MODS and UPGRADES – Air attemperation

Protecting HRSGs against damage at low loads

Most owner/operators of gas-turbine-based generating assets—combined cycle and cogeneration—are painfully aware of the beating their heat-recovery steam generators take in cycling service. Until recently, most HRSGs were designed for base-load service because that’s the way the plants were supposed to operate. If you need proof that this generally is not the case, read “Changing duty cycles ….

In today’s competitive generation market, operating flexibility and ancillary services are what the grid operator generally is willing to pay a premium for; kilowatt-hours are “table stakes” in many areas of the country.  And this operating paradigm is unlikely to change given the increasing need for fast-start GT assets to back up intermittent renewables.

Also noteworthy is that combined-cycle owner/operators traditionally have focused their resources on the gas turbine—the so-called “money machine.” If you need proof of this, count the number of user groups focusing on specific GT models and compare it to the one user group addressing HRSG concerns.

Gas turbines certainly deserve the attention they get from the asset owner. Miss seeing a blade crack during an inspection and you could easily be looking at a $10-million bill to repair part of the rotor, plus some serious forced-outage time.

By contrast, a few million dollars can buy years worth of weld repairs for your HRSG. Plus, a tube failure is unlikely to cause much, if any, collateral damage and unlikely to dictate an immediate shutdown. No wonder HRSGs often don’t get the respect they deserve; steam turbines neither.

The years have taken their toll on HRSGs and steamers. It’s only recently that many plants installed during the bubble of 1999-2005 have been taking a hard look beyond the gas turbine. Wear and tear and, in some cases, poor operating practices have caused cracking of piping downstream of attemperators, “spaghetti” tubes in high-temperature harps, severely fouled tubes in LP and economizer heat-transfer sections, etc. Performance has suffered and owner/operators must take corrective to remain competitive.

What to do? There’s no quick fix, no aspirin, for what may ail your HRSG. First step is to decide what to do to re-establish design performance and to assure that the unit can accommodate future service demands. A proper course of action cannot be charted without a thorough inspection of both the gas side and water side and investigation of the root causes of any problems uncovered.

Many plants are challenged by the need to operate at low loads to avoid overnight shutdowns and the cycling damage they inflict on major equipment and plant thermal networks. A goal you often hear at user group meetings for a 2 x 1 combined cycle is having the ability to remove one GT from service late in the day and operate the other at 50% of its rated output, allowing the steamer to remain synchronized at low load. This, of course, depends on the capability of the GT to operate at low load within permitted emissions limits. Several owners have upgraded their turbines to achieve this goal (access “Klamath gets better with age”).

However, users may find low-load operation detrimental to the health of their HRSGs. Reason is that for some gas turbines—such as the popular GE 7FA—exhaust temperature increases at low loads. Specifically, the exhaust from a 7FA can be as high as 1200F at loads from about 30% to 70% of rated output. Operation in this range can cause over-spray in interstage desuperheaters as the control system calls for more water to reduce excessive steam temperature. Over spray is conducive to fatigue damage in large-bore piping downstream of the desuperheaters if the pipe undergoes a large, sudden temperature drop associated with the over-spray event.

It is difficult to prevent over spray in some HRSG designs. The control system signals attemperators to quickly reduce steam to its saturation temperature, thereby causing unevaporated water to enter the steam pipe. The water quenches the pipe and initiates thermally induced low-cycle fatigue. Should this phenomenon occur frequently—defined as less than 1000 cycles—fatigue failure of superheater and reheater components is likely. This puts a 10-yr lifetime on critical system components for a plant that cycles 100 times annually.

Also keep in mind that 1200F exhaust gas causes oxidation and creep damage to HRSG inlet-duct non-pressure-parts—such as flow distribution plates, duct liners, tube ties, etc.

Forget the water, use air

The engineers at HRST Inc, Eden Prairie, Minn, known worldwide for their HRSG engineering know-how, had seen enough over-spray damage at the hundreds of plants they inspect regularly and came up with the idea of air attemperation to minimize the need for water to control steam temperature.

Analysis Manager Amy Sieben, HRST’s thought leader on air attemperation, told the editors that the idea is relatively simple: Inject significant amounts of ambient air into the transition duct between the gas turbine and the HRSG inlet duct to reduce the temperature of the exhaust stream entering the boiler. The amount of air flowing into the system is adjusted to maintain a specific range of superheat for both main and reheat steam.

The temperature of the air/exhaust gas mixture is set to maintain metal temperatures in the HRSG and steam turbine, and the interconnecting piping, within a “safe” range without having to use water for attemperation.

When the GT achieves 80% of its rated output, control of final steam temperature reverts to the spray-water attemperators better suited for service at near design conditions.

QuenchMaster™ is the name of the patent-pending system enhancement. Don’t confuse this with HRST’s recently announced economizer upgrade, ShockMaster™, designed to prevent tube cracking and other issues caused by damaging thermal gradients.

The air injection system is simple in design, and modular, for quick installation. Main components are main air fan, ductwork, damper, damper seal-air fans, piping, and controls (Fig 1). Construction time is about three weeks for foundation and setting of equipment; tie-in can be completed within a week.

Sieben recommended a front-end cycling study for each prospective customer to determine the system’s applicability and benefits given the plant’s configuration and operating paradigm. She said this takes about six weeks to plan and execute because dispatch coordination is required. Lead time on equipment is about 12 weeks.

HRST did CFD and performance modeling on its first commercial installation—a 3 x 1 7FA-powered combined cycle—and nearly pinpoint design accuracy was verified by field measurements. The first application “worked right out of the box.” Sieben said she didn’t think CFD analysis generally was necessary for follow-on projects.

The plant that installed the first QuenchMaster is owned by a major utility (Fig 2). The editors did not have permission to attribute the following information on the installation without completing the company’s review process, which was not possible by press time. Here are the basic facts as the editors understand them:

QuenchMaster is programmed into the plant’s DCS as a permanent part of the facility’s operation. It is used during every startup from the time each GT reaches 20 MW until it achieves 140 MW. Before air attemperation was installed, the GTs couldn’t operate below 110 MW because of balance-of-plant equipment limitations.

Specifically, the boiler-feed pumps were maxed out (100% of rated speed) providing water for desuperheaters and inlet-duct water sprays, as well for maintaining drum levels. Note that the wasteful practice of spraying high-quality water directly into the exhaust gas stream also was tearing up the inlet duct liner. At 110 MW before QuenchMaster, the reheat attemperator was open to 45% and the main steam attemperator to 80%.

With air attemperation, the turbine can operate down to 90 MW with the feedpumps at 45% speed, the reheat attemperator closed, and the main-steam attemperator open only 15%. The wear and tear on pumps and spray-water control valves is reduced dramatically.

Long-term the benefits of QuenchMaster are potentially far more significant. Sieben estimated that the service life of main-steam piping—located about 80 ft above grade at this plant—may be three times as long with air attemperation than without. Experience at similar plants spraying to saturation, she continued, indicates you can expect failures within 10 years of service.

Regarding the lifetime of harps, failure to prevent moisture ingress means virtually instant failure of tubes receiving water. Therefore, eliminating over spray prevents it as a cause of tube failure. Of course, you can still get water ingress from leaking control valves, failure of the control system to stop attemperation on a trip, and buildup of condensate. CCJ