Design – Jasper Generating Station

Reliable backup for instrument air system

Jasper Generating Station
South Carolina Electric and Gas Co

910-MW, gas-fired, 3 × 1 combined cycle located in Hardeeville, SC
Plant manager: Steve Palmer
Key project participants: Tim Glover, operations superintendent Kevin Croft, E&I supervisor Rusty Mezel, maintenance superintendent

Challenge.

A loss of instrument air pressure is a major reliability concern because instrument-air-dryer purge valves were prone to malfunction, causing a rapid decrease in instrument air header pressure and subsequent equipment upset. There is also the risk that the backup air compressor can fail to start upon the loss of the running unit, putting the facility at risk. The air compressors and dryer systems are located at the far end of the plant making rapid response and correction challenging.  A reliable, cost-effective backup system was desired.

Solution.

After an engineering review, plant staff decided to modify and configure the existing, GE-supplied, air processing units (APUs) for use as backup to the instrument air system when 7FA gas turbines are online and operating in Mode 6 (Fig1).  APUs take compressor discharge air, regulate, and cool it for pulse cleaning of the turbine inlet filters. The instrument-air-system header previously had been tied into the 7FA inlet filter cleaning system (supplanting the APUs) to supply clean, cleaning because the plant had excess compressed-air capacity and was experiencing ongoing maintenance problems with the APU regulators.

The existing APU control logic was reconfigured into the Ovation DCS to “open” the corresponding APU blocking valve and allow its compressor discharge to provide backup to the instrument-air-system header when header pressure decreases to 90 psig and the respective gas turbine is operating in Mode 6.Excess moisture is removed by an existing inline drain trap on the 7FA side of each APU air cooler and a modified inline filter with draining capability on the header side of each APU cooler. Hot compressor discharge air also is cooled by the existing cooler on each APU prior to admission into the instrument air header (Fig 2). Existing APU moisture-separation towers are bypassed, as the moisture drains are sufficient for short-term, emergency service.

Results.

The facility’s instrument air system is backed up by each gas turbine’s compressor discharge in the event of a low air-header pressure condition when the gas turbine is operating in Mode 6, eliminating the risk of a plant trip and/or equipment upset. This design change also allows a safe, measured response to air compressor and dryer malfunctions when personnel are not in the immediate area. Modifications were implemented by plant personnel with some minor material costs. Since implementation, one plant trip was averted when a dryer purge valve stuck open and the idle air compressor failed to start.