1. Objective
At the end of this session you will able to describe
What is pump
Application of pump
Classification of pump
Centrifugal pump
Reciprocating pump
Rotary pump
Pump cavitation
2. What is Pump?
A pump is a mechanism that is used to
transfer a liquid from one place to another by
imparting energy to the liquid being
transferred.
The Hydraulic machines which convert the
mechanical energy into hydraulic energy are
called pumps. The hydraulic energy is in the
form of pressure energy.
3. Application of Pump
Pumps are employed to move materials ranging
from molten metals at very high temperatures, to
cryogenic materials at extremely low
temperatures. They are used to generate
pressures so small as to be barely perceptible, to
pressures so high that the liquid being pumped is
capable of cutting through material as though it
were a saw. Also. they are designed to supply
quantities from as small as one drop per day, to
four billion liters per day. They have power
requirements from a few watts to nearly 75
megawatts.
4. Application of Pump
Some of the more common types of pumps required in industrial plants
are:
Boiler feedwater pump - supplies the boiler with feedwater as
required. It must be capable of forcing this water into the boiler against
the pressure existing in the boiler.
Fuel oil pump - used in oil-fired boilers to pump fuel oil to the burners.
Lubricating oil pump - used to circulate oil to the bearings of a
machine such as a turbine, engine, pump or compressor.
Circulating water pump - also called a cooling water pump. It is used
to pump water through a heat exchanger such as a condenser or an oil
cooler.
Chemical feed pump - small capacity units are used to pump
chemicals into boilers: larger units are used as process pumps.
Fire pump - used to supply water to plant fire lines.
Domestic water pump - used to supply water to plant washrooms, etc.
5. Classification of Pump
Pumps fall into two main categories:
1. Those that use liquid velocity to create pressure.
2. Those that use positive displacement to create
pressure.
Pumps are classified, according to their method of operation,
as:
Reciprocating Pump
Centrifugal Pump
Rotary Pump
7. Centrifugal Pump
A centrifugal pump may be defined as a pump that uses
centrifugal force to develop velocity in the liquid being handled.
The velocity is then converted to pressure when the liquid
velocity decreases. As kinetic energy is decreased, pressure is
increased
8. Centrifugal Pump
OPERATING PRINCIPLE
The working principle of a
centrifugal pump is shown
diagrammatically in Figure 1.
Rotation of the impeller causes
any liquid contained in it to flow
towards the periphery because of
the centrifugal force generated.
The center or eye of the impeller is
thus evacuated and liquid from the
suction line then flows in to fill the
void.
9. Centrifugal Pump
Centrifugal pumps can be subdivided into the following types:
volute,
diffuser,
axial flow,
mixed flow
regenerative.
10. Volute Centrifugal Pump
Basically, the volute centrifugal
pump consists of an impeller,
made up of a number of vanes,
which rotates in a volute
stationary casing. The term
"volute" refers to the gradually
increasing cross-sectional area
of the spiral casing.
11. Volute Centrifugal Pump
The liquid being pumped is
drawn into the center or eye
of the impeller. It is picked
up by the vanes, accelerated
to a high velocity and
discharged into the casing
by centrifugal force. As the
liquid travels through the
volute casing to the
discharge, its velocity energy
is converted into pressure
energy. Since the liquid
between the vanes is forced
outward, a low pressure area
is created in the eye and
more liquid is drawn in
through the suction inlet. As
a result, the flow of liquid
through the pump is constant
12. Diffuser Pump
In the diffuser centrifugal
pump, the high velocity
liquid leaving the impeller
passes between a number of
vanes in a stationary diffuser
ring. These vanes are
shaped in such a way that
the channels between them
gradually increase in area.
As the liquid passes through
these channels, its velocity
energy is converted into
pressure energy. The liquid
is then discharged either into
a volute casing or into a
concentric casing where
farther velocity to pressure
conversion takes place.
13. Diffuser Pump
As these diffuser vanes are
spaced uniformly around the
impeller circumference there
is no radial imbalance
developed. In addition, in the
diffuser pump the velocity
energy of the liquid is more
completely converted into
pressure energy than it is in
the volute pump. As a result,
the diffuser pump is
commonly used for high
capacity, high pressure
service.
