Fire Pump
Theory-2
Centrifugal Pumps
Information and slides provided from RFD
Objectives


Define Centrifugal pump
Discuss the three basic
characteristics of the centrifugal
pump:



Pump speed
Pump discharge (GPM)
Pump pressure
Objectives





Find the number of pump stages on
your pumper?
Define Parallel
Define Volume
Define Pressure or Series
Define “change over”
Objectives

Given a type 1 pumper:




Find the pump performance plate.
Determine the number of pump stages.
Find the transmission/impeller gear
ratio.
Find the primer and main fire pump.
Centrifugal Pumps

Non-positive
displacement pump


Does not pump a definite
amount of water with each
rotation.
Impeller imparts velocity
to the water and converts
that velocity to pressure
within the pump.
Nearly all modern fire apparatus
use centrifugal pumps as the
main fire pump.
Pump Performance (continued):

Centrifugal pumps depend on the
velocity of water to move water
through the pump.


Cannot pump air- not self-priming
Generally require a positive
displacement pump to prime or draft

Positive displacement pump removes air
from the centrifugal pump. Atmospheric
pressure pushes water into the
centrifugal pump.
Principles of Operations

Centrifugal force:

the tendency of a body to move away
from the center when rotating in a
circular motion.
The impeller acts as a rotating disk.
 Confining the water creates pressure.

Pump Performance and PDP
New GPM = New Pump Speed x Old GPM
Old Pump Speed
Pump Performance and PDP

There are three factors that
influence a centrifugal pump’s
discharge pressure:



Amount of water being discharged
Speed at which the impeller is turning
Pressure of water coming into the
pump;

a hydrant, tandem, relay or tender
operation.
Three factors of PDP (continued):


The greater amount of water being
flowed, the lower the discharge
pressure.
For a given impeller speed:
If a 2 1/2’ discharge outlet is fully open
the pressure build up is low.
 If the same discharge outlet is partially
closed the pressure build up is higher.

New GPM = New Pump Speed x Old GPM
Old Pump Speed
Three factors of PDP (continued):


The greater the speed of the impeller, the
greater the pressure developed.
Given discharge remains the same.
 Double the speed of the impeller and the
pressure is increased four times if flow is kept
constant.
 Impellers rotate between 2000 and 4000
rpm.
 Check the pump gear ratio on your
apparatus.
(
)
New PSI = New Pump Speed 2
Old Pump Speed
x Old PSI
Three factors of PDP (continued):

Intake pressure influences PDP

No mechanical blockage in a centrifugal
pump between intake and discharge:



100% slippage
Water will flow through the pump even if the
impeller is not turning.
Any movement by the impeller increases
water velocity and pressure buildup within
the volute.
Intake pressure and PDP
(continued):

intake pressure:
Adds directly to the pump discharge
pressure.
 is reflected in the discharge pressure.

Rated Capacity - Performanc

Rated pump capacity:

The flow of water at specific pressures a
pump is expected to provide.
Rated Capacity (continued):

Pump performance plate indicates:


rated capacity at draft
single or two stage pump
Classroom Drill

Question 1


Given that a pump is discharging 500 gpm at 100
psi with a pump speed of 1000 rpm.
What is the new gpm if the pressure stays the
same (100 psi), but pump speed is increased to
1500 rpm?
New GPM = New Pump Speed x Old GPM
Old Pump Speed
Classroom Drill

Answer to Question 1
New GPM = New Pump Speed x Old GPM
Old Pump Speed
New GPM = 1500 RPM x 500 GPM
1000 RPM
= 1.5 x 500 GPM
= 750 GPM
Classroom Drill

Question 2


Given that a pump is discharging 500 gpm at
a pressure of 100 psi while turning at a
speed of 1000 rpm.
At what new pressure will the pump
discharge water if quantity remains 500 gpm
and pump speed is increased to 1500 rpm ?
(
)
New PSI = New Pump Speed 2 x Old PSI
Old Pump Speed
Classroom Drill

Answer to Question 2
New PSI = New Pump Speed 2 x Old PSI
Old Pump Speed
(
)
New PSI = 1500RPM 2 x 100 PSI
1000 RPM
(
2
(1.5)
= 225 PSI
)
x 100
Single Stage Centrifugal Pumps

Single impeller


Used in front mount pumps, power takeoff, skid mount pumps, and midship
mount.
High capacity pumps require double
suction impellers.

Reduce lateral thrust of water entering the
eye.
Two-Stage Centrifugal Pumps

Two impellers mounted in a single
housing.


