Maintenance practices that cause seal and bearing problems
MAINTENANCE PRACTICES THAT CAUSE SEAL AND BEARING PROBLEMS GT004
Maintenance departments seldom return savings to the company management. They fear that if they do not spend this year’s budget next year’s allocation will be reduced. Management views maintenance savings as bottom line money and works at reducing maintenance manpower and inventory costs. Here are some of the maintenance practices that increase the pump failure rate:
We will start with problems with pump maintenance that can cause excessive shaft movement and deflection. This shaft deflection or displacement is a major cause of premature seal and bearing failure.
- Failure to align the pump and driver. Misalignment will cause the mechanical seal to move excessively, increasing the chance for the seal faces to open and fail the seal.
- Pipe strain is another cause of misalignment between the seal’s stationary and rotating faces. Wear ring damage is common if pipe strain is present.
- Failure to dynamically balance the rotating assembly can result in “whip, wobble, and run-out problems.” Shops commonly balance the impeller but not the entire rotating assembly that includes the impeller, shaft, sleeve, mechanical seal, coupling, drive keys, pulleys, etc.
- Damage to the shaft and bearings during the sleeve removal process. Banging on the sleeve with a large hammer or heating the shaft with a torch are common methods used to remove sleeves. Needless to say the seal and bearings stand a good chance of being destroyed in the process along with the shaft that will be bent or warped.
- Damage to the impeller during the removal process. Many impellers do not have a convenient tightening nut that can be loosened.
- Trying to use the coupling to compensate for misalignment. A coupling is designed to transmit torque and compensate for axial growth of the shaft, nothing else! It cannot compensate for misalignment between the pump and its driver. You must do an alignment to solve that problem.
- Trimming the impeller without dynamically balancing it. The impeller casting is not homogeneous; it must be re-balanced after any machining operation has taken place.
- Throttling the pump discharge to stop a cavitation problem. The more you pump the more net positive suction head available (NPSHA) you need, so throttling does work, but you may be now operating off the pumps’ best efficiency point (BEP) resulting in shaft deflection.
- Failure to machine the stuffing box square to the shaft will result in excessive seal movement unless you are using stationary mechanical seals. You can prevent excessive movement of rotating seals by going to a self-aligning design.
- Failure to level the pump. Without leveling it is almost impossible to maintain the correct bearing lubrication level.
- Repairing the cutwater to the wrong length can cause a cavitation problem known as the “Vane Passing Syndrome” that will damage the tips of the impeller blades and damage the volute just beyond the discharge nozzle.
- Failure to properly adjust the open impeller clearance or letting the closed impeller wear ring clearance become excessive can make the pump run inefficiently and vibrate.
- Turning down a shaft and repairing fretting or packing damage with a polymer material will weaken the shaft making it more sensitive to deflection forces. That practice was common with packed pumps, but should be avoided when mechanical seals are being used.
- Substituting a globe valve for a gate valve will throw the pump off of its best efficiency point (BEP), causing shaft deflection.
- Any alteration in the piping system or failure to prevent solids “build up” in the lines will have the same affect.
- Mounting the pump and motor on too light a foundation. The foundation should be at least five times the mass of the equipment sitting on it or vibration will become a problem. Proper grouting is also necessary to mate the base of the pump to the foundation.
- Check that the cut-waters are 180 degrees apart in double volute applications. Wear and improper repair can cause one of the cut-waters to be “off,” causing shaft deflection.
Seal handling practices can also lead to premature seal failure.
- Some of the problems occur during installation.
- The seal is installed at the wrong length.
- No print was available so the old set-screw marks were used.
- The shaft sleeve moved after the impeller was tightened on the shaft.
- The mechanic did not compensate for thermal growth when he set the face load.
- The mechanic used the shaft shoulder instead of the stuffing box face as his reference dimension.
- The seal was installed before the impeller setting was made or an impeller adjustment was made without resetting the mechanical seal. In most cases this will cause the seal faces to open prematurely.
- The wrong lubricant was used on the dynamic rubber part causing it to be chemically attacked. Petroleum grease on ethylene propylene O-rings is a good example of this problem. In salt water applications zinc oxide should be used on all rubber parts and metal components that clamp together.
- The shaft or sleeve is out of tolerance. This can cause serious problems with those seal designs that have a dynamic elastomer sliding on the shaft (most original equipment seals fit into this category).
- The sleeve was hardened to resist packing wear causing the seal set-screws to slip and the faces to open.
- The elastomer (rubber part) exceeded its shelf life. This is a real problem with the Buna “N” material found in most rubber bellows seals.
