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Part 3 of 6:
SEALS, GASKETS AND VALVES
Table of Contents
1
2
Definition ........................................................................................................ 2
Functions of Seals ........................................................................................... 2
2.1 Examples of seals ..................................................................................... 4
2.1.1 Lip seals............................................................................................. 6
2.1.2 Piston rings ....................................................................................... 9
2.1.3 Metal-to-metal joints ....................................................................... 11
2.1.4 Gaskets ............................................................................................. 11
2.1.5 Sealing washers ................................................................................14
2.1.6 O-rings .............................................................................................14
2.1.7 Further examples ............................................................................ 18
3 Pipes and Pipe Fittings ................................................................................. 18
3.1 Plastic pipes ........................................................................................... 18
3.2 Copper pipes ...........................................................................................19
3.3 Steel pipes .............................................................................................. 24
4 Valves ............................................................................................................ 26
4.1 Types of valve......................................................................................... 27
4.1.1 Gate valves ...................................................................................... 28
4.1.2 Globe valves .................................................................................... 29
4.1.3 Poppet valves .................................................................................. 30
4.1.4 Butterfly valves ................................................................................31
4.1.5 Needle valves................................................................................... 32
4.1.6 Safety valves .................................................................................... 33
4.1.7 Pressure relief valves ...................................................................... 34
4.1.8 Pressure regulating valves .............................................................. 35
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Mechanical engineering very often involves the design, construction, use and
maintenance of MACHINES. A machine implies movement, if any useful task is to be
performed. There are very many different machines, performing very many
different tasks. One example is the petrol engine which drives a car or motor cycle.
The car itself is in fact a more complex machine which incorporates a number of
simpler machines or mechanical systems such as engine, gearbox, differential, etc.
Another example is a refrigerator in which refrigerant is pumped round and round a
series of pipes, radiators, valves etc. to perform its cooling function. The
refrigerator includes a number of sub-systems such as the pump for the refrigerant,
the motor to drive the pump, the control valves, etc.
In both examples given above, and in many other common examples, the
machines work (and continue to work satisfactorily) only because of the use of
effective SEALS serving many different functions at many different locations in these
machines.
1
Definition
SEALS are usually defined as components or assemblies which prevent the passage
of fluids between the moving parts of a machine. Note that "fluid" may refer to
liquid, vapour or gas.
Where the need arises to prevent the leakage of fluid between two stationary
parts, different sealing components, often called GASKETS, are used. These are
described later in this Project.
2
Functions of Seals
Seals have a number of quite distinct functions. Not all seals perform all the
functions listed below. The great diversity of seal types is in fact a result of the wide
variation of seal requirements.
Some of the functions seals may be required to perform are to:
1.
Seal working fluid into its desired location.
e.g. In a car engine the piston rings seal the compressed gas in the space above
the piston.
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2. Prevent escape of lubricant.
e.g. In a car engine seals prevent loss of lubricating oil where the ends of the
crankshaft protrude from the engine assembly to drive the flywheel at the rear
and the accessory drive belts at the front.
3. Prevent contamination.
e.g. Seals in a food processing machine prevent grease from the working parts
from contaminating the food product.
4. Prevent the ingress of dirt.
e.g. It is vitally important to "seal out" abrasive dust from the steering joints
and driveshafts on a tractor.
5. Prevent pollution and environmental damage.
e.g. Seals on a CFC-based automotive refrigeration system prevent the
refrigerant escaping to the atmosphere.
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2.1
Examples of seals
Figure 3-1 Figure 3-1 is a very simplified freehand sketch of a single-cylinder
petrol engine. The view shown is a section cut vertically through the centreline of
the cylinder and piston, with all seals and bearings omitted. Can you see any major
fault with the configuration as drawn? 1
1
If you look carefully, it is obvious that there is insufficient height to allow the piston to rise as
the crankshaft rotates, nor is there room for the crankshaft webs to clear the crankcase.
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Figure 3-2 The sketch in Fig 3-1 with bearings shown in green, piston rings in red,
gaskets in purple, lip type seals on the crankshaft in blue, metal-to-metal joint on
the poppet valve in pink and sealing washers in yellow.
Fig 3-2 shows the use of two different types of SEALS to prevent the passage of fluid
between moving parts. The ends of the rotating crankshaft must project from the
engine so that it can be used to do useful work. The crankshaft runs in BEARINGS
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which must be lubricated with a continuous flow of oil to reduce friction and
prevent the bearings from seizing. The flow of oil also helps to keep the bearings
cool. SEALS must be used to prevent the lubricating oil from escaping from the
outer ends of the crankshaft bearings, otherwise the oil supply soon runs out and
the engine is at risk of major damage.
