Hot gas expander power recovery and control

Abstract

A method and apparatus are disclosed for preventing overspeed of a hot gas expander and its resultant load when the load (or a portion of it) is shed suddenly. In this situation, power applied by the expander to the shaft must be reduced, but normal feedback to the speed controller may be too slow to be effective. The load rejection may be sensed by a feedforward system before a significant speed increase is detected by the speed control loop. For this approach, the expander can be instrumented with either a flow measurement device, a downstream pressure, or a downstream temperature. Inlet pressure and inlet temperature are required. The method uses the characteristic map describing the expander (shaft power versus mass flow rate) along with a feedforward control action to anticipate the speed increase. By using dimensional analysis, the characteristic curves will collapse into single curves describing the parameters of reduced flow, reduced power, and pressure ratio. Speed increase can now be largely avoided by employing the resultant values from any of the pairs of reduced power relationships (reduced power versus reduced flow rate squared, reduced power versus pressure ratio, and reduced power versus temperature ratio). The reduced flow rate needed to keep the expander and its loads from increasing speed can now be correlated to a valve position resulting in the reduced flow required to maintain speed. This valve position setpoint is applied as an open loop step change.