- Installing a stationary seal on a cartridge will cause the rotating face to “cock” when the set-screws are attached to the shaft.
- The seal is installed at the wrong length.
- An environmental control was lost while the seal was installed in the pump. Typical environmental controls include:
- Clean flushing liquid to keep solids away from the moving seal parts. Be sure to check that the clean flushing fluid is coming into the bottom of the stuffing box or seal gland and not the top
- Controlling stuffing box temperature with a cooling or heating jacket. If the circulating water is “hard” condensate may have to be substituted to prevent the cooling jacket from becoming coated with calcium and other solids that will interfere with the heat transfer.
- Barrier or buffer fluid is used to circulate between two mechanical seals. Sometimes the circulation is done by simple convection, but pumping rings and forced circulation are common also. Check to see if your convection tank has to be pressurized. This is a common problem with many original equipment seals. Feel the convection lines to make sure the convection is taking place in the right direction. Newer seal designs sometimes use inert gas as the barrier fluid.
- A steam quench is often used to remove dangerous vapors and to keep the seal area warm when the pump is shut down. Metal bellows applications use the steam quench to cool down hot oil to prevent unwanted “coking”.
- A stuffing box vent should be connected from an area above the seal faces to the suction side of the pump, or some other low-pressure area to prevent air from being trapped at the seal faces.
- A discharge recirculation line and a bushing in the end of the stuffing box are often used to pressurize the stuffing box to prevent the product from vaporizing at or between the lapped seal faces.
- Is there enough clearance between the seal outside diameter and the inside of the stuffing box? Solids build up in the stuffing box can interfere with the free movement of the seal.
- The seal was installed with unidentified materials making troubleshooting almost impossible.
- Which carbon seal face is being used? There are a hundred available and they are not all alike.
- Which elastomer was selected? Do you know both the material and the grade?
- What material are the metal components manufactured from? Not all stainless steel grades are alike, and stainless steel springs or metal bellows should never be used because of potential problems with chloride stress corrosion.
- There are many hard seal faces in use. All ceramics, silicon carbides and tungsten carbides are not alike.
- The outside springs were painted on a dual or double seal when the pump area was refurbished.
- The pump discharge recirculation line is handling abrasive solids. They are being directed at the lapped seal faces or at the thin metal bellows.
- If the open impeller is adjusted backwards (this can be a common problem if a facility has both Duriron and Goulds pumps) it can create a vacuum in the stuffing box as the impeller “pump out vanes” are running too close to the back plate.
- Do not shut off the stuffing box cooling jacket when a metal bellows seal is installed. The stuffing box is cooling down the shaft as well as the seal area. Shaft cooling is necessary to prevent heat conduction to the bearings.
Poor bearing maintenance practices are a major cause of premature bearing failure.
- If the oil level is too high or the bearings are over greased the low specific heat of the lubrication and its poor conductivity will cause the bearing area to over heat.
- The inside of the bearing case must be protected against rust when it is stored as a spare. The bearings should be coated with an appropriate grease because they can rust also.
- During storage or while in a standby condition, nearby equipment that is vibrating can induce vibration into the static bearings causing false brinneling or hardening of the bearing balls and races.
- If the oil becomes contaminated with water you will experience a very rapid bearing failure. The water can enter through the grease or lip seals from several sources:
- Leakage through the packing or mechanical seal.
- From the water hose that is used to wash down the base plates and pump area.
- From moisture in the air. We call this aspiration.
- From the steam or water connected to the quench gland used on some mechanical seals.
- The bearing was installed improperly:
- The shaft outside diameter has the wrong tolerance. Remember that the tolerance is given in tenths of thousands of an inch or thousands of a millimeter.
- Too much pressure was put on the arbor press during the assembly sequence.
- The bearing was heated in contaminated oil that has deposited the contaminates in the bearing races
- The oil was over heated and varnish particles are now in the bearing raceways.
- The bearing was pushed too far up a tapered shaft.
- A simple snap ring is retaining the thrust bearing. During operation the shaft thrust is usually toward the volute and against this thin ring.
Other maintenance practices that can cause problems.
- Installing gages with the wrong redout. Operators are used to looking at gages that should run in the middle of the face.
- Do not let insulation people cover a dischage recirculation line. Once hidden it can cause problems if you try to regulate stuffing box temperature.
- Do not install seals on hardened shafts. The set screws can slip.
- Do not replace steel bolts with stainless steel bolts. You might run into a chloride stress corosion problem. Unless you know what you’re doing it is a bad idea to substitute materials.
- On February 18, 2018