林氏三兄弟单级离心泵设计-翻译原文

2021年1月19日发(作者：史佚)



CENTRIFUGAL
PUMPS
IN
THE
CHEMICAL
INDUSTRY

Abstract

:

A
centrifugal
pump
converts
the
input
power
to
kinetic
energy
in
the
liquid
by
accelerating
the
liquid
by
a
revolving
device
-
an
impeller.
The
most
common
type
is
the
volute
pump.
Fluid
enters
the
pump
through
the
eye
of
the
impeller
which
rotates
at
high
speed.
The
fluid
is
accelerated
radially
outward
from
the pump chasing.
A
vacuum
is created at
the
impellers eye
that continuously draws
more fluid into the pump . This article stresses on a series of centrifugal pumps
，
From
a brief introduction to the principles.
Keywords

:

centrifugal pump ,Introduction ,Working

principle ,


Cavitation
，

Mechanism of Cavitation
，
Solution and Remedies
1. Introduction



Pump ,device
used to
raise ,transfer ,or compress
liquids and
gases .Four
general
classes
of
pumps
for
liquids
are
described
below .In
all
of
them
,steps
are
taken
to
prevent
cavitation
(the
formation
of
a
vacuum)
,which
would
reduce
the
flow
and
damage
the
structure
of
the
pump .Pumps
used
for
gases
and
vapors
are
usually
known as compressors .The study of fluids in motion is called fluid dynamics.



Water pump ,device
for
moving water
from one
location to another ,using tubes or
other
machinery .Water
pumps
operate
under
pressures
ranging
from
a
fraction
of
a
pound
to
more
than
10,000
pounds
per
square
inch .Everyday
examples
of
water
pumps range
from small
electric pumps
that circulate and aerate
water
in aquariums
and
fountains
to
sump
pumps
that
remove
water
from
beneath
the
foundations
of
homes .



One
type
of
modern
pumps
used
to
move
water
is
the
centrifugal
pump .Early
version
of
the
centrifugal
pump
,the
screw
pump
,consists
of
a
corkscrew-shaped
mechanism
in a pipe
that ,when rotated ,pulls water
upward .Screw pumps are often
used
in waste-water
treatment plants because
they can
move
large amounts of
water
without
becoming
clogged
with
debris
.In
the
ancient
Middle
East
the
need
for
irrigation of farmland was a strong inducement to develop a water pump .Early pumps
in
this
region
were
simple
devices
for
lifting
buckets
of
water
from
a
source
to
a
container
or
a
trench .Greek
mathematician
and
inventor
Archimedes
is
thought
to
have
devised
the
first
screw
pump
in
the
third
century
BC .Later
Greek
inventor
Ctesibius
develop
the
first
lift
pump .During
the
late
17
th

and
early
18
th

Centuries
AD
,British
engineer
Thomas
Savery
,French
physicist
Denis
Papin
,And
British
blacksmith
and
inventor
Thomas
Newcomen
contributed
to
the
development
of
a
water
pump
that
used
steam
to
power
the
pump’
piston
.The
steam-powered
water
pump’s
first wide
use was
in pumping
water out of
mines .Modern
-day examples of
centrifugal
pumps
are
those
used
at
the
Grand
Coulee
Dam
on
the
Columbia
River .This pump system has the potential to irrigate over one million acres o
f land .



Also known as rotary pumps ,centrifugal pumps have a rotating impeller ,also known
as a blade ,that
is
immersed
in the
liquid .Liquid enters
the pump
near the axis of the
impeller ,and the rotating impeller sweeps the liquid out toward the ends of the impeller
blades at high pressure .The impeller also gives the liquid a relatively high velocity that
can
be
converted
into
pressure
in
a
stationary
part
of
the
pump
,known
as
the
diffuser .In
high-pressure pumps ,a
number of
impeller
may be
used
in series ,and the
diffusers
following
each
impeller
may
contain
guide
vanes
to
gradually
reduce
the
liquid
velocity
.For
lower-pressure
pumps
,the
diffuser
is
generally
a
spiral
passage ,known as a volute ,with
its cross-sectional area
increasing
gradually
to reduce
the velocity efficiently .The impeller must be primed before it can begin operation ,that
is
,the
impeller
must
be
surrounded
by
liquid
when
the
pump
is
started .This
can
be
done by placing a check
valve
in
the suction
line ,which
holds
the
liquid
in the pump
when the
impeller
is
not rotating .If this
valve
leaks ,the pump
may
need
to be primed
by the
introduction of
liquid
from an outside
source such as the discharge reservoir .A
centrifugal
pump
generally
has
a
valve
in
the
discharge
line
to
control
the
flow
and
pressure
.For
low
flows
and
high
pressures
,the
action
of
the
impeller
is
largely
radial .For
higher
flows and
lower discharge pressure ,the direction of the
flow
within
the pump is more nearly parallel to the axis of the shaft ,and the pump is said to have an
axial
flow .The
impeller
in
this
case acts as a propeller .The
transition
from one set of
floe
conditions
to
the
other
is
gradual
,and
for
intermediate
condition
,
the
device
is
called a mixed-flow pump .

