Understanding Shaft Alignment: Thermal Growth
VibrAlign,
Inc.
Posted 9-3-03
Part two of a four-part series that will cover alignment
fundamentals and thermal growth, and highlight the importance
of field measurements through two case studies.
Machine conditions change from the time the machine is off
line to when it is running under normal operating conditions.
Some of these changes are due to process forces (e.g., fluid
pressures, airflow, etc.). The most notable of these changes
is the change in the temperature of the machine bearings and
supports. This is called the machine's thermal growth.
Thermal growth is the change in the length of a particular
metal as a result of the change in temperature of that metal.
Typically, when a metal bar is heated, it will get longer.
These changes can be very small (0.0005 in.) or they can be
very large, depending on the length of the piece of metal and
its coefficient of linear expansion.
Formula for thermal growth
The formula used for this calculation is often referred to
as the T x L x C formula. T represents the change in the material's
temperature in degrees Fahrenheit, L represents the length
in inches of the material, and C represents the material's
coefficient of linear expansion. Different materials have different
C values. Using the formula, we can anticipate the change in
a machine's shaft alignment based on the expected changes in
machine temperature. Fig. 1 is a chart
of the most common machine materials and their C values.
Consider the following example: A motor with a starting temperature
of 70 F is perfectly aligned to the pump shaft it will be driving.
For this exercise, the temperature of the pump will not change;
however, the temperature of the motor will increase to 120
F under normal operating conditions. The motor end bell's material
is cast iron with a C value of 0.0000059. The distance from
the bottom of the motor feet to the center of the shaft is
15 in. We now can calculate the change in position of the motor
from off line to running by multiplying the T, L, and C values.
T x L x C = growth (120 F - 70 F) x 15 in. x 0.0000059 = 0.0044
in.
Based on this information, the motor will grow 0.0044 in.
or 4.4 mils. If the growth of the motor is the same for both
ends, the result will be a change in the offset alignment of
4.4 mils but the angular alignment will not change. This motor
shaft should be aligned 4.4 mils lower than the pump shaft
which will allow the machine to grow into an aligned condition.
Temperature changes unequally
That was a fairly simple example and does not accurately
reflect what will happen to an actual machine. In reality,
the temperatures of all the machine supports will change; however,
they will almost never change equally.
Using the above machine example, consider the change in shaft
alignment if the outboard end (OE) bearing temperature changed
by 20 F and the drive end (DE) bearing temperature changed
by 50 F. The drive end bearing would grow by 4.4 mils; however,
the outboard bearing would grow only by 1.8 mils. The result
will be a change in both the offset and angular alignment.
If the motor feet are 20 in. apart, the change in the angular
alignment will be 0.13 mil/in. [(4.4 - 1.8)/20 = 0.13] open
at the top of the coupling. Changes in the temperature of machines
from off line to running can have a significant impact on the
shaft alignment.
These changes in the shaft alignment can be accommodated
in a few different ways. One way is to align the machines to
zero and then remove or add the amount of shim under the machine
feet as determined by the temperature data. Another way is
to gather the alignment data, graph the results, and predetermine
the actual shim corrections based on the graph.
With today's modern laser alignment technology, accounting
for thermal changes at the machine feet is actually a simple
evolution. Most alignment systems on the market today have
within them a function that allows the user to program the
foot targets of the machine being aligned. For the previous
example, the front foot target would be -4.4 mils and the back
foot target would be -1.8 mils. After programming the determined
foot target values at the machine feet, the user aligns the
machines to zero on the display unit. The shaft alignment system
will automatically calculate the required foot corrections
to leave the feet at the prescribed positions. As the machine
heats up, the shaft centerlines will grow into a properly aligned
condition.
Gearboxes are difficult
Thermal changes in gearboxes can be especially difficult
to calculate. Often the input shaft temperatures will be different
from the output shaft temperatures. This causes the gearbox
shaft alignments to change in the horizontal plane as well
as the vertical plane.
Force-lubricated systems with an oil cooler also can have
an effect on the final alignment condition of a machine. Higher
oil temperatures out of the cooler will result in a hotter
operating condition of the machine, therefore creating a more
drastic change in the running alignment condition. A 10 F change
in the operating temperature of a turbine from 105 F to 115
F can change the foot positions as much as 2-4 mils. The alignment
condition of turbines and compressors that operate at very
high speeds can be adversely affected by these relatively small
temperature changes.
Pipe strain
Another condition that changes is the increase or decrease
in temperatures of the suction and discharge piping attached
to pumps and compressors. Some compressors may actually form
ice on the suction end while the discharge piping is too hot
to touch. Conditions such as these can force major changes
in the operational alignment condition of machines.
While original equipment manufacturers might be able to anticipate
the nominal changes in operating temperatures of a piece of
equipment, they cannot accurately anticipate the effects of
the piping configurations of the final machine installation
or the changes in the temperature of the piping runs. Piping
runs are typically very long and can have a tremendous impact
on the change in the shaft alignment from off line to running
condition. In addition, piping connections act as fixed (or
restraining) points with respect to the tendency of machines
to move/grow when on line. The effect of these fixed points
on the final position of the machines is almost impossible
to calculate or predict.
Depending on the piping configuration, these changes may
be in the vertical plane or in the horizontal plane and are
extremely difficult or impossible to accurately calculate based
on the TLC formula above.
Consider two identical boiler feed pumps (BFP) as shown in Fig.
2. BFP #1 feeds boiler #1 which is 20 ft away and BFP
#2 feeds boiler #2 which is 60 ft away. The length of the
discharge piping on BFP #2 will be approximately three times
longer than that on BFP #1. This will result in the two "identical" machines
showing drastically different alignment changes from off
line to running. A great deal of care must be taken when
calculating the changes in the alignment condition of these
machines. Just because two machines appear identical and
serve the same function does not ensure they will exhibit
the same operational characteristics.