It is clear that there must be very significant differences between the seals used
on the two ends of the crankshaft. With the crankshaft as sketched, a seal of annular
configuration (a ring) can be slipped over the left-hand end of the shaft. On the
right-hand end, the crankshaft drive flange clearly makes this impossible. For the
remainder of this section, the discussion will be on seals of annular configuration
suitable for use where the seal can be slipped axially over its shaft. The generic
name for seals of this type is LIP SEALS.
2.1.1 Lip seals
The need for this type of seal, between a rotating shaft and a stationary housing,
occurs on many machines. Such seals are often referred to as SHAFT SEALS or OIL
SEALS or LIP SEALS. Leather was used initially as the seal material, but this has
been superseded by synthetic rubber seals, using various polymers for different
applications. Seals of this type perform their sealing function by means of a lip
pressing against the rotating surface of the shaft with effectively line contact. In
some seals, the elasticity of the rubber of the seal provides adequate contact with
the shaft. In more difficult conditions (e.g. where the shaft bearing has some
clearance, allowing the shaft to move radially) the natural elasticity of the rubber
may be supplemented by a GARTER SPRING. The garter spring sits in a recess
within the body of the seal and outside the lip of the seal, as seen in Fig 3-3.
Figure 3-3 An example of a LIP TYPE SEAL which uses a GARTER SPRING to
increase contact pressure between the stationary seal and the rotating shaft.
http://1-800-seal-911.com/pages/TFE/FWS/dk351.html
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In many applications, a lip type seal of the general configuration shown in Fig 3-3
provides an adequate sealing solution. In some applications, the garter spring is not
needed, with the lip of the polymer seal providing sufficient elasticity to hold the
sealing lip in contact with the shaft.
As may be seen from Fig 3-4a below, there is a very wide range of “general
purpose” lip seals. Much of the variety of configuration lies in the design of the
outer part of the seal, the part which fits into and is located by some sort of recess or
housing.
Figure 3-4a Examples of cross-sections of what are described as general purpose
OIL SEALS. The top row shows SINGLE-LIP SEALS while the lower row shows
DOUBLE-LIP SEALS. http://www.fos-oilseal.com.tw/upload_files/general-purpose-type.pdf
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Figure 3-4b Examples of cross-sections of a more extensive range of DOUBLE-LIP
OIL SEALS. http://www.fos-oilseal.com.tw/upload_files/pdf/dual-lip-type.pdf
As shown in the text in Fig 3-4b, one of the main functions of double lip seals is
to ensure that fluids in adjacent parts of a machine do not mix, e.g. the automatic
transmission fluid in some machines is in a compartment adjacent to the
differential which uses different oil and a double lip seal is used to prevent leakage
and mixing. Note that most of these seals are fitted with two garter springs
If the engine in Fig 3-2 operates in dusty conditions, the crankshaft seals may
be required to perform two distinct functions: seal the oil in the engine; and seal
dust and dirt out of the bearings. This sort of condition occurs frequently on
machines such as tractors and earth-moving machinery. Seals of this type are the
DOUBLE-LIP SEALS or DUPLEX SEALS such as those seen in Fig 3-4b. The outer
sealing lip prevents dust from becoming wedged in the tapering gap outside the
inner sealing lip. This dust can rapidly wear the shaft so severely that both the seal
and the shaft need to be replaced.
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Figure 3-5 An example of how a seal might be mounted to seal one end of a shaft
assembly supported on rolling contact bearings. In this example, the shaft bearing
is a deep-groove ball bearing rather than the sliding contact bearings shown in Fig
3-2, and the seal is mounted in an END PLATE or COVER PLATE rather than directly
into the engine block, but the sealing principle is the same. In the particular seal
shown in the centre assembly of Fig 3-5 ((b) commercial seal), the rubber seal is
encased within a steel shell and is generally similar to those in Fig 3-4a. The steel
shell is pressed into the end plate with sufficient interference fit to prevent lubricant
leaking between the outside of the seal and the end plate. However, many modern
seals, including some in Fig 3-4a, encase the steel shell in a polymer outer layer
which makes the diameter tolerances less critical. In Fig 3-5, diagram (a) Felt seal
shows the cross section of a typical felt seal while diagram (c) Labyrinth seal
attempts to seal by using a series of steps as a barrier to leakage. Both felt seals and
labyrinth seals were less effective than modern lip seals and are now rarely used.
