- ▸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
Bottom pumps convey residue at 350–380°C, and resin-impregnated graphite carbonizes above 300°C — so the material answer for hot oil pump seals is nearly singular: antimony-impregnated graphite. But material is only one third of the answer: the face liquid film flashes easily at high temperature, so API 682 Plan 23 self-circulation is needed to pull seal chamber temperature down, and a balanced-type design controls face pressure. This article walks material, structure and flush plan in three steps, backed by refinery field cases.
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 M106D antimony-impregnated graphite in 380℃ residue oil service shows: average life of resin-impregnated M106K is 4200 hours, while Sb-impregnated M106D 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 M106D + 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.