Factors That Affect the Mechanical Seal Leak Rate—And What You Can Do About It

Mechanical seals depend on a small amount of leakage of process or barrier fluids to function properly. The proper mechanical seal leak rate of process fluid or barrier gas across seal faces lubricates and cools the seal faces. Several critical factors have to work together to achieve and maintain the proper leak rate. When they do, a mechanical seal shows no visible external leakage. But when one or more factors go awry, you’ll probably run into a mechanical seal leak rate that’s unacceptable. 

California’s stringent environmental regulations prohibit leakage of toxic or hazardous fluids. In those instances, even negligible leakage can be problematic. These fugitive emissions can bring sanctions from Cal/OSHA or BAAQMD. With multiple factors having to work together to ensure mechanical seal reliability, diagnosing the reason for the leak can be a challenge. Whether you’ve discovered a pump with obvious leakage or one with emissions detected by meter, I’ve found that a categorical approach focusing on process conditions, mechanical issues, and seal support functions can help diagnose unacceptable mechanical seal leak rates. 

Check for Changes in Process Conditions

When you install a new centrifugal pump, the pump’s mechanical seal and seal support system are designed to support a specific pumping process—fluids, pressure, temperature, cycles. One thing that often catches maintenance personnel by surprise are changes in process conditions. Ask yourself: Have process fluids become more viscous, contain a higher concentration of particulates or H2S (sour crude), or being processed at higher pressures or temperatures? If so, there’s a good chance these changes exceed the operating specs of the mechanical seal and seal support system and lead to an unacceptable mechanical seal leak rate.

When process changes are the source of the problem, the best strategy is to consult with your mechanical seal vendor who can recommend the appropriate mechanical seal design for the process conditions. Once you’ve settled on the mechanical seal, then contact your local seal support system supplier for guidance on upgrading or designing a new seal support system to meet the process conditions.

Mechanical Issues Associated With Pump Operations and Components

The critical gap between seal faces—5 to 50 microinches—can easily be affected by pump operations. Vibration is the major factor to consider. A loose pump or motor mount, misaligned shaft, damaged impeller, or burned-out bearing induce vibrations that undermine mechanical seal integrity and reliability. Any of these sources can cause excessive radial or axial movement of the impeller shaft that widens the seal gap and brings the mechanical seal leak rate to an unacceptable level. 

As I mentioned above, secondary seals—O-rings, V-rings, and other elastomeric components—don’t have the resilience and lifespan of metal, alloy, or ceramic components. Even when seal the seal support system maintains the required seal chamber environment, secondary seals can wear out—and when they do, you’re dealing with an unacceptable mechanical seal leak rate. To get the longest life from secondary seals, make sure the seals are chemically compatible with the process fluids.

Mechanical Seal Support System Functions 

The third factor to consider in diagnosing mechanical seal leak rates is the seal support system. You have the right pump installed and the mechanical seal was specifically selected for the process conditions. But if your seal support system isn’t doing what it’s meant to do, you’re bound to experience an increase in the mechanical seal leak rate. When this happens you’ll want to check your mechanical seal support system for these possible problems.

• Control valves accidentally misadjusted cause the process, buffer, or barrier fluid to be delivered at incorrect flow or pressure.
• Scaling in the heat exchanger diminishes cooling capacity, leads to higher seal face temperatures than can distort seal faces.
• Inadequate barrier gas supply causes process fluid to migrate across the inboard seal face and potentially across the atmospheric seal. Check for damage to gas supply piping, leakage at connections, and inadvertent closure of an upstream supply valve.
• Buffer gas oversupply won’t result in a visible leakage, but you’ll be wasting buffer fluid as it migrates across the inboard seal into the process fluid.

That’s not a comprehensive list of seal support system problems for which to check. And if you’re baffled by mechanical seal leak rates that don’t have an obvious cause, a call to a seal support system supplier with industry-specific experience is often the quickest and most cost-effective means to diagnose and remedy the problem. 

Mechanical Seal Leak Rate: Other Factors to Consider

There are some situations where leakage varies with pump operations. Awareness of these situations can save you a lot of headaches. Seals are more prone to leakage in a running pump. A static, pressurized pump may not show any indications of leakage, but there’s no guarantee it’ll maintain an acceptable mechanical seal leak rate when it’s running.

In other situations, a mechanical seal leak rate can be higher at the initial pump startup, then lessen once the mechanical seal is broken in. Understanding pump performance through all its operating phases will help you determine when a mechanical seal leak rate is truly problematic.

Mechanical Seal Leak Rate: It’s Often Multiple Factors  

Effective mechanical seal functioning depends on many factors working together and therein lies the biggest challenge in determining why a seal is leaking. In older facilities, where it seems like there’s always a ticket for some pump with an unacceptable leak rate, I’ve recommended prioritizing these aging, problem assets and working with your local mechanical seal and seal support system suppliers to remedy the problems. 

Your mechanical seal supplier will ensure you’ve got the right seal pumping process. A seal support system supplier like Swagelok will then design, fabricate, and test the support systems that are specifically tailored to your mechanical seal and pumping conditions. With the highest quality components, design experience gained through decades of helping Northern California and Western Nevada industries boost rotating asset reliability—and outstanding technical support via phone, email, and on-site—there’s no better mechanical seal partner in the area.

To find out more about how Swagelok Northern California can provide the seal support systems that minimize the risk of mechanical seal leaks, contact our team today by calling 510-933-6200.

About Paul Lesnau | Sales Manager, Business Development Manager, and Field Engineer

Paul holds a B.S. in Mechanical Engineering from North Dakota State University. Before joining Swagelok Northern California, he was the West Coast Regional Sales Manager for an organization focused within the pneumatic and hydraulic industry where he supervised product distribution throughout the western United States, Canada, and Mexico. While in this role, he was able to help provide technical and application-specific expertise to customers and distribution to drive specifications.