14. Axial Flow Pump
Axial flow pumps, also referred
to as propeller pumps, use
impellers with blades similar to
those of an aircraft propeller.
The pump head is developed by
the propelling or lifting action of
the blades on the liquid.
The arrangement of the pump is
usually vertical as in Figure but
horizontal and inclined shaft
arrangements are also
available. For the smaller
pumps, fixed blade type
impellers are used. Larger
pumps may use impellers with
adjustable or variable-pitch
blades which can be used to
maintain efficiency at loads that
differ from the design load.
15. Impeller Types
Impellers vary considerably in design. They can be
classified according to specific speed, the way the
liquid is drawn into the eye, vane design and pump
application
16. Impeller Types
The open impeller, A, has vanes attached to a central hub with a
relatively small shroud on one side. It is of end suction or single-inlet
design, thus the water enters the eye from one side only. B shows a semi
closed single-inlet impeller. A full shroud closes off one side. An
enclosed, single-inlet impeller is shown in C. The liquid passages
between the vanes are closed off by the shrouds on both sides. Impeller
D is also enclosed but it has a double-inlet, thus water enters the eye
from both sides. Design E is used in paper-stock pumps handling liquids
containing solids. F is a propeller type impeller while impeller G is used in
mixed-flow pumps.
17. Multistaging
Pumps may be either single
or multistage design. In-
general, single stage pumps
are used for heads of 120 m
or less while the multistage
design is usually necessary
for heads above 120 m. To
obtain these higher heads,
centrifugal pumps are
equipped with two or more
impellers operating in series.
That is, the discharge of one
impeller is connected to the
suction of the next impeller.
These pumps are known as
multistage pumps.
18. Axial Flow Pump
The advantages of axial flow
pumps are their compact
size and the ability to
operate at high speeds,
while their disadvantages
include low suction lift
capacity and relatively low
discharge bead capability.
They are used mainly for low
head, high capacity
applications and are
available in the singlestage
design or the multistage
19. Mixed Flow Pumps
Mixed flow pumps combine
some of the characteristics
of the volute and diffuser
pumps together with some
axial flow pump features.
The head developed by this
pump is produced partly by
centrifugal force and partly
by the lift of the impeller
vanes on the liquid.
The mixed flow pump shown
in Figure has a single-inlet
impeller. The flow enters the
pump in an axial direction
and leaves the pump in a
direction somewhere
between axial and radial.
20. Mixed Flow Pumps
The mixed flow pump,
combines some of the
characteristics of the radial
flow and axial flow pumps. It
develops its discharge head
by using both centrifugal
force and lift of the vanes on
the liquid. The pump is built
for vertical and horizontal
applications and it is
commonly used for low
head, high capacity
operation.
21. Regenerative or Turbine Pump
The regenerative
pump or turbine
regenerative pump as
it is also called,
features an impeller
with a double row of
vanes cut in the rim,
as illustrated in Figure
22. Both the suction
and the discharge
connections are
located in the casing
at the periphery of the
impeller. The liquid
circulates almost 360
degrees before being
discharged
22. NPSH
The NPSH required by a pump is the head of the liquid pumped,
measured at the suction nozzle of the pump, necessary to
overcome all energy requirements at the inlet of the pump
(these included friction losses, acceleration, heating effect of
internally circulated liquid etc.) and thereby avoid any
vaporization of liquid in the pump suction. The NPSH required is
thus the head of the liquid required at the pump suction nozzle
above the vapour pressure of the liquid at that point.
for centrifugal pump The NPSH required is expressed in terms
of head of liquid pumped, and not pressure while for positive
displacement pump NPSH are not always expressed in terms of
head of liquid. In some cases-as in the case of Reciprocation
Pumps- NPSH is expressed as a pressure increment above the
vapour pressure of the liquid
23. NPSH
NPSHA (Available) = Terminal Pressure in the vessel (in
gauge)
(+) Static Head of fluid above pump centre line .
