STEAM T URBINE V ERSUS P RESSURE REDUCING V ALVE O PERATION

Abstract

A question arising frequently in steam system design relates to thebenefits and drawbacks associated with passing steam through a pres-sure reducing valve or a steam turbine to supply a low pressure steamdemand. The most appropriate analysis of the economic benefits of op-erating the steam turbine utilizes an incremental systems approach.A case study analysis based on a systems approach was used todemonstrate the role of incremental fuel and electricity costs, boilerand turbine efficiencies, as well as steam flow and thermodynamicproperties on the economic performance of the turbine-generator andpressure reducing valve options. An example analysis was performedfor a boiler producing superheated steam at 600 psig and 800 °F to sup-ply a low pressure steam demand of 30,000 lbm/hr. Overall energybalances were computed for a turbine with an isentropic efficiency of40%, a 90% efficient generator and an isenthalpic pressure-reducingvalve as an alternative scheme. For a fuel unit cost of $3.00/10 6 Btuand an electricity unit cost of $0.035/kWh, it is shown that 408 kW ofelectricity can be produced while supplying the steam demand. Use ofthe turbine-generator requires the total steam flow rate to increase tosatisfy the process heating demand. The steam flow through the tur-bine would be 31,261 lbm/hr or about a 4% increase over the low-pressure process base load. Assuming that the modest additional high-pressure steam demand can be met, a net purchased energy saving of$70,000/yr. could be realized. This analysis demonstrates that a sub-stantial plant-purchased energy cost saving may be achieved for typi-cal system operating conditions when a turbine-generator is used to produce low-pressure process steam rather than a pressure reducing valve.