June 20, 2016 – Combined Cycle Journal
Presentations and discussion at meetings of the gas-turbine users’ groups, such as the 7F, often call into view problems owner/operators are having with the liquid-fuel systems on their dual-fuel engines—in particular, unreliable starts and unreliable fuel transfers from gas to oil. A couple of years ago, the sure solution for some users rarely called to operate on oil was to disable or remove their “unnecessary” liquid-fuel systems. A benefit of this approach was less-complex and less-costly annual inspections.
But yesterday is not today in the continually evolving business of electric generation. Many owner/operators, now unable to operate profitably on energy sales alone, must provide the grid ancillary services to boost revenue. Some of these services require dual-fuel capability to assure reliable power production on oil should gas supplies become tight, or unavailable—as happened in New England a couple of winters ago. Thus some users are re-commissioning their liquid-fuel systems; others are installing liquid-fuel capability on gas-only engines, and liquid-fuel systems are a headline item—once again.
Information shared at user-group meetings points to two ways to improve the reliability of dual-fuel gas turbines (GT) on oil starts and fuel transfers: Continuous circulation of oil through liquid-fuel-system components, and active cooling—including the use of water-cooled valves, manufactured by JASC (Tempe, Ariz), to prevent coking of oil in critical check valves and assure proper seating.
The coking problem many users experience with standard liquid-fuel check valves occurs after switching from oil to gas. Oil remaining in check valves, which is located close to the combustion section of the unit, is exposed to high temperatures. Above about 250F that oil is oxidized. The resulting coke coats check-valve internal surfaces (and fuel lines as well) and restricts the movement of valve parts and the flow of oil.
Once this occurs, a check valve will not open and close properly until it is overhauled, which requires special equipment and skilled technicians to assure its better-than ANSI Class 6 seal in the reverse-flow direction. The most common trip during fuel transfer is on high exhaust-spread temperature—caused almost exclusively by check valves “hung-up” on coked fuel.
To get a first-hand look at the issues faced by owner/operators with dual-fuel GTs, the editors recently spoke with a 7FA user who shared experience from a station equipped with five simple-cycle 7FAs and one 2 × 1 7FA-powered combined cycle. All engines at the facility are equipped for dual-fuel firing and have DLN2.6 combustion systems. Plus, all have JASC water-cooled liquid-fuel check valves—14 per GT (one per combustor). JASC valves were installed on the first engine at this site in 2006. Worldwide, more than 500 industrial gas turbines worldwide now are equipped with JASC water-cooled liquid-fuel valves…