(+) Atmospheric Pressure
(-) Vapour Pressure of liquid at pumping temperature
(-) Friction loss in suction piping up to pump centre line
consisting of the following
NPSHR (required):
The net positive suction head required is a function of the pump
design at the operating point on the pump performance curve
At any fixed speed, the NPSH required by a centrifugal pump
will increase with increase in flow from rated flow. At
substantially increased flow from design flow the increase in
NPSHR is very rapid
24. ADVANTAGES AND DISADVANTAGES
OF CENTRIFUGAL PUMPS
The advantages of centrifugal pumps include simplicity,
compactness, weight saving, and adaptability to high-speed prime
movers. One disadvantage of centrifugal pumps is their relatively poor
suction power. When the pump end is dry, the rotation of the impeller,
even at high speeds, is simply not sufficient to lift liquid into the pump;
therefore, the pump must be primed before pumping can begin. For this
reason, the suction lines and inlets of most centrifugal pumps are placed
below the source level of the liquid pumped. The pump can then be
primed by merely opening the suction stop valve and allowing the
force of gravity to fill the pump with liquid. The static pressure of the liquid
above the pump also adds to the suction pressure developed by the
pump while it is in operation. Another dis- advantage of centrifugal
pumps is that they develop CAVITATION. Cavitation occurs when the
velocity
25. Cavitation
Cavitation is defined as phenomenon of formation of vapour
bubbles of a flowing liquid in a region where the pressure of the
liquid falls below its vapour pressure and collapsing of these
vapour bubbles in a region of higher pressure. When the vapour
bubbles collapse and very high pressure is created, the metallic
surface above which the liquid is flowing is subjected to these
high pressures which cause pitting action on the surface, thus
cavities are formed on metallic surface and also considerable
nose and vibration created.
formation of vapour bubbles of flowing liquid takes place only
whenever the pressure in any region falls bellow vapour
pressure, at this time liquid starts boiling and vapour bubbles
forms, these bubbles carried along with the flowing liquid to the
higher pressure zone where this bubbles condense and bubbles
collapse due to sudden collapsing og the bubbles on metallic
surface high pressure is produced and surface subjected to high
local stress.
26. Cavitation
Precaution against cavitation
The pressure of the flowing liquid in any part of the hydraulic
system should not be allowed to fall below vapour pressure
( NPSHA>NPSHR).
The special material or coating such as aluminum bronze and
stainless steel should be used.
27. Positive Displacement Pump
By definition, positive-displacement (PD) pumps displace a
known quantity of liquid with each revolution of the
pumping elements. This is done by trapping liquid between
the pumping elements and a stationary casing. Pumping
element designs include gears, lobes, rotary pistons,
vanes, and screws.
PD pumps are found in a wide range of applications --
chemical-processing; liquid delivery; marine;
biotechnology; pharmaceutical; as well as food, dairy, and
beverage processing. Their versatility and popularity is due
in part to their relatively compact design, high-viscosity
performance, continuous flow regardless of differential
pressure, and ability to handle high differential pressure.
28. RECIPROCATING PUMP
In a reciprocating pump, the pumping action is produced by
the to and fro (reciprocating) motion of a piston or plunger
within a cylinder or by the flexing action of a diaphragm.
Piston pumps may be either single-acting or double-acting.
A single-acting pump will deliver liquid when the piston
moves in one direction only and a double-acting pump
when the piston moves in either direction. Plunger and
diaphragm pumps are usually single-acting.
Reciprocating pumps are referred to as simplex pumps
when they have one piston or plunger, duplex when they
have two pistons, triplex when they have three pistons, etc.
29. RECIPROCATING PUMP
In one revolution of the crankshaft, a single-acting simplex pump will deliver one pulse
of liquid. A triplex double-acting pump will deliver six pulses of liquid in one crankshaft
revolution. The crankshaft throws (eccentrics) on a triplex pump are set 120º apart.
When one stroke takes 180º to complete, it must follow that when one piston is nearing
the end of its delivery stroke, another piston is just beginning its delivery. The advantage
of this type of pump is that it not only increases the volume of output, but it also reduces
the surging and vibration caused by intermittent flow in a simplex pump. Some high-
pressure pumps further reduce the surging by installation of a surge accumulator on the
pump discharge line.
Surge accumulators may be referred to as "bumpers" or air chambers, as they may be
filled with air. Some are designed with a diaphragm to separate the chamber from the
liquid being pumped. This cushion above the diaphragm may be filled with air or
nitrogen.
Reciprocating pumps can be divided into two classes according to the pump drive:
Direct-acting in which the the piston rod of the driver is connected directly to the piston
rod of the pump. In a direct-acting, steam-driven pump, the movement of the piston in
the water cylinder is produced by a steam piston in a steam cylinder.
