SUBJECT : Centrifugal pump vibration
It 's necessary to be concerned about vibration because it has a
major affect on the performance of your pump. At least six components
are seriously affected by vibration :
- The life of the mechanical seal is directly related to shaft
movement. Vibration can cause carbon face chipping and seal face
opening. Drive lugs will wear, and metal bellows seals will
fatigue. In some instances, the shaft movement can cause the
rotating seal components to contact the inside of the stuffing
box, or some other stationary object, causing the seal faces to
open and allowing solids to penetrate between the lapped faces.
Vibration is also a major cause of set screws becoming loose and
slipping on the shaft, causing the lapped seal faces to
- Packing is sensitive to radial movement of the shaft. You'll
not only experience excessive leakage, but excessive sleeve or
shaft wear also. Additional flushing will be required to
compensate for the heat that'll be generated by the high friction
- Bearings are designed to handle both a radial and axial load.
They were not designed for the vibration that can cause a
brinneling (denting) of the bearing races.
- Critical dimensions and tolerances such as wear ring clearance
and impeller setting will be affected by vibration. Bearing
internal clearances are measured in tenths of thousands of an
inch. (thousands of a millimeter)
- Pump components can be damaged by vibration. Wear rings,
bushings and impellers are three examples.
- Bearing seals are very sensitive to shaft radial movement.
Shaft damage will increase and the seals will fail prematurely.
Labyrinth seals operate with a very close tolerance. Excessive
movement can damage these tolerances also.
- Pump and motor hold down bolts can become loose.
The vibration comes from a number of sources that include :
causes of vibration
- Unbalanced rotating components. Damaged impellers and non
concentric shaft sleeves are common.
- A bent or warped shaft.
- Pump and driver misalignment.
- Pipe strain. Either by design or as a result of thermal
- The mass of the pump base is too small.
- Thermal growth of various components, especially shafts.
- Rubbing parts.
- Worn or loose bearings.
- Loose hold down bolts.
- Loose parts.
- Product attaching to a rotating component.
- Damaged parts.
causes of vibration
- Operating off of the pump's best efficiency point (BEP)
- Vaporization of the product
- Impeller vane running too close to the pump cutwater.
- Internal recirculation.
- Air getting into the system through vortexing etc..
- Turbulence in the system ( non-laminar flow).
- Water hammer.
causes of vibration.
- Harmonic vibration from nearby equipment.
- Operating the pump at a critical speed. Watch out for this
problem in variable speed and pulley driven pumps.
- Seal "slip stick" at the seal faces. This can occur if you are
pumping a non-lubricating fluid, a gas or a dry solid.
- A pump discharge recirculation line aimed at the seal
can read the vibration a variety of ways:
- Spike Energy
- Acoustic emissions
Many systems read vibration by recording acceleration. The problem
with this method is that if you do not know the frequency the
readings are not very meaningful. Because of this most systems read
an average of all of the frequencies involved and recommend taking
action when this average reading doubles in a particular location. If
bearings are your primary concern high and low electronic filters can
be used in some equipment to filter out frequencies below 55 Hz. and
above 2500 Hz. These filters will help the operator zero in on those
frequencies normally associated with bearing problems.
Unfortunately, most vibration data references bearing operation.
There is little to no information available about mechanical seal
vibration modes. The problem is further compounded by:
- The large variety of seal materials in use.
- Major differences, in design between popular brands of single
and multiple seals.
- Availability of vibration damping in these seal designs.
- The wide spread use of environmental controls.
- The variety of fluids surrounding the seal
The vibration readings almost always means that the equipment has
started to destroy its self. Most companies are trying to collect
enough data to predict the remaining life before total destruction
The obvious solution to all of this is to adopt good maintenance
practices that 'll eliminate most of the vibration and then try to
install hardware that can live with the vibration you have left.
Recording vibration makes sense only after good maintenance practices
are in force.
- Balance all of your rotating equipment. If you do not have
dynamic balancing equipment in your plant there are contractors
and vendors anxious to work with you. Balance is always a problem
when you're pumping abrasives, or a slurry, because the rapid wear
always destroys balance. In the higher speed pumps this wear can
be very severe.
- Bent shafts are a problem. If you can straighten them go ahead
and do it, but most attempts are unsuccessful. In the majority of
cases you are better off replacing the shaft.
- Do a proper pump/ driver alignment using either a laser or the
reverse indicator method. Upgrading the pump power end to a "C" or
"D" frame motor adapter is a more sensible and economical
decision. Once the conversion is made misalignment ceases to be a
concern. These adapters are available for most motors and will
maintain the proper alignment as the equipment goes through its
normal temperature transients.
- Always pipe from the pump suction to the pipe rack, never the
other way. There are some more piping practices that you should
- If you are experiencing pipe strain because of thermal
growth at the suction, you might convert to a "centerline"
design wet end and solve the problem. Center line designs make
sense any time you are pumping a fluid in excess of 200 degrees
Fahrenheit (100 Centigrade)
- Try to use at least ten diameters of pipe between the pump
suction and the first elbow.
