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

Abstract

A question arising frequently in steam system design relates to the
benefits and drawbacks associated with passing steam through a pres-
sure reducing valve or a steam turbine to supply a low pressure steam
demand. 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 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 sup-
ply 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 $3.00/10 6 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 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.