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
hazardous
fluid
work
power
liquid
operate
force
dangerous
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.
3
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
4
precautions, cylinders and motors of both types can operate in highhumidity 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 competitive
6 Most pneumatic circuits are below 3 hp
7 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?
5
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 liquid
4)
the measure of a force traversing through a distance
5)
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
6
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
7
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
8
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 closetolerance 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.
9
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 out
3 When pressure against the piston area becomes high enough to
overcome the load resistance, the load forces the oil to move upward
4 Pumps give pressure
5 A common measure of power is horsepower
6 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
10
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
11
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.
12
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
13
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.
14
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
15
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 watercontent fluid (or HWCF). The common mixture is 95% water and 5%
16
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 antifreeze) 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 fireresistant 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
17
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,
18
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 source
2 Fire hazard situation can be eliminated by fireproof fluids
3 High water-content fluid contains no oil
4 Water glycol fluids have better lubricating qualities than mineral oil
does
5 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
19
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
The filter cart shown in
20
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.
21
The most common type of dynamic pump is the c e n t r i f u g a l pump.
This classification is subdivided according to head and discharge
characteristics into: a x i a l -flow (low head and high discharge), m i x e d f l o w (moderate head and moderate discharge), and r a d i a l - f l o w (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 multiplestage; single- or double-suction (mixed flow only); open or closedimpeller; self-priming or nonpriming; fixed- or variable-pitch; and fixedor 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
BETWEEN BEFORE
WITH WITH TO TO TO TO TO AS
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.
23
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 headdischarge 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
Flow
curve can be generated. If the elevation of
24
Fig. 3.1 Typical system head curve.
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 Head
Before 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
25
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.
26
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
14
0
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
0
100 200 300 400 500 600
700 800 900
a normal characteristic
Capacity (gpm)
curve, operating near
Fig. 3.3 Pump rating curve for low specific speed pump.
the design point, the
head can be related to
the discharge by
Hp = H0-ClQ-C2Q2
(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 Power
The horsepower delivered by a pump to the fluid, referred to as water
horsepower, is calculated by:
(3.4)
27
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
total input horsepower to the motor is
=
whp / bhp). The
(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
28
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)
29
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 Speed
Specific 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.
30
Suction Specific Speed
This 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 Inertia
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. 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)
31
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 = 4 H 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 . 2 5 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 –
32
Several –
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.
33
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.
34