Fluid Mechanics for Chemical
Arif Hussain (Lecturer)
Pumping (Basic Terminology)
• In a pumping system, the objective is to
transfer a liquid from a source to a destination.
This may be filling a reservoir at a higher level
or circulating liquid as in a heating system.
• In either case a pressure is needed to make
this happen. This is generally referred to as
• Static Head
• Friction Head
It is the vertical distance that the liquid has
to be lifted in order to achieve useful work.
The friction head, which may be due to the materials of the
pipe, the size of the pipe, is basically increasing as a square
of the increase in flow.
The friction head, which is basically increasing
as a square of the increase in flow.
System Head/ System Curve
When we put these together, and we add a static head
and friction head, we end up with a system curve.
System curve is where we need to look if we are going
to save money and save energy with a pumping system
System Head/ System Curve
Where the static head is relatively high compared to the friction
head there is actually less capability of saving money but you are
still going to actually save some.
Pumps are used to force
a liquid to flow from a
point of low pressure to
one of higher pressure.
There are two general
classifications of pumps.
Centrifugal Pumps (Basic Pump Parts)
A typical centrifugal pump
has five basic parts
• Seal or Packing
• Visible part of the pump
• Other parts are enclosed within it
• Usually made of cast iron, steel, plastic etc
• In oilfields, casing on pump operating at a
pressure below 1000 kPa made of cast iron.
• Higher pressure operating pump generally will
have a casing made of steel.
• Causes liquid pressure to rise.
• Firmly attached to the
shaft, rotates inside the case at
the speed of the shaft.
• Most oilfield impellers made
up of cast iron.
• Closed vane develops higher
pressure but has a lower
• Open vane develops lower
pressure but has a higher
• The shaft rotates inside the case at the speed
of the driver.
• It usually made of steel.
• The portion of shaft exposed to the seal or
packing may have a sleeve made of hard
metal, such as tungston carbide, to resist
corrosion or wear at that point.
Bearings serve two functions on a pump:
• To hold the shaft so that it does not wobble
inside the pump casing.
• To prevent lateral movement of the shaft so
that the rotating parts do not touch the pump
Seal or Packing
• The seal or packing is used
to prevent liquid under
pressure inside the pump
from leaking out the pump.
• Mechanical seal is often
used in oilfields centrifugal
pumps which has two basic
• A stationary ring.
• A rotating ring.
Seal or Packing
Packing often is used in low
pressure service, or in pumps
handling abrasive liquids such as
mud or slurries.
Packing is composed of a series of
pliable rings contained in a
Mechanical seals generally requires
much less maintenance than
packing, so they are use
whenever possible. When they
are use liquid must be free of
sand, dirt or other solid particles
that can scratch the seal faces
and cause leakage.
• The pump shaft connects to the driver with a
PRINCIPLES OF CENTRIFUGAL PUMPS
• Liquid enters the pump at the centre or eye of
• Usually impellers rotates at a speed of 1200-3600
• The speed of the impeller creates a centrifugal
force that throws the liquid to the outer edge at a
• It leaves the impeller at high velocity and enters
the volute, which is enlarged chamber where the
velocity is quickly reduced. This velocity reduction
results in pressure increase.
PRINCIPLES OF CENTRIFUGAL PUMPS
•The amount of pressure an impeller will develops depends upon
its diameter and speed at which it rotates.
•The large diameter impeller operating at a higher speed will
develop a highest pressure.
•The pressure developed by the impeller is limited by the
materials of which the impeller is made.
•If a single impeller will not develop the pressure required, two or
more impellers can be installed in series to increase the pressure
rise across the pump. A pump with three impellers can be
compared with three pumps which operates in series.
•There is no theoretical limit to the number of impellers which
can be installed in a pump. However, horizontal pumps seldom
have more than eight impellers in one casing. If this is not enough
to produced a desire pressure, a second pump will be used.
Submersible pumps can have 50 or more impellers.
• The purpose of the pump is to raise the
pressure of the liquid.
• The amount of pressure rise is called the head
pressure or simply head.
• Head pressure = discharge – suction (pressure)
HOW CAN YOU DETERMINE
In an existing system, just read the suction
gauge then subtract the vapor pressure.
Is that simple
“It’s the pressure above vapor pressure”
For a new system being designed, you have to calculated.
Take the pressure in your suction vessel add the static
height of the liquid or subtract it in the case of lift subtract
the friction loss on the suction side and then subtract the
vapor pressure. Simple!
Cavitation and Vapor lock
• Cavitation and vapor lock are terms often
used interchangeably to describe the pump
failure due to presence of vapor in it.
Although cavitation and vapor lock both
occur when gas is present in a pump, they
each have different effects on the operation
of the pump.
• Cavitation occur when the liquid entering a
pump contains a few bubbles of gas. The gas
flows through the impeller with the liquid and
its pressure is increased in the pump, some or
all of the gas liquifies (the vapor bubbles
collapse.) A high centripetal force results from
this collapse and may cause severe vibration
and possible pump damage. The pump will
continue to pump liquid, but it will be noisy
and may vibrate.
• Vapor lock occur when gas enters the pump
with liquid and separates from the liquid
inside the pump and fills all or a part of the
pump. The pump will compress the gas a
slight amount, but not nearly enough for the
gas to flow out the discharge line. The trapped
gas prevents liquid from entering the pump.
The effect is that no liquid flows through the
• When a pump vapor locks, the discharge
pressure gauge reads about the same as
suction pressure while the pump is running. In
order to clear the condition, the vapor must
be removed from the pump. In some
cases, this can be done by opening the vent
valve while the pump is running. Quite
often, the pump must be shutdown and the
casing vented until liquid flows out the vent
• Some pumps are more prone to vapor lock than others.
A procedure for starting these pumps is
1. Close a valve in the discharge line.
2. Open valve in suction line.
3. Open casing vent valve until a steady stream of liquid
4. Start the pump and observe the discharge pressure. It
should rapidly increase and then level off.
5. Slowly open the valve in the discharge line.
6. Close the valve in the vent line.
Observe the discharge pressure during stpe-4. If it drops to suction
pressure, the pump has vapor lock again, and you will have to shut it
down and start over.
Cavitation and vapor lock occur when gas is present in the pump. A
few gas bubbles will cause cavitation. More will cause vapor lock. This
can be done by raising the suction pressure to the pump, or raising
the level of the liquid in the vessel that is being pumped.