Shigley JE and Mischke CR, Mechanical Engineering Design, McGraw Hill,6E p 728. Diagrams
courtesy of New Departure-Hyatt Division, General Motors Corporation.
2.1.2 Piston rings
The second type of seal between moving parts which has been used in the engine
shown in Fig. 3-2 is between the moving piston and its cylinder. The engine
produces useful power by burning the mixture of petrol and air in the space above
the piston, using the increased pressure resulting from combustion to force the
piston down, thereby turning the crankshaft. In the case of the piston, there is no
rotation about its own axis and the seal is required to slide up and down the
cylinder.
The seals in this particular case are known as PISTON RINGS. They must
operate under very severe conditions of temperature and pressure, with very poor
lubrication. They are very often made of good quality CAST IRON which, due to its
high carbon content, possesses good self-lubricating properties.
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Figure 3-6 Left: A real-life piston about to be inserted into its cylinder. Three
piston rings are visible, the upper ring being a COMPRESSION RING, the middle ring
a second COMPRESSION RING and the lower slotted ring an OIL CONTROL RING.
Right: Illustrations of two ways in which the oil control ring returns excess oil to
the engine sump.
http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Graphics/Piston%20Rings.jpg
Figure 3-7 A piston with three ring grooves and the three rings which fit into
those grooves. The lower oil-control ring appears to be of the composite type,
comprising two thin steel rings with a wavy steel segment between the two. The
wavy segment is a commonly used method to ensure the oil-control ring fills the full
width of the ring groove, since sideways clearance is known to cause an engine to
burn oil. http://www.cbperformance.com/catalogimages/1060.jpg
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The piston rings actually serve two purposes: they seal the compressed gas
mixture above the piston; and they scrape the oil present in the lower part of the
engine from the cylinder wall. Without the second function, the engine would
rapidly burn its lubricating oil. The lower piston rings are sometimes referred to as
OIL CONTROL rings or SCRAPER rings, whilst the top ones are called COMPRESSION
rings. Oil rings are designed with wide slots so that oil scraped from the cylinder
wall has a clear return path to the lower part of the engine (refer to Fig 3-6 right).
As previously mentioned, compression rings are usually made from cast iron. Oilcontrol rings also can be made of cast iron, but are sometimes made of heat-treated
and tempered steel segments, several of which are put together to form one oil
control ring, as seen in Fig 3-7.
The design of compression rings is such that gas pressure is admitted to the
space behind the ring. This gas pressure significantly increases the pressure of the
piston ring against the cylinder wall and greatly assists the gas-sealing process. A
similar feature will be noted when discussing O-rings later in this document.
2.1.3 Metal-to-metal joints
Figure 3-2 also gives an example of another type of sealing function. The inlet
VALVE shown at the top of the engine, usually described as a POPPET VALVE, is
required to move down at predetermined times in the engine cycle to admit the
petrol-air mixture to the cylinder. For the remainder of the engine cycle, the valve
must prevent the same gas mixture from escaping. To do this, the VALVE FACE is
held in contact with its VALVE SEAT by a spring. The valve face and the seat are
both machined to the same angle of taper, often 45°. The resulting METAL-TOMETAL JOINT is adequately gas-tight.
The valve is caused to open intermittently to admit a fresh petrol-air charge,
using a CAM on a rotating shaft called a CAMSHAFT. Details of the camshaft are not
shown in Fig 3-2.
If carefully and accurately machined, metal-to-metal joints perform quite
satisfactorily in many engineering applications, e.g. the top and bottom sections of
some water and air valve bodies (see gate and globe valves later in this document)
are screwed together and seal with a metal-to-metal joint. Many pipe fittings, as
discussed later in this document, also use this principle.
2.1.4 Gaskets
Figure 3-2 shows two examples of joints where there is no movement between the
two parts. At the top left of the figure, the fuel/air inlet pipe is joined to the top
section of the engine which is called the CYLINDER HEAD, with the joint sealed by a
GASKET. The second example in Fig. 3-2 is the larger and more critical joint
between the CYLINDER and the CYLINDER HEAD. In these joints, a reasonably
compliant piece of flat material, called a GASKET, is placed between the two
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components before they are bolted together. The function of the gasket is to
conform to any surface irregularities or slight misalignment between the two
surfaces and thereby to provide a complete seal.
Gaskets are generally of reasonably complex shape, although they are usually
made from sheet material of uniform thickness. Often the bolts which secure the
joint members pass through holes in the gasket, and the gasket usually covers the
entire surface to be joined.
Figure 3-8 Examples of gaskets, cut from flat sheet material.
