Steam Turbine Versus Pressure Reducing Valve Operation

Abstract

ABSTRACT A question arising frequently in steam system design relates to the benefits and drawbacks associated with passing steam through a pressure reducing valve or a steam turbine to supply a low pressure steam demand. The most appropriate analysis of the economic benefits of operating the steam turbine utilizes an incremental systems approach. A case study analysis based on a systems approach was used to demonstrate the role of incremental fuel and electricity costs, boiler and turbine efficiencies, as well as steam flow and thermodynamic properties on the economic performance of the turbine-generator and pressure reducing valve options. An example analysis was performed for a boiler producing superheated steam at 600 psig and 800°F to supply a low pressure steam demand of 30,000 lbm/hr. Overall energy balances were computed for a turbine with an isentropic efficiency of 40%, a 90% efficient generator and an isenthalpic pressure-reducing valve as an alternative scheme. For a fuel unit cost of $3.00/106 Btu and an electricity unit cost of $0.035/kWh, it is shown that 408 kW of electricity can be produced while supplying the steam demand. Use of the turbine-generator requires the total steam flow rate to increase to satisfy the process heating demand. The steam flow through the turbine would be 31,261 lbm/hr or about a 4% increase over the lowpressure process base load. Assuming that the modest additional highpressure steam demand can be met, a net purchased energy saving of $70,000/yr. could be realized. This analysis demonstrates that a substantial plant-purchased energy cost saving may be achieved for typical system operating conditions when a turbine-generator is used to produce low-pressure process steam rather than a pressure reducing valve.