Heat Recovery Steam Generators (Hrsg) and Treatment Chemistries

8/12/2019 Heat Recovery Steam Generators (Hrsg) and Treatment Chemistries

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VI. HEAT RECOVERY STEAM GENERATORS

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Heat Recovery Steam Generators (HRSG)

HRSG Steam-Raising Waste Heat Boilers. A typical

oil refinery may have 30 or more HRSGs.

HRSGs commonly employed in smaller electricalpower generators using diesel engines or gas turbine

.

Combined-Cycle and Cogeneration applications where

heat from the exhaust of a combustion turbine is usedto produce steam for additional power production or

other industrial purposes.

- ,burn natural gas or oil, and easily installed in a short

period.

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Heat Recovery Steam Generators (HRSG)

HRSGs a key component in a Combined-Cycle or

Co eneration units.

Several HRSG variations: Vertical Tube Drum-type, inwhich a single drum boiler is placed within the gas path.

Natural or forced circulation, and once-through boilers

are also available.

those of multiple pressures, make boiler water

treatment, chemistry and control a critical and

c a eng ng as High cycling HRSGs are particularly vulnerable to

roblems such asFAC.

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Erosion, Flow Accelerated Corrosion (FAC),

Erosion dama e resultin from water steam and/or

particulates on a material. Erosion evident as etching, distinct lines, or the

wa ow ng ou o a cer a n area; c em s ry an ve oc y

can be causative; often misdiagnosed as Flow

Accelerated Corrosion (FAC).

FACFlow Accelerated (or Assisted) Corrosion is

defined as a process in which the normally protective

-into a stream of flowing water or steam-water mixture.

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Erosion, Flow Accelerated Corrosion (FAC),

, .

FAC can occur in sin le- and in two- hase re ions

predominantly in the LP Section of the HRSG. Chrome steels are required in this section of the

.

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Differences Between Erosion, FAC, Erosion

orros on, an av tat on ros on

chemistry. Two phase FAC can be differentiatedbetween Cavitation by the evidence of tiger stripes or

chevrons.

FAC, often generically classif ied within the industry as- , .

EPRI defines Erosion-Corrosion (EC) as the

degradation of material caused by both mechanical

and chemical processes.

FAC is often mislabeled as Erosion-Corrosion, even

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orros on, an av tat on ros on

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and corrosion mechanisms, while the term FAC is veryspecific.

It would not be incorrect to call FAC erosion-

corrosion. However, FAC refers to a specific set of- .

While general industry practices in identifies FAC as

Erosion-Corrosion, there are no mechanical

processes associated with FAC.

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orros on, an av tat on ros on,cont .

Cavitation Erosion CE occurs downstream of a

directional change or in the presence of an eddy, andis identified by round pits; often misdiagnosed as

.

CE also involves fluids accelerating over the surface

of a material but unlike erosion, the actual fluid is notdoing the damage.

Cavitation results when small bubbles in a liquid

.pressure of a fluid drops below the vapor pressure

(the pressure at which a liquid becomes a gas); when

these bubbles strike the surface, they collapse or

implode. 8


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orros on, an av tat on ros on,cont .

A single bubble imploding does not carry much force

u , over me, e sma amage cause y eac

bubble exhibits cumulative effects.

formation of pits.

Similar to erosion, the presence of chemical corrosionenhances the damage and rate of material removal.

Cavitation is not a property of a material, but of the

.

size and shape of the vessel, not the material. While a

stronger material can be highly resistant to cavitation,

no me a s mmune.

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Combined Cycle Utility HRSG

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Water Tube HRSG from Gas Turbines

Waste Heat Recovery

o ers use w as

Turbines provide 1 -15 MW, u to 40 MW

with a steam pressure

up to 60 bar and

up to 450C.

HRSG is a strategic part of a Combined Heat & Power(CHP) industrial system recovering the waste heat of

. Benefits: High Thermal Efficiencies; Reduced Thermal

Emissions; Improved Overall Energy Economics. 12


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Typical Design Features of GT HRSG

Drums: The upper and lower drums are installed

crosswise to the GT flue gas flow. The internal steam

drum provides for high steam purity due to a wide steam

scrubber and free end-flow, to minimize turbulence and

Primary Steam Generation: Uses a convection Steam

Heater (SH) consisting of sections of fully drainable,vertical-finned tubes or smooth-type coils with multi-

pass steam flow and gas cross-flow.

,section of the boiler, is shielded by a number of screen

tubes in order to avoid creep fatigue failure due to

exposure at high temperatures over a long period.

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Typical Design Features of GT HRSG,contd.

Convection Section: The HRSG tube bank consists of a

series of tubes properly bent and welded to the external

headers. The outer tubes forming the external wall are

membraned, providing a continuous gas enclosed

Economizer: The HRSG Economizerrecovers waste

energy from the flue gases downstream of theconvection section and transfers it to the boiler

feedwater, increasing thermal efficiency and reducing

thermal emissions.