Those illustrated on the left are probably for applications like joining two lengths of
pipe together by flanges while those on the right are from a number of different
applications, including three rubber seals at lower right. The type of material varies
according to the gasket application. In former days, various forms of asbestos was
frequently used but has been replaced by a number of proprietary materials.
http://www.americansealandpacking.com/sheetgasketing.htm
http://www.americansealandpacking.com/cutgaskets&sheet.htm
Gaskets are often made of special paper-like or cardboard-like material of the
general type seen in Fig 3-8, but are sometimes made of soft ductile metals like
copper or aluminium. Cork, used in earlier years, is seldom used now.
COMPOSITE GASKETS were popular for many years for difficult sealing tasks,
but are now often superseded by less expensive construction combined with sealing
compounds. One popular composite gasket used two thin sheets of copper with an
asbestos interlayer to seal the cylinder heads of automotive and other engines. The
use of asbestos is, of course, no longer permitted.
In Fig. 3-2, the inlet pipe gasket would probably be made from a thick paperlike or thin cardboard-like material. In former times, the cylinder head gasket
might have been a copper-asbestos composite gasket. In modern production
engines, it will probably still be of composite construction, such as the gaskets seen
in Fig 3-9, the greater part of which will be made of a special heat resisting
proprietary compound, the composition of which is not made public, reinforced
with metal rings in critical locations.
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Figure 3-9 Examples of modern composite cylinder-head gaskets using fibrous
material for the bulk of the gasket with metal or other reinforcing rings at critical
locations. http://www.automotive-technology.com/contractors/accessories/rajdhani/
Figure 3-10 Examples of gaskets for such applications as oil pans (sumps) and
valve covers of motor vehicle engines. Some such gaskets are made from carefully
chosen polymer. http://www.automotive-technology.com/contractors/accessories/rajdhani/
Figure 3-11 Examples of modern gaskets for inlet and exhaust manifolds of motor
vehicle engines. Steel reinforcing rings are commonly used in critical locations.
http://www.automotive-technology.com/contractors/accessories/rajdhani/
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2.1.5 Sealing washers
The spark plug at the top of the engine in Fig. 3-2 is screwed into the cylinder head
using a standard V thread. It is possible to seal the spark plug by a metal-to-metal
joint (generally using a conical seat), and this is done on many late-model engines,
but the seal has traditionally been made by a gasket of simple annular shape, such
as that seen in Fig 3-12 left, which is then called a SEALING WASHER. A sealing
washer will also be used on the sump-drain plug in Fig. 3-2, which allows the oil to
be drained when necessary.
Figure 3-12 Left: A sealing washer made from sheet copper filled with some sort
of fibrous material which compresses when the washer is subjected to loading.
Right: Specialised composite sealing washers made from the various materials
indicated.
http://www.google.com.au/search?client=safari&rls=en&q=copper+asbestos+gasket+pictures&ie=
UTF-8&oe=UTF-8&redir_esc=&ei=-v6eS-3rPM6IkAXNmoG5DA
http://www.calchiefs.org/%5Citems%5CEMS_Sealing_Washer.jpg
2.1.6 O-rings
An O-RING is a special sealing component which provides a very effective seal for a
wide range of operating conditions. Its use in industry is now so widespread that it
deserves special mention.
O-rings consist of an annulus of solid circular cross-section. They are usually
made from synthetic rubbers of various types, to resist oils, acids, bases, solvents,
etc, and the effects of temperature. Square-section O-rings are made for special
applications, e.g. piston seals in disc-brake calipers on motor cars.
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Figure 3-13 Top: Examples of some O-rings which are commercially available.
The colour of the material is often used to indicate its composition and therefore its
properties. Lower: The correct use of an O-ring in its machined groove in a shaft or
piston. The O-ring must be deformed from its circular cross-section if it is to be an
effective seal.
http://www.americansealandpacking.com/orings.htm
http://www.americansealandpacking.com/O-Rings/Design/2.html
http://commons.wikimedia.org/wiki/File:FloDynamix_O-Rings.jpg
O-rings are often used in static sealing applications (i.e. as gaskets) as well as
the sealing arrangement of Fig 3-14 below. They are often used to seal a
reciprocating piston in a hydraulic machine. O-rings are not usually used where
one of the components rotates.
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Figure 3-14 An illustration of an O-ring groove which has been machined too
large for the chosen size O-ring. Pressure can thereby by-pass the O-ring.
http://www.americansealandpacking.com/O-Rings/1.html
Figure 3-15 Illustration of the behaviour of an O-ring under excessive pressure.
