Key Takeaways
- Investigate the cause of a seal failure before deciding on the solution. Misdiagnosing the issue can lead to another failure.
- Some failures stem from the seal itself, such as installation errors or mishandled parts. Others stem from the pump system, such as dry running and thermal distortion.
- Look at the seal to determine the cause. Damage to the faces, elastomer, or sleeve can all point to different issues.
- To prevent future failures, specify each replacement to your exact operating conditions, verify the equipment before installation, and confirm that the seal support system works.
A mechanical seal is often the first component in a pump system to fail, causing costly downtime. While repairing or replacing the seal might seem like the most straightforward solution, it might not resolve the underlying issue, and you may find yourself having to call your maintenance team back in after just a few short months. That’s because the root of many seal failures starts somewhere other than the seal itself.
In this blog post, we’ll cover the most common causes of pump seal failure, how to recognize them, and how to prevent them, so you make more informed (and lasting) maintenance decisions the next time you run into trouble.
12 Common Causes of Pump Seal Failure
Mechanical seal failures are often attributed to the seal itself, and that assumption is sometimes correct. Selecting the wrong face or elastomer for the process fluid, installation errors, component mishandling, or a specification that does not match the application are just a few reasons why a seal might fail on its own.
Just as often, though, the source of the failure is the pump system around the seal. Shaft misalignment, vibration, dry running, and an inadequate flush will all damage even a properly selected and installed seal. If the underlying condition goes unaddressed, the replacement seal will eventually fail, too. Therefore, it’s critical to thoroughly investigate and diagnose the underlying issue before taking any remedial action.
Below, you’ll find a list of issues, both seal-specific and system-related, to look for the next time you experience a pump seal failure.
Improper Seal Selection
Every mechanical seal is designed for a specific set of operating conditions. Operation outside those conditions will cause the seal to fail, even if it was properly installed.
Examples of this include:
- Material incompatibility: An elastomer that swells, softens, or hardens in the process fluid will not hold a seal, and a carbon face that runs against abrasive solids will wear down quickly.
- Fluid properties: Above 190°F, hot water no longer lubricates and cools the seal faces adequately, and the resulting friction between the faces can lead to premature failure. Boiler feedwater applications are frequently affected by this issue.
Selection can also fall out of alignment over time. A seal correctly specified several years ago may be unsuited to the product a plant currently processes.
To check for improper seal selection, check the O-rings first. Swelling, a gummy texture, or hardening all point to chemical incompatibility within the process fluid. Corrosion or pitting on springs, sleeves, or gland components indicate a metallurgical problem. Heavy wear across the entire face track after a short period in service suggests the face pairing was too soft for the fluid being pumped.
Installation Errors and Mishandling
Seal faces and O-rings are precision components, and it takes very little to damage them. A dropped face, a nicked elastomer, or debris left on a face during assembly can all shorten a seal’s service life.
Most installation-related seal failures are the result of simple mistakes, such as removing the setting clips before the set screws are tightened, which leaves the seal without a fixed working length, or tightening the gland bolts unevenly, which pulls the stationary face out of square with the shaft. Component seals are more susceptible to these errors than cartridge seals because they require additional measurements and assembly steps at the pump.
Common signs of installation errors and mishandling include faces with no wear track, wear concentrated to one side of the track, and clean cuts or missing sections on elastomers.
Improper Storage
Several components within a seal can be damaged before installation as a result of improper storage conditions. Elastomers are the most vulnerable part of an assembly and will degrade when exposed to sunlight, ozone, and heat. An O-ring left compressed inside an assembled seal for an extended period can take a permanent set that prevents it from sealing once the seal enters service. Lapped faces present a separate concern: Two faces left in contact for too long can stick together or lose their flatness, which is why assembled seals held in inventory need their faces rotated against each other periodically.
Secondary Seal Degradation
Although the primary faces often receive the most attention, a seal can leak from any O-ring, gasket, or wedge in the assembly. These components need to be flexible enough to move with the seal while maintaining a static seal against the sleeve, gland, or seal chamber.
