Pumping
chemicals
MM
Kulkarni, RS Dipali and SK Kundu lay down the factors that influence
the selection of appropriate pumps and bolster their analysis with case
studies
Pumps
have existed for long and are in widespread use. They can be found in
an endless variety of types and sizes and find use in myriad fields.
Our
aim will be to examine the basic types of pumps that exist today and
explore the ways for making a rational decision while buying them, with
respect to the chemical industry and power plants.
In
this article we are also sharing our experiences for a few specific
projects for different types of pumps (including special types in some
of the projects) both during engineering and execution stage.
Classification
of pumps
Pumps
may be classified into two general types based on the principle by which
energy is added to the fluid being pumped, the means of implementation
of these principles and the specific geometries employed.
a)
Dynamic pumps such as centrifugal or special effect pumps are those
in which energy is continuously added to the fluid pumped by means of
impeller or propeller moving on shaft. Velocity energy is converted
to pressure energy by subsequent reduction in velocity within or beyond
the pump.
b)
Displacement (reciprocating and rotary type) pumps are those in which
energy is periodically added by application of force to one or more
movable boundaries of any desired number of enclosed fluid containing
volumes resulting in direct increase in pressure up to the value required
to move the fluid through valves or ports into the discharge line.
Relative
duty fields or different pump types
Specific
speed is a useful index in getting a general idea of the type of pump
to be chosen. This dimension less number is defined as below.
Here
Ns is specific speed, N is speed, Q is capacity or flow and H is head.
When capacity is expressed in gallons and head in feet, centrifugal
pumps have specific speeds that range from about 400 to 11,000 depending
on the type of impeller design.
Smaller
dynamic pumps such as regenerative turbine pumps are in the specific
speed ranging from about 100 to 1,200. Rotary and reciprocating pumps
have even lower values.
Looking
at it in practical terms, the relative duty fields of operation can
be explained as below
In
the centrifugal pump the liquid flows through the rotating impeller,
and best discharge rate for the pump in relation to power input, is
a product of the impeller diameter and the speed and the passage cross-section.
From manufacturing considerations there is a limit to how small the
passage are can reasonably be made in relation to the diameter, therefore
to how small the discharge rate can be in relation to head for good
efficiency. In the positive pump there is no such limitation as a moving
element with very small volume displacement can impart very high pressure.
On the other hand, for large flows, the dynamic pump is much more compact
than the positive, since for example in the reciprocating pump, the
tolerable average velocity is lower, and greater flow area therefore
necessary for the same capacity.
Thus
purely from head and flow aspect, positive pumps suit low flow high
pressure duties, and rotodynamic pumps the converse. Between reciprocating
and rotary pumps, the former is better suited to extra high pressures
on account of the absence of side thrusts. In reciprocating pimps there
is a possibility of smaller casing diameter owing to the whole cross-section
being a flow area. In rotary pumps only a small proportion may so be.
These considerations result in division of the total duty field between
the different types of pump.
It
is worth noting here the large field of operation ordinarily occupied
by the centrifugal pumps.
....CONTD
