Machinery Condition Assessment
Big Creek 1, Unit 4 Penstock Safety Assessment
This
Southern California Edison’s Big Creek 1 project contains a high-head (1,960
feet), single-nozzle, single-overhung impulse turbine that had not been
thoroughly tested for many years. The owner wanted to know if pressure
transients occurring during normal operation were still within the original
design parameters. The owner asked Devine Tarbell & Associates, Inc.’s (DTA)
staff to evaluate worse-case pressure surges during load rejection and to
determine if this was within an allowable stress range, given the age and
construction of the penstock.
This project involved a thorough investigation of the penstock of Unit 4 at Big Creek 1, including functional testing of the governor and determination of governor and control system parameters using digital data acquisition. Load rejection testing was to be made only up to 50 percent load. A computer simulation was to be prepared to model the transient conditions and predict pressure rise at 100 percent load rejection. Determination of worse-case pressure rise and implications for the project’s unusual forge-welded penstock were to be evaluated.
DTA’s staff conducted tests at the site to determine governor permanent droop and other governor parameters. Load rejection tests were conducted up to a maximum of 50 percent load using a 16-channel digital data acquisition system to document events during testing. A computer model was prepared and calibrated using the results of the load rejection testing, in order to more accurately predict pressure rise for load rejections above 50 percent. A detailed examination and a discussion of the forge-welded penstock were conducted in the context of the expected pressure rise.