An elastomer that loses its flexibility can no longer perform either function. Heat is the most common reason for elastomer degradation, hardening the material until it no longer conforms to the parts it seals against. Chemical exposure has the opposite effect, softening or swelling the elastomer until it deforms out of position. Excessive pressure forces the material into the clearance around it, and the extruded portion tears away as the seal continues to move.
Leakage that persists even after a face inspection reveals nothing unusual indicates a problem with the secondary seal. Look for O-rings that have lost their round cross-section, are brittle enough to crack when flexed, or are torn along one edge where the material was forced into a clearance.
Loss of Axial Movement
A mechanical seal has to travel along the shaft to keep the faces closed as the carbon wears down and the shaft moves axially. The floating element that enables that travel needs clear space beneath it to move. Small amounts of process fluid can leak into that space and, depending on the product being pumped, that fluid can solidify, cementing the floating element to the sleeve. The seal can no longer follow the faces as they wear, so the faces separate, causing additional leakage.
Hardened deposits caked onto the sleeve beneath the floating element are clear evidence of this issue. Faces that show less wear than service hours would account for are another indicator, since a seal that seized early stopped closing against its mating face.
Spring, Bellows, and Drive Component Issues
Over time, corrosion, fatigue, and cracking can weaken the springs and metal bellows responsible for holding the faces together, causing the seal to leak. The drive pins and notches that hold the rotating element in position wear down in a similar fashion, shearing or going oval under the torque generated by friction at the faces, until the element shifts far enough to allow the faces to separate.
Sheared drive pins and drive notches worn into an oval shape could signal a torque problem at the faces. Springs that have lost their free length or exhibit corrosion pitting can no longer deliver the load the seal requires. On a bellows seal, cracks at the convolution welds indicate fatigue, and a fatigued bellows no longer generates enough force to hold the faces closed.
Inadequate Seal Support or Flush Plan
The seal support system delivers clean, cool liquid to the seal chamber and transfers heat away from the faces. A failure within that system removes the conditions under which the seal was designed to operate.
There are a few ways piping plans can fail:
- A flush line fed from pump discharge can get clogged with the same solids it was designed to exclude.
- Cooling water to a heat exchanger can be shut off during a turnaround and never turned back on.
- The barrier fluid reservoir on a dual seal can run low, or the pressure differential that holds barrier fluid between the faces can drop below what the design requires.
- In some cases, the plan was unsuited to the service from the start.
Heat damage or contamination on the faces can point to these issues. Either finding warrants an inspection of the flush line for blockages and confirmation that the flow reaches the seal chamber. On a dual seal, discolored barrier fluid in the reservoir can indicate that an inner seal is leaking process fluid into the barrier circuit.
Dry Running
Seal faces require a thin film of process fluid to lubricate them and remove the heat that friction generates. Without this film, the faces make direct contact and can crack, blister, or glaze. This can occur when a mechanical seal operates without liquid in the seal chamber, a condition known as dry running.
Dry running can happen during equipment startup, when a pump that hasn’t been properly ventilated traps vapor in the seal chamber, and the faces are dry from the first revolution. Suction issues caused by a closed valve, a plugged strainer, or an empty tank produce the same result during operations.
The most common signs of dry running include fine radial cracks across a face, discoloration on the faces, and hardened elastomers. A seal that fails within hours of startup with clear evidence of heat damage almost certainly points to dry running.
Thermal Distortion
Even with liquid in the seal chamber, the faces can reach temperatures high enough to fail. Friction between the faces generates heat continuously, and the flush must remove it faster than the seal can produce it. High process temperatures, insufficient flush flow, or a face combination that generates more friction than allowed for in the design will all tip that balance.
Heat distorts the faces by causing them to expand unevenly, concentrating contact at the edge of the track rather than across its full width. It also hardens elastomers until they no longer seal. When the film that lubricates the faces becomes hot enough to vaporize, it disappears, and the faces make direct contact.
