Any device is considered as failed when it is unable to perform satisfactorily as it was supposed to perform or designed with the intention to perform. Similarly, a mechanical seal is considered to have failed when leakage past the seal assembly causes either:
- Excessive loss of fluid in the system being sealed
- Excessive reduction of pressure in the system being sealed
- Excessive addition of barrier fluid into the system being sealed in case of dual pressurized seal
- Addition of product fluid in the buffer fluid in case of dual unpressurized seal
- Any physical damage which affects the functioning of the seal
Now, if we come to the word Excessive it is very difficult to define it and will vary with the application. In some applications litres of fluid per hour will be acceptable depending upon the size, e.g., sealing some non-hazardous and non-expensive fluids. But for critical applications like sealing hazardous and expensive fluids such as H2S, Methane, fuels, etc. small drip is unacceptable and will be considered excessive. Therefore, practical seal failure definition is mostly dependent on operator experience.
The Mechanical seal failure may result from the following:
- Wrong selection of the seal design or seal materials for the particular application such as speed, pressure, temperature, and fluids.
- Improper installation of the mechanical seal such as the improper setting of the seal.
- Improper handling of the seal during assembly or installation causes scratching or chipping of the seal components.
- Upsets in the pump operation such as a pump are operating above the pressure limit or working below the minimum flow condition.
- Contamination of the seal components during the installation or assembly.
- Operating the seal without proper seal support or wrong selection of the seal support system.
- Improper equipment conditions such as excessive shaft runout, deflection, vibration, or worn bearings.
Knowing the symptoms of mechanical seal failure gives the best indication of action required to increase seal life expectancy in a particular application. An indication of the causes of seal problems can often be obtained by analysing the symptoms experienced in service. Following are the external symptoms related to mechanical seal failure,
Seal is making high pitched noise during operation:
- There is an insufficient amount of liquid to lubricate the seal faces
- Fluid is not lubricating in nature
- Hard vs hard face combination is chosen for dry running
- Provide the seal flushing plan, if already provided check for blockage in the orifice of the flushing line
- If the fluid is not in lubricating nature provide external flushing fluid
- There is carbon dust accumulating around the dynamic secondary seal because of the high wear of the carbon faces
- Liquid film vaporizing between the seal faces in some cases leaves some residue which gets deposited around the dynamic secondary seal
- Quenching plan is not given on the outside of the seal faces
- Check for the condition of the seal faces.
- Provide quenching media for cleaning dirt, dust, and wear debris from around the dynamic secondary seals.
Seal leaks steadily
- If there is heavy continuous leakage this indicated that the leakage is most probably from the seal faces and if this continuous leakage is small, then this indicates that the leakage is from Secondary seals such as O-rings or packings
- Seal faces are not flat or distortion of the seal faces due to pressure or thermal stress
- Faces are cracked during handling or transportation or chipped which made a radial path in the seal face.
- Improper compression or there are nicks or cracks in the O-rings, gaskets, or packings
- Stuffing box is not square with the shaft
- Check for seal face flatness and condition of the seal faces
- Check for physical damage in the O-rings if present replace it with a new one
- Check for the equipment condition such as Runout, squareness, and also the upsets in equipment pressure
Seal failed after a very short period
- Equipment condition is very poor such as equipment is mechanically out of line.
- Seal design selected is not suitable for the application or insufficient data was provided during seal selection.
- High upsets in equipment operating parameters
- Check for equipment conditions before installing the seal
- Select proper seal as per application
- Maintain and check the equipment condition regularly because Studies have shown that more than 40% of short seal life has been the result of operational problems therefore, Proper pump commissioning plays a crucial role in increasing the life of the seal.