O&M Business: Granite Ridge
Hot-reheat warm-up lines reduce cold-start time
Granite Ridge Energy
Owned by Granite Ridge Energy LLC, Operated by NAES Corp730-MW, gas-fired, two-unit, 1 × 1 combined cycle located in Londonderry, NH
Plant manager: William Vogel
Key project participant: Larry Hawk, plant engineer; Jim Golles, control room operator; Dan Jorgensen,maintenance manager
Challenge.
Since commissioning, Granite Ridge Energy (GRE) has been challenged by a significant hold up in getting the hot-reheat (HRH) steam conditions within parameters prior to coupling the steam turbine during cold starts. A historically low frequency of cold starts militated against improving this condition. However, the recent change in the regional bulk electric system’s needs suggested that GRE reconsider its position.
Solution.
A study of the situation was performed to determine the best actions to undertake for improving cold-start time. Suggestions considered included the addition of larger drain lines immediately upstream of the steam-turbine induction valves at the existing drip pots and back-feeding HP steam to the HRH line to achieve the desired steam conditions faster.
GRE decided that the best course of action was to install additional drain lines leading from the HRH steam lines to the steam-turbine drains tank. Atmospheric venting and routing to the condenser was dismissed. Design of the drains tank was reviewed to ensure it could take the additional pressure and thermal loading and was found to be acceptable.
Next was to determine necessary line size. GRE determined that a 30-min minimum cold-start improvement would be necessary to justify the expense of the upgrade. A local engineering firm was hired to determine necessary line size. Calculations determined that a 2-in. line from each HRH inception line would suffice.
Necessary considerations to line routing included the need to allow for thermal movement—including steam-turbine thermal growth, materials, valve selection, safety concerns, valve placement to platform location, proper support, and negotiating existing piping runs and support steel. A final plan was designed and implemented during a routine maintenance outage. Operating procedures have been altered and will be finalized as greater experience with the upgrade occurs.
Results.
The installation of these lines has provided a safe and effective method for minimizing cold-start time. GRE has experienced relative saving of upwards of 50 minutes of time which equates to approximately $10,000 of startup gas. If future cold-start frequency mirrors that of the first quarter of 2010, this upgrade may allow complete return of investment within half of a year and significant savings onward.
