The Missing Link in Combined-Cycle Efficiency Testing

By Rob Swanekamp and Thamarai P. Chelvan

WHEN STARTUP teams conduct overall-plant efficiency testing of new combined-cycle projects, they often overlook one of the necessary correction factors.

When they’re looking within the system boundary of the overall plant, the Startup team typically does a thorough job–closely monitoring, and correcting for such gas-turbine variables as firing temperature, ambient temperature, ambient humidity, and elevation.

When they’re looking within the system boundary of the GT, however, some Startup teams don’t do as thorough of a job. Specifically, they don’t always monitor, and correct for tube fouling in the heat-recovery steam generator (HRSG), because they assume that a brand-new HRSG isn’t going to have any significant fouling.

But experience in the HRSG User’s Group–the combined-cycle industry’s original and longest-running user group–proves otherwise.

Our experience shows that even newly erected HRSGs without a single operating hour can have one to two inches, water column, of tube fouling.

That’s what we’re calling “the missing link” in combined-cycle efficiency calculations and we’re reminding Startup teams of it in this article.

HOW CAN A BRAND NEW HRSG HAVE THAT MUCH FOULING?

Contamination starts depositing on HRSG tubes “right from the get go,” meaning it starts all the way back during initial fabrication, because the original equipment manufacturers (OEMs) aren’t meticulous enough in their shop procedures.

We realize that’s not entirely the OEMs’ fault. Today’s competitive-market pressures–which for the most part are established by the customers’ specifications–require the OEMs to produce the lowest-cost product compliant with minimum design codes, and to deliver that product in the shortest possible time.

So the customers–the HRSG users, ourselves– are just as much to blame as the OEMs.

Even more contamination deposits on the HRSG tubes occurs during their long, punishing trek from the OEM’s shop to the project site.

During this trek, the HRSG tubes are exposed to contaminants during (1) the loading onto trucks, rail cars, or sometimes ocean-going Container Ships, (2) during the shipping across hundreds or sometimes thousands of miles, and (3) during the off-loading at the project site.

AND THE CONTAMINATION OF HRSG TUBES DOESN’T END, EVEN THEN!

Once it’s off-loaded at the project site, the HRSG gets stored outdoors and exposed to the elements–often for as long as a year because of construction delays or contractual conflicts.

Storing the HRSG anywhere other than in an indoor, climate-controlled facility causes most of the tube fouling in a pre-operational HRSG–some 70 to 80 percent of it.

A slightly lower amount–perhaps 20 to 25 percent–occurs during shipping. And the remaining 10 to 15 percent occurs during initial fabrication.

All of that fouling will get removed during the pre-operaional cleaning, right?

In theory, yes. But in actual practice, no!

The pre-operational cleaning of HRSGs often gets short-changed these days, because Startup Schedules are so aggressive, and because each Startup team member is so focused on the hefty bonus and penalty clauses in their particular contract.

You’ve probably heard the old saw that goes, “A fine wine takes time.” In a similar vein, we’re reminding Startup teams that “cleaning HRSGs to their peaks takes weeks.”

Unfortunately, some of today’s Startup teams try to rush through the pre-op HRSG cleaning in a matter of days.

We realize that’s because their long, arduous commissioning project is nearly over by the time HRSG cleaning roll around on the schedule. So it’s a natural tendency for them to, like marathon runners, lean forward as they’re nearing “the Finish Line.” That’s a natural tendency, alright, but it’s letting momentum overcome judgement. Remember, the startup of a new, $300-million combined-cycle project isn’t merely a footrace, is it?

Rob Swanekamp, PE, is director of the HRSG User’s Group Conference and Expositon. Thamarai P. Chelvan is the engineering manager of Boiler and HRSG Services for Siemens Energy Inc.