elevation change and significant friction losses. The shape of the system head curve depends on the relative magnitudes of the elevation change versus friction losses.

Before a pump can be selected for a particular application, information must be provided from which the system head curve can be generated. If the elevation

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of either reservoir is a variable, then there is not a single curve but a family of curves corresponding to the various reservoir elevations. In addition to supplying the system head curve information, it is necessary that the desired operating range be identified before any pump or combination of pumps can be selected.

With the system head and discharge characteristics and the approximate operating point determined, selection of a pump is possible. Proper selection of a pump requires that it not only provide the required head and discharge, but that it operate near its rated conditions, which is its best efficiency point (bep), and function free of cavitation, vibrations, and any other undesirable characteristics. The entire piping system should also be analyzed from the standpoint of any hydraulic transients which may be generated by start-up, shutdown, and any other normal or abnormal changes in the flow.

Total Dynamic HeadBefore discussing pump characteristics, several parameters used to

describe pump performance should be defined. The pump head H p

discussed in connection with Eq. 1 ( ) is usually referred to as the total dynamic head of the pump. It is the change in the energy grade line at the pump. An explanation of what it represents is obtained by considering how one would experimentally measure the total dynamic head for a pump installed in a pipeline. Assume that the pump is installed with a straight section of suction pipe and a straight section of discharge pipe both of sufficient length to develop uniform flow. Assume further that piezometer taps are installed in the suction pipe several diameters upstream from the pump inlet and in the discharge pipe at a sufficient distance that uniform flow exists at the piezometers. Rewriting Eq. 1 (neglecting H t , the turbine head) and solving for total dynamic head H p produces:

(3.2)This equation represents the total increase in energy created by the

pump, expressed in feet or meters of fluid between section 1 and 2. This

total dynamic head includes 1) any increase in dynamic head

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created by having the discharge pipe smaller than the suction pipe, 2) increase in the pressure head , 3) any elevation change between the suction and discharge piping (z2 – z 1 , and 4) the pipe friction losses which occur between the piezometers in the suction and discharge pipes (H f ) . If H p is calculated using Р 1 and P 2 from the hydraulic grade line projected back to the suction and discharge sides of the pump, then H f

in Eq. 3.2 would be zero. If the pump is supplying water between two reservoirs and points 1 and 2 are selected at the surface of the reservoirs, the equation reduces to Hp = z 2 - z 1 + H f . In this case, H f includes all friction and minor losses for the entire system.

Equation 3.1 shows that the pump head is related to the square of the velocity or discharge. If there is a valve in the discharge piping, it is possible to vary the flow through the pump, which results in a corresponding change of pump head. By measuring the total dynamic head at different discharges, one generates what is referred to as a pump rating curve. Typical rating curves (or characteristic curves) for constant speed centrifugal pumps are shown in Fig. 3.2. Different characteristic curves can be generated by changing the speed of the pump or impeller diameter. At zero flow, the total dynamic head is referred to as the shutoff head.

Curve A in Fig. 3.2 is called a stable or normal rising pump characteristic. As the flow is reduced, the head continually increases. Curve В is an example of an unstable or drooping characteristic because below some flow the head reduces as the flow decreases. Such a pump is unstable because at low discharges the flow can oscillate between two values. Points 1 and 2 on curve В of Fig. 3.2 represent two such flow rates. When the pump tries to operate at a flow below that corresponding to point 3 on curve B, the flow can be unstable. This results in fluctuations in the electrical load and creates pressure surges in the pipeline. Such situations should be avoided either by selecting pumps that have stable characteristics or by making provisions that prevent an unstable pump from operating near its unstable zone. The third type of characteristic shown in Fig. 3.2 is called a steeply rising characteristic (pump C). This type of characteristic is useful when the system pressure varies significantly.

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A sample set of pump characteristic curves for a centrifugal pump is shown in Fig. 3.3. Data are shown for three impeller diameters, labeled as curves A, B, and C. The figure includes information on head, flow, efficiency, net positive suction head, and brake horsepower. Each of these terms will be discussed in the next section. The best efficiency point (bep) or normal operating point would be near the middle of the area of 85% efficiency.

For computational purposes, especially for computer applications, it is convenient to express the head – flow part of the pump rating curve as an equation. For a centrifugal pump with a normal characteristic curve, operating near the design point, the head can be related to the discharge by

H p = H 0 -C lQ -C2Q 2 (3.3)

in which H 0 is the shutoff head (head at Q = 0), and C1 and C2 are constants evaluated for each pump curve. The constants are evaluated by substituting H 0 and two sets of Q and H values scaled from the curve near the design point into Eq. 3.3 and solving simultaneously for C1 and C2. The equation can also be solved simultaneously with the system equation to evaluate the pumps flow rate.

Mechanical and Electrical PowerThe horsepower delivered by a pump to the fluid, referred to as water

horsepower, is calculated by:

(3.4) 27

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0 100 200 300 400 500 600 700 800 900Capacity (gpm)

Fig. 3.3 Pump rating curve for low specific speed pump.

in which Q is the flow rate in cubic feet per second, γ is the specific weight of water, and H p is the total dynamic head in feet, evaluated from Eq. 3.2.

