Manually throttling a valve to reduce the boiler steam pressure to the pressure required in the process, leads to excess steam consumption.
Steam is required in the process as a heating media. The major component of heat transfer during the process is the latent heat of evaporation and this heat content varies with the pressure of the steam.
Many plants opt for manual control of steam pressure by throttling a valve. It is common practice to “set” these valves assuming a constant boiler pressure. However boiler pressure varies and a manual valve is not capable of responding to the variation in the boiler pressure, with the result the downstream pressure either reduces or increases. With a higher steam pressure the quantity of steam required increases because the latent heat content of the steam decreases with an increase in the steam pressure. And with a lower steam pressure the process temperature requirements may not be met.
Thus to reduce steam consumption and to ensure steam is delivered at the right pressure, a pressure reduction station should be opted for.
Consider a heating application where 15kl of water needs to be heated from ambient to 80degC. Let the required steam pressure be 2.0Kg/cm2. Via a valve the pressure is manually set to 4kg/cm2. Let this process run for 12 hrs a day for 300 days in a year.