- ▸Atmospheric-vacuum distillation bottom pumps handle 350-380℃ residue oil; conventional resin-impregnated graphite carbonizes and fails above 300℃, requiring antimony-impregnated graphite (rated to 500℃)
- ▸At high temperatures the face liquid film easily vaporizes; API 682 Plan 23 (self-flush circulation with cooling) or a Plan 21+23 combination is needed to bring seal chamber temperature below 200℃
- ▸Balanced-type construction (load coefficient B=0.7-0.8) reduces face specific pressure; paired with SiC mating material, service life reaches 12000-16000 hours
- ▸Huahao Sealing extended seal life from 4000 to 14000 hours in a Sinopec 8 million ton/year atmospheric-vacuum unit revamp
- ▸Three selection factors: media temperature gradient, solid particle content, and start-stop frequency — together they determine the impregnation type and flush plan combination
The bottom pumps of atmospheric-vacuum distillation units in petroleum refineries are the "heart" of the unit. The atmospheric residue they convey is typically 350-380℃, and vacuum residue exceeds 380℃. Under such conditions, mechanical seal reliability directly determines whether the unit can achieve a four-year turnaround cycle. Drawing on years of field experience serving Sinopec and PetroChina refineries, this article systematically covers selection and design of carbon graphite seals for high-temperature oil pumps.
1. Operating Conditions and Failure Mechanisms
1.1 Sealing Challenges of High-Temperature Residue
Atmospheric-vacuum bottom pump media exhibit "three-high one-containing" characteristics: high temperature (350-380℃), high viscosity (100-500 mm²/s), high saturation vapor pressure (prone to flashing), and containing solid particles (coke powder, rust). High temperature carbonizes the impregnation resin of ordinary carbon graphite above 300℃; high viscosity increases face friction torque leading to thermal cracking; pressure drop at the face causes flashing that destroys film lubrication; coke particles entering the face accelerate wear.
1.2 Typical Failure Modes
Field statistics show 70% of high-temperature oil pump mechanical seal failures fall into three categories: graphite ring surface thermal cracking (35%), impregnant carbonization failure (22%), and spring fatigue jamming (13%). The first two are directly related to carbon graphite material selection.
2. Material Selection: Antimony-Impregnated Graphite
2.1 Why Not Resin Impregnation
Thermosetting resin-impregnated graphite (phenolic, furan) has a long-term service temperature ceiling of 200-220℃, or 250℃ short-term. In 350℃ residue oil, the resin carbonizes progressively within 500-2000 hours. After carbonization the volume shrinks 8-12%, porosity rebounds from 2% to above 8%, and the sealing face develops permeation leakage.
2.2 Advantages of Antimony Metal Impregnation
Antimony (Sb) metal-impregnated graphite has a temperature ceiling of 500℃ (antimony melting point 630℃). After impregnation, graphite porosity drops below 1%, and the metal impregnation layer does not decompose or shrink at high temperature, remaining stable long-term. Antimony's thermal conductivity (24 W/(m·K)) is far higher than resin (0.2 W/(m·K)), aiding face friction heat dissipation.
Huahao Sealing's M106H-Sb antimony-impregnated graphite in 380℃ residue oil service shows: average life of resin-impregnated M106H is 4200 hours, while Sb-impregnated M106H-Sb averages 13800 hours — a 3.3x life improvement.
2.3 Mating Material Selection
For high-temperature residue oil, pair carbon graphite with silicon carbide (SiC). SiC thermal conductivity reaches 120-150 W/(m·K), three times that of hard alloy, rapidly conducting face friction heat away. Avoid alumina ceramic (thermal conductivity only 30 W/(m·K)), which is prone to thermal stress cracking at high temperature.
3. Structural Design Points
3.1 Balanced Load Coefficient
High-temperature service must use balanced-type construction with load coefficient B of 0.7-0.8. Unbalanced type (B≥1) creates excessive face specific pressure in 350℃ residue oil, squeezing out the thinned high-temperature film and causing dry friction.
3.2 Metal Bellows Instead of Springs
For high-temperature oil pumps, Inconel 718 metal bellows are recommended over conventional cylindrical helical springs. Bellows serve as both elastic element and auxiliary seal, eliminating O-rings (FKM O-rings at 350℃ last only 2000-3000 hours) and offering overall temperature resistance above 400℃.
4. Flush Plans: API 682 Plan 23
4.1 Plan 23 Self-Flush Circulation Cooling
Plan 23 is the preferred solution for high-temperature oil pumps: media is drawn from the seal chamber, cooled by a cooler to below 200℃, then returned to the seal chamber forming a closed loop. Cooling water consumption is about 0.5-1.0 m³/h, reducing chamber temperature from 350℃ to 180-200℃, fundamentally eliminating vaporization risk.
4.2 Plan 21+23 Combination
For vacuum residue with higher particle content, Plan 21 (clean wax oil from pump discharge, pressure-reduced through an orifice, injected into seal chamber) can be paralleled with Plan 23 — a dual-flush plan that both cools and blocks particles.
5. Huahao Sealing Case Study
A Sinopec 8 million ton/year atmospheric-vacuum unit bottom pump (flow 450 m³/h, temperature 365℃, media containing coke powder) originally used an imported brand mechanical seal averaging 4000 hours life, with three leakage shutdowns per year. After switching to Huahao Sealing's M106H-Sb + SiC + Inconel bellows + Plan 23 solution in 2019, average life reached 14000 hours. Over 4 years and 56000 cumulative hours, only 4 seal sets were replaced, saving about 800000 CNY in spare parts.
Selection requires weighing three factors: media temperature gradient, particle content, and start-stop frequency. Large temperature swings favor bellows; high particle content favors Plan 21+23; frequent starts/stops should reduce face specific pressure to 0.3-0.4 MPa.