How pump works

A detailed understanding of the design principles behind centrifugal
pumps is not essential to this book, though a grasp of the fundamentals
is.
Real Impellers have a series of blades attached to a rotating disk. The blades normally slope backward, away from the
direction of rotation. As the impeller rotates, the curvature of the blade
pushes the liquid out into the casing that surrounds it. This collects the
displaced fluid and directs it into the piping. These are called centrifugal
pumps. The centrifugal force arises because the radius at which an
individual particle leaves the impeller is larger than that at which it
enters.
As liquid is displaced out into the
casing, more liquid is pushed in to
replace it.1 This process takes place
continuously to create a pumping
action.
Basic elements of a single stage
centrifugal pump. Impeller [here with seven
vanes] and single volute collector
One member of the pump family
displaces the liquid axially. These
impellers look more like propellers,
a name which is probably more apt.
The force that displaces the liquid is
only due to the hydrodynamic vane
action. Here, liquid leaves the
propeller at more or less the same
radius as it enters, so there can be no
centrifugal forces involved. In all but
the pure axial flow impeller, head is
being generated by both centrifugal
force and vane action. At the axial
flow end of the family almost all theCentrifugal pumps are the most preferred pumping devices in the hydraulic world. In this article we will have a conceptual overview of working of centrifugal pumps. At the heart of the system lies the impeller. It has got a series of curved vanes, fitted inside shroud plates. The impeller is always immersed in water. When the impeller is made to rotate, it makes the fluid surrounding it also rotate. This imparts centrifugal force to the water particles and the water moves radially out. Since rotational mechanical energy is transferred to the fluid, at discharge side of the impeller, both pressure and kinetic energy water will rise. At the suction side water is getting displaced, so a negative pressure will be induced at the eye. Such a low pressure helps in sucking freshwater stream into the system again. and this process continues. This is the reason why priming is important for centrifugal pumps. If no water is present initially. the negative pressure developed by the rotating air at the Eye of impeller will be negligibly small to suck fresh stream of water. Impeller is fitted inside a casing. So the water moving out will be collected inside it, and will move in the same direction of rotation of impeller, to the discharge nozzle. Here you can note one speciality of casing. It has got increasing area along the flow direction. Such increasing area will help in accommodating, newly added water stream and will also help in reducing exit flow velocity. Reduction in flow velocity will result in increase in static pressure, which is required to overcome resistance of pumping system Here you can see more details of veins inside impeller. They are backward curved vanes with state-of-the-art Eye configuration. If pressure at suction side impeller goes below vapor pressure of water, a dangerous phenomenon could happen. Water will start to boil forming vapor bubbles and spoil impeller materials over time This phenomenon is known as cavitation. More the section head, lesser should be the pressure at the suction side, to lift the water. This fact puts a limit to maximum suction head a pump can have. Careful pump selection is required to avoid the problem of cavitation. The current impeller type is enclosed, semi-open and open impellers are also in use depending upon the application. If the working fluid is cloggy in nature, it is preferred to use open kind of impeller. But they are slightly less efficient. Mechanical design of centrifugal pump is always challenging. A shaft is used to connect between the impeller and motor Since water pressure inside the casing is huge, a proper sealing arrangement is imperative in arresting water leakage through the shaft-casing clearance. Mechanical seal or stuffing box based mechanism is used for this purpose. Impeller is mounted on bearings, but at the suction side impeller, it is not advisable to fit a bearing, since it will block the flow. So bearings have to be fitted at the other end. This means impeller is mounted like a cantilever. For high flow rate pumps, a bearing housing with cooling oil is necessary for improving the life for bearings.head is generated by vane action. At the centrifugal flow end, most, but
not all, of the head comes from centrifugal force. In between these two
extremes are the impellers known as mixed flow. Here the head from
the vane action and centrifugal contributions are comparable.
The centrifugal pump is by far the most prolific member of the pump
family, so it is most useful to concentrate on these. How does a
centrifugal impeller work?
One simple mental experiment can demonstrate the principle upon
which centrifugal pumps are based.
9 Imagine an open bucket, half-filled with water, and suspended on
the end of a short piece of rope,
m Next, imagine swinging that rope around in a vertical plane. The
centrifugal forces acting on the water keep it in place inside the
bucket. It does not fall out as long as the bucket is being swung fast
enough. The centrifugal force pushes the water into the bottom of
the bucket, even when it is upside down. The centrifugal force
pressurises the liquid in the bucket. The faster the bucket spins the
greater the centrifugal force and the greater the liquid in the base of
the bucket is pressurised. In other words, the liquid tries to push the
base off the bucket.
What would happen if there were small holes in the base of the bucket?
Well, the centrifugal force would continuously push liquid out of the
bucket, Each particle of water is thrown off the bucket-
impeller at a tangent, just as do particles flung off from a wet spinning
bicycle wheel. The casing would then normally direct the particles into
a pipe, or, for illustration, into a fountain.
The faster the particles are flung off, the higher the casing can direct itas a fountain. Bigger impellers fling particles faster and hence higher
than small impellers running at the same speed. On the other hand
spinning the impeller faster will do the same thing. It simply remains to
provide a continuous feed to the bucket and all the elements of a
centrifugal pump are in place for this mental experiment.