Discoloration on the faces and metal components nearest to them is the primary sign of thermal distortion, along with elastomers that have hardened or cracked. Wear concentrated at the inside or outside edge of the face supports the same conclusion. In hydrocarbon services, hardened deposits on and around the faces indicate the product coked at temperature.
Contamination and Abrasives
Abrasive particles that reach the seal chamber will grind away at the faces until they can no longer seal. The fluid film that separates faces is thinner than most of the particles a process stream contains, so those particles either wedge between the faces or embed themselves in the softer carbon, where they act like cutting tools against the harder mating ring. This results in scoring and grooving across the face track, and you can often find the particles still lodged in the carbon during teardown.
Slurries present an obvious risk, but they’re far from the only source of contamination. Corrosive products can travel down from upstream piping, and solids can settle during a shutdown and get stirred back into circulation at the next startup. A flush line fed from pump discharge can deliver process fluid straight into the seal chamber, which is why a Plan 11 flush on a service containing solids can shorten seal life rather than extend it.
Shaft Misalignment, Runout, and Vibration
The seal faces must remain flat against each other while one of them rotates. Any movement in the shaft that the seal cannot follow separates the faces briefly on every revolution, and even slight separation allows fluid to escape and disrupts the lubricating film. Several equipment conditions produce that movement, including misalignment between the pump and driver, worn bearings, a bent shaft, or an unbalanced impeller.
If you suspect shaft misalignment is behind your pump seal failure, look for fretting on the sleeve behind the dynamic elastomer, which will appear as a polished or grooved band where the seal has moved back and forth against it. A wear track wider than the face itself, worn drive notches, or fatigued springs each indicate the faces were not tracking consistently.
Off-Design Pump Operation
Every centrifugal pump is designed to move a certain amount of fluid. As long as it stays close to that volume, the pressure inside the casing remains balanced around the impeller. Move too far above or below, and that pressure becomes uneven, pushing the shaft sideways and bending it slightly under the load. The seal faces must follow that movement.
A throttled discharge valve, a plugged line, or an incorrectly sized pump can cause a plant to operate below that volume. Restricting flow this way also heats the fluid, since the same liquid recirculates inside the casing instead of moving out through the system. That heat then reaches the seal chamber while the bent shaft is already disturbing the faces. Insufficient pressure at the pump suction point creates a separate problem, forming vapor bubbles that collapse inside the pump and shake it hard enough to disturb the seal faces continuously.
Both conditions damage the seal mechanically. Look for fretting on the sleeve, an irregular wear track on the faces, and fatigue in the springs and drive components. A pitted impeller alongside a failed seal points toward issues on the suction side of the pump.
How to Prevent Mechanical Seal Failures
Most seal failures are preventable by following these best practices:
- Provide complete details about the pump’s operating conditions to your manufacturer, including fluid composition, temperature, seal chamber pressure, shaft speed, and solids content, so they can specify a seal to those exact conditions.
- Verify the equipment before the seal goes in. Shaft runout, shaft axial movement, seal chamber face runout, and gland register measurements should all fall within the OEM’s tolerances. A seal installed in a pump outside those limits will fail no matter how carefully it’s handled. Correct any misalignment between the pump and the driver at this time.
- Where possible, choose a cartridge seal. Cartridge seals arrive preassembled and preset, which eliminates most of the inaccurate field measurements that lead to installation errors.
- Never start a pump without venting the seal chamber first. Trapped vapor can lead to dry running and high temperatures that cause faces to crack or glaze.
- Avoid throttling the discharge valve to reduce output. Restricting the flow this way can push the shaft sideways under uneven casing pressure and heats the fluid at the same time, both of which can damage the seal. Address flow requirements at the pump, instead.
- Confirm that the seal support system is actually working. Check for flow through the flush line, that cooling water is restored after every turnaround, and that there’s adequate level and pressure in the barrier fluid reservoir on dual seals.