Power-driven in which a crankshaft is driven by a separate power source, such as an
electric motor.
30. SINGLE-ACTING PUMPS
Referring to Fig. when the plunger moves
from right to left, the pressure in the
cylinder drops below the pressure in the
suction line and liquid is drawn into the
cylinder through the suction ball check.
The high pressure in the discharge line
keeps the discharge ball check firmly on its
seat. At the end of its travel, the plunger
reverses direction and starts moving from
left to right. This raises the pressure in the
cylinder above the pressure in the
discharge line and the liquid is forced out
via the discharge ball check. The suction
ball check is forced shut while the
discharge valve is forced open by the
pressure of the liquid. Liquid is discharged
only when the plunger is moving from left
to right, hence the name single-acting. The
movement of the plunger in one direction
is called the stroke of the plunger. The
distance the plunger moves in and out of
the cylinder is the length of stroke
31. DOUBLE-ACTING PUMPS
The double-acting pump in Figure
4 has two discharge valves, D.A.
and D.B.and two suction valves,
S.A. and S.B. When the piston
moves from left to right as shown
in Fig. a), the liquid will be drawn in
through suction valve S.A. while at
the same time liquid, on the other
side of the piston, is being forced
out through the discharge valve
D.B. When the piston reverses and
moves from right to left as in Fig. b
liquid is drawn in through the
suction valve S.B. and at the same
time liquid is forced out through the
discharge valve D.A. With this
arrangement, liquid is discharged
when the piston moves in either
direction, hence the name double-
acting
32. DIAPHRAGM PUMPS
The diaphragm pump differs from the piston or
plunger-type reciprocating pump in that the fluid
being pumped is completely isolated from the
reciprocating mechanism by a diaphragm, thereby
eliminating leakage along the piston rod and plunger.
The diaphragm is a flexible membrane which acts as
the liquid displacement component. The diaphragm
can be made of flexible metal or nonmetallic
materials such as plastic, rubberor neoprene,
depending on the fluid being pumped.
33. DIAPHRAGM PUMPS
A cross-sectional view of a
mechanically actuated
diaphragm pump is shown in
Figure. The diaphragm, D, is
attached to the piston guide,
P, by the disc, B. An
eccentric is used to produce
the reciprocating motion of
the guide, P, causing the
diaphragm to move to and
fro, resulting in pumping
action.
34. ROTARY PUMP
Rotary pumps are positive displacement pumps. Instead
of imparting high velocity to the liquid due to centrifugal
force, as in a centrifugal pump, rotary pumps trap the
liquid between a close fitting casing and either gears,
lobes, vanes or screws. The liquid is pushed from the
suction to the discharge of the pump. Unlike reciprocating
pumps, the flow of liquid through rotary pumps is
continuous and the discharge is smooth, without pressure
fluctuations.
Gear pump (external, internal gear), lobe pump (two lobe
three lobe), vane pump( sliding, flexible vane) screw
pump, progressive cavity pump are examples of rotary
pump
35. External Gear Pump
The most common rotary pump is probably
the external gear pump, also known as the
gear pump or the spur gear pump.
The external gear pump shown in Fig.
consists of a housing containing two gears;
the driving gear at the top and an idler or
driven gear at the bottom. As the gears
rotate, the liquid fills the voids between the
gear teeth. The rotation carries these voids
or pockets to the discharge side of the pump
where the teeth of opposite gears mesh,
filling the pockets and displacing the liquid
carried in them. For this reason, the gear
pump is called a positive displacement
pump, as the gear tooth and the liquid
cannot exist in the same pocket at the same
time. The empty pocket continues its rotation
to the suction side of the pump and as the
teeth disengage, the void created is filled
with liquid from the suction line. The small
clearance between the gears and the pump
casing minimizes the amount of leakage
from the discharge side to the suction side.
36. External Gear Pump
The rotation and flow arrows
indicate the path of the liquid. A
common misconception is that the
liquid is somehow squeezed
through the teeth to produce
pumping action.
The top or driving gear is keyed to
the shaft which may be driven
directly by a motor or indirectly by
means of belts or gears.
