points to consider while selection of a Pump are
Pump Material of construction
2. Various other features as governed by liquid
to be pumped and pumping conditions
3. NPSH, and
4. Working temperatures and cooling or flushing
media plans size
1. Knowing the duty point i.e.
head and capacity, find from graph, the near about
pump size suitable. In case one or more sizes
are suitable, try to offer the economical (normally
2900 RPM) size.
2. Refer the exact curve corresponding
to the size selected from the family graph and
find exact duty point. Note the following;
Head / capacity
Apply the corrections corresponding
to vapour pressure, viscosity, specific gravity
before arriving the at exact duty point and B.H.P.
efficiency corrections as mentioned in para 3.
3). Referring to data sheet find
out the most suitable material of construction.
If all Iron pumps are suitable in addition to
others, it would be always be economical. However,
following are limitations for all C.I. pumps
A) The impeller peripheral speed
should not exceed 135 ft/sc. Or 41.2m/sec.Corresponding
to head of 105 m (275 mm dia. Impeller trimmed
from 315mm. Normal size) at 2880 rpm. When is
C.I.(is 210 GR. 25 )
that case select cast steel or cast alloy steel
correction factor -for Cast Steel, Alloy steel impellers.
By 3.5% upto 12.5 mm width (1/2")
By 2.5% upto 16.0 mm (12.5-16mm) or (½"-5/8")
By 1.5% upto 20.0 mm (3/4")
By 1% above 20 mm impeller width.
The overall performance is derated in case of viscous
The limiting temperatures and pressures are as under
for C.I. and bronze construction.
construction / bronze construction.
200 deg. C -pressure 12.5 bar.
120 deg. C -pressure 16 bar.
Steel & Stainless Steel construction.
315 deg. C -pressure 16 bar.
Temp. 150 deg. C-pressure 20 bar.
For higher pressure and temperatures
of order of 26 kg /sq. cm and 320 deg. C. use
of SG Iron or cast steel IS GRADE 3038 is recommended.
D). Therefore, in case of heating
chambers or centerline supports which are recommended
above 170°C-175°C liquid temperatures
normally cast steel or cast alloy steel pumps
are recommended and bronze/C.I. pumps are not
E). Bearing cooling is offered
in case it is expected that the bearing temperatures
due to surrounding, condition of heat along shaft
and generated heat due to driving friction, would
rise above 75°C. The oil temperature in the
housing are kept between 75°C to 80°C
by carrying away that (liquid temperature above
F). External gland cooling is
recommended when the temperature at gland is expected
to be 100°C and the above. In that case the
cooling is adjusted to keep gland temperature
between 100°C to 150°C (liquid temperature
G). While selecting a prime mover,
check surrounding conditions, if it is necessary
to have flame -proof motor to avoid accidents
(inflammable, volatile liquids-use mechanical
seals to stop dripping of liquids) find the max.
H.P. required at 10% lower heads than specified
and allow for 15% extra power to take care of
process variations considered for viscosity and
are the important factors taken in to consideration
while selection chemical pumps
Capacity and discharge head
expected in usual terms
Suction process pump may be
required to lift the liquid from a lower level
or liquid coming under pressure at the suction
end. It is, therefore, necessary to know the working
pressure and the operating head. For example,
if the working pressure of the pump is 25 Kg/cm2
and the inlet pressure is 10 Kg/cm2 the operating
head can be only upto 15 Kg/cm2.
One of the most important parameters
to be taken into consideration is the Net Positive
Head. Generally speaking NPSH is defined as pressure
at the suction inlet of the pump, less vapour
pressure. It is known that atmospheric pressure
can support water column to a height of 34 ft.
In practice, this is not true for several reasons.
The suction pressure should not go below vapour
pressure of the liquid. Otherwise the liquid will
get converted into vapuor form and the flow will
be discontinued. The pump is not designed to handle
vapour. There is some loss of pressure due to
friction in the pipe line, foot valve, bend etc.
Some head gets converted into velocity head. As
a result the head available may be given as 34-ft.
vapour pressure -velocity head -friction head
+inlet pressure. This can be described as Net
Positive Suction Head available. Every pump by
its design characteristic requires certain NPSH
at the inlet for pumping liquid. This is called,
as NPSH available must be more than NPSH required.
While selecting chemical pump therefore, both
the values must be known. Generally NPSH required
by Chemical Process pump is 2.5 metres. However,
it differs with different models of pumps and
also it differs with speed.
