Years ago, most pump shafts were sealed using rings of soft packing compressed by a packing gland. 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 restrains 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, it typically evaporates and is not noticeable.
To avoid potential pump downtime, 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. Seals can also fail faster than anticipated even if it’s the correct one for the job.
When this happens, it’s imperative to quickly identify the cause of leakage. Every minute of downtime results in lost production and maintenance fees. Fortunately, leaking face seals often show contact patterns that help identify the cause of the leaks. Among the most common of these is the full contact pattern.
Observation
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.
Symptom
The seal drips steadily whether the shaft is rotating or stationary and 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