Centrifugal Pump

The
centrifugal
pump
is
by
far
the
most
widely
used
type
in
the
chemical
and
petroleum
industries
.It
will
pump
liquids
with
very
wide
ranging
properties
and
suspensions
with
a
high
solids
content
including
,for
example
,cement
slurries
,and
may be constructed from a very wide rang of corrosion resistant materials .The whole
pump
casing
may
be
constructed
from
plastic
such
as
polypropylene
or
it
may
be
fitted
with
a
corrosion-resistant
lining .Because
it
operates
at
high
speed
,it
may
be
directly coupled to an electric motor and it will give a high flow rate for its size .


In
this
type
of
pump
,the
fluid
is
fed
to
the
centre
of
a
rotating
impeller
and
is
thrown
outward
by
centrifugal
action .As
a
result
of
the
high
speed
of
rotation
the
liquid
acquires a
high kinetic energy and the pressure difference between
the suction
and delivery sides arises from the conversion of kinetic energy into pressure energy .



The impeller consists of a series of curved vanes so shaped that the flow within the
pump
is as smooth as possible .The
greater the
number of
vanes on
the
impeller ,the
greater
is
the
control
over
the
direction
of
the
liquid
and
hence
the
smaller
are
the
losses due to
turbulence and circulation between
the
vanes .In the open
impeller ,the
vanes are fixed to a central hub ,whereas in the closed type the vanes are held between
two
supporting
plates
and
leakage
across
the
impeller
is
reduced .As
will
be
seen
later
,the
angle
of
the
tips
of
the
blades
very
largely
determines
the
operating
characteristics of the pump .





The
liquid enters the casing of the pump
，
normally
in an axial direction
，
and
is
picked
up by
the
vanes of
the
impeller
．
In
the simple
type of centrifugal pump
，
the
liquid discharges into a volute
，
a chamber of gradually increasing cross
—
section with
a
tangential
outlet
．
A
volute
type
of
pump
is
shown
in
Fig.(a)
．
In
the
turbine
pump[-Fig
．
(b)]the
liquid
flows
from
the
moving
vanes
of
the
impeller
through
a
series of fixed vanes forming a diffusion ring
．


This
gives
a
more
gradual
change
in
direction
to
the
fluid
and
more
efficient
conversion
of
kinetic
energy
into
pressure
energy
than
is
obtained
with
the
volute
type
．
The angle of the leading edge of the fixed vanes should be such that the fluid is
received without shock
．
The liquids flows along the surface of the impeller vane with
a
certain
velocity
whilst
the
tip
of
the
vane
is
moving
relative
to
the
casing
of
the
pump
．
The
direction
of
motion
of
the
liquid
relative
to
the
pump
casing--and
the
required angle of the fixed vanes
—
is found by compounding these two velocities
．
In
Fig
．
c
，



























c.
u
v

is the
velocity of the
liquid relative to
the vane and
u
t

is the tangential
velocity
of
the tip of the
vane
；
compounding
these
two
velocities
gives the resultant velocity
u
2
of the liquid
．
It is apparent
，
therefore
，
that the required vane angle in the diffuser
is dependent on the throughput
，
the speed of rotation
，
and the angle of the
impeller
blades
．
The pump will
therefore operate at
maximum
efficiency only over
a
narrow
range of conditions
．





Virtual head of a centrifugal pump
The maximum pressure is developed when the whole of the excess kinetic energy
of
the
fluid
is
converted
into
pressure
energy
.
As
indicated
below
．
the
head
is
proportional to the square of the radius and to the speed
，
and is of the order of 60m for
a
single
—
stage
centrifugal
pump
；
for
higher
pressures
，
multistage
pumps
must
be
used
．
Consider
the
liquid which
is rotating at a distance of between r and
r+dr
from
the centre of the pump(Fig
．
d)
．























d













The mass of this element of fluid dm is given by 2πrdrdρ
，
where ρ is the density of the
fluid and 6 is the width of the element of fluid
。