If the clearance at the ends of the
gears and between the gears and
the casing, is small and the suction
lift is not too great, a gear pump
can prime itself by discharging the
air trapped in the casing.
37. External Gear Pump
Advantages
High speed.
Medium pressure.
No overhung bearing loads.
Relatively quiet operation.
Design accommodates wide variety of materials.
Disadvantages
Four bushings in liquid area.
No solids allowed.
Fixed End Clearances.
38. Applications
Industrial and mobile applications
Fuel and lubrication
Metering
Mixing and blending (double pump)
Hydraulic applications
OEM configurations
Precise metering applications
Low-volume transfers
Light or medium duty
39. INTERNAL GEAR PUMP
The internal gear pump has an
externally-cut gear which meshes
with an internally-cut gear on one
side of the casing. The internal gear
is separated from the external gear
on the opposite side by a crescent-
shaped partition which prevents
liquid from passing back from the
discharge to the suction side. Liquid
from the suction side fills the spaces
between the teeth of both gears
when they un mesh and is forced out
of these spaces into the discharge
when the gears mesh again.
40. Internal Gear Pumps Advantages
Two moving parts
One stuffing box
Positive suction, no pulsating discharge
Ideal for high viscosity liquids
Constant and even discharge regardless of varying pressure conditions
Low NPSH required
Easy to maintain
Disadvantages
Low speeds usually required
Medium pressure
One bearing runs in pumped product
Overhung load on shaft bearing
41. Applications
Barge, tanker, and terminal loading and unloading.
Filtering.
Circulating.
Transferring.
Lubricating.
Booster.
General industrial.
Marine applications.
Petrochemical.
Light, medium, or heavy-duty service
42. LOBE PUMP
The lobe pump (three lobe pump) in
Figure has similarities to the
external gear pump. Liquid is
pushed from the suction side of the
pump to the discharge side in a
similar path to that of the gear
pump. The pockets are larger and
the quantity of liquid handled is
greater (up to 12 000 liters/min), but
the head is reduced to a maximum
of about 750 kPa. The gear pump
has a driver and an idler gear. This
is not possible in the lobe pump as
the drive lobe might slip out of
synchronization with the idler lobe
when working against a head
pressure. External synchronizing
gears drive the lobes to avoid this
problem.
43. LOBE PUMP
Clearances in positive
displacement pumps have to
be kept to a minimum. Some
lobe pumps have wear strips
on the tips of the lobes,
which can be renewed when
wear occurs.
Lobe pumps and gear
pumps are usually
unsuitable for pumping
liquids containing abrasives
as wear would reduce their
efficiency. A two lobe pump
is illustrated in Figure
44. LOBE PUMP
Advantages
Pass medium solids.
No metal-to-metal contact.
Superior CIP/SIP capabilities.
Positive suction, non pulsating discharge.
Disadvantages
Requires timing gears.
Requires two seals.
Reduced lift with thin liquids.
45. Applications
Food processing.
Beverages.
Dairy Produce.
Personal Hygiene Products.
Pharmaceutical.
Biotechnology.
Chemical.
Industrial.
Medium and heavy duty cycles
46. SLIDING VANE PUMP
The sliding vane pump has a rotor with
slots spaced evenly around it. Vanes in
these slots are free to slide in and out to
meet the casing wall. The rotor is
eccentrically mounted in the pump cavity
with only enough clearance to avoid
contact with the casing at the closest point.
The vanes are forced out against the
casing wall by centrifugal force and the
liquid trapped between the vanes is carried
around from suction to discharge. As the
vanes pass the suction port they are
rotating into an increasing void. Liquid from
the suction line fills the void. As the vanes
continue to rotate they pass the point of
maximum clearance between the rotor and
housing and the void then begins to
diminish. As the volume decreases, the
trapped liquid is forced out the discharge
side of the pump.
47. SLIDING VANE PUMP
Advantages
Medium capacity
Medium speed
Thin liquids
Sometimes preferred for solvents, LPG
Can run dry for short periods
Can have one seal or stuffing box
Develops good vacuum
Disadvantages
Can have two stuffing boxes
Complex housing
Not suitable for high pressures
Not suitable for high viscosity
Not good with abrasives
48. SCREW PUMPS
The screw pump in Fig. features
a power rotor situated between
two idler rotors. The liquid is
drawn into both ends of the rotor
where it is trapped in the pockets
formed by the threads. The liquid
is carried between the screw
threads along the axes of the
screws as in a screw conveyor.
