combined-cycle hrsg shutdown, layup, and startup chemistry control -power

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POWER ENGINEERING

Combined-Cycle HRSG Shutdown, Layup, and Startup Chemistry Control

08/01/2012

By Brad Buecker, Contributing Editor, and Dan Dixon, Project Engineer, Lincoln Electric System

Damage that can occur in steam generators due to in-leakage of contaminants during normal

operation is a subject of much discussion. However, very serious damage is possible in systems that

cycle on and off but are not shut down, laid-up or started up properly. Combined-cycle plants are

particularly susceptible to these problems because of typical numerous startups and shutdowns.

This article examines the most important issues with regard to off-line chemistry.

Oxygen is the Darth Vader of Poor Layups

Home More Coal Industry News Combined-Cycle HRSG Shutdown, Layup, and Startup Chemistry Control

of 15Combined-Cycle HRSG Shutdown, Layup, and Startup Chemistry Control - Power Engin...

7/26/2015http://www.power-eng.com/articles/print/volume-116/issue-8/features/combined-cycle-hrs...

Both conventional and heat recovery steam generators (HRSG) are a complex maze of waterwall

piping, superheater and reheater tubing, boiler drums, and other equipment. When a unit is taken

off-line due to reduced load requirements or other issues, the water inside the circuits contracts in

volume. This volume reduction induces a slight vacuum within the system, which in turn draws in

outside air. Now, a stagnant condition with oxygen saturation, at least at water-air interfaces, has

been established.

Oxygen attack can be extremely serious for several reasons. The corrosion mechanism itself can

induce severe metal loss in those areas of high oxygen concentration.

The attack often takes the form of pitting, where the concentrated corrosion can cause through-wall

penetration and equipment failure in a short period of time. Also of major importance is that off-line

oxygen attack will generate corrosion products that then carry over to the steam generator during

startups. Deposition of iron oxides in the waterwall tubes leads to loss of thermal efficiency and,

most importantly, establishes sites for under-deposit corrosion. These mechanisms may include the

very insidious hydrogen damage, [1] acid phosphate corrosion in improperly treated units, and

caustic gouging.

Oxygen corrosion in a steam generator tube.

Source: www.power-eng.com.



Another method by which oxygen can infiltrate steam generators is at startup when stored

condensate or fresh demineralized water is needed for filling or boiler top-off. Quite often, high-

purity water is stored in atmospherically-vented storage tanks. The water absorbs oxygen and

carbon dioxide, and may even become saturated with these chemicals. When the makeup is injected

into a cold steam generator, additional attack will occur.

Nitrogen Blanketing at Terry Bundy

At the Lincoln Electric System (LES) Terry Bundy combined-cycle plant, utility personnel have

implemented several of the most effective techniques to prevent oxygen ingress and corrosion. We

will examine these techniques plus some alternatives that can also be effective.

First and foremost is nitrogen blanketing during the last stages of shutdown and subsequent short-

term layups. Experience has shown that introduction of nitrogen to key points in the system before

the pressure has totally decayed will minimize ingress of air. Then, as the system continues to cool,

only nitrogen enters, not oxygen-laden air. Key points for nitrogen protection in HRSGs include the

evaporator, economizer, and feedwater circuits.

At Terry Bundy, primary power is produced by two GE LM 6000 combustion turbines and two Nooter-

Eriksen dual pressure HRSGs (no reheat) feeding a 26 MW steam turbine. Feedwater conditioning is

all-volatile treatment oxidizing [AVT(O)], with ammonium hydroxide injection to maintain feedwater

pH within a range of 9.6 to 10. High-pressure evaporator chemistry is based on EPRI’s phosphate

continuum guidelines, with tri-sodium phosphate as the only phosphate species and control within

a 1 to 3 parts-per-million (ppm) range. The HP evaporator pH control range is 9.5 to 10. Free caustic

concentrations are maintained at or below 1 ppm to minimize the risk of caustic gouging.

Following early operation of the combined-cycle units, plant personnel discovered oxygen pitting in

one of the high-pressure evaporators. The first step to mitigate this issue was installation of a

nitrogen blanketing system in 2005. One question that often arises is how best to store or generate

nitrogen. Certainly it can be provided from nitrogen bottles provided by local gas-supply or welding

firms, and liquid nitrogen is another possibility. LES personnel selected a different method, nitrogen

generation via a pressure-swing adsorption (PSA) system.

The process utilizes a carbon molecular sieve (CMS), which, when compressed air is introduced at

high pressure, adsorbs oxygen, carbon dioxide, and water vapor, but allows nitrogen to pass

through. Obviously, the nitrogen can then be collected in receivers for use as needed. At a pre-

selected interval, pressure is released from the unit allowing O2, CO2, and H2O to desorb from the

material, at which time these gases are vented off to the atmosphere. The table below outlines

nitrogen purity from this system as a function of production rate.



