Premature seal failure is a major problem that leads to expensive downtime as well as the potential for equipment damage, environmental impact, and even physical harm. When a seal does fail prematurely, it is important to track down the root cause of failure to prevent it from happening again with a new seal.
All seals are going to experience normal wear and eventually reach the point where they need to be replaced. The signs of normal wear include an even, glossy circumferential pattern on the seal lip and on the hardware and a relatively small protrusion on the heel of the seal at the extrusion gap.
When high pressures are involved, normal wear also entails circumferential wear patterns (which will be over a larger area) and a protrusion at the extrusion gap (which matt also be larger). In addition, grooving may be present on the sealing surface. While normal wear cannot be prevented, it can be minimized through proper installation and maintenance.
Shaft Surface Hardness
If the hardness of the contact surface of the shaft is not sufficient, excessive wear is likely to occur. In most cases, the shaft surface hardness should be at Rockwell 30C at minimum. In situations where there can be issues of the shaft being nicked or scratched prior to installation, operating speeds in excess of 15 fps, or the potential for abrasive contamination exist, then the minimum hardness should be 45C. In addition, the surface hardness should penetrate to a depth of 0.3 mm. If the shaft cannot be hardened enough, then a wear sleeve should be considered.
Signs of surface hardness issues include radial grooving with embedded metal filings and either axial or circumferential scratches on the dynamic surface. This type of damage can be prevented by ensuring that the shaft surface meets the appropriate hardness requirements for the application and seal jacket material.
Shaft Surface Finish
Another potential cause of premature shaft failure lies in the surface finish of the shaft because the sealing lip makes direct contact with the shaft. For most applications, the surface finish of the shaft should be between 10 and 25 𝝁in Ra but that is highly dependent on the seal material chosen and the shaft material. For example, PTFE is a dry-running material that needs a certain level of roughness on the shaft in order to create a low friction barrier between the materials and serves as an additional seal barrier.
In addition, there should be no machine lead (helical scoring or spiral lines). The presence of a lead can not only abrade the seal lip but can essentially act as a pump and lead to leaking. A lead essentially serves as a leak path, which is never a good thing.
The signs of having too rough a mating surface include radial grooving with metal filings embedded, axial nicks on the seal lip, and/or axial scratches on the dynamic surface. The appropriate surface finish and removal of the machine lead can both be accomplished through careful plunge grinding,
It is extremely important that the seal material chosen is compatible with the media it will be exposed to during operation. And when the media is changed, that can often necessitate a change in seals. Furthermore, media includes not just what is being sealed but the type of lubricant used with the seal. Signs of chemical incompatibility include cracks or holes in the seal jacket, pilling, corrosion, and/or circumferential grooving of the dynamic surface.
When issues with chemical compatibility arise, the best solution is to choose a seal jacket material that is chemically compatible with the media and the lubricant. While the base fluid may be compatible, lubricant additives may cause problems. If lubricants are the problem, keep in mind that materials such as PTFE and PEEK are both highly chemically compatible and self-lubricating so that no lubricant is needed.
Compression set occurs when a seal has become less elastic, resulting in leaks even at low pressures. If this is a regular occurrence, it might be wise to choose a different polymer for the seal jacket. The primary sign of the compression set is a flat-sided seal cross-section where the flat side corresponds to contact with the mating surface.
High Pressures Extrusion
At high pressures, seals can be forced into the extrusion gap where the seal will experience excessive wear and may eventually be torn apart. The best approach to preventing damage resulting from high pressure is to use a thermoplastic seal in place of an elastomeric seal or to use a BUR (Back-Up Ring) to prevent extrusion.
One of the most common causes of premature seal failure is improper installation. Using the right tools will help prevent common issues such as installing the seal backwards or damaging the seal during installation.
Another common source of seal failure that is typically related to installation is misalignment and runout. This is caused when either the shaft or seal is out of alignment. In some cases, the alignment issues may not be apparent until the shaft is rotating. The signs of damage related to misalignment include an even pattern of wear on the seal lip with one part that is more heavily worn, heavy wear on one side of the seal, an offset wear pattern, or a combination of high and low wear spots.
The key to preventing failure associated with installation is quite straightforward:
- Follow any instruction provided by the manufacturer
- Use appropriate tools
- Verify that the seal is being installed in the right direction
- Check for any sharp areas that could damage the seal
- Ensure that the shaft and the seal are properly aligned
Premature seal failure does not have to be a repetitive cycle if the root cause is detected and addressed. However, just replacing a seal without tracking down the cause will simply mean high M&O costs and unnecessary downtime. By inspecting a failed seal, it is possible to narrow down what caused the failure and take steps to prevent it from happening again.