Orthophosphoric acid (H₃PO₄) pumping through chemical pumps requires the use of mechanical seals that are resistant to corrosion and chemical attacks, but unlike more aggressive acids such as sulfuric or nitric acid, orthophosphoric acid is not as strong of an oxidizing agent. However, it still requires special materials for the durability and reliability of equipment.
1. Seal materials:
When pumping orthophosphoric acid, it is necessary to choose materials that have good resistance to phosphoric acid, its aqueous solutions, and high temperatures.
Seal faces:
Ceramic (silicon carbide or aluminum oxide): These are ideal materials for aggressive chemicals, such as orthophosphoric acid, as they have excellent wear resistance and high chemical resistance.
Graphite (or graphite with metallic coatings): Graphite has good resistance to phosphoric acid, but it should be protected from mechanical wear and damage.
Stainless steel (316L or 904L): Stainless steel, especially grade 316L or 904L, has good resistance to orthophosphoric acid, especially at low and medium concentrations. However, for high acid concentrations or in aggressive conditions, additional protection may be required, such as special coatings or additives.
PTFE (Teflon): PTFE has high chemical resistance and is used for sealing rings, as well as other parts that come into contact with chemicals. PTFE is resistant to orthophosphoric acid and suitable for working with this liquid in moderate concentrations and temperature ranges.
Elastomers:
FKM (fluoroelastomer, Viton): This material has good resistance to most acids, including orthophosphoric acid. However, its use is limited by the temperature range and acid concentration.
EPDM (ethylene-propylene-diene monomer): EPDM is used at low concentrations of orthophosphoric acid. It works well at cold and moderate temperatures.
PTFE (Teflon): Can be used for gasket seals, as with other chemicals. PTFE will be resistant to the effects of orthophosphoric acid, but it is important to consider that its mechanical strength is limited.
2. Types of seals:
Mechanical seal faces: These are the most suitable type of seals for chemical pumps pumping orthophosphoric acid. They provide tightness, preventing leakage of chemicals, and allow maintaining pressure in the system.
Wet seal faces: Used to prevent overheating of seals under high load or temperature. This is important if orthophosphoric acid is heated or the pump operates at elevated temperatures.
Dry seal faces: Can be used if cooling is not necessary, and if the pump operates under conditions of low temperature or lower loads.
3. Design and additional mechanisms:
Dust caps and protective rings: Protect the seal from particles that can damage the seal face. This can be important if solid particles are present in the system.
Lubrication and cooling systems: If the pump operates at high temperatures or high acid concentrations, a cooling system for the seals may be required to prevent overheating and ensure long-term operation.
Wear monitoring: In some cases, sensors should be used to monitor the condition of the seal and replace it in a timely manner if necessary.
4. Operating conditions:
Temperature: Orthophosphoric acid can operate at temperatures from -10°C to 85–90°C, depending on the concentration. Seals must withstand these temperature fluctuations.
Acid concentration: Orthophosphoric acid is usually used in concentrations of 20–85%. The higher the concentration, the higher the material resistance requirements.
Pressure: It is important that the mechanical seal is designed for the operating pressure. Orthophosphoric acid can be pumped at moderate pressures, but it is important that the seal can withstand these loads.
5. Recommendations:
Ceramic, PTFE, or graphite are excellent materials for seal faces, providing resistance to orthophosphoric acid.
FKM (fluoroelastomer) or EPDM can be used for elastomers if the acid concentration is not too high and the temperature is within the working range.
Stainless steel (316L or 904L) is suitable for metallic seal parts at low and medium concentrations of orthophosphoric acid.
Conclusion:
For pumping orthophosphoric acid, the most suitable materials for mechanical seals are ceramic, PTFE, graphite, and stainless steel for metallic parts. It is important to choose a seal considering the temperature, concentration, and pressure of the working environment. The seal should provide tightness and long-term operation in conditions of chemical activity and possible temperature fluctuations.