The liquid is discharged at the
middle of the rotors.
The idler rotors are driven by
liquid pressure and there is no
metal to metal contact between
idlers and power rotor. This
design may operate at pressures
up to 7000 kPa and at speeds up
to 7000 r/min.
49. SCREW PUMPS
A variation of the screw
pump is the two rotor screw
pump shown in Fig. This
pump has two rotors, each
with opposing helical
threads. The liquid is trapped
between the screw threads
and the pump casing and is
conveyed axially by the
meshing of the rotors, until it
is discharged in the middle.
The two rotor screw pump
uses timing gears to keep
the rotors synchronized
50. Advantage of Screw pump
Wide range of flows and pressures
Wide range of liquids and viscosities
Built-in variable capacity
High speed capability allowing freedom of driver selection
Low internal velocities
Self-priming with good suction characteristics
High tolerance for entrained air and other gases
Minimum churning or foaming
Low mechanical vibration, pulsation-free flow, and quiet operation
Rugged, compact design -- easy to install and maintain
High tolerance to contamination in comparison with other rotary
pumps (Fraser, et. al., 1986)
51. Disadvantage of Screw pump
Relatively high cost because of close tolerances and running
clearances
Performance characteristics sensitive to viscosity change
High pressure capability requires long pumping elements
(Fraser, et. al
52. PROGRESSIVE CAVITY PUMP
Another variation of the screw pump is the single
screw pump also called progressive cavity pump. It is
composed of a spiraled metal rotor which fits inside a
flexible helical liner. The liner may be made of rubber
or some other elastomer.
53. Some Other Design
Cam and Piston
Also called rotary-
plunger type, a cam
and piston pump
consists of an
eccentric with a
slotted arm at the top.
Shaft rotation causes
the eccentric to trap
liquid in the casing,
discharging it through
the slot to the outlet
54. Some Other Design
Swinging Vane
These pumps have a
series of hinged vanes
which swing out as the
rotor turns through the
eccentric cavity. Liquid
is trapped and forced to
the discharge side of
the pump
55. Some Other Design
Shuttle Block
Shuttle block pumps
have a cylindrical rotor
turning in a concentric
casing. The rotor
includes a shuttle block
and piston reciprocated
by an eccentrically
located idler pin,
producing suction and
discharge.
56. Do's and Don'ts
Do's
Install the pump as close as possible to the supply tank.
Leave working space around the pumping unit.
Use large, short, and straight suction piping. "Short and fat" pipes
are excellent.
Install a strainer in the suction line.
Double-check alignment after the unit is mounted and the piping is
hooked up.
Provide overpressure protection for the discharge side of the pump,
either in-line or on the pump.
Extend service life with preventive maintenance procedures such
as periodic lubrication, adjustment of end clearance, and
examination of internal parts.
Obtain, read, and keep the maintenance instructions furnished with
your pump.
57. Do's and Don'ts
Don'ts
Run a pump at faster than approved speeds.
Run a pump at higher than approved pressures.
Run a pump at temperatures at higher than approved
temperatures.
Use extra large, extra long suction line with a suction
lift.
58. Pump Seal
A pump has power delivered to its shaft to cause
pumping action. At some point, the liquid being
pumped must be restricted from leaking out of the
pump and air should be prevented from entering.
The seal used to restrict this flow must be a dynamic
seal. In other words, the shaft must be able to turn or
reciprocate and the seal should allow this motion
without allowing uncontrolled loss of liquid.
59. PUMP SHAFT SEALING
In order to minimize leakage around the pump shaft where it
passes through the casing, stuffing boxes and mechanical seals
are used.
Stuffing Boxes
A stuffing box consists of a cylindrical recess around the shaft
that holds a number of packing rings. The packing is a soft,
pliable yet durable material which bears against the pump shaft
and the stuffing box walls and reduces leakage around the shaft.
Packing is made from a wide variety of materials, some of
which are asbestos, nylon, flax, Teflon, lead, copper and
aluminum. Frequently, a lubricating material such as graphite or
grease is incorporated into the packing material.