Determining alignment changes
In the past, there have been several methods used to attempt
to measure the changes in the shaft alignment of two or more
machines. One of these methods involves measuring the changes
in machine temperatures at each machine support and performing
the target alignment based on mathematical calculations.
Another method relies on tooling balls mounted on machine
bearings. Typically an optical transit (scope) is used to measure
the off line positions of the tooling balls. Once the machine
is at operating conditions, another set of measurements is
made; the positional changes are compared to the "stationary" tooling
balls. These changes are triangulated to calculate the change
in the position of the shafts.
There is a variant to the above technique, the Acculign method,
which involves installing tooling balls in the foundation as
well as at the bearings. The distance between the fixed tooling
balls (mounted in the foundation) and the bearing-mounted tooling
balls is measured off line and then on line. Precise measurements
of the distances and angles are required to make the calculations
of the growth.
Doing hot alignment checks
Another way to gather this data is to perform a hot alignment
check of the affected piece of equipment. The procedure for
this is relatively simple. The machine is aligned off line
and the results of the alignment are documented (horizontal
angularity, horizontal offset, vertical angularity, and vertical
offset). The machine then is placed on line and allowed to
reach normal operating conditions. At this point, the machine
is shut down and allowed to stop rotating.
The alignment system is remounted on the machine and the
shaft alignment is remeasured and documented. Now the machine
may be aligned hot by reshimming and repositioning the moveable
machine as quickly as possible. One drawback of this method
is that the machine will begin to cool as soon as it is shut
down, adversely affecting the accuracy of the hot alignment
check.
If the two sets of alignment readings were documented, a
set of cold alignment targets can be calculated. Alignment
results (hot) - alignment results (cold) represents the change
in the alignment condition of the machine from cold to hot.
The alignment targets for this machine will be the opposite
of the changes in the alignment parameters.
While this is currently a widely used method of hot-aligning
machines, it will measure only the changes in the shaft alignment
due solely to the changes in the machine's temperatures. Discharge
pressure, shaft torque, multiple machines operating in parallel,
electrical loading of a generator, etc., also can play a large
role in the change in the alignment condition from off line
to running. These changes most often will be seen in the horizontal
plane, but could affect the vertical alignment as well.
Yet another factor to consider is the location of the machine.
If a machine is located indoors in a controlled environment,
the operating characteristics should be relatively constant
throughout the year. A machine that operates outdoors and is
exposed to large changes in temperature also could exhibit
extreme changes in its shaft alignment as the temperature changes
(as in the change of seasons).
On line positional change measurements
One method used to measure the change in the alignment of
two pieces of machinery is to document their bearing cap positions
in both the horizontal and vertical planes relative to some
fixed points in space while the unit is off line. After the
data has been documented, the machine is started and placed
on line. When the machine has reached its normal operating
temperature, the positions of the bearing caps are measured
again and compared to the points that are stationary. The movement
of the machines and the changes in the shaft alignment then
can be either calculated or graphed.
In the past, there have been problems obtaining on line readings
using this method. A nominal amount of vibration can make an
optical scale very hard to read through a transit or theodolite.
Care must be taken that the scale is placed back in the exact
location for each measurement at each point. Bearing caps are
not typically precision machined on the outside surfaces. A
very small deviation in the position of the detector can lead
to a very large error if the surface that is being measured
is not flat and smooth.
Modern laser-based measurement systems designed to measure
flatness and surface parallel also can be used in this manner.
One benefit of the laser-based positional measurement systems
is that the data can be averaged, eliminating the potentially
large errors when measuring machines that are running. When
the laser beam strikes a vibrating detector surface the data
will appear to bounce slightly. A simple function in the display
unit will sample the data for the desired amount of time, locate
the maximum and minimum values on the detector, and average
the data accordingly. Since vibration, by definition, is cyclic
and repeatable, very good results can be obtained.
Laser measurement systems
In the 1980s, a laser-based system became available that
mounted to the drive end bearings of a machine to monitor the
changes in the machine's alignment from cold to hot or from
hot to cold. Two laser transmitter/detectors are mounted on
the stationary machine drive end bearing. One of these transmitter/detectors
must be positioned in the 12 o'clock position (to monitor vertical
changes) and the other must be positioned in the 3 o'clock
position (to monitor horizontal changes). The transmitter/detectors
are positioned coaxially with the stationary shaft centerline
and level. Corresponding prisms are mounted on the moveable
machine drive end bearing. They are positioned to reflect the
laser beam back to the detectors mounted on the stationary
machine.
The transmitter/detectors are hooked up to a computer running
the measurement software. The user now can program the alignment
monitoring equations into the software and have the system
monitor all four alignment parameters simultaneously. The values
are auto-zeroed and the data collection begins. When the machine
is started and the alignment changes, it is recorded in the
system software. When the machine reaches normal operation,
the data collection is stopped and the alignment changes calculated.
The results are displayed as a graphical trend.
The cold alignment targets will be opposite of the measured
change in the machine alignment if the data collection was
started when the machine was cold. If the cool down was monitored,
the targets are equal to the values displayed in the software.
While this system can be very effective for diagnosing alignment
problems, it also can be very time consuming and frustrating
to set up and monitor. Any change in the bracket position during
the data collection will introduce errors into the results.
This system also requires the user to purchase a PC to use
for the data collection.
Contributors to this article include Rich Henry, Ron
Sullivan, John Walden, and Dave Zdrojewski, all of VibrAlign,
Inc., 530G Southlake Blvd., Richmond, VA 23236; (804)
379-2250; e-mail info@vibralign.com.
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