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TwoStage Combined CycleGas Turbine and

HRSG Steam Turbine Electricity Generation

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TwoStage Combined Cycle

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HRSG Operation Problems

Most large new combined cycle plants in the US were

designed under the assumption that they would be

ase- oa e , or a eas n requen y cyc e . s

basic assumption has proven to be far from the actualoperating modes for most new plants as gas energy

prices have fluctuated widely in recent years.

Two-Shift cycling is now common and is differentiatedfrom Seasonal Duty, where plants are run essentially

base-load but only for a few months of the year.

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HRSG Operation Problems,contd.

Since combustion turbine ramp rates and startup

procedures directly affect HRSG component

empera ure ramp ra es, e pus o rap om us on

Turbine (CT) startups results in greater ramp rates inHRSG hot section components than was assumed in

plant design analyses.

Larger thermal stresses result with significantimplications for the fatigue life of affected

components such as drums, thick section headers,

and tube-to-header welds.

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Additional HRSG Operational Problems

Cycling produces a rapid thermal response, which results

in more condensate accumulation during startups and a

piping metal temperatures.

The extreme conditions that are caused b c clin

operations sometimes result in water-hammer in affected

piping systems, thermal quenching of hot component

, ,

pressure boundary at tube-to-header welds, riser piping to

drums, crossover (connecting) piping, and drain

.

Cold weather operations also provide a different challenge

with the need to maintain tem erature to revent header

failure from freezing conditions.

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Additional HRSG Operational Problems,contd.

Many new combined-cycle plants have experienced premature

failures leading most operators to pursue a more aggressive

approach to assure that HRSG component integrity is verified

by periodic inspections more detailed than statutory boiler

inspections). For example:

. sua nspec on o gas pa componen s: u es,

headers and their supports, crossover piping, risers,

drains, gas baffles, acoustic baffles and related structuralcomponents.

2. Ultrasonic Testing (UT) of wall thickness for selected (high

establish the condition of HRSG components early in their

service. Drum baffle plates and cyclone separator can

c nesses are a so measure .

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Additional HRSG Operational Problems,contd.

3. Visual inspection of accessible HRSG water-side

components including: primary and secondary steam

and blowdown penetrations; and baffle plates and theirmechanical restraints (bolting and/or welds).

Plants with a history of HRSG component damage may also

schedule:

cracking.

Radiographic Testing (RT) of tube-to-header welds

susp c on o we e ec s or su -sur ace crac s . Thermography of HRSG casings to identify hot spots and

more thorough examination of older units with more

accumulated operating hours.

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Dewpoint and Chloride Stress Corrosion

Cracking Combined Cycle Cogeneration Facility

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Fail HRSG Ga Baffl

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Failed Casing Seal Weld Around Reheat

onnector to ower an o

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B ckl Lin r Plat in Firin D ct

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C clin nit: Kink in R h at r T

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B w R h at T D t Wat rHamm r

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Cycling HRSG: Fatigue Failure of 304H

ta n ess tee e eater u e tu

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Cycling HRSG: Stress Corrosion Cracking

a ure n ee water u e

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Cycling HRSG: Leaking 16 Reheater

rossover n u e

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Water Chemistry: Sticky Deposits (pH 3.0)

on conom zer u es

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HRSG: Flame Impingement

on ownstream u es

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HRSG Water Chemistry Control

In some multi-pressure HRSGs, the low-pressure(LP) boiler serves as a feedwater heating source forintermediate- (IP) and high-pressure (HP) boilers.

An All-Volatile Treatment (AVT) would probably bethe best choice for the LP boiler, so that the waterremained low in dissolved solids.

Congruent or Coordinated Phosphate would be a

reasonable choice for the IP boiler and possibly theo er, epen ng upon e sever y o eou .

Combined-cycle units are often cycled frequently.

chemistry than any other aspect of operation.Cycling also has more influence on the frequency of

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HRSG Water Chemistry Control, contd.

Each steam generating network in a multiple-boiler HRSGoperates at a different pressure. Intermediate pressuresma ran e from 200 to 500 si with low- ressureboilers operating from as low as saturation pressure to

around 100 psig. The different pressures requiredifferent chemistry control regimes.

The low-pressure boiler may directly supply feedwater tothe intermediate-pressure and high pressure steam

generators. This factor dictates additional chemical- .

The deaerator may be integral to the low pressure boiler.

Flow rates in HRSG s are generally higher than inconventional boilers. This increases the possibil ity offlow-assisted corrosion.

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Blowdown may be cascaded from high-pressureboilers to low-pressure boilers as an energy saving

.

Makeup water requirements can be quite large forcogeneration units, where part or all of the steammay e consume n an n us r a process.

Heat fluxes in HRSG boilers are lower than those in- ,

boilers are not exposed to radiant heat.

Lower heat fluxes reduce scaling and corrosion

potent a . owever, t s a vantage may e o set yincreased cycling.