The packing is held in place by a gland. The gland can be
adjusted with tightening nuts, compressing the rings in order to
obtain the desired fit. The bottom or inside end of the stuffing box
may be formed by either the pump casing itself or by a bottom
bushing
60. The gland must not be
tightened too much, as
there needs to be a small
amount of leakage along
the shaft to lubricate and
cool the packing. If the
packing is too tight, the
friction created by the
shaft turning against it
causes the shaft and
packing to heat up. If left
unchecked, the friction
can cause gouging of the
shaft and the packing to
smoke
61. Stuffing Box With Lantern Ring
The lantern ring (also called seal cage) is a
metal ring with channels machined in its
inside and outside perimeters, giving a
modified "H" cross section to the ring. The
inner and outer channels are connected by
radially drilled holes. The lantern ring
serves to distribute sealing liquid under
pressure to the packing, thus preventing air
infiltration and providing lubrication. This
sealing liquid is usually provided by the
high-pressure section of the pump casing,
either through an external connection,
Figure 3or through an internally drilled
passage in the casing.
Lantern rings are also used on pumps
handling liquids containing sand, grit or
other abrasives, which could damage the
shaft and shorten the life of the packing
when allowed to enter the stuffing box.
Clean sealing liquid provided by a separate
source or by the discharge side of the
pump via a filter or separator will then keep
the gritty substances out of the stuffing box.
62. MECHANICAL SEALS
Leakage from stuffing boxes is objectionable on pumps handling liquids
such as gasoline, acids and ammonia. Instead, these pumps are
equipped with mechanical seals which reduce leakage to a minute
amount. They are also used on pumps such as high pressure pumps
where stuffing boxes cannot offer adequate leak protection.
Mechanical seals have the following advantages over packing rings:
They require much less maintenance.
They do not wear shafts or shaft sleeves as do packing rings.
They reduce leakage to a minimum.
They can be designed to work under very high temperatures and
pressures
Basically, a mechanical seal consists of two flat rings, each with a
polished sealing surface. The rings are perpendicular to the pump shaft;
the sealing faces rotate on each other. One of the rings is called the
sealing ring and it is held in position by a spring. The other ring, its face
in contact with that of the sealing ring, is called the mating ring.
Mechanical seals may be divided into two general types: the rotating
seal and the stationary seal.
63. Rotating Mechanical Seal
The basic design of a rotating seal is
illustrated in Figure 5. The mating ring
is held stationary in a recessed part of
the pump housing or the seal housing
cover. An O-ring provides a seal
between ring and casing to prevent
leakage. A shell secured to the shaft by
set screws holds the sealing ring so that
it turns with the shaft. Leakage between
shaft and sealing ring is prevented by a
second O-ring. As the pump shaft turns,
the sealing ring is held against the
mating ring by a number of small
springs contained in the shell, thus
preventing leakage between the faces.
The springs allow the sealing ring
enough flexibility to maintain full face
contact with the mating ring at a
constant pressure during slight shifts in
shaft position.
64. Stationary Mechanical Seal
In this type of seal, the shell
containing the springs and
sealing ring is held stationary
in the annular space of the
pump housing as sketched
in Figure 7. The mating ring
is fastened rigidly to the
shaft, usually against a
shoulder, so that it rotates
with the shaft. The springs
force the sealing ring against
the mating ring so leakage
between the faces is
prevented. O-rings are used
to prevent leakage between
sealing ring and shell and
between mating ring and
shaft.
65. Care of Mechanical Seals
When a pump is equipped with mechanical seals, the following precautions
should be taken before and during operation:
Never run the pump unless it is completely filled with liquid.
Vent all air out of the seal housings before start-up.
Make sure an adequate flow of quenching or cooling liquid is flowing to the
seals.
It is extremely important that the seals never run in dry condition because this
causes the faces to score and become grooved. Dry running seal faces are
often indicated by a squealing sound, but absence of this sound should not be
interpreted as an indication that sufficient liquid is supplied to the seals.
A leaking seal may be caused by:
Seal faces that are scored or grooved.
Distortion of the rings due to unevenly tightened bolts on the seal housing gland.
O-ring or other type gaskets that are cut or nicked during installation.
Misalignment of piping resulting in distortion of pump parts.
Excessive pump shaft vibration.
Improper adjustment of the spring tension.
