API Plan 21

Product recirculation from discharge through flow control orifice and heat exchanger to seal chamber.  

1. Improves pressure margin over vapor pressure.  
2. Improves temperature margin to meet secondary sealing element limits, to reduce coking / polymerizing & improve lubricity.  
3. Self venting plan.  
4. Provides sufficient pressure difference to allow proper flow rate.    

1. For high temperature applications e.g. hot water application (temperature > 80ºC), hot hydrocarbons etc.  In hot non-polymerizing fluids.    

1. Always ensure that cooler is placed after the orifice.   
2. Check pressure difference between discharge and seal chamber.  
3. Cooler duty is high leading to fouling on waterside.
4. Potential plugging on process side if fluid viscosity gets high quickly.