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Pump systems, pipes, valves and seals
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16
Mechanical Technology — September 2015
The Pump Guy
M
any engineers, technicians,
pump users and even pump
company employees mis‑
understand the concept of
head. The pump industry is guilty of not
properly explaining this concept to the
pump users, while engineering textbooks
and professors of fluid mechanics are
guilty of leaving too many engineering
students with doubts regarding the exact
meaning of ‘head’. This is what happens
after industry downsizes: older engineers
are let go and mentoring as a teaching
tool is abolished.
Pump companies rate their pumps in
metres of head because the pump com‑
pany normally doesn’t know the liquid
that will move through the pumps. And
what the pump company calls ‘metres of
head’ is a measure of the specific energy
available to the pump, ie, the energy per
unit weight of fluid displaced. And this
In this pump guy article, Larry Bachus defends the statements: “the term head is the
constant for the pump manufacturer”; “a pump that generates 30 m of head can elevate
any liquid to 30 m above the surface level of the liquid’s source”; and “if I use the word
‘head’, the liquid’s density is not important”.
Total head and density
specific energy is, therefore, independent
of density. Let me explain this with an
example:
A pump manufacturer in Joburg sells
20 centrifugal pump models to their
distributor in Durban. The 20 pumps are
rated for the chemical process industry.
Durban has many applications for mid-
frame, back pullout chemical process
pumps.
The 20 pumps accommodate impel‑
lers up to 250 mm in diameter. The 20
pumps each have a shaft that measures
50 mm in diameter and bearings that can
handle the loads generated by the shaft
and impeller. The performance curve for
each pump is rated at 1 450 rpm and
indicates best efficiency at 78%, when
pumping 24 m at 125 m
3
/hr.
The sales manager at the pump
distributorship in Durban wants to give
good service to the customers. He put
these pumps on his shelf for
immediate delivery. The sales
reps are under instructions
to promote these pumps as
much as possible.
One sales rep sells three
pumps to the water bottling
plant in Durban to pump
potable water. Water has a
specific gravity of 1.0. The
pumps will elevate potable
water 24 m, or overcome
24 m of resistance in the
pipes. At best efficiency, the
differential pressure across the
pumps will be about 240 kPa
(24 m ×10×1.0=240 kPa).
The pump manufacturer in
Joburg doesn’t know three
pumps were sold to pump
water in Durban.
Another sales rep in
Durban sells two pumps to
a dairy to pump milk. Whole
milk has a specific gravity of
1.07. The pumps will elevate
milk 24 m, or overcome 24 m.
of resistance in the pipes. The
differential pressure across the
pumps at best efficiency will
be about 257 kPa (24 m ×10×1.07 =
257 kPa). The pump manufacturer in
Joburg doesn’t know two pumps are in
Durban moving milk.
Another sales rep sells three pumps
to the petrochemical plant in Durban
to pump sulphuric acid. Sulphuric acid
has a specific gravity of 2.0. The pumps
will elevate sulphuric acid 24 m, or
overcome 24 m of resistance in the
pipes. The differential pressure across
the pumps will be about 480 kPa
(24 m ×10×2.0=480 kPa).
With a specific gravity of 2.0, these
pumps will require a motor with twice
the power (kW) rating. The technician
will mate and align larger motors to these
pumps before shipping the pumps to the
customer. In Joburg, the pump manufac‑
turer might never know that three pumps
are moving sulphuric acid in Durban.
Another sales rep sells a pump to
the local paint factory to pump paint
thinner. The paint thinner has a specific
gravity of 0.87. The pump will elevate
the paint thinner 24 m, or overcome
24 m of resistance in the pipes. The
differential pressure across the pump
at best efficiency will be about 209 kPa
(24 m×10×0.87=209 kPa). The pump
manufacturer in Joburg will never know
this, unless there is a problem while the
pump is under warranty.
Let’s return to the dairy in Durban.
The pumps were bought to pump milk
and whole milk has a specific gravity of
1.07. However, skim milk, chocolate
milk, half and half, evaporated milk,
coffee cream and ice cream mixes all
have different specific gravities. If a pump
moves more than one liquid, the pres‑
sures and the motor’s power (to drive the
pump) will vary according to the specific
gravity of the liquid.
But 24 m of specific energy is always
24 m. And frequently, this is all the pump
manufacturer knows. So, the pump
manufacturer prints a curve that shows
metres of head.
I ask those of you in doubt to review
your university ‘Fluid mechanics’ text‑
book. Also, review some recent pump