HRSG User's Group: Improving steam-plant reliability, durability and profitability    

How to Operate Baseload-Designed HRSGs in Cycling Service

Thamarai P. Chelvan—Sr. Key Expert, Fired Boiler, HRSG & BOPs, Siemens O&M Technical Support

Most of today’s heat-recovery steam generators (HRSGs) were designed for steady-state baseload duty, but are forced by market demands to operate in the cycling mode.  As a result, HRSGs are failing at an alarming rate.  

To understand these alarming failures, and brainstorm ways to prevent them, a vocal crowd of HRSG users, manufacturers, and service providers attended this presentation. Chelvan began with a comprehensive explanation of the different damage mechanisms occurring in the different HRSG components. Chief among them, he explained, is thermal-fatigue in the Superheaters and Reheaters, which is caused by transient temperature differences during startup, shutdown, and plant upsets.  For each damage mechanism, Chelvan then gave a specific modification that users and manufacturers can install to combat it. 

To combat thermal-fatigue, he recommended installing an operable stack damper and front-end closure upstream of the gas-turbine inlet, which will minimize the transient temperature differences during shutdowns. The next damage mechanism Chelvan discussed was condensate-quench, which occurs when large quantities of condensate form in the lower headers of Superheaters during the HRSG’s prestart purge. If that condensate is not fully drained prior to the initiation of high-pressure-steam flow on a hot HRSG, the Superheaters will experience thermal shock, and will crack (Fig 1).

Figure 1. Thermal-fatigue is a leading cause of HRSG failures, particularly in tube-modules with tight tube bends and in tube-to-header welds.

“The customary solution to this problem is to retrofit larger, automated drain valves, particularly with a ball valve fitted with pneumatic quick open/close actuator,” Chelvan explained. “But a more innovative solution is to eliminate the pre-start purge from the plant’s startup procedure.” When Chelvan recommended this solution, a dozen or so hands shot up from the audience. “Isn’t the purge required by NFPA [National Fire Protection Association] code?” one attendee asked. It was, indeed, required in the past, Chelvan explained. But the latest revision of NFPA 85 allows the purge to be eliminated, if the plant establishes and maintains certain “purge credits” between restarts. Chelvan then gave the specifics of these NFPA-85 purge credits—using triple block-and-vent valves with pressurized pipe sections, then filling the pipe sections with an inert gas or air at sufficient pressure to prevent fuel gas from entering. Continuous monitoring of valve positions and pressurized pipe section pressures is required.

The next innovative mod that Chelvan recommended was retrofitting automated, on-line water-chemistry monitors, which provide more consistent, and more accurate results, compared to the traditional method of batch-sampling and manual monitoring. Most cogen plants don’t have an automated system, and the few that do, have models designed for the lowest capital cost, and therefore need to be upgraded.

Another mod Chelvan recommended was even more innovative, judging by the vigorous Q & A that it sparked. He called this mod ‘a wet Reheater,’ which means, changing your startup procedure, to initiate steam flow into the Reheater as soon as possible after the gas turbine is started. That’s the exact opposite of the standard startup procedure followed by most HRSG users. Most HRSG users run their Reheater dry during startup, which significantly shortens HRSG life, Chelvan said.

Chelvan listed some of the other specific items that need to be monitored and controlled to minimize the effect of cycling service on key HRSG components, including: 

      • HP drumlife
      • Backend corrosion
      • Frontend fatigue and creep
      • Thermal shock to cold inlet header
      • HPSH and Reheater Attemperation (spray) performance
      • SCR pluggage and fouling
      • Pinch and approach performance monitoring
      • Condensate flooding monitoring
      • Duct burner performance monitoring

For each of these key monitoring items, Chelvan presented the existing and few minimal new instruments needed at locations shown in Fig. 2 for monitoring and detailed the design criteria/rule basis for their safe limit operation.  Chelvan also presented typical HRSG failure items that are found during visual inspections of yearly scheduled maintenance and how to repair them.

Fig 2. HRSGs can be “souped up” for cycling, with some savvy modifications.

In his concluding remarks, Chelvan gave the HRSG users a mini-MBA lesson about financially justifying these modifications. “Each plant should do a cost/benefit analysis, he said, to determine which of the mods make economic sense for its age, its material-condition, and its financial standing,” he said. “And when you’re doing your analysis,” he continued, “don’t count only the short-term costs. Be sure to use the long-term, or ‘lifecycle’ costs of cycling your plant, too—the penalties you incur by the wear-and-tear on equipment, the loss of revenue during forced outages, and so on.”

There are many digital tools are on the market to help users perform this cost-benefit analysis, Chelvan pointed out. For instance, Siemens offers Asset Performance Management (APM) for Power Plants. Chelvan invited attendees to the Siemens booth to see a demo of his APM software.