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 location.
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 bushing.
- 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 diameter.
- The impeller and the volute casing or the pump back plate.
- 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
Observation – 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 shut.
- The viscosity of the liquid has changed.
- The impeller has been trimmed.
- The discharge piping or a fitting on the discharge has been damaged.
- The motor is running at the wrong speed. This could be caused by a change in the specific gravity of the pumped fluid.
- 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 deflection.
- 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 the driver.
- 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 temperatures.
- A protruding piece of stationary hardware is contacting the rotating part.
- A fitting is protruding into the stuffing box through the lantern ring connection.
- A gasket on the gland face is extruding into the stuffing box.
- A recirculation line aimed at the seal will give the appearance of rubbing marks if there is a lot of abrasives in the re-circulating fluid.
- 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.
- 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 removed.
- 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 assembly.
- 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, etc.
- 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.
- 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 jacket.
- 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.
- 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.
- Someone has hit the piece with a hammer.
Additional pump troubleshooting subjects:
- The pump is not putting out enough capacity 10-10
- The pump is not producing enough head 10-9
- The pump is consuming too many amps.10-11