If
the
fluid
is
traveling
with
a
velocity
u
and
at
an
angle
θ
to
the
tangential
direction
．
The angular momentum of this mass of fluid

















= dM (urcosθ)





The
torque
acting
on
the
fluid
dτ
is
equal
to
the
rate
of
change
of
angular
momentum with time
，
as it goes through the pump









Dτ = dM α/αt(urcosθ)=2πrbρdrα/αt(urcosθ)




The volumetric rate of flow of liquid through the pump
：










Q=2πrbα/αt










Dr =Q ρ d(urcosθ)

The total torque acting on the
liquid
in
the pump
is
therefore obtained
integrating
dτ between the
limits denoted by suffix
1 and suffix 2
，
where suffix 1 refers to
the
conditions
at
the
inlet
to
the
pump
and
suffix
2
refers
to
the
condition
at
the
discharge
．



Thus
，
τ=Q ρ(
u
2
r
2
cos
?
2
-
u
1

r
1
cos
?
1
)


The advantages and disadvantages of the centrifugal pump


The main advantages are
：



(1)

It
is

simple

in construction and can
，
therefore
，


be
made

in a

wide
range of materials


(2)There is a complete absence of valves
．



(3)It operates at high speed(up to 100 Hz)and
，
therefore
，
can be coupled directly to

an electric motor.

In general
，
the higher the speed the smaller the pump and motor
for a give n duty
．



(4)It gives a steady delivery
．



(5)Maintenance costs are lower than for any other type of pump
．



(6)No damage is done to the pump if the delivery line becomes blocked
，
provided it
is not run in this condition for a prolonged period
．



(7)It is much smaller than other pumps of equal capacity
．
It can
，
therefore
，
be made
into a sealed unit with the driving motor and immersed in the suction tank
．



(8)Liquids containing high proportions of suspended solids are readily handled
．




The main disadvantages are
：



(1)The single
—
stage pump will not develop a high pressure
．
Multistage pumps will
develop
greater
heads bat they
are
very
much
more expensive and cannot readily be
made
in
corrosion
—
resistant
material
because
of
their
greater
complexity
．
It
is
generally
better
to
use
very
high
speeds
in
order
to
reduce
the
number
of
stages
required
．





(2)It
operates
at
a
high
efficiency
over
only
a
limited
range
of
conditions;
this
applies especially to turbine pumps
．







(3)It is not usually self-priming.


(4)If a non- return valve is not incorporated in the delivery or suction line, the liquid
will run back into the suction tank as soon as the pump stops
．



(5)Very viscous liquids cannot he handled efficiently
．



3. Cavitation

in centrifugal pump

（
1
）
The
term
‘cavitation’
comes
from
the
Latin
word
cavus,
which
means
a
hollow

space
or
a
cavity
.
Webster’s
Dictionary
defines
the
word

‘cavitation’
as
the
rapid
formation
and
collapse
of
cavities
in
a
flowing
liquid
in
regions
of
very
low pressure.
In
any
discussion
on
centrifugal
pumps
various
terms
like
vapor
pockets,
gas
pockets,
holes,
bubbles,
etc.
are
used
in
place
of
the
term
cavities.
These
are
one
and
the
same
thing
and
need
not
be
confused.
The
term
bubble
shall
be
used
hereafter in the discussion.
In
the
context
of
centrifugal
pumps,
the
term
cavitation
implies
a
dynamic
process
of
formation
of
bubbles
inside
the
liquid,
their
growth
and
subsequent
collapse as the liquid flows through the pump.
Generally
, the bubbles that form inside the liquid are of two types: V
apor bubbles or
Gas bubbles.


1.V
apor
bubbles
are
formed
due
to
the
vaporisation
of
a
process
liquid
that
is
being
pumped.
The
cavitation
condition
induced
by
formation
and
collapse
of
vapor bubbles is commonly referred to as V
aporous Cavitation.

bubbles
are
formed
due
to
the
presence
of
dissolved
gases
in
the
liquid
that
is
being
pumped
(generally
air
but
may
be
any
gas
in
the
system).
The
cavitation
condition
induced
by
the
formation
and
collapse
of
gas
bubbles
is
commonly referred to as Gaseous Cavitation.