SUBJECT : Troubleshooting rub marks in a
centrifugal pump 5-05
When a centrifugal pump is disassembled there are a couple of
things visible to the trained trouble shooter. He can see either
corrosion, or evidence of rubbing, damage or wear. Whenever a
rotating piece of hardware hits a stationary piece it leaves a mark
that is clearly visible and capable of being analyzed for cause.
This type of rub mark should never be confused with the dull
appearance we see on a piece of metal that has been rotating in an
abrasive slurry. In strong corrosive applications the rub mark may
not be visible. The contact will cause an increase in the metal
temperature causing rapid chemical attack. This condition is easy to
identify because the corrosion is localized at the rubbing
Shaft fretting is another
common rub mark that should not be confused with the rub marks we
will be discussing in the following paragraphs. Fretting is visible
between the dynamic elastomer in the mechanical seal and the sleeve
or shaft that the elastomer is sealing against. You will also observe
this type of damage immediately under the grease or lip seals that we
find being used to seal most bearing applications.
There are five possible rubbing combinations that can be seen:
- All around the rotary and one spot on the stationary.
- All around the stationary and one spot on the rotary.
- All around both the rotary and stationary.
- One spot on both the rotary and stationary.
- One spot on the rotating component.
You should look for the rub marks on those pieces that normally
come in close contact. Common sense will dictate that the further the
hardware is located from the bearings, the more likely the contact
will occur. Here are some likely candidates for rubbing when the pump
experiences shaft deflection, or any other type of radial
displacement. Look for contact between :
- The stationary and rotary parts of the wear rings that are
installed in most closed impeller pump designs.
- The shaft or sleeve and the mechanical seal stationary
face inside diameter.
- The shaft or sleeve and the bottom of the pump stuffing
box, or stuffing box restrictive bushing.
- The shaft or sleeve and the API gland disaster
- The outside diameter of the mechanical seal rotating element
and the inside diameter of the stuffing box. You will need a
mirror and flashlight to see the stuffing box inside
- The impeller and the volute casing or the pump back
- The outside diameter of the rotating seal, and a protruding
gasket or fitting.
In the following paragraphs I will list the observations, explain
the causes and where practical list some of the conditions that can
initiate the problem with centrifugal pumps. If you would like to
learn more about how to trouble shoot the rubbing marks we normally
find in ball bearings, please refer to the bearing
paper in this series
- All around the rotary, one spot on the stationary. The shaft
is being deflected from its true position or the hardware surrounding
the rotating piece is being forced into the rotary unit.
- The pump is operating off of its B.E.P. The stationary mark
will be visible at either 240° or 60° from the discharge
"cut-water" as measured in the direction of shaft rotation.
- Some one has throttled the pump discharge valve.
- The capacity has increased.
- The discharge lines have a solids build up on the I.D. or
there is a restriction in the discharge piping.
- The tank is being filled from the bottom. The head is
increasing as the tank fills.
- The discharge by-pass line is not functioning.
- You have the wrong size pump.
- Two pumps are piped in parallel. The larger pump is
shutting the discharge check valve of the smaller pump.
- The pump speed has changed.
- The system has been altered. Piping and fittings have been
added or removed.
- The pump was started with the discharge valve fully open or
- The viscosity of the liquid has changed.
- The impeller has been trimmed.
- The discharge piping or a fitting on the discharge has been
- The motor is running at the wrong speed. This could be
caused by a change in the specific gravity of the pumped
- The suction head has changed & the discharge head
changed to compensate.
- An in-line filter is clogged.
- The shaft is pulley driven. The off-set driver is causing the
- Misalignment between the pump and the driver.
- They never were aligned.
- Thermal growth.
- Vibration has loosened the hold down bolts.
- The seal was changed and the pump was not realigned.
- A universal joint has been installed between the pump and
- Pipe strain
- Thermal growth - no expansion joints.
- During the installation process the piping was forced to
the pump suction&emdash; instead of piping from the suction to
the pipe rack.
- A center line design pump was not specified for elevated
- A protruding piece of stationary hardware is contacting the
- A fitting is protruding into the stuffing box through the
lantern ring connection.
- A gasket on the gland face is extruding into the stuffing
- A recirculation line aimed at the seal will give the
appearance of rubbing marks if there is a lot of abrasives in the
- The mechanical seal gland has slipped and is now contacting
the rotating shaft.
- A bad foot bearing on a mixer.
- The stationary seal face was not centered on the shaft and now
the inside diameter of the seal face is rubbing on the shaft. A
severe cocking of the seal face can cause the same problem.
- All around the stationary, one spot on the rotary.
- The shaft is bent.
- It never was straight.
- The shaft was damaged when it was dropped.
- The shaft was overheated and warped when the sleeve was
- The rotary unit is out of balance. You must balance everything
that rotates with the shaft such as the impeller, sleeve, sleeve
gasket, drive key, seal, bearings, coupling, motor etc.
- It never was balanced.
- Cavitation damage.
- Some of the product has attached it self to the rotating
- The impeller is the most logical place to look for un
balance problems, especially in the balancing holes.
- Erosion can remove metal from the rotating parts.
- Corrosion can do the same.
- Temperature distortion.
- A non concentric sleeve, seal, impeller, coupling,
- The impeller was trimmed and not re&endash;balanced.
- A piece was damaged during the installation process.
- The rotary unit is dragging something around with it.
- A piece left over from the last seal change. No one notices
that one of the springs has fallen out and is resting in the
bottom of the stuffing box, getting ready to be picked up by
the new seal.
- A piece of the seal has come loose. Look for set screws,
springs, drive lugs and all of the obvious seal parts.
- The pump is running at a critical
speed, or it has passed through a critical speed.
- The seal or sleeve is not concentric with the shaft.
- The mark is all around both units.
- Look for a combination of the first two discussed. This is a
very common condition.
- Thermal expansion.
- The shaft usually expands faster than restriction bushings
placed in the bottom of the stuffing box.
- Hot oil applications use a thermal bushing in the bottom of
the stuffing box to gain more efficiency from the cooling
- Steam is often used as a quench with an A.P.I. gland. This
gland has a close fitting disaster bushing that can be
overheated by the quench temperature.
- Excessive vibration.
- Bad bearings or a loose bearing fit.
- Cavitation - there are five
- Harmonic, from nearby equipment.
- Seal "slip
- One spot on both the stationary and rotary units.
- This is caused by a momentary deflection of the rotary unit.
Just about the only time it happens is when some one drops the
pump while it's being transported.
- One spot on the rotating unit.
- Someone has hit the piece with a hammer.
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