POSITIVE DISPLACEMENT PUMP - is a mechanical device
designed to move fluid by trapping a fixed volume of the
fluid and then forcing it into a discharge pipe or outlet.
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fluid is moved by the action of its internal
components, such as pistons, diaphragms, gears,
lobes, or screws.
THE MAIN ADVANTAGES OF POSITIVE DISPLACEMENT
PUMPS INCLUDE:
Precise Flow Control - they offer accurate and consistent
flow rates, making them suitable for applications that
require precise metering or dosing of fluids.
Handling Viscous Fluids - positive displacement pumps
are well-suited for pumping viscous fluids or fluids
containing solids, as they can provide the necessary force
to move such substances.
Self-Priming - many positive displacement pumps are
capable of self-priming, meaning they can evacuate air
from the system and start pumping without the need for
external priming mechanisms.
Constant Flow - the flow rate is not affected by changes in
system pressure, making them suitable for systems with
varying pressure conditions.
Can Generate High Pressure - some positive displacement
pumps can generate high pressures, making them useful
for applications requiring high-pressure fluid delivery.
DISADVANTAGES OF POSITIVE DISPLACEMENT PUMPS:
Limited Flow Rates at High Pressure - positive
displacement pumps have limited flow rates at high
pressures compared to centrifugal pumps. As the pressure
increases, the flow rate tends to decrease, which can limit
their efficiency in certain high-pressure applications.
Sensitivity to Viscosity Changes - positive displacement
pumps can be sensitive to changes in fluid viscosity.
Variations in viscosity can affect the pump's ability to
deliver a consistent flow rate and can lead to inefficiencies
or reduced performance.
Slippage at High Speeds - at very high speeds, positive
displacement pumps can experience slippage, where a
portion of the fluid leaks back from the high-pressure side
to the low-pressure side due to the clearance between
moving parts. This can affect pump efficiency and output.
Complex Maintenance - positive displacement pumps
often have more complex maintenance requirements
compared to centrifugal pumps. They may require more
frequent inspections, component replacements, and
tighter tolerances to maintain optimal performance.
Potential for Seal and Packing Issues - positive
displacement pumps commonly use seals, gaskets, and
packing materials to prevent leakage. These components
can wear over time and need replacement, potentially
leading to maintenance challenges and fluid leakage if not
addressed promptly.
Higher Initial Cost - positive displacement pumps can
have a higher initial cost compared to some other pump
types. Their more intricate design, precision
manufacturing, and use of specialized materials can
contribute to higher upfront expenses.
Pulsating Flow - positive displacement pumps create a
pulsating flow due to their cyclical nature. This pulsation
can create pressure and flow fluctuations downstream,
which might require additional system design
considerations to mitigate.
Limited Use for Non-Newtonian Fluids - Non-Newtonian
fluids, which do not have a constant viscosity, can pose
challenges for positive displacement pumps. Their varying
viscosity can affect pump efficiency and cause flow
irregularities.
Energy Consumption - positive displacement pumps may
require higher energy consumption compared to some
other pump types, especially at high flow rates and
pressures.
Size and Weight - positive displacement pumps can be
bulkier and heavier compared to other pump types with
similar flow rates. This might be a consideration in
applications with space limitations.
Limited Self-Priming Ability - some positive displacement
pumps may have limited self-priming capabilities,
requiring careful attention to priming and initial setup.
TYPES OF POSITIVE DISPLACEMENT PUMP
1. Reciprocating Pumps - these pumps use a
reciprocating motion, usually driven by a piston or
diaphragm, to create pressure and move fluid.
Examples include piston pumps and diaphragm
pumps.
Piston Pumps - these pumps use a reciprocating piston
within a cylinder to create pressure and move fluid. They
can be single-acting (fluid is moved in one direction) or
double-acting (fluid is moved in both directions).
Diaphragm Pumps - diaphragm pumps use a flexible
diaphragm that moves back and forth, creating a vacuum
on one side and pressure on the other to move the fluid.
2. Rotary Pumps - rotary pumps use a rotating
mechanism to move fluid. Examples include gear
pumps, vane pumps, and screw pumps. Rotary
pumps are known for their smooth flow
characteristics.
Gear Pumps - gear pumps consist of two intermeshing
gears that trap and move fluid between the gear teeth
and the pump casing.
Lobe Pumps - lobe pumps consist of lobes (cogs) that
rotate and mesh together to create chambers that move
fluid from the inlet to the outlet.
Vane Pumps - vane pumps use sliding vanes that are
pushed in and out by an eccentric cam, creating chambers
that move fluid from the inlet to the outlet.
Screw Pumps – screw pumps utilize rotating screws or
helical rotors within a stator to trap and move fluid along
the screw threads.
3. Peristaltic Pumps - also known as hose pumps or
tube pumps, these pumps use rollers to compress
and release a flexible tube, creating a peristaltic
(wave-like) motion that propels the fluid.
4. Progressive Cavity Pumps - these pumps use a
helical rotor within a stator to create a series of
cavities that move fluid from the inlet to the
outlet. The fluid is moved progressively along the
length of the rotor