Ammonia Refrigeration Compressors: Graphite Seal Ring Media Compatibility and the No-Copper Red Line

1. Why Ammonia Eats Copper but Not Graphite

Anhydrous ammonia corrodes copper through complexation: NH₃ forms soluble [Cu(NH₃)₄]²⁺ with copper ions, continuously pulling the surface into solution. Trace water accelerates it (ammonia solution ionizes to OH⁻ and dissolved oxygen speeds electrochemistry). Hence the long-standing "no copper" rule for ammonia piping, valves, bolts and gaskets.

Carbon graphite behaves oppositely:

• Carbon is chemically inert in ammonia; neither anhydrous ammonia nor weak ammonia solution (pH ≈ 11) reacts with it
• Graphite is inherently self-lubricating and can run briefly without an oil film
• Strength is stable from ambient to ~300°C depending on grade, covering most ammonia compressor duties

That is why everything from old open-type stuffing boxes to modern semi-hermetic and open screw-compressor mechanical seals uses carbon graphite primary rings. The catch is the impregnant: copper-impregnated graphite embeds copper in the ring itself; ammonia attacks the metal phase through the pores, strength drops, porosity grows, and the ring eventually breaks apart. The first selection question is always the impregnant.

2. Impregnant Selection

| Impregnant | Ammonia compatibility | Typical compressive | Typical use |

|---|---|---|---|

| Pure carbon (unimpregnated) | Fully compatible | Lower (grade-dependent) | Light-load bearings, isolating rings |

| Resin (phenolic/epoxy) | Compatible (resists weak alkali) | 90–100 MPa | Primary and secondary seal rings |

| Antimony | Compatible | ~190 MPa | High-load or hot oil side |

| Copper / copper alloy | Prohibited | ~260 MPa | —— |

Practical selection for ammonia compressor shaft seals:

• Primary (rotating) ring: resin- or antimony-impregnated carbon graphite against SiC or high-chrome stainless. SiC is fully inert to ammonia and hard; it is the standard mating face today
• Sleeves / bearing parts: pure carbon or antimony-impregnated. Where carbon graphite bearings serve sliding surfaces (screw-compressor gear supports, capacity-control slides), account for oil carryover changing lubrication
• Secondary O-rings: EPDM or butyl. NBR handles oil but not ammonia; EPDM is the ammonia-system standard and stays elastic at -40°C evaporating temperatures

3. Mechanical Seal Arrangements

3.1 Single vs Double Seals

Ammonia is toxic and flammable, so sealing strategy targets zero leakage:

• Small semi-hermetic screws / recip compressors: single mechanical seal plus an external oil-thrower, leakage managed at trace level
• Large open screws / centrifugals: double seals recommended, barrier fluid being compatible refrigeration oil or glycol solution at 0.05–0.1 MPa above ammonia side, reversing leakage inward so external leakage tends to zero
• Hazardous installations (enclosed machinery rooms, large charge): double seals with seal-fluid reservoir and leakage collection, plus low-level alarm on the barrier system

3.2 Barrier Fluid and Flush

Ammonia dissolves in water (700:1 by volume) but is largely immiscible with mineral oil, while PAO is mutually soluble. Consequences:

• If refrigeration oil is the barrier fluid, oil enters the ammonia circuit and coats evaporator tubes, degrading heat transfer over time — the practical cost of ammonia seal-fluid management. Size flush/barrier flow for "minimum effective," not maximum
• External water flush (Plan 32) introduces water and oxygen and accelerates ammonia electrochemistry; it is generally not recommended in ammonia service unless water quality is controlled and drained regularly
• The usual approach is Plan 11 (self-circulation) plus jacket cooling, holding seal-chamber temperature 10–20°C above ammonia boiling point to prevent flashing and dry running at the faces

3.3 Dry-Running Windows

Two classic dry-friction moments:

1.Startup vacuum pull-down: crankcase evacuation drops seal-chamber pressure; if oil has not arrived, faces run semi-dry

2.Shutdown let-down: ammonia backflow or barrier pressure loss removes the film after stop

Countermeasures: establish barrier/oil pressure before starting the compressor; stop the compressor before the oil pump; select rings rated for brief dry running (graphite's self-lubrication helps here); measure face wear at overhaul and replace beyond 0.5 mm (once face-width margin is gone, face pressure rises sharply).

4. Installation and Maintenance

| Stage | Requirement |

|---|---|

| Cleanliness | Pickle and degrease seal chamber and piping; no copper tools in the chamber |

| O-ring lube | Ammonia-compatible silicone or special grease; no ordinary mineral grease |

| Bolts and gaskets | All stainless or zinc-plated; no copper alloys; flexible-graphite composite or copper-free spiral-wound gaskets |

| Anti-rotation parts | Stainless pins — legacy copper pins must be replaced |

| Pressure test | Pneumatic with nitrogen at 1.0–1.5× design pressure, soap-water leak check; do not substitute hydrostatic for tightness |

| Leak detection | Fixed ammonia detectors alarming at 25 ppm; dye paper or combustion methods as backup |

5. Failure Modes

| Symptom | Root cause | Action |

|---|---|---|

| Ring fragmented, fracture dark green/blue | Copper-impregnated graphite corroded by ammonia (or copper contamination) | Replace with antimony/resin grades; audit system for copper |

| Fast uniform face wear | Frequent dry-running windows; slow barrier buildup | Revise start/stop sequence; widen face |

| O-ring hardened and cracked | NBR or ammonia-incompatible elastomer used | Switch to EPDM |

| Radial marks on SiC seat | Particles from ammonia side; carbonized oil | Check filtration and oil cleanliness |

| Leakage rising with load | Seal chamber too hot; ammonia flashing | Add cooling/jacket; maintain subcooling |

Conclusion

Graphite seal selection for ammonia refrigeration compressors reduces to one sentence: carbon graphite does not fear ammonia — it fears copper carried in with it, and dry running. Limit impregnation to resin or antimony, use SiC faces, EPDM elastomers, and start/stop sequences that establish the film first, and shaft seals will hold for a full overhaul interval (typically 8,000–15,000 hours). Any copper in the system is a liability — including the inconspicuous anti-rotation pin and gasket.