- Pipe supports and hangers should be installed at unequal
- Use lots of hangers to support the piping.
- Use lots of loops and expansion joints in the piping
- After fabrication and testing, remove all supports and lock
pins from the spring hangers, loosen pipe flanges and adjust
the system to free the pump from pipe strain.
- Reference the "Hydraulic Institute Manual", or a similar
publication to learn the proper methods of piping several pumps
from the same suction source to prevent vortexing etc..
- The mass of the pump concrete foundation should be five times
the mass of the pump, base plate and other equipment being
- The foundation should be three inches ( 75 mm) wider than the
base plate, all around, up to 500 horsepower (375 KW) and six
inches (150 mm) above 500 horse power (375 KW).
- Imaginary lines, extended downward 30 degrees to either side
of a vertical through the pump shaft, should pass through the
bottom of the foundation and not the sides.
- Every inch of stainless steel grows 0.001 inch for every
100° Fahrenheit rise in temperature (0,001 mm/mm/50°
Centigrade) This thermal growth can cause the impeller to rub the
pump casing as well as cause rubbing in many close tolerance
clearances such as the wear rings. Carbon steel grows about 30%
less than stainless steel.
- Any time the shaft moves there is the danger of parts rubbing.
Thermal imaging equipment can detect this rubbing easily. When
ever you set tight tolerances be sure to allow for thermal growth
and, in the case of A.N.S.I. pumps, impeller adjustment.
- Worn or loose bearings are caused by improper installation or
allowing water to enter the bearing cavity. Labyrinth seals or
positive face seals are the easiest solution to the water problem.
Install bearings by using a proper induction heater to prevent
contamination during the installation process.
- The answer to loose hold down bolts is obvious and requires no
- You may be able to increase or reduce the impeller diameter to
get close to the pump B.E.P., but if this is not practical your
best bet is to reduce the L3/D4 by going to
a solid shaft or upgrading the power end to a larger shaft
diameter. In some instances you can install a support bushing in
the bottom of the packing stuffing box and install a mechanical
seal closer to the bearings. Split seals are ideal for this
conversion. In a few instances, changing the shaft speed will
solve the problem. A closed loop system with a high system head is
an ideal candidate for a variable speed pump
- Insure that you have enough NPSH for your application. If
there isn't enough, an inducer or booster pump might solve the
- An impeller, running too close to the pump cutwater will cause
vibration and damage. An impeller tip to cutwater clearance of 4%
(of the impeller diameter) in the smaller impeller sizes (to 14
inch/355 mm) and 6% in the larger sizes will solve this problem.
This becomes a problem with most self priming pumps and the only
solution is to contact your pump supplier for his recommendation,
if he has one. Repaired impellers sometimes experience this
- Internal recirculation problems can be solved by either
adjusting the open impeller or replacing the closed impeller with
an alternative design. This problem was discussed in another
volume of this technical series.
- Air can get into a system through valves above the water line
or flanges, but the easiest way for air to enter a system is
through the stuffing box of a packed pump. The simplest solution
is to replace the pump packing with a balanced o-ring seal. If
vortexing is the problem, consult the "Hydraulic Institute Manual"
for information on vortex breakers and proper piping layouts to
prevent turbulence in the lines, and at the pump suction.
- Water hammer is not very well understood by our industry, but
we know how important it is to keep air out of the piping
- It's good practice to use one size larger suction pipe and
then use a reducer to connect the piping to the pump. Do not use
concentric reducers. Eccentric types are much better, as long as
you do not install them upside down.
to other types of vibration
- The pump, or one of its components, can vibrate in harmony
with another piece of equipment located in close proximity.
Isolation, by vibration damping, is the easiest solution to this
problem. This is a big problem with many metal bellows seal
designs because they are lacking an elastomer that functions as a
- Critical speed operation is not a common problem unless you're
operating with a variable speed drive. Changing the speed is the
obvious solution. If that's not practical, changing the impeller
diameter is another solution.
- Seal "slipstick" is a problem with non lubricants such as hot
water or most solvents. If you're using o-ring seals, the o-ring
is a natural vibration damper. Metal bellows seals require that a
separate vibration damper be installed; usually in the form of a
metal component vibrating and sliding on the shaft.
- Pump discharge recirculation lines can cause a vibration every
time the impeller passes the recirculation line "tap off". This
vibration will affect the mechanical seal and like all vibration,
can be recognized by chipping on the outside diameter of the
carbon face and worn drive lugs.
Most of us can not stop all of the vibration that is causing our
seal, packing, bearing, and critical clearance problems, so our only
solution is to live with it. Unfortunately the standard pump and
original equipment seal is not prepared to handle vibration without
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