Predictive Maintenance by Condition Monitoring through Vibration Monitoring of Pumps. Mantenimiento preventivo de bombas por medio de Sistemas de Monitoreo de las Vibraciones

1. Vibration Monitoring
of Pumps
Applications
A pump is a machine that moves a liquid or
gas, or can propel a liquid or gas to a higher
level or pressure. Pumps are the second most
common machines in the world1 and are used in a
nearly all industries including food and beverage,
wastewater, pulp and paper, textiles, agriculture,
electronics, steel, oil and gas, chemical, and
metals.
Types
There are six basic types of pumps: direct lift,
displacement, velocity, buoyancy, impulse and
gravity. This white paper will focus on one type
of velocity pump, a centrifugal pump.
A centrifugal pump functions with a rotating
element, called an impeller, that adds energy to
the flow of a liquid through rotation, thereby
increasing the liquid’s velocity and pressure.
Causes of Failure
Common causes of pump failure are misalignment,
imbalance, contamination or improper lubrication
or running conditions.
Failures
Excessive vibration seen on the volute (the
casing that surrounds the impeller) could indicate
an imbalance or misalignment of the impeller
which may reduce the pump’s efficiency and/or
capacity. Also, excessive vibration, along with
crackling and popping noises, coming from the
volute could indicate cavitation. Cavitation occurs
when the pressure of the liquid in the pump drops
below a threshold and causes the liquid to vaporize
creating tiny bubbles that, when they pop, throw
tiny, destructive jets of water onto the impeller.
Not only can cavitation be an issue for the impeller,
but it can be destructive to bearings as well.

2. bearing failure. For horizontally mounted
centrifugal pumps the sensor should be installed
perpendicular to the shaft on the bearing case.
On a vertically mounted pump two sensors
should be installed 90 degrees from each other
and perpendicular to the shaft on the bearing
case. For axial measurements for a vertically
mounted pump, a sensor can be installed on a
location near the pump casing.
Analysis
Figure 1: Cavitation damage on an impeller.
Cavitation is not only a problem in and of itself, but
is also an incation of poor pump performance. If
cavitation is not caught, it can shorten the pump’s
life, increase other maintenance requirements, and
threaten the pump’s reliability. (Photo source:
“Solving a Cavitation Problem,” http://jacpump.
wordpress.com/2011/04/17/solving-a-cavitationproblem/)
Vibration monitoring of the stuffing box can
detect seal lubricants changing from a liquid
state to a gas or solid which can lead to seal
failure. This is especially a potential problem
in hot water applications.
What Should Be Measured
Centrifugal pumps have multiple areas
of interest for monitoring in a predictive
maintenance program. The first area of
interest for monitoring is the volute.
The second area of interest is the stuffing
box/seal area which is the joint that prevents
fluid in the pump from coming out of the
pump between the volute and pump shaft.
The third area of interest is the bearing
case, which can be monitored to detect
Centrifugal pumps vibrate at multiples of
the motor running speed and of the blade pass
frequency (BPF) which is defined as the number
of blades multiplied by the pump’s running speed
in hertz. When a gearbox is present between the
motor and pump, the BPF is the running speed
times the gear ratio times the number of blades.
In general, the maximum vibration levels occur at
the BPF. Typical centrifugal pumps have running
speeds of 1,500-1,800 RPM (4-pole motor) or
3,000-3,600 RPM (2-pole motor).
Savings
Efficiencies around 80 percent are possible
for centrifugal pumps, but studies show that the
average pump is running at an efficiency below 40
percent, and more than ten percent of pumps run
below ten percent efficiency. While the reasons
vary, it is clear that there is dramatic room for
improvement with enhanced awareness of the pump
systems. Unexpected failures can be disruptive
and sometimes catastrophic. Take for example
a fire that broke out at an oil refinery. The fire
forced a three-day shut down of the refinery and
cost $1.5 million in damages and lost production.
It was later discovered that the cause of the fire
was a pump that was being run inefficiently. Since
the fire, the refinery has installed a monitoring
system and two years after installing the system,
the refineries pump system has yet to require
unplanned maintenance.1
KCF Technologies
336 South Fraser Street
State College, PA 16801
Phone: +1 814-867-4097
E-mail: sales@kcftech.com
www.kcftech.com

3. Figure 2: Diagram of a centrifugal pump.
Additional Things You May Want to Know
The motor bearing frequencies are calculated based on the running speed and the bearing geometry.
For rolling element bearings, the following formulas are used:
u Pass Frequency Outer Race (BPFO) = Nb/2 x S x (1 – (Bd/Pd * cos (th))
Ball
u Ball Pass Frequency Inner Race (BPFI) = Nb/2 x S x (1 + (Bd/Pd * cos (th))
u Fundamental Train Frequency (FTF) = S/2 x (1 – (Bd/Pd x cos (th))
u Ball Spin Frequency (BSF) = Pd/2Bd x S x (1 – (Bd/Pd * cos (th))
Where:
u
u
u
u
u
Nb = number of rolling elements
S = speed (revolutions per second, in Hz)
Bd = ball diameter
Pd = pitch diameter
th = contact angle (degrees)
The following guidelines can be used as a quick reference:
u
u
u
u
Ball Pass Frequency Outer Race (BPFO) = Nb x S x 0.4
Ball Pass Frequency Inner Race (BPFI) = Nb x S x 0.6
Fundamental Train Frequency (FTF) = S x 0.4
Ball Spin Frequency (BSF) = S x 1.6
KCF Technologies
336 South Fraser Street
State College, PA 16801
Phone: +1 814-867-4097
E-mail: sales@kcftech.com
www.kcftech.com

4. For example, a centrifugal pump with 8 blades operating at 1,800 RPM with 19 rolling elements in
each bearing will have the following frequencies:
Fundamental Train Frequency (FTF)
1X Motor Speed
Ball Spin Frequency (BSF)
2X Motor Speed
Ball Pass Frequency Outer Race (BPFO)
Blade Pass Frequency (FPF)
Ball Pass Frequency Outer Race (BPFI)
12Hz
30Hz
48Hz
60Hz
228Hz
240Hz
342Hz
The vibration spectrum will display a peak at each frequency noted above. The actual frequency will
be slightly lower as the speed slows under load with motor slip of a few percent. The analysis should be
performed over a frequency band accommodated this range of speed variation.
For variable-speed pumps, the motor speed will vary over a range of speeds based on the required
pressure or flow rate, or peak efficiency.
Each peak is analyzed for a trend in amplitude against pre-set warning and alarm levels. Specific
information may be available from individual manufacturers or through operational specifications. The
following guidelines provide a useful start for analyzing the FFT max amplitudes in each band for a pump:
General (rotation and bearing locations including motor speed, FTF, BSF, BPFO, BPFI)
Level
Warning
Alarm
Shutdown
Amplitude (max)
0.10 in/s
0.25 in/s
0.62 in/s
Kernan, Daniel. “Pumps 101: Operation, Maintenance, and Monitoring Basics.” gouldspumps.com. ITT Corporation, n.d. Web. 22 June 2012.
1
KCF Technologies
336 South Fraser Street
State College, PA 16801
Phone: +1 814-867-4097
E-mail: sales@kcftech.com
www.kcftech.com