Diagnosing the Cause of a Mechanical Seal Leak - Full Contact Ring

Why a mechanical seal drips steadily whether the shaft is rotating or stationary, and also fails the allowable emission limits?

November 6, 2019 | 2 minute read


Years ago, most pump shafts were sealed using rings of soft packing compressed by a packing gland, but this type of shaft seal required a fair amount of leakage to lubricate the packing and keep it cool. Then came the development of the mechanical seal, which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces―one stationary and one rotating. Even though these mechanical seal faces also require some leakage, this leakage typically evaporates and is not noticeable.

Because of the delicate components used for this new sealing method, mechanical seal failures are the greatest cause of pump down time. To avoid this, it's essential to apply the right seal for the desired and appropriate function.

A seal can be exposed to a wide variety of operating conditions—sometimes very different from conditions the seal was intended for—which can cause issues down the line. However, even if your seal is the right one for the job, there can be times when it fails faster than anticipated.

When this happens, it’s imperative that you act quickly to identify the cause of leakage. Every minute of downtime results in lost production and maintenance fees.

Fortunately, leaking seal face seals often show telltale contact patterns that help identify the cause of the leaks. Among the most common of these is that of the full contact pattern.


Full contact is the typical and desired contact pattern for a mechanical seal. While there is full contact on the mating ring and primary ring surface through 360 degrees, there is little to no measurable wear on either seal ring.


The seal drips steadily whether the shaft is rotating or stationary, and also fails the allowable emission limits.

Possible Causes

  • Secondary seals nicked or scratched on installation
  • Damaged or porous secondary seal surfaces
  • Compression set of O-rings
  • Chemical attack of secondary seals
  • Not a low emission seal or arrangement
  • Materials not conducive to low emissions

Corrective Procedures

  • Replace secondary seals
  • Check secondary sealing surfaces
  • Check with seal manufacturer for proper materials
  • Check for proper lead in chamfers, burrs, etc.
  • Change seal to low emission design, materials or arrangement

Want a hands on learning experience? Check out our 3 day Engineered Sealing Solutions training to diagnose seals that failed in real life applications. 

Scroll to top