The sizing of condensate return lines cannot be an exact science because there are variables to contend with.
- The start up (warm up) load can be at least twice that of the running load.
- On a running condition flash steam will form within the condensate pipe.
- Air and other incondensible gases will be discharged into the condensate line from thermostatic air vents fitted to the steam traps and to the process vessels.
- It is normal practice to double the calculated running load and apply that to the pipe sizing chart in order to cater for the above three variables.
Table: Carrying capacity of condensate return lines.
The table shown above indicates carrying capacities which are 50% of the actual pipe capacity and can be taken as running load figures which will cater for a start up load on pipe size with a frictional resistance of 1.4 mbar (140 Pa) per meter of pipe on a fully loaded start up condition.
Condensate should flow by gravity
The ideal situation to aim for in any condensate return system is that condensate should flow by gravity from the steam trap on any heat exchange process equipment, the gradient should be about 200:1. i.e. condensate return lines should not have any lift after the trap. By doing this, there is no back pressure on the steam trap thereby giving maximum pressure differential across it and maximizing its performance, and thus giving a good thermal performance on the process application.
Unfortunately in a practical situation it is not possible to achieve this “ideal” and if gravity flow cannot be attained then when choosing the size of trap required. Consideration should be given to the amount of back pressure which will exist and apply this to calculate the actual pressure differential and to the steam trap sizing chart. Remember that 1 meter rise in a vertical condensate pipe will impose a back pressure of 0.1 bar. If it is not possible to arrange for the condensate to gravitate back to the boiler feed tank then it is essential for optimum performance of the heat exchange process that the condensate should gravitate to the vented receiver of a Pressure Powered Pump.