Note how the applied pressure, within the O-ring’s rated pressure range, assists in
sealing process.
http://commons.wikimedia.org/wiki/File:FloDynamix_O-Rings.jpg
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If an O-ring is to be successfully used as a seal, it must be located in a specially
machined recess or groove, which must be machined to quite small tolerances. This
may add to the cost of the component. With O-ring seals, the pressure of the fluid
being sealed helps to press the O-ring against its groove (Fig 3-15), thereby
contributing to its sealing effect. The use of fluid pressure in this way is similar to
that described for piston compression rings. Fig 3-14 illustrates the lack of sealing
which may occur when the groove is not of the correct size.
Figure 3-16 As seen by referring to Item 5 of the exploded view of a vacuum pump
from a diesel truck, O-rings can be used as gaskets. Note that the two Items 10 of
sealing-washer like construction are also referred to as O-rings, so terminology is
not always precise.
Mazda T2600 Workshop Manual, 6/89, page P24.
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2.1.7 Further examples
For more information on gaskets, sealing rings and O-rings, go to
http://www.iqsdirectory.com/gaskets/ or other websites.
3
Pipes and Pipe Fittings
Whilst dealing with seals in general, it is appropriate to make at least some mention
of a different area of sealing – joining pipes in a leakproof fashion.
One point to ponder is the multiplicity of pipe jointing methods which have
been used at various times and for various purposes. Why are there so many
different systems in use? The answer probably lies in the wide range of pipe
applications, from small bore, light duty plastic, through copper and steel pipe in
various sizes up to say 25 mm, on to large, high-pressure steel pipes.
3.1
Plastic pipes
Figure 3-17 Plastic pipes are now being joined successfully using material-specific
adhesives. PVC rainwater and wastewater pipes are a good example of this type.
The pipes to be joined are simply cut square and to length, treated with primer, then
smeared with PVC adhesive and pushed together. The adhesive grips after just a
few seconds and forms a strong permanent joint.
http://www.rd.com/advice-and-know-how/stepbystep-pictures-and-instructions-for-gluing-andjoining-plastic-pvc-pipe/article118256-3.html#slide
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3.2
Copper pipes
Joining of many copper and steel pipes is achieved by variations of metal to metal
joints, in which the surfaces are clamped tightly together (compressed), often taking
advantage of the self-centring and wedging action of opposing conical surfaces.
Figure 3-18 Left: A joint between a brass T-piece and three copper pipes. This
type of joint is sometimes referred to as a DOUBLE CONE COMPRESSION fitting. In
the left-hand photograph, sealing of each pipe is achieved by an OLIVE, a ring of
brass which looks somewhat like a wedding ring, which slips over the pipe and is
compressed when the nut is tightened to seal the joint. Right: The right-hand
diagram indicates the mode of compression between the union and the nut. That
diagram is incorrectly drawn - the “bulge” in the pipe is actually the separate OLIVE,
as seen in the left-hand photograph, but the principle of compression between the
two conical surfaces is correctly drawn.
http://www.diydoctor.org.uk/project_images/copper_compression_joints/Compression%20T%20j
unction.jpg
Nylon tubes are often joined by a fitting similar to that described in Fig 3-18.
The loose piece or OLIVE which is slipped onto the pipe may be made of plastic or
metal, depending on the application. Its function is to clamp tightly onto the
outside diameter of the pipe as it is squeezed between the two screwed fittings.
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Figure 3-19 A further example of a compression fitting using an OLIVE, here
referred to as a FERRULE.
http://www.managemylife.com/mmh/articles/authored/using-compression-fittings
Figure 3-20 In this fitting, the end of the pipe, which may be either copper or
steel, is FLARED to a conical shape. The conical section is then clamped between
conical sections of the body and nut.
http://machinedesign.com/content/hydraulic-tube-fittings2-0602
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Figure 3-21 This connection is a variation on the olive type, using a shaped
FERRULE with a sharp conical section to wedge onto the pipe. Note the black O-ring
on the left-hand end of the connection to seal the thread.
http://machinedesign.com/content/hydraulic-tube-fittings3-0602
Figure 3-22 An O-ring in a machined recess in the body of the joint is compressed
to form a FACE SEAL. Note the O-ring on the thread on the right-hand end to
prevent leakage from the thread.
http://machinedesign.com/content/hydraulic-tube-fittings4-0602
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Figure 3-23 The 60° cone fitting again uses a metal to metal seal with the conical
section to centre the components and increase the clamping force.
