API Plan 31

Overview:

• API Plan 31 utilizes a cyclone separator to prevent abrasive particles from contacting the mechanical seal.
• High pressure fluid from the discharge of the pump is routed through the cyclone separator where centrifugal force works to separate abrasive particles present in the fluid. The particles “drop out” of the fluid and are routed back to the suction of the pump, through the bottom of the cyclone separator. The clean (i.e. particle-free) fluid is routed to the mechanical seal, through the top of the cyclone separator, to provide cooling and lubrication for the seal.

Advantages:

• Relatively low cost – only tubing or piping and a cyclone separator are needed.

Disadvantages:

• Not all particles can be separated with API Plan 31. The particle weight must be at least double that of the fluid for the particles to effectively separate in the cyclone separator.
• Not suitable for extremely dirty applications, such as slurries. 
• The long term reliability of API Plan 31 has not been proven. An alternative for API Plan 31 is to use API Plan 11 in combination with a seal that has all “hard faces” (i.e. silicon carbide or tungsten carbide). 

Best For:

• Single mechanical seals
• Fluids containing particles with relatively heavier (i.e. double) specific gravities than that of the fluid (e.g. water with sand). 
• Process fluids not susceptible to polymerization or “setting-up” at ambient or lower temperatures

Best Practices:

• Calculate the pressure differential between the pump discharge pressure and the seal chamber pressure to ensure positive flow into the seal.
• Calculate the required flow rate through the cyclone separator to determine the required size of the cyclone separator.
  Isolation valves should not be installed in the API Plan 31 flush piping. If such a valve is installed in the flush line, there poses the risk of it being closed and preventing flow to the seal. In such an event, the seal life will be drastically reduced and subsequently fail due to the lack of cooling and lubrication.
• Routine Maintenance
  • Check the piping or tubing temperature against that of the pump. If the Plan 31 flush piping is appreciably cooler than the pump temperature, there may be an obstruction in the flush piping.
  • Additionally, check the temperature both upstream and downstream of the cyclone separator. If the temperature downstream of is appreciably cooler, the cyclone separator may be obstructed. Immediate action is needed to clear the obstruction. If found obstructed, the seal is likely damaged and needs to be replaced.
  • Where practical, such as during a pump or seal repair/replacement, the interior of the cyclone separator should be visually inspected. Over time, the particles within the fluid wear the interior of the cyclone separator considerably, resulting in decreased separator performance, potential separator failure, and subsequent mechanical seal failure.