Best Practices: Newington

Shift from base-load to cycling duty requires mods to protect HRSG, reduce ‘standby’ costs

Essential Power Newington LLC, a 525-MW, 2 ×1 F-class dual-fuel combined cycle was designed to operate base-load—and did for about the first six years of its existence following commercial start in 2002. The plant averaged approximately 7500 hr/yr and 13 starts annually during that time.

A tolling agreement and cyclic operation began in June 2008, when Newington was acquired by Essential Power LLC. For the next five years, the facility averaged about 4800 service hours and 200 starts annually (Fig 1). However, hours/starts declined dramatically in 2013 to 1800/125. Among the reasons: The tolling agreement ended in June 2013 and gas supply in the Northeast became challenging.

Nitrogen mitigates HRSG corrosion, inerts gas lines during maintenance

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Challenge. During the shoulder months, Newington now can be offline for several days in a row, allowing the heat-recovery steam generators (HRSGs) to depressurize and permit the entry of air into the units.

Problems associated with improper layup include general corrosion, which contributes to elevated iron levels in boiler water, and pitting corrosion conducive to the cracking of pressure parts and waterside leaks. Improper layup also can force operators to extend startups to satisfy chemistry specifications.

For a few years the site relied on tube trailers for bulk nitrogen supply to the HRSGs, but that became costly as demurrage charges compounded and the uncertainty of needing a trailer became relevant.

In addition to HRSG wet-layup protection, there were many occasions where pipe purging of the fuel headers was necessary for routine and corrective maintenance. Previously, several 230-liter liquid-nitrogen dewars were used for this purpose. But they proved problematic because of product off-gassing, limited capacity, and difficulties associated with maneuvering the dewars into positions required for pipe inerting.

Solution. In 2013, a nitrogen generator using state-of-the-art dual-bed pressure swing adsorption (PSA) technology was installed. The adsorption bed converts compressed air, readily available from plant supply, into a concentrated nitrogen output stream. Proper use of the integrated PLC-based control system assures the plant of nitrogen having a purity of 95.00% to 99.95%; the purity setting depends on the purge requirement.

The stationary generator provides a nitrogen blanket capable of preventing oxygen ingress and for improving the overall cycle chemistry of the plant’s two HRSGs. EPRI recommends a minimum of 99.6% nitrogen purity for wet layup of an HRSG. This easily accomplished by the system installed at the flow rate required.

During periods of fuel-pipe inerting, the generator can easily be set to purge any pipe in the plant, eliminating the need for several liquid nitrogen dewars. A quick connection to the nitrogen-generator outlet with a standard hose is used to reach any fuel-pipe tap necessary to inert the entire system prior to maintenance.

Results. The nitrogen generator has improved overall cycle chemistry by reducing iron deposition in the HRSG caused by surface corrosion. Plus, startup chemistry specifications are achieved much faster following a boiler layup with a nitrogen cap than without. Other benefits achieved include the following:

• Reduced nitrogen commodity costs with onsite production.
• No demurrage charges are incurred for rented bulk-nitrogen trailers or cylinders.
• No rental charges are incurred for liquid nitrogen dewars formerly used in pipe-inerting activities.
• The potential for employee injuries associated with handling cylinders and dewars has been eliminated.

Project participants:

Joshua Leighton, operations manager
Ted Karabinas, maintenance technician

Mod to auxiliary cooling system reduces station back-feed

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Challenge. With the change in Newington’s operating profile, a challenge surfaced with regard to adequate cooling of essential services with minimum electrical back-feed charges. The plant was constructed with a 10-cell induced-draft saltwater cooling tower. Cooling water (CW) for steam condensing and other plant uses is circulated by two 50% centrifugal pumps, each driven by a 2450-hp (4160 V) motor.

The plant also was equipped with a closed-loop cooling water (CCW) system to remove sensible heat from essential systems—including the steam- and gas-turbine lube-oil systems and generator hydrogen coolers. CCW, a blend of demineralized water and polypropylene glycol, is circulated by a 500-hp centrifugal pump. A full-size backup pump is in reserve.

Plant designers did not provide for auxiliary off-line cooling with either the CW or CCW systems. Thus, during periods where the plant is not dispatched, or during maintenance outages, the only method of cooling available is continued use of the main CW and CCW pumps (total of 2950 hp)—far and above that required for removing sensible heat generated by lube-oil systems under off-line conditions. Use of these systems when the plant isn’t dispatched results in excessive electrical back-feed charges from the local utility.

Solution. Newington personnel approached the retrofit of auxiliary cooling capability in two phases. The first phase involved installation of an auxiliary CCW pump. Calculations revealed that the amount of sensible heat to be removed from the CCW system when the plant was offline was only about 500,000 Btu/hr. As a frame of reference, the two CCW plate-and-frame heat exchangers each are sized for 100 times this amount of heat-transfer duty.

Even though the heat-removal requirements were quite small, plant staff recognized the importance of ensuring that CCW flowed where required and in sufficient quantity to remove heat from the system (Fig 2). During design of the auxiliary system, plant personnel identified an unused 10-hp demineralized-water transfer pump.

After reviewing its capability and construction, they decided to design the piping system to accommodate the surplus pump, rated 700 gpm at 40 ft TDH, for CCW duty. However, during warm ambient conditions, the auxiliary CCW circulation was insufficient to cool the turbine lube-oil systems—this despite uninsulated CCW piping.