http://machinedesign.com/content/hydraulic-tube-fittings5-0602
Figure 3-24 Using a straight thread, there is a leak path along the helix of the
thread. Sealing is achieved by an O-ring in a machined recess. The recess prevents
the O-ring from being forced out under pressure.
http://machinedesign.com/content/hydraulic-tube-fittings6-0602
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Figure 3-25 In this design, sealing is achieved by the tapered thread on the fitting
wedging into the straight thread in the housing. It is common practice to apply a
small quantity of sealant such as Loctite to ensure complete sealing.
http://machinedesign.com/content/hydraulic-tube-fittings7-0602
Figure 3-26 Some of the range of brass fittings available for different applications.
http://machinedesign.com/content/hydraulic-tube-fittings-0602
Students interested in learning more about pipe fittings and joints will find
useful information at http://machinedesign.com/article/hydraulic-tube-fittingsdeliver-leak-free-performance-and-long-life-0602.
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Figure 3-27 Relatively new fittings known as Sharks teeth, in which the pipe to be
joined and sealed is simply pushed into the fitting. The teeth visible inside the
fittings grip the pipe to prevent it slipping out under pressure and the rubber
provides a seal. http://www.copper.com.au/cdc/article.asp?CID=49&AID=288
3.3
Steel pipes
Steel pipes bring jointing problems which are different from copper pipes. Sizes
may be much larger, the material is harder and stronger, hence the pressures to be
sealed may be much greater. Several different jointing methods are available,
including threaded pipe ends screwed into threaded sleeves, sealed by sealant such
as Loctite or, in past years, “red lead”. Flanges may be either screwed or welded to
the pipe, and may be sealed by either gaskets or O rings.
Figure 3-28 Some examples of methods of attaching flanges to steel pipes by
welding or threading.
http://www.google.com.au/imgres?imgurl
http://www.wermac.org/flanges/flanges_general_part3.html
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Steel pipes frequently have long "runs", i.e. long overall lengths formed by
joining a number of standard lengths of pipe together. This may cause two
problems. First, if the temperature of the pipe changes (due to the fluid flowing
through it, e.g. steam) the pipe length may change significantly and provision must
be made for expansion and contraction. Secondly, it is prudent to design the
pipeline and its fittings so that maintenance can be carried out easily, e.g. can one
leaking joint be repaired without dismantling the whole pipeline?
Figure 3-29 Some years ago, the School of Mechanical and Manufacturing
Engineering updated its fire hose equipment. The new system uses 100 mm
galvanised steel mains. Joints are formed by split clamps which locate into
machined recesses close to the end of each pipe. Standard pipe lengths come onsite with these recesses already machined. However, where a pipe needs to be cut to
length, a new recess can be machined on-site. Sealing of the joint is achieved by a
special rubber seal, held in place by the split clamps. An example of this type of
fitting, trade marked VICTAULIC, is on view in the Laboratory.
http://www.power-technology.com/contractors/thermal_insulation/victaulic/victaulic1.html
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Note some of the advantages of the Victaulic system for fire hose installation.
1.
Pipes have no protrusions and pass readily through 100 mm clearance holes
drilled in concrete floors, etc. (cf. pipes with welded-on flanges).
2. Any individual section of pipe can be removed without disturbing the rest of the
line.
3. Pipe lengths can be cut on-site to suit the job in hand.
4. Sealing is simply and effectively achieved by the rubber seals.
5. The pipe lengths do not abut and there is sufficient axial space to cater for
longitudinal expansion and contraction.
Figure 3-30 Fittings of a type generally used on steel pipes for very high pressure
applications, e.g. supplying fuel to engine injectors in diesel engines. The ends of
the pipes are SWAGED to form external cones, which make metal-to-metal joints in
internal cones. http://news.thomasnet.com/images/large/453/453814.jpg
4
Valves
VALVES are used to control the flow of fluids in hydraulic and pneumatic systems.
Many systems which might appear to be purely mechanical actually have hydraulic
or mechanical sub-systems using valves, so that valves are an important group of
mechanical engineering components. For example, the engine sketched in Fig. 3-1
3-2 has a valve to admit the petrol/air mixture to the cylinder at the proper time.
This particular valve is called a POPPET VALVE. The engine will also have a
carburettor which incorporates a THROTTLE VALVE to control engine speed. The
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lubrication system on the same engine (not shown in Fig. 3-1) will have an oil pump
and a PRESSURE RELIEF VALVE to ensure that the oil pressure cannot exceed a safe
level. As a further example, the automatic transmissions fitted to many cars are
hydraulic devices which use a number of carefully designed valves to change the
drive ratios to suit driving conditions.