CHEMICAL COMPOSITION OF THE LIQUID
In order to ascertain the reaction
of the liquid on the wetted parts of the pump,
it is necessary to know the chemical composition
of the liquid, it is also desirable to know the
pH valve which indicates the acidic or basic nature
of the liquid. Pure water has a PH valve of 7.
Between 0 to 7, the liquid is acidic and between
7 to 14 the liquid is basic.
SSU. It can be used viscous liquid
from 500 SSU to1500 SSU with due correction. However,
in this range the pump has to be heavily departed.
It results in selecting a big pump for relatively
smaller capacity and head. It is not advisable
to use centrifugal High viscosity of liquid can
cause high friction loss. As a result the pump
develops less head and less capacity. The power
consumption increases. In other words the pump
required to be derated. Chemical process pump
can safely handle viscosity up to 500 pump for
viscous higher than 500 SSU.
affects the pump in following way;
a) In case of certain chemicals
corrosion action depends upon the temperature.
Usually hot boiling liquid has more corrosive
B) If the temperature raises,
heat is transferred from gland packing through
the shaft to the bearing. Such a transfer of heat
to the bearing reduces bearing life considerably.
It may even seize the bearing. In such cases,
it is necessary to make sufficient cooling arrangement
at the gland, so that heat is carried away. It
may be down either by use of direct injection
or cooling water in to gland or by providing cooling
jacket for the cold water circulation.
c) Temperature affects viscosity.
Many times the viscosity is given at certain laboratory
temperature. However, If the working temperature
is quite different, the viscosity figure given
is of no use. In such a case, the knowledge of
actual temperature will be quite useful in getting
the correct figure for the viscosity.
d) At high temperature, the expansion
of metal parts takes part in different proportion.
This factor must be taken in to consideration
when offered a pump for high temperature
e) Bedside temperature, it is
necessary to know whether it is required to preserve
heat (or to maintain the low temperature as the
case may be). Such a condition may be necessary
to maintain the flow condition or to achieve operation
economy. In such cases, steam jacketing may have
to be used to maintain high temperature of the
while passing through the pump or proper insulation
to be provided to maintain the low temperature.
Chemical process pump can operate
in the range of minus 50 deg c to plus 300 deg
c. if the temperature is above170°C, it is
necessary to provide cooling arrangement at the
gland. A stuffing box jacket is provided through
which cooling liquid can be passed. Also in such
cases, centerline mounting foot support is to
be provided. Volute casing is mounted on external
feet (not integral with casing) and the support
is at the level of shaft centerline. Cold water
is circulated through the external feet. This
design takes into account uneven expansion of
As soon as the earlier vapour
pressure of the liquid influence the working of
the pump on the suction side, it is the pressure
at which a given liquid vapourises. The vapour
pressure again depends upon the temperatures.
Higher the vapour pressure, lower the NPSH available.
If pressure is more than the available NPSH, cavitation
occurs. In case of high vapour pressure, the liquid
is supplied to the pump with some suction head
created by keeping the pump at a lower level thereby
creative positive head at the suction head. The
other solution is to reduce the temperature of
the liquid before it enters the pump.
Specific gravity directly influences
the power consumption. Hence the liquid with higher
specific gravity requires proportionately high
H.P. It should be noted that there is no change
in the capacity delivered and the head developed,
if the specific gravity is higher or lower. Specific
gravity also affects the NPSH available. All the
while we have taken the figure of 34 ft. of water
column while calculating NPSH available. However,
if the liquid is heavier than water, the atmospheric
pressure again support that liquid to a higher
of 34 ft. as is amply clear from the fact that
with a heavier specific gravity like mercury in
the liquid form, the height is only 32".
If therefore one is required to calculate NPSH
available, specific gravity has to be taken into
The solid contents of the liquid
influences the type of impeller and gland arrangement.
The solid may be in two forms. One in the form
of fully dissolved state. In this case, it will
only affect the specific gravity and the viscosity
of the liquid. If the solid remains in undissolved
form, it is necessary to know the type of solid,
such as hard, soft, abrasive or nonabrasive. A
centrifugal pump can normally handle between 10
to 15% by volume.
Hard and abrasive solids wear
out the pump parts rapidly. To protect the costlier
parts, the manufacture may have to provide suitable
renewable liners. The manufacture will also have
to take care to prevent the entry of solid in
the gland region, so that the life of the gland
ring and the shaft protecting sleeve is not shortened.
In such cases, double seals may be used. It will
also be possible to provide circulation through
the seal to carry away abrasive particles. Generally
when there are solids in suspension, semi-open
impeller, is offered.