- ▸Ammonia synthesis unit carbamate pumps and liquid ammonia pumps face harsh service: NH₃ + CO₂ + H₂O forms Carbaugh liquid with pH 10-11, strongly corrosive to copper alloys and fluororubber
- ▸Carbon graphite seals should use furan resin-impregnated graphite (NH₃-corrosion resistant); avoid Babbitt metal-impregnated graphite containing copper
- ▸High-pressure service (15-25 MPa) requires tandem double mechanical seals with barrier fluid pressure 0.2-0.3 MPa above media
- ▸Barrier fluid uses ISO VG 46 anti-wear hydraulic oil, with diaphragm pump pressure-stabilized make-up to prevent NH₃ atmospheric leakage
- ▸Huahao Sealing extended carbamate pump seal life from 2000 to 8000 hours in a 300000 ton/year ammonia synthesis plant revamp
Carbamate pumps and liquid ammonia pumps in ammonia synthesis units convey media containing ammonia (NH₃), carbon dioxide (CO₂), and water, forming highly corrosive Carbaugh liquid (ammonium carbamate solution). Drawing on Huahao Sealing's service experience at Yuntianhua and Xinlianxin fertilizer plants, this article introduces carbon graphite seal selection for NH₃ media service.
1. Service Characteristics
Carbamate pumps convey ammonium carbamate aqueous solution at 70-90℃, 15-25 MPa, pH 10-11, strongly corroding copper alloys and fluororubber. Liquid ammonia pumps convey -33℃ to +40℃ liquid ammonia at 1.5-3.0 MPa; liquid ammonia viscosity is only 0.25 mPa·s (one-quarter of water), making face film extremely easy to disrupt.
Failure statistics: impregnant corrosion loss (38%), O-ring swelling (25%), spring corrosion fatigue (18%), face thermal cracking (12%). The first three are directly related to material selection.
2. Material Selection
2.1 Furan Resin-Impregnated Graphite
For NH₃ media, furan resin-impregnated graphite M106K is preferred, with excellent alkali resistance, stable long-term in pH 10-11 carbamate solution, temperature rating 200℃. Comparative tests: furan-impregnated graphite immersed in 80℃ carbamate solution for 1000 hours sees porosity rise only from 1.8% to 2.1%; phenolic-impregnated graphite rises to 4.5%, no longer meeting seal requirements.
2.2 Prohibited Materials
Avoid antimony metal-impregnated graphite (antimony undergoes complexation dissolution in NH₃, losing 40% in 3 months) and Babbitt metal-impregnated graphite containing copper (copper corrodes rapidly in ammonia).
2.3 Mating Material
Sintered silicon carbide (SSiC) is recommended, with better corrosion resistance than reaction-bonded SiC and thermal conductivity 120 W/(m·K). Avoid hard metal (cobalt binder undergoes selective corrosion in ammonia).
3. Tandem Double Seal Structure
Carbamate pump high-pressure service uses tandem double mechanical seals: the inner seal bears media pressure, the outer seal bears barrier fluid pressure. Even if the inner seal fails, the outer seal still blocks NH₃ atmospheric leakage.
Barrier fluid pressure exceeds media by 0.2-0.3 MPa, ensuring media-side face reverse pressure differential. Barrier fluid uses ISO VG 46 anti-wear hydraulic oil, with diaphragm pump pressure-stabilized make-up, pressure fluctuation within ±0.05 MPa.
O-rings must use Aflas or Kalrez, with better NH₃ resistance than FKM. FKM swells 15% in liquid ammonia within 500 hours; Aflas swells less than 2% in 1000 hours.
4. Liquid Ammonia Pump Cryogenic Supplement
Liquid ammonia pumps recommend AM350 semi-hardened stainless steel bellows, avoiding Inconel 718 (low-temperature elastic modulus drop). Bellows require no O-rings, avoiding low-temperature embrittlement. Face specific pressure drops to 0.3-0.4 MPa (standard 0.5-0.6 MPa), preventing low-viscosity liquid ammonia film extrusion.
5. Huahao Sealing Case Study
A fertilizer plant 300000 ton/year ammonia synthesis unit carbamate pump (flow 12 m³/h, pressure 22 MPa, temperature 85℃) originally used an imported brand seal averaging 2000 hours life. After switching to Huahao Sealing's M106K + SSiC + Aflas + tandem double seal + diaphragm pump barrier fluid solution in 2020, average life extended to 8000 hours, saving about 450000 CNY in spare parts over 3 years while eliminating NH₃ leakage hazards.
Three selection factors: media pH determines impregnant (alkaline uses furan), pressure rating determines structure (high-pressure uses tandem double seal), temperature range determines auxiliary seal (cryogenic uses Aflas/bellows).