Mounted on a single shaft and driven by
a single driveshaft.
Both impellers are identical.
Two-Stage Centrifugal Pumps
(continued):

Versatility and efficiency of the two
stage pump is based on:


Connecting two stages in series for
maximum pressure output.
Connecting two stages in parallel for
maximum volume output.

A transfer valve moves the pump from series
to parallel positions.
Pumping in Volume (Parallel)

Each impeller takes
water from a source
and delivers it to the
discharge.


Each impeller delivers its
own rated capacity.
Total pump capacity is
the sum of both
impellers.
If a pump is rated at 1000gpm at 150 psi NPDP,
then each impeller would provide 500 gpm at
150 psi NPDP.
Pumping in Volume (Parallel)
(continued):

Remember:


Pumps are rated at 100% capacity at 150 psi NPDP.
Increasing pressure above 150 psi NPDP in
volume will reduce the output of the pump.
Pumping in Pressure (Series)

Water from the intake manifold is
directed into the eye of the first
impeller.

First impeller increases pressure and
discharges 50 to 70 % of the volume
capacity through the transfer valve and into
the eye of the second impeller.
Change over

Historic rule of thumb:


Modern two stage pumps:


Pump in pressure until more than half
the rated capacity is required.
Pump in pressure until two-thirds rated
capacity is required.
Always consult the manufacture.
Rule of thumb: max of 75 psi NPDP
for any changeover.
Summary


Define Centrifugal pump
Discuss the three basic
characteristics of the centrifugal
pump:



Pump speed
Pump discharge (GPM)
Pump pressure
Summary (continued):




Determine the number stages on
your pumper?
Define Parallel or Volume
Define Pressure or Series
What is a change over and when
should we do one?
Fire Pump Theory2
Written and Class Drill
Fire Pump Theory-2
Question # 1

Which of the following is true about
centrifugal pumps:


A. Impellers impart pressure to the
water and convert that pressure to
velocity within the pump.
B. They Do not pump a definite amount
of water with each rotation.
Fire Pump Theory-2
Question # 2

There are three factors that influence a
centrifugal pump’s discharge pressurewhich is not a factor:




A.
B.
C.
D.
Amount of water being discharged
Speed at which the impeller is turning
Pressure of water coming into the pump
Positive displacement pump
Fire Pump Theory-2
Question # 3


A pump is discharging 150 gpm at 170 psi
with a pump speed of 1200 rpm.
What is the new gpm if the pressure stays
the same (170 psi), but pump speed is
increased to 1500 rpm?
New GPM = New Pump Speed x Old GPM
Old Pump Speed
Fire Pump Theory-2
Question # 4

Where is the transfer valve setting in a
two stage pump when each impeller
takes water from a source and delivers
it to the discharge?





A. Volume
B. Parallel
C. Series
D.Pressure
NOTE: there are two correct answers.
Fire Pump Theory-2
Question # 5

Where is the transfer valve setting
in this two stage pump. All water
from the intake manifold is
directed into the eye of the first
impeller.





A. Volume
B. Parallel
C. Series
D.Pressure
NOTE: there are two correct answers.
Fire Pump Theory-2
Question # 1

Which of the following is true about
centrifugal pumps:


A. Impellers impart pressure to the
water and convert that pressure to
velocity within the pump.
B. They Do not pump a definite amount
of water with each rotation.
Fire Pump Theory-2
Question # 2

There are three factors that influence a
centrifugal pump’s discharge pressurewhich is not a factor:




A.
B.
C.
D.
Amount of water being discharged
Speed at which the impeller is turning
Pressure of water coming into the pump
Positive displacement pump
Fire Pump Theory-2
Question # 3


A pump is discharging 150 gpm at 170 psi
with a pump speed of 1200 rpm.
What is the new gpm if the pressure stays
the same (170 psi), but pump speed is
increased to 1500 rpm?
New GPM = New Pump Speed x Old GPM
Old Pump Speed
New GPM = 1500 x 150
1200
GPM = 187.5
Fire Pump Theory-2
Question # 4

Where is the transfer valve setting in a
two stage pump when each impeller
takes water from a source and delivers
it to the discharge?





A. Volume
B. Parallel
C. Series
D.Pressure
NOTE: there are two correct answers.
Fire Pump Theory-2
Question # 5

Where is the transfer valve setting
in this two stage pump. All water
from the intake manifold is
directed into the eye of the first
impeller.





A. Volume
B. Parallel
C. Series
D.Pressure
NOTE: there are two correct answers.
Fire Pump Theory-2
Class Drill

For your apparatus:




Find pump specification/performance
plate.
Determine the number of pump stages.
Find transmission/impeller gear ratio.
Find the primer, and main fire pump.