Most centrifugal pumps are purchased to stimulate flow. In the process,
they also pressurise the liquid. This pressurisation is useful in
overcoming any resistance to flow. Finding the correct combination is
the task of the pump designer.
A minority of pumps are purchased to pressurise a process, and here any
flow is largely incidental.
In any event, the relationship between pump flow and pump generated
head/pressure is a very important one and must be known in order to
ensure good pump scheme flowknown as the pump characteristic
curve, the pump performance
curve, or mostly, 'the curve'. This
is basically, what manufacturers do
on their test bed, in order to
determine the pump performance
curve. [though they use different
techniques]
The curve derived from this
method will generally steadily fall
to the right.
In some rarer cases, the curve may
show a point of discontinuity at Range of curve shapes likely to be
about 40 to 70% of the pump encountered
design flow. This is typical of very
high flow- low head pumps. Operation below the point of
discontinuity is possible, but generally discouraged since it will be
associated with unsatisfactory pump running. In general operation of
ANY pump at low flow is to be discouraged, though more common
pumps would not have such a restrictive level as 70%.
Occasionally pumps appear with characteristics that do not steadily fall
to the right. Although there is widely held negative view of such curves,
they are really only of consequence if the pump has to operate in
parallel with another, or the static head is unusually high. Pumps with
this sort of characteristic are physically smaller than a pump with a
steadily falling curve, As a result, they may find favour in applications
where space is at a premium, and the two mentioned constraints do not
apply. Mine dewatering is a typical case.
Pump head and pressure
Pump engineers often interchange the words pressure and head when
discussing pump performance. Pump head is a constant property, while
pump pressure depends upon the liquid specific gravity,
Important as the performance curve is, other associated data is taken to
give a more complete picture of the pumps behaviour characteristics.
This data should be taken almost simultaneously with the curve data.Impeller fundametals
There are of course many nuances
to the geometry of a pump
impeller, most of which arc only of
real interest to the designer. To
the pump owner/user three
parameters need to be
understood,
9 Eye diameter, DI: In con-
junction with the pump speed, the eye diameter will largely Three main impeller design
decide how much flow the parameters
pump can accept, as well as the
minimum suction pressure requirements to avoid cavitation.
Increasing D 1 allows the impeller to accept more flow. Alternatively,
without increasing the flow, an increase in D1 [Up to a point]
reduces the minimum suction pressure requirements. When D1
approaches D2, there is too little radial space to install efficient
vanes. Such extreme designs might be associated with operation
'bad habits'
9 Impeller outlet diameter, D2: In conjunction with the pump speed,
the outlet diameter will largely dictate how much head the pump
generates. Increasing D2 will also increase the pump head/pressure,
and vice versa
9 Impeller outlet width, b2: In conjunction with pump speed, this will
decide how much flow the pump can discharge. In combination
with D2 this will also decide the slope of the Flow-Head curve and
the shape of the Flow-power curve. Generally enclosed impellers
where b2 is less than 2% of D2 are quite difficult to manufacture.
At this level it is sufficient to note, without further discussion, that the
geometry of the stationary casing components can also play a role in
pump performance.
However, the three items referred to above are the most critical in
terms of achieving, and maintaining performance. In that respect, it is
worth recording these dimensions whenever a pump is dismantled, and
verifying them if ever replacement spare parts are ordered.
Manufacturers often have a range of different impellers for the same
pump casing, and occasionally supply errors occur. These can easily be
avoided by keeping a record of these check dimensions. Other data
worth recording is the number of impeller vanes.Basic impeller
Designers call this basic impeller a single stage -single entry impeller.
This single impeller will have a certain characteristic curve, depending
upon its geometry, Performance of a pump with a single basic impeller. This basic impeller has only one
"eye", or entry annulus
Using this basic impeller as a benchmark, flow can be increased simply
by increasing D1, & b2. At some point, D1 approaches D2, with the
result that there is barely enough radial space for the vanes to do work
on the liquid.
Impellers that approach this limit often have 'bad habits' associated
with them. For example their band of useful operation might be quite
limited, and they are susceptible to hydraulically excited vibrationOf course a single impeller may be operated at an increased speed, and
this may result in more flow, see Appendix C. But in doing this, the
suction pressure requirements of the pump also increase. With a fixed
level of sitc suction pressure, there will be limits to which the speed can
be increased-even if the pump is capable of it.
In these cases it is necessary to conceptually combine two impellers so
that they operate in parallel. Such impellers are lmown as double entry
designs, because they have two 'eyes'. This concept is described next.
Impellers in parallel- more flow
In this case, 2 single stage impellers are arranged back to back, and in
parallel. This doubles the pump flow but the head stays the same. These
are known as double-entry single stage designs. Sometimes the pump
does actually use two separate impellers, but more usually they are
manufactured as a single entity,
Their main purpose is to increase the flow limitations of single entry
designs. A side benefit is that, at the same flow, double entry impellers
would normally require less suction pressure than impellers of single
entry because the flow per eye has been halved.