4.1
Types of valve
It is perhaps worth summarising the different functions performed by the main
classes of valves which are generally available.
Function
Examples
1.
Off-on
Gate valve
Globe valve
Poppet valve
2.
Regulating
Globe valve
Butterfly valve
Needle valve
3.
Non-return
Flap valve
Ball valve
4.
Safety
Spring valve
Deadweight valve
5.
Pressure stabilising
Pressure relief valve
Pressure regulating valve
P3-28
4.1.1 Gate valves
Figure 3-31 Gate valves are made in both small and large sizes, up to the size used
on large water mains. A characteristic is that, when the valve is open, the GATE
which is the green component in the right-hand diagram is withdrawn completely
from the flow path, so unobstructed flow is obtained and there is little pressure loss
through the valve. Gate valves are generally intended to be either fully closed or
fully open. They are generally not used for flow regulation. Note also the use of
another type of seal, the PACKING on the valve spindle to prevent leakage from the
top of the bonnet. If the packing wears and begins to leak, there is provision to
“squeeze” it down to renew its sealing effect. Note also that the packing is not
subjected to fluid pressure when the valve is closed.
http://www.brassvalve-manufacturers.com/photo/828055e2f390bad6c890604ffe04fb68/GateValves.jpg
P3-29
4.1.2 Globe valves
Figure 3-32 Globe valves take their name from their shape, which is required to
allow the valve disc to be moved into contact with its seat to shut off the flow.
The construction results in some impediment to flow and some pressure loss
through the valve. These valves are often used for flow regulation. A common
example is the garden tap. Note the need for packing on the valve spindle to
prevent leakage from the top of the valve bonnet, and the provision of a means of
squeezing the worn packing to renew the seal.
http://www.energy.gov.kw/data/site1/images/doha/Illustrations/globe%20Valve.gif
Figure 3-33 Further examples of globe valves. They are designed for flow only in
the direction shown. Since the disc or plug may be mounted on a ball or spherical
surface so that it can conform to its seat, the flow may become unstable if it is
reversed.
http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/6/1/Fig_6_1_2.gif
P3-30
4.1.3 Poppet valves
Figure 3-34 Left: An example of a valve assembly from an automotive engine.
This is similar to the POPPET VALVE shown in Fig 3-2. For most of the engine cycle,
the spring holds the valve tightly in metal-to-metal contact with its seat. When the
valve is required to open to admit air and fuel or to discharge exhaust gases, the end
of the valve is pushed downwards by a CAM.
FOR KEEN STUDENTS
Fig 3-34 Right: Two poppet valves showing the cam mechanism used to open the
valves by means of a rotating CAMSHAFT. This illustration is a desmodromic
mechanism used in some high speed engines to positively control both the opening
and closing of the valve. If normal engines are driven at too high a speed, valve
springs cannot control the inertia forces quickly enough and the valve does not close
quickly enough. This is called VALVE FLOAT or VALVE BOUNCE.
http://www.google.com.au/search?client=safari&rls=en&q=poppet+valve+pictures
P3-31
4.1.4 Butterfly valves
Figure 3-35 Examples of small butterfly valves used in automotive carburettors.
Depressing the accelerator opens the valve and a spring automatically closes the
valve when the accelerator is released. Butterfly valves are usually circular discs,
although they can be square with rounded corners.
http://www.mustangandfords.com/techarticles/mufp_0603_ford_carburetors/photo_16.html
http://www.mustangandfords.com/techarticles/mufp_0603_ford_carburetors/photo_36.htm
http://en.wikipedia.org/wiki/Valve
Figure 3-36 Large butterfly valves used on water mains and similar applications.
Used for flow regulation, they cause little pressure drop when fully open.
P3-32
4.1.5 Needle valves
http://www.sealexcel.com/gifs/needle-v.gif
http://www.tubefittings.in/valve-fittings.html
http://www.google.com.au/imgres?
Figure 3-37 Examples of NEEDLE VALVES which are usually used for precise
regulation of small flows, usually by screw adjustment of a tapered needle.
P3-33
4.1.6 Safety valves
http://en.wikipedia.org/wiki/File:Deadweight_safety_valve_section_(Heat_Engines,_1913).jpg
http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/9/1/fig9_1_2.gif
Figure 3-38 Top: A DEAD-WEIGHT SAFETY VALVE in which the force from the
pressure acting over the small inner pipe area must exceed the dead weight for the
valve to open and release pressure. Bottom: SPRING SAFETY VALVES in which the
dead weights have been replaced by a metal spring.