- Do not reuse elastomers, springs, or mating rings during a rebuild, as each part has already conformed to one set of conditions and will not perform the same way again.
- Track seal life by pump. Knowing how long seals typically last in a given pump is the only way to tell whether a seal failure occurred due to normal wear, or if there’s a bigger issue at play. It also helps you identify and remediate pumps with chronic issues.
- Examine every failed seal before ordering a replacement. Use the following checklist during your examination:
- Look at the wear track for its width, evenness, and position on the face.
- Check the faces for heat checking, discoloration, scoring, or embedded particles.
- Inspect the elastomer for hardening, swelling, cuts, and flattening.
- Check the sleeve beneath the seal for fretting or hardened deposits.
- Note how long the seal lasted, and whether anything in the process or equipment changed since the last repair.
Should You Repair or Replace a Failed Seal?
After identifying the root cause of a mechanical seal failure, the next step is deciding whether to repair the existing seal or install a new one. Both options can return the pump to service, but the right choice depends on the condition of the components and whether the original seal was suited to the application.
When Repair Makes Sense
Repair is appropriate when the seal was correctly specified, and the failure resulted from a condition that’s since been corrected. A professional repair will restore the seal to its original working length and closing force, using the original drawing as the reference.
Good candidates for repair include:
- Faces with enough material left to be lapped back to flatness
- Metal components free of corrosion, fretting, or erosion damage
- Seal designs with an established service record in the application
When Replacement Makes Sense
Replacement is the better option when damage has compromised components that cannot be restored, or when the original seal was never suited to the service. Rebuilding a misapplied seal reproduces the conditions that caused the failure.
Good candidates for replacement include:
- A face pairing that’s too soft for the fluid
- A cracked bellows or a sleeve worn through beneath the dynamic elastomer
- An obsolete seal design with limited parts availability
- Root cause analysis that identified the seal itself as the problem
- A single seal in a service that calls for a dual arrangement
If you’re still unsure which approach is best, consult your seal manufacturer. Provide them with the full operating conditions, and let their engineering team recommend how best to proceed.
Put an End to Repeat Seal Failures
Every Flexaseal engagement starts with a full seal failure analysis, so you learn what caused the failure and how to correct it before deciding whether to repair or replace. We repair all makes from every major OEM, with reverse engineering, machining, and lapping performed in-house. If, however, you decide to replace it with a Flexaseal mechanical seal, our engineers can modify materials, face combinations, and arrangements to suit your equipment’s unique operating conditions.
Request a repair quote today, and a member of our team will be in touch in 48 hours or less.
FAQs
How long should a mechanical seal last?
A mechanical seal operating at the limits of its published pressure, speed, and temperature ratings should last at least two years, and three years for designs that conform to API 682. Seal life shorter than this points to an underlying problem with the seal itself or with the pump and process around it.
How often should mechanical seals be replaced as preventative maintenance?
Mechanical seals are not typically replaced on a fixed calendar interval. Because seal life varies depending on the process fluid, operating temperature, and mechanical condition of the pump, most facilities replace seals during scheduled pump overhauls or when ongoing monitoring reveals a potential problem. Tracking how long seals last in each individual pump yields a more useful replacement interval than any general recommendation.
Will upgrading to a more expensive seal prevent seal failures?
An upgraded seal will only prevent failures if the upgrade actually addresses the root cause of the failure. One published reliability case study documented a seal upgrade that failed within the same short interval as the original, because the specification behind both failures was never properly identified. A more capable seal installed into a pump system with persistent issues, such as a misalignment problem or an inadequate flush, will fail the same way.
How much does it cost to repair a mechanical seal vs. replace it?
Repairing an existing mechanical seal generally costs less than replacing it, though the exact cost varies with the extent of the damage and the design of the seal. Lapping the faces and replacing the elastomer typically falls at the lower end of the cost spectrum, while full face replacement and sleeve refurbishment can approach the cost of a new unit. The cost of downtime exceeds both figures, which makes lead time an essential part of your calculation.
