Shaft displacement and the original equipment seal (O.E.M.) 11-7
The following picture describes a typical original equipment seal (O.E.M.) used in the process industry. It should run trouble free until the carbon face (B) wears away.
To experience long life with this type of seal:
- It must be installed with the proper face load.
- Stationary face “A” must be perpendicular or square to the rotating shaft.
- The seal components must be chemically compatible with the fluid in the stuffing box and any cleaners or solvents that might be flushed through the lines.
- The seal must be designed to handle the pressures and temperatures experienced by the product and cleaners it is sealing.
- The seal must be able to handle the shaft speed.
- Nothing must interfere with the ability of the seal components to follow shaft movement while the flat seal faces stay in contact. Solids, crystallizing fluids and high viscosity fluids are examples of fluids that will clog sliding components and cause seal faces to separate with excessive shaft movement.
- Excessive shaft displacement could cause a series of sealing problems that would include:
- Fretting and shaft damage between the Teflon wedge (2) and the sleeve (c).
- Excessive movement of the springs that could them to work harden and fatigue them quickly.
- A high risk of the seal faces opening allowing solids to penetrate between the faces, imbed into the softer carbon, and cause face damage.
- A change in the spring load on the faces that could either open the faces prematurely or cause excessive heating between the lapped faces.
- The seal case or carbon face could come into contact with the inside diameter of the stuffing box, either causing damage to a seal component, or opening the lapped seal faces.
- The rotating shaft or sleeve could come into contact with and damage the stationary seal face.
Just about all original equipment seals (O.E.M.) leak long before the carbon faces wear out because this type of seal is very sensitive to shaft displacement. Here is a list of some of the common causes of the shaft and seal components to be axially and radially displaced:
Causes of axial displacement of the rotating shaft :
Remember that sleeve bearings allow a lot of axial movement. Precision bearings limit axial movement to thousands of an inch (hundredths of a millimeter), but can still allow enough radial movement to open lapped mechanical seal faces.
- Up to 65% of its efficiency a centrifugal pump thrusts towards the thrust bearing. Beyond 65% of its efficiency the shaft thrusts towards the volute.
- Attaching a mechanical seal to the shaft adds to the axial thrust because of the stuffing box pressure working on the seal area attached to the shaft or sleeve . This thrust is normally towards the bearings
- Thermal growth causes shaft axial growth that should be partially compensated for in the coupling internal clearances.
- Impeller adjustment of open and semi-open impellers can move the shaft towards or away from the volute depending upon the pump design. In the United States, the Duriron pump company is unique in that impeller adjustment moves the shaft in the direction of the bearings. Remember that there is an inital impeller setting and “on going” settings that have to be made for casing and impeller wear.
Causes of a radial displacement of the rotating shaft
- Operating off the best efficiency point (BEP) causes the shaft to deflect in a radial direction. The deflection is normally 60° or 240° from the pump cutwater, measured in the direction of shaft rotation, if you are using conventional Francis Vane impellers with a specific speed between 1500 and 4000.
- Dynamic unbalance of the rotating assembly, especially the impeller.
- A bent shaft.
- A non concentric shaft sleeve.
- Set screwing a mechanical seal to a shaft or sleeve will cause the seal to run non-concentric with the rotating shaft or sleeve.
- Misalignment between the pump and its driver.
- Pipe strain.
- Upward thermal growth in a non-centerline design pump.
Both radial and axial shaft displacement
- Bad bearing.
- Bad bearing fit.
- Cavitation. There are five types to consider.
- Water hammer.
- Running at or passing through a critical shaft speed.
The shaft is not centered in the stuffing box.
- A bolted on stuffing box has slipped.
- The pillow block bearing of a double ended pump are not on the same centerline as the pump stuffing boxes.
- Mechanical causes of vibration
- Unbalanced rotating components. Damaged impellers and non concentric shaft sleeves are common.
- A bent or warped shaft. This often occurs during the sleeve removal process.
- Pump and driver misalignment. Remember that these components must be aligned when the pump and driver are hot and all expansion has taken place.
- Pipe strain. Either by design or as a result of thermal growth.
- 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.
- There is not enough mass in the pedestal. If you weigh the pump and its driver there should be a least five times that mass in the pump pedestal.
- The pedestal is not wide enough. If you drop a vertical line from the center of the motor two lines radiating out thirty degrees from this center line should pass through the base, not the sides of the pedestal.
- Hydraulic causes of vibration
- Operating off of the best efficiency point (B.E.P.) of the pump.
- Vaporization cavitation.
- 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.
- Other 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 often happens when you are seal a non-lubricant.
- A pump discharge recirculation line aimed at the seal faces.
- Loss of power to the pump
- A parallel pump is closing the check valve on the problem pump.
Along with the seal problems just mentioned, excessive radial movement of the shaft could cause contact between:
- The impeller and the volute casing or backplate. A clearance of about 0.015″ (0,5 mm) is typical.
- The stationary and rotating wear rings you find in closed impeller pumps. A clearance of 0.003″ per inch (0,03 mm/ 10 mm) diameter of wear ring is a typical clearance.
- Between the inner and outer portions of the labyrinth seals find in many bearing seal applications.
The excessive shaft displacement could also:
- Overload the shaft bearings causing excessive heat.
- Put an uneven load on the grease seals you find in most bearing seal applications.