P3-34
Students are sometimes confused over the two quite distinct functions of SAFETY
VALVES and PRESSURE RELIEF VALVES. A safety valve is a precaution against an
unforeseen or unlikely failure. For example, if the pressure in a boiler should ever
rise to a dangerous level, the SAFETY VALVE will operate automatically and allow the
pressure to drop to a safe level. The safety valve operates only in an emergency and
may never operate within the life of the boiler.
4.1.7 Pressure relief valves
Pressure relief valves are designed to open automatically at a pre-determined level
of system pressure and to regulate system pressure at that level despite changes the
system operating conditions.
Figure 3-39 An application of a pressure relief valve used in an automotive
engine. The diagram shows an OIL PUMP, used to provide oil under pressure to
engine bearings of a light truck engine. Oil is drawn into the pump through the
strainer, with the strainer located at the bottom on the ENGINE SUMP, also called
the OIL PAN. If the pressure from the pump exceeds set limits, the SPRING RELIEF
VALVE opens and drops the pressure.
Toyota 4YE Engine Repair Manual, August 1985, page LU8
Consider the oil pump shown in Fig 3-39. This pump must provide a flow of oil
to lubricate the engine bearings under all operating conditions. When the oil is very
hot, its viscosity drops, so that it flows very easily through the bearings. The pump
must therefore be designed to have a large flow rate. When the engine is first
started, the oil is cold, its viscosity is relatively high and it does not flow easily
through the bearings. Since the pump still has a large flow rate, the pressure under
these conditions may rise high enough to damage the pump. This risk is overcome
by providing a PRESSURE RELIEF VALVE to open at a pre-set pressure. This is not
an emergency procedure; it is part of the valve's normal function.
P3-35
Figure 3-40 Spring loaded pressure relief valves.
http://1.bp.blogspot.com/_YOjepBEvjc8/RtFJB1RGcKI/AAAAAAAAAfk/KxnSkQyz5ww/s320/valve101.gif
http://www.google.com.au/imgres?imgurl=http://www.valve-world.net/images/specials/srv/2_fig2.gif
The simplest and most reliable type of pressure relief valve is the spring-loaded
design (Fig 3-40) where a spring force opposes the system pressure acting on the
area of the valve disc. Valves of this type may be used to control the pressure of
systems using either liquids or gases.
When the system pressure rises sufficiently, it overcomes the spring force and
the valve opens to reduce the pressure by allowing some of the flow to be ‘dumped’
through the outlet pipe to a storage tank or, in the case of a compressed air system,
vented to atmosphere.
It is usual to provide a means of SETTING or adjusting the spring pressure so
that the same valve can be used for a variety of applications.
P3-36
FOR KEEN STUDENTS
4.1.8 Pressure regulating valves
Figure 3-41: A schematic diagram 2 of the layout of a pressure
regulating valve used on hydraulic systems.
J. Carvill, The Student Engineer's Companion, Butterworths, 1980. page
35.
This type of valve does much more than the simple pressure relief valves of
Figs 3-39 and 3-40. Oil at unregulated pressure enters the inlet port of the
valve. If the pressure is higher than the ‘set’ pressure, some of the oil can be
discharged back to the storage tank, dropping the outlet pressure.
Dumping oil is achieved by forcing the main piston in the centre of the valve
body to move to the left, off its conical metal-to-metal seat. The desired
outlet pressure is set by adjusting the length of the pilot-valve spring and
hence the force holding the pilot valve onto its seat.
When the valve is working, pressure at the inlet tends to push the main
piston to the left and off its conical seat, but movement is resisted by the
main spring tending to push the piston to the right. Inlet pressure is ‘bled’
through a small diameter hole in the piston and, if the pressure to the left of
the piston becomes too high, the small pilot valve opens slightly and drops
the pressure in the cavity to the left of the piston. The drop in pressure
causes the main piston to move to the left, causing oil to be discharged to
the tank until the pressure drops to the set pressure. In practice, the system
usually stabilises with a small, slightly varying flow through the pilot valve.
_________________
2
This is intended to be a simple diagrammatic illustration of the function of the valve.
You may notice that the valve, as drawn, cannot be assembled because the main piston
cannot be inserted into its bore.
P3-37
Figure 3-42 A sectioned view of a commercial pressure regulating valve.
The mode of operation of this valve is less clear than the example in Fig 341. It is based on a diaphragm rather than a pilot valve. The large area of
the diaphragm means that small pressure variations can easily be detected
and controlled. The ‘set’ pressure is adjusted by means of the threaded
stem and nut under the top cover.
http://www.swagelok.com/regulators/pressure_reducing_regulators.htm