Diagnosing Mechanical Seal Faces When Vibration is the Symptom

Vibrations were good for the Beach Boys, but with few exceptions (shaker tables, for example), there are no good vibrations in most manufacturing processes. Vibration is one of the most frequently discussed and studied areas of maintenance and when proactively addressed, most vibration issues can be mitigated to reduce maintenance costs. 

A particular case in point is mechanical seal face vibration. Seal faces are machined to exceedingly tight tolerances (flatness measured in wavelengths). Excessive, prolonged mechanical seal face vibration can severely impact performance. But when properly installed and maintained, mechanical seals prevent leakage from centrifugal pumps.  

The Problem With Mechanical Seal Face Vibrations

A mechanical seal has two smooth seal faces that can be the same or dissimilar materials—silicon carbide, carbon, ceramic. One seal face is stationary, attached to the pump casing. The other, attached to the impeller shaft, rotates. These seals require uniform radial movement and a lubricating fluid film (process fluid or barrier fluid provided by a seal support system) across the seal faces. Maintaining a consistent flow of lubricating fluid at the right pressure between the seal faces prevents process fluid leakage. 

Vibrations are the result of deflections, however slight, in the impeller shaft. The deflections cause the rotating seal face to wobble relative to the stationary seal face and produce an impact load to the seal faces with each shaft rotation. As a result, the lubricating fluid is no longer uniform. Friction builds heat across the seal faces. Damage is imminent. 

In some instances, the shaft itself is the root cause of the problem. But more than likely, the root cause is found elsewhere. Let’s take a few minutes to review common root causes for vibration and discuss mitigation strategies.

Shaft Misalignment: Get It Straight From the Start 

All too often when working with Northern California refineries, utilities, and semiconductor fabrication plants, I hear about mechanical seal failures occurring soon after a new pump has been installed or after extensive planned maintenance has been completed. The problem is usually shaft misalignment. Misalignment of the drive shaft, coupling, or impeller shaft, producing imperceptible vibrations at first, eventually led to seal failure. 

The finest pump, reliable motor, and carefully matched seal support system can’t overcome the deficiencies of poor installation. Vibration monitoring at the conclusion of the installation or planned maintenance would have diagnosed the problem immediately.

Bearings Take the Heat and Other Abuse

Bearing failure is a leading cause of pump breakdown. Failures are due to inadequate lubrication, overheating, wear, corrosion, and contamination. That shouldn’t be a surprise in any industrial setting.

Ineffective pump seals can lead to damaged bearings. Packing glands especially are prone to eventual fluid leakage that can damage bearings. In the case of wastewater pumping, you have the damaging effect of contaminants. Another source: a steam quench to an atmospheric side seal can expose bearing housings to water vapor, gradually reduce bearing lubrication and result in a burned-out bearing that induces shaft vibration.

In extreme cases bearing damage is obvious; it can be seen and heard. For more subtle degradation, a regular planned pump maintenance schedule that includes vibration monitoring should be able to identify bearing issues well before they induce mechanical seal face vibrations.

A Firm Foundation: Tight Bolts Reduce Little Jolts 

Inadequate pump and drive foundations can undermine pump performance. Foundations need to be designed to absorb or dampen—not amplify—vibrations resulting from pumping stresses. Pumps and drive motors need to be solidly anchored and anchor bolts checked during planned maintenance. Thicker, heavier anchor plates may be the most straightforward way to remedy foundation vibration issues that appear immediately after a new installation. For older installations, consider replacing worn-out vibration-damping motor mounts.

Impeller Choices Fend Off Vibration

Over its lifetime, an impeller is subject to significant stresses. Pumping conditions that involve high-particulate concentrations or slurries will eventually damage impeller vanes and lead to hydraulic imbalance that induces shaft vibration. In the worst cases, not only will you experience mechanical seal failure, you’ll likely have to replace bearings. 

While you can’t completely prevent impeller degradation, your choice of the impeller can help prolong impeller life and mechanical seal integrity. Compared to cast impellers, machined impellers are more precisely balanced. They undergo more uniform wear in harsh pumping conditions. You’ll likely pay more for a machined impeller, but that investment can boost pump reliability and lower maintenance costs.  

Operating Outside a Pump’s Best Efficiency Point 

Vibration can occur when operating a pump outside of its best efficiency point (BEP). Ideally, a pump and its mechanical seal support system are chosen and installed to efficiently handle a specific process condition. Over time, process conditions may have changed—higher temperatures, higher pressures, increased process fluid viscosity—stressing the pump beyond its rated capacity and inducing vibration. 

Alternatively, if process conditions haven’t changed, running a pump at a higher RPM has been shown to induce vibration. Any imbalance in an impeller and shaft can become more noticeable at higher RPMs. The easiest remedy for this problem is simply to reduce pump speed. 

Check for Sources of Mechanical Seal Face Vibration First

Many of the blogs I’ve previously posted focus on mechanical seal problems associated with seal support systems. Often, however, the damage from mechanical seal face vibration is the result of circumstances beyond the control of the seal support system. Operations and maintenance personnel wrestling with problematic centrifugal pumps need to focus on root cause analysis to isolate the problem. Sophisticated vibration monitoring and diagnostic tools in the hands of experts can be used to quickly isolate the source and remedy the problem. 

Before you suspect the problem is your mechanical seal support system, ensure you’ve eliminated vibration as an issue. Once you’ve ruled those out, then it’s time to consult with an experienced, local seal support vendor who can provide an on-site diagnosis of the problem and recommend the appropriate steps to enhance seal support system performance, upgrade components, or replace outdated systems with ones designed, assembled, and tested to help ensure pump reliability. 

To find out more about how Swagelok Northern California can assist you in diagnosing and remedy mechanical seal face failures associated with inadequate seal support systems, contact our team today by calling 510-933-6200.

About Malik Durojaiye | Custom Solutions Engineer, Assembly Services

Malik Durojaiye began his Swagelok career in 2019 as a Custom Solutions Engineer in our Assembly Services group. Prior to Swagelok, Malik developed as a design engineer as well as a manufacturing engineer for 6 years serving Kentucky and California with Altec Industries; a leading provider of products and services to the electric utility, telecommunications, tree care, lights and signs, and contractor markets.