Positive Displacement Vs Centrifugal Pump
Positive Displacement Vs Centrifugal Pump
Positive Displacement Vs Centrifugal Pump
begins to press the fluid resulting in going out of the pump with pressure
rise.
Install a foot valve in the suction piping to insure the liquid will not drain
from the pump casing and
suction piping when the pump
stops. Keep in mind that these
valves have a nasty habit of
leaking.
Evacuate the air in the system
with a positive displacement
priming
pump
operating
between the pump and a closed
discharge valve.
Fill the pump with liquid prior
to starting it.
Convert the application to a self
priming pump that maintains a
reservoir of liquid at its suction.
As the pressure developed is
related to the head by the
equation head
=
pr
/
sp.
When to use a
Positive Displacement Pump
When to use a centrifugal or a Positive Displacement pump (PD Pump) is not always a
clear choice. To make a good choice between these pump types it is important to understand
that these two types of pumps behave very differently.
VERSUS PRESSURE
VERSUS
NET
HEAD
VISCOSITY
POSITIVE SUCTION
REQUIREMENTS
Another
consideration is NPSHR. In
a
centrifugal the NPSHR
varies as a
function of flow, which is
determined by pressure and viscosity as discussed above. In a PD pump, NPSHR varies as a
function of flow which is determined by speed. The lower the speed of a PD pump, the lower
the NPSHR.
HIGH VISCOSITY
OPERATING
AWAY FROM
THE MIDDLE OF
THE CURVE
Centrifugals do not operate well when being run too far off the middle of
the curve. At best, this results in reduced efficiency which would require
larger motors and higher energy costs. At worst, this can result in cavitation
damage, shaft deflection, and premature pump failure. PD pumps on the
other hand can be run at any point on their curve without damaging the
pump or greatly affecting the efficiency.
VARIATIONS IN
PRESSURE
The first graph above clearly illustrates the effect that even modest changes
in pressure can have on the flow rate of a centrifugal pump. Additional
restrictions such as debris in a filter, corroded / rough piping, or a valve left
too far closed (or too far open) can have a dramatic effect on a centrifugal
pumps flow rate and efficiency. PD pumps maintain their flow rate and
efficiency even with significant changes in pressure.
VARIATIONS IN
VISCOSITY
HIGH
PRESSURES
While some centrifugals can be run in series to boost their pressures, none
can compete with PD pumps for high pressure applications. Pressure limits
will depend on the design of each pump, but pressures of 250 PSI (580 feet)
are not unusual for a PD pump, with some models going over 3,000 PSI
(7,000 feet). The capability for a PD pump to produce pressure is so great
that some type of system overpressure protection is required.
SHEAR
SENSITIVE
LIQUIDS
SUCTION LIFT
APPLICATIONS
By their nature, PD pumps create a vacuum on the inlet side, making them
capable of creating a suction lift. Standard ANSI centrifugals do not create a
vacuum and cannot create a suction lift. There are self-priming centrifugal
designs that can lift liquid an average of 15 feet when partially filled (13
hg vacuum). Many dry PD pumps can pull that or better and wetted PD
pumps (a pump that is not full of liquid but with some liquid in it) can often
reach vacuums of 25 to 28 hg. PD pumps are the logical choice when a
suction lift is required.
PROPERTIES
CENTRIFUGAL PUMP
POSITIVE DISPLACEMENT
PUMP
CAPACITY
Medium/high
low
Low/medium
High
100000 + GPM
10,000+ GPM
6000 psi
100,000+ psi
REQUIRE RELIEF
VALVE
No
Yes
FLOW TYPE
Variable
Constant
PRIMING
required
Self-primed
FLOW
CHARACTERISTIC
S
Smooth
Pulsating
SPACE
CONSIDERATION
INITIAL COST
Lower
Higher
MAINTAINANCE
COST
lower
Higher
ENERGY COSTS
higher
lower
LIQUID
RECOMENDATION
PRESSURE
(HEAD)
MAXIMUM FLOW
RATE
MAXIMUM
PRESSURE
HIGH VISCOSITY
OPERATING
AWAY FROM
THE MIDDLE OF
THE CURVE
VARIATIONS IN
PRESSURE
VARIATIONS IN
VISCOSITY
HIGH
PRESSURES
SHEAR
SENSITIVE
LIQUIDS
SUCTION LIFT
APPLICATIONS