How does a steam trap vary from a steam valve? Why Manual Valve can't be used in place of steam trap?

It is sometimes believed that the load of condensate can be regulated with a regular valve instead of a steam trap by simply adjusting the valve opening manually to match the amount of condensate generated.
Theoretically, this is possible. However, the range of conditions necessary to achieve this are so limited that in practice it is not a realistic solution.
The largest problem with this method is that having the valve opening set to discharge a fixed amount of fluid means that fluctuations in the load of condensate cannot be compensated for. Indeed, the amount of condensate generated in a given system is not fixed. In the case of equipment, the load of condensate at start-up differs from that during normal operation. Fluctuations in the product load also result in differences in the amount of condensate generated. Similarly, in the case of steam transport piping, the load of condensate may differ depending on outdoor air temperature or as a result of heavy rain or snow.
If the device can't respond to fluctuations in condensate load, condensate that should be discharged will instead pool inside the equipment/pipe and heating efficiency will suffer. On the other hand, when the condensate load lessens, steam leakage will occur and steam will be wasted.