- ▸The leak location itself is a clue: dripping along the shaft points to the secondary seals, face sling-out points to the friction pair, leakage at the gland points to the stationary ring seal
- ▸Installation misalignment is the top human-error cause: the stationary face needs 0.0009mm-class flatness, or the fluid film cannot form
- ▸O-ring aging accounts for a large share of hot-service leaks: standard NBR O-rings age rapidly above 80-100°C — change the material to match media temperature
- ▸Cavitation-induced leakage is often misdiagnosed as a seal quality problem: check suction conditions first before replacing the seal
- ▸Verify material selection before installing a new seal: running a resin-impregnated graphite ring above 200°C is a selection error, not a quality defect
Mechanical seal leakage is one of the most frequent pump failures. In most cases the cause is not "a bad seal" but installation, operating conditions or selection. This article lists 8 common causes in checking order, each with a field-executable method.
1. Read the Leak Location First
1.Dripping along the shaft, intermittent: check the secondary seals (O-rings) and shaft surface first
2.Sling-out around the face, large volume: check the friction pair faces first
3.Leakage at the gland-to-cover joint: check the stationary ring gasket and gland gasket
Once located, work through the following 8 causes.
2. The 8 Common Causes and How to Check Each
2.1 Installation Misalignment
A misaligned stationary or rotating ring gives uneven face pressure and local opening. Check: disassemble and inspect the face contact pattern — a normal face shows a continuous, closed narrow bright band; a broken or one-sided band means misalignment. The stationary ring should have 0.0009mm-class flatness, re-verified with an optical flat before assembly.
2.2 Spring Failure
A broken, jammed or weakened spring leaves the compensating ring unable to track. Check: measure free height and working compression; replace if force has decayed more than 10%; inspect the spring cavity for media crystallization jamming the bellows or springs.
2.3 O-Ring Aging
Heat, ozone or media swelling destroys the secondary seal. Check: look for hardening, cracking or permanent compression set. NBR serves long-term at about 80-100°C; hot service calls for FKM or perfluoroelastomer, and above 200°C consider flexible graphite packing.
2.4 Face Wear
Once graphite ring wear exceeds limits, the film balance collapses. Check: measure wear depth (see our September 6 article) and inspect for radial scoring and heat cracking. Replace the friction pair as a set when wear nears the limit.
2.5 Cavitation
Low suction pressure vaporizes the face film and the seal runs dry. Check: leakage accompanied by noise and vibration, with honeycombed pitting on the face, points to cavitation. Fix suction conditions first (raise liquid level, clean the strainer) before touching the seal.
2.6 Excessive Axial Float and Vibration
Axial movement beyond the spring compensation range (a fraction of the typical 5-10mm compensation) opens the faces cyclically. Check: measure pump axial float and radial runout; worn bearings are the usual root cause.
2.7 Flush and Cooling Failure
In seal system designs related to API 682 and GB/T 566, the flush plan carries away friction heat. Check: blocked flush lines, insufficient flow, or incorrectly operated quench. Even graphite with 70-150 W/(m·K) conductivity will thermally crack once the flush fails.
2.8 Wrong Material Selection
Media temperature or corrosivity outside the material's range. Check: match temperature to impregnation — resin impregnation tops out near 150-200°C, beyond which choose antimony-impregnated M106D (density 2.20 g/cm³, compressive strength 190 MPa, service above 400°C); for strongly corrosive media, verify the impregnating resin's chemical resistance spectrum.
3. Summary and Actions
Recommended order: locate by leak position → secondary seals → friction pair → operating conditions. Most leaks are found within the first four causes. Huahao Sealing was founded in 2006 with its factory in Lu'an, Anhui, offering made-to-drawing customization and selection support. If the same leak keeps recurring, send media, temperature, pressure and speed parameters plus leak-location photos to the manufacturer for a selection review.