When using Q in gpm and specific gravity (Sg), water horsepower is calculated by

(3.5)The horsepower required to drive the pump is referred to as brake

horsepower and is denned as

(3.6)

in which e p is the efficiency of the pump ( e p = whp / bhp). The total input horsepower to the motor is

(3.7)in which e m is the efficiency of the motor.Electrical power consumption rate is expressed in kilowatts (kW) and

is related to horsepower by kW = hp • 0.746 kW/hp. Total power consumption, which is the method used to compute power charges, is expressed in kilowatt-hours (kW – h).

A graphical representation of the variation of brake horsepower with flow rate for a low specific speed centrifugal pump is shown in Fig. 3.3. For a low specific speed pump, the brake horsepower typically increases with discharge. For high specific speed pumps, the horsepower near the shutoff head increases rapidly.

To determine the water horsepower and select the best operating point for the pump, it is necessary to specify the efficiency of the pump (also shown in Fig. 3.3). From data such as shown in Fig. 3.3, it is possible to predict the input and output horsepower for any discharge, determine the bep, and decide if the pump is stable. The point of maximum efficiency is also called the design point and identifies Hdes and Qdes. When selecting a pump, it is desirable to have it operate near its design point or bep.

When selecting a pump for a particular application, it is usually possible to select from several impeller diameters, such as curves A, B, and

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С in Fig. 3. Curve A is for the largest impeller diameter. The option of different impeller diameters allows more flexibility in choosing a pump that meets the system requirements and operates near its design point. Each impeller has a separate bhp line, but usually a common NPSHr (net positive suction head required) line.

Net positive suction head (NPSH)Satisfactory pump performance requires that adequate attention be

given to cavitation. Pumps can be forced to cavitate by reducing the suction pressure. Cavitation has two general effects on pump performance. First, the cavitation can cause erosion damage, which wears away the impeller and other parts of the pump and eventually degrades the pump performance. Second, for advanced stages of cavitation, even before erosion has had time to occur, the pump performance can be degraded by large quantities of vapor.

The pressure necessary at the suction side of the pump to prevent cavitation from deteriorating the pump performance is referred to as the net positive suction head required (NPSHr). The NPSHr is determined from pump tests. It is essential that the net positive suction head available (NPSHa), which depends upon the system, exceeds the required NPSHr with a reasonable margin of safety to ensure satisfactory operation.

For a pump connected to a suction reservoir, the NPSHa is calculated from

NPSHa = H b – H v a + z s – H f (3.8)

in which H b is the minimum expected absolute barometric pressure head, z s the elevation from the centerline of the pump suction to the water surface elevation in the suction well (negative if the water surface is below the pump), H f the friction head loss and any local losses in the suction piping, and H v a the absolute vapor pressure of the liquid at the maximum expected water temperature. All units in Eq. 3.8 are expressed in feet (or m) of fluid. In the case of pumps where there is no suction well, and therefore no water surface elevation for reference, the quantity z s – H f i s replaced by the gauge pressure head P s / y at the centerline of the pump suction plus the suction velocity head, so

(3.9)

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The definition of NPSHa and the relationship between the two equations (3.8 and 3.9) is illustrated in Fig. 3.4. Note that the NPSHa is the vertical distance between the absolute vapor pressure line and the energy grade line (EGL).

Specific SpeedSpecific speed is a parameter which correlates pump capacity, head,

and speed as follows:

(3.11)in which N s is the specific speed, N the rotational speed of the pump

in revolutions per minute (rpm), Q the flow rate in U.S. gallons per minute (gpm), and H p is the total dynamic head in feet. Q and H p are for optimum efficiency (bep).

Pumps are divided into three general classes depending on the nature of the flow pattern inside the pump and the magnitude of the specific speed. Radial flow or turbine pumps produce large heads and small discharges and have small values of N s (500 – 2,000). Mixed-flow pumps produce modest head increases and reasonably large flows and have intermediate values of N s (2,000 – 7,000). Pumps that produce large amounts of discharge at relatively low head have large values of N s (7,000 – 15,000) and are referred to as axial flow or propeller pumps. For a given pump design, the specific speed can be altered by changing the pump speed. Typical values of pump speed are 450, 900, 1,800, and 3,600 rpm. The high speeds are associated with smaller pumps.

When selecting the speed, two opposing factors must be considered. A larger N produces a larger N s and for N s < 2,000 improves the efficiency. The higher speed also results in a smaller pump and less cost. The disadvantages to higher speed are faster wear (especially if there are suspended solids in the water) and increased problems with cavitation and transients. The wear is approximately proportional to the square of the shaft speed, so doubling N may increase wear by four times, which increases maintenance costs.