The Terry Bundy N2-generator applies nitrogen, at a pressure of 5 psig, to the LP and HP drums

during wet layups, and the nitrogen is utilized to “push” water from an HRSG during dry layup

draining. A nitrogen pressure of 5 psig is maintained during the dry layup, provided no major tube

work is required. An obvious major concern with nitrogen blanketing, and the reasoning behind its

rejection at some plants, involves safety. Of course, elemental nitrogen is not poisonous, as it

constitutes 78 percent by volume of our atmosphere. However, an individual who enters a confined

space where nitrogen has not been purged may pass out nearly instantaneously due to lack of

oxygen. Death can occur within minutes.

An alternative to pressure swing adsorption is gas separation by membrane technology. In these

systems, compressed air flows along specialty hollow fiber membranes. The material allows oxygen

and water to pass through each membrane, but N2 does not penetrate and can be collected at an

outlet port. The literature indicates that this process can produce 99.5 percent pure nitrogen.

Another important point with regard to wet layup chemistry is periodic water circulation. This

minimizes stagnant conditions that can concentrate oxygen in localized areas to cause pitting.

Both Terry Bundy HRSGs have circulating systems installed on the high-pressure and low-pressure

circuits for use during wet layups. Each circuit utilizes one of two redundant pre-heater recirculation

pumps, which normally are in service during HRSG operation to mitigate acid dew point corrosion of

external circuits. Each pump provides approximately 100 gpm of flow per circuit. Valves and piping

have been added to provide to allow for seamless transition from layup circulation to normal

operation. Sample/injection systems are available to allow operators to test the layup chemistry for

pH and dissolved oxygen (using colorimetric ampules), and to inject ammonium hydroxide if the pH

needs to be raised. Also, modifications made in each boiler drum allow the layup water to bypass

the drum baffle, promoting circulation and minimizing short-circuiting via the downcomers. The

pumps are typically started once drum pressure is less than 50 psig, and remain in service for the

duration of the layup.



A PSA nitrogen generator.

Source: www.balstonfilters.com

Dissolved Oxygen Removal from Condensate and Makeup Water

Very often, demineralized water is stored in atmospherically-vented storage tanks. Thus, oxygen-

laden water enters the steam generator during normal operation and even more critically during

boiler filling operation. In the latter case, the influx of cold, oxygen-saturated water can cause

severe difficulties. A possible method to minimize this problem is to limit oxygen ingress to storage

tanks, but this is typically a difficult proposition. Terry Bundy personnel selected another gas

transfer membrane technology to treat condensate return and makeup water.

The process is similar to the gas-gas transfer membrane process outlined above, but in this case the

carrier is water. As the liquid flows along the hollow fiber membranes in the vessel, gases pass

through the membrane walls but the water is rejected. The technology is capable of reducing

dissolved oxygen concentrations to less than 10 parts-per-billion (ppb). Most importantly, the

system eliminates introduction of air-saturated makeup (where the oxygen concentration may be

7.5 ppm, which is 75 times the recommended limit) during boiler fills.

Don’t Forget the Steam Turbine



During my (author Buecker) 30-plus years in or affiliated with the power industry, I have seen many

instances where the condenser hotwell was allowed to remain moist, or even contain standing

water, during outages where the condenser vacuum was broken and air entered the condenser and

LP turbine. The combination of a moist-laden atmosphere and the salt deposits that collect on LP

turbine blades during routine operation can be quite damaging. Pitting and stress corrosion

cracking (SCC), two very harmful mechanisms, are two of the potential outcomes.

A very practical method to combat this corrosion, and one that has been adopted at Terry Bundy, is

desiccated air injection to the condenser during all but short-term layups (<72 hours).

This system is capable of providing 700 standard cubic feet per minute (SCFM) of 100oF air at 10

percent humidity to the condenser and low-pressure turbine. This flow can lower the relative

humidity from nearly 100 percent to less than 30 percent in just a few hours.

Results

In 2005, drum inspections showed significant pitting. It was after this inspection that the changes

outlined above were implemented. A repeat inspection in 2008 showed no new pitting.

Iron level monitoring, via particulate collection on 0.45-micron filters showed a significant decrease

in samples from the condensate pump discharge (CPD) and both LP and HP drums. Quicker startups

are now commonplace. Main steam cation conductivity drops to within the recommended guideline

(0.2 µS/cm) up to 1.5 hours sooner, and CPD cation conductivity consistently remains below 0.1

µS/cm, whereas in the past it might climb as high as 0.45 µS/cm. Of significant importance is that

the units can be left in wet layup for extended periods, which saves the plant six hours (over dry

layup) to reach full load per a dispatchers request.

References

1. Cycle Chemistry Guidelines for Shutdown, Layup and Startup of Combined Cycle Units with Heat

Recovery Steam Generators, EPRI, Palo Alto, CA: 2006, 1010437.



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combined-cycle hrsg shutdown, layup, and startup chemistry control -power

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