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Suction Specific SpeedThis is another parameter used to describe the cavitation

characteristics of an impeller. It is defined as

(3.12)

This parameter relates the cavitation potential of a pump to its speed and discharge. N is the motor speed in rpm, Q the flow in gpm a t the point of maximum efficiency, and NPSHr the required NPSH in feet at the point of maximum efficiency. Large values of S indicate more severe cavitation conditions. A typical upper limit of S for centrifugal pumps with good cavitation performance is 9,000 (30). Many commercial pumps have values of S between 5,000 and 7,000. Boiler-feed and condensate pumps have values between 12,000 and 18,000 (30). Upper limits of the suction specific speed are also given by the Hydraulic Institute standards (25). It is best to obtain values of S or NPSHr for each specific pump from the pump manufacturer if they are available.

Rotational InertiaThe quantity WR2 is a parameter describing the moment of inertia of

the rotating parts of the pump and motor about the axis of rotation. R is the radius of gyration in feet and W is the weight of the rotating parts (including the water inside the pump) in pounds. The WR2 of the pump is calculated or determined experimentally by the equipment manufacturers. The parameter is used to calculate the required starting torque of the motor and for determining its coast-down speed when the power is turned off to the motor. The latter is used for hydraulic transient analysis to determine the severity of waterhammer generated when the pump is shut off.

Similarity Laws

Scaling head and discharge data from one pump to a geometrically similar pump of a different size, or predicting performance at a different speed can be done with the following similarity equations:

(3.13)

(3.14)

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These equations are easily derived by dimensional analysis. They neglect viscosity but ensure similarity of the velocity vector diagrams at the impeller.

To demonstrate application of these equations, assume that it is desired to perform model tests of a large pump to evaluate its characteristics. The prototype design conditions are represented by D 1 , N 1 , Q 1 , and H p 1 . For the model, it is necessary to set two of the variables and determine the other two from Eqs. 3.13 and 3.14. The process may involve iteration if Q 2 and H p 2 are selected as the independent variables and D 2 and N 2 determined from the equations. The reason is that normally only synchronous motor speeds are used. If D 2 and N 2 are selected as the independent variables, then Q 2 and H p 2 are calculated directly by

(3.15)The equations are also useful for evaluating the influence of changing

speed or impeller diameter on Q, H p , and whp. For example, determine the effect of doubling pump speed on Q, H p and whp. With D 2 = D 1 , Eq. 3.15 gives Q2 = 2Q1, and H p 2 = 4H p 1 . This causes the horsepower (Eq. 3.4) to increase 8 times.

Next, consider the case of increasing the impeller diameter 25%. From Eq. 3.15 with N1 = N2, Q2 = 1.253 Q 1 , H p 2 = 1 .25 2 H p 1 and the water horsepower increases by (1.25)5.

Exercise 3 Give synonyms to the following words.

Liguid –Damage –Hydraulic system –Shape –Entire –Obtain –Occur –Rise –Fluctuation –Compute –Determine –Opt –Several –

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Approximately –

Exercise 4 Insert the necessary preposition from the list given below.

WITH BY IN IN IN FOR OF TO TO TO

1 With good circuit design, the power source, valves, and actuators will run with little maintenance _____________ extended times

2 _____________ most hydraulic systems, the fluid must have good lubrication qualities.

3 This fire hazard situation can be eliminated _____________ several different choices of fluids.

4 When huge oil deposits were discovered, mineral oil replaced water because _____________ its additional benefits.

5 This equation shows that the pump head is related _____________ the square of the velocity or discharge.

6 If there is a valve in the discharge piping, it is possible to vary the flow through the pump, which results _____________ a corresponding change of pump head.

7 By measuring the total dynamic head at different discharges, one generates what is referred _____________ as a pump rating curve.

8 Typical rating curves (or characteristic curves) for constant speed centrifugal pumps are shown _____________ Fig. 15.

9 Satisfactory pump performance requires that adequate attention should be given _____________ cavitation.

10 The high speeds are associated _____________ smaller pumps.

Exercise 5 Decide whether the following statements are true or false

1 With the system head and discharge characteristics and the approximate operating point determined, selection of a pump is impossible.

2 Pumps can be forced to cavitate by reducing the suction pressure.

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3 The pressure necessary at the suction side of the pump to prevent cavitation from deteriorating the pump performance is referred to as the net positive suction head available.

4 Pumps are divided into three general classes depending on the nature of the flow pattern inside the pump and the magnitude of the net positive suction head.

5 The quantity WR2 is a parameter describing the moment of inertia of the rotating parts of the pump and motor about the axis of rotation.

Exercise 6 Continue the sentences.

1 Selecting a pump for a particular service requires matching the system requirements and …

2 The horsepower delivered by a pump to the fluid, referred to as water horsepower, is calculated by:

in which Q is … , γ is … , and H p is the total dynamic head in feet.3 Pumps can be forced to cavitate by reducing ...4 Pumps are divided into three general classes depending on the

nature of the flow pattern inside … and the magnitude of the specific …5 The quantity WR2 is a parameter describing the moment of inertia

of the rotating parts of ...

Exercise 7 Make up questions to the text so that they could be used as a plan for retelling.

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