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Hydrazine

Several difficult ies have arisen from using Hydrazine.The most troublin roblem is Flow Assisted Corrosion(FAC). In completely deoxygenated environments,

flowing water wil l erode carbon steel, particularly at pipebends and other flow disturbances. Several catastrophic

- , ,occurred within the past decade.

FAC is greatly minimized if a slight oxygen residual is,protective magnetite layer is maintained on the pipewalls. A dissolved oxygen residual of 1 to 2 ppb isrecommended. For this reason Oxygenated Treatments

(OT) have developed. FAC in HRSGs may be even more severe, due to the

higher flow rates. This can be combated by fabricating

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elbows out of higher grade materials, such as the 1.25 Cr-0.5 Mo or 2.25 Cr- 1.0 Mo steels.


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Alternate Oxygen Scavengers

They are more expensive and will break down in

boilers to form organic acids and carbon dioxide,which in turn will carry over to turbines and other

- .

Debate goes on regarding the corrosive effects on

turbine blades of the carr over roducts.

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Oxygenated Treatment (OT)

OT in which ox en is deliberatel in ected into thefeedwater system, was developed in Germany about

25 years ago for replacement of all-volatile treatment(hydrazine/ammonia) in once-through steamgenera ng un s.

The treatment involves the controlled injection of

feedwater system. The controlled feed causes themagnetite layer on the pipe walls to becomeoverlayed and interspersed with an even tighter film

of ferric oxide hydrate (FeOOH). This compact layeris more stable than magnetite and releases very li tt ledissolved iron or suspended iron oxide particles to

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Oxygenated treatment (OT)

Some once-through utilities that switched from AVTo ave repor e a sso ve ee wa er e

concentrations, which were often 10 ppb or higheron oxygen scavenger programs, dropped to as low

established.

In the most popular OT program, known asom ne a er rea men , oxygen s oseto maintain a 50 to 150 ppb residual. Ammonia isadded to raise the pH to 8.0-8.5. Typically, 20 to 70

.

OT is a very real consideration for once-throughHRSGs. One drawback is that OT requires extremely

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,use of a condensate polisher.


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pH Control, contd.

Ammonia and organic neutralizing amines are the pH.

Amines will decompose to produce ammonia infeedwater. Whether the ammonia comes from directammon a ee or am ne ecompos on, a generarule of thumb recommends that ammoniaconcentrations be limited to 0.5 ppm in systems

- .too high where excess air in-leakage may occur.

In systems with turbines, decomposition of aminescan po en a y n ro uce unwan e organ c ac sand CO2 into the turbine. Some experts recommendammonia over all other pH conditioning chemicals,

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Phosphate Treatment

Phosphate hideout and incongruent precipitation ofa3 4 on u e wa s can e pro ems.

TSP is most soluble at 250 F, but solubil ity rapidlyfalls off and becomes ver low at 600 F. The effectmay become very pronounced in units above 2,000psi and/or are subjected to frequent load changes.

Hideout depletes the boiler water of the chemicales gne o con ro c em s ry. e p osp a ecompounds often precipitate incongruently, withdeposit sodium-to-phosphate molar ratios of 2:1 or

" , .

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Phosphate Treatment, contd.

Hideout can be at its worst in a cycling unit.u owns an s ar ups, an even oa c anges,

cause precipitation and dissolution of phosphates.This in turn causes wide fluctuations in boiler water.

dissolution process have been known to force boilerwater pH below 7 in cycling units.

osp a e eou appears o e ur er n uenceby the cleanliness of the boiler tubes, and becomesmore severe with increased deposit loading,

.

sodium phosphates form a sodium-iron-phosphatecomplex with the magnetite layer. Periodic boilerchemical cleanings can potentially reduce hideout

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by minimizing the presence of porous magnetitedeposits.


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Phosphate Treatment, contd.

Lower heat fluxes in HRSGs tend to reduce POdeposition. Frequent cycling is a problem.

Equilibrium Phosphate Treatment (EPT) is designed.

maintained within a range of 0.2 to 2.5 ppm, and pHwithin a range of 9.0 to 9.7.

phosphate with supplemental addition of caustic,generally at unit startup. This treatment produces a

- - .

greater, with most of the control range at 3:1 orabove. Caustic alkalinity is from 0 to 1 ppm.

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HRSG W t Ch i t C t l td


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Caustic Treatment

In a caustic treatment program, boiler water pH isma n a ne w n a range o . o . y a on o

caustic to maintain an NaOH level of 1.0 to 1.5 ppm. Feedwater chemistr must be well controlled to

prevent excessive deposit formation in the boiler,which might lead to under-deposit caustic corrosion.

Sodium nitrate is frequently added with caustic topreven s ress corros on crac ng o o er u es.

As is the case with EPT, the higher levels of causticin the boiler water may lead to greater carryover of

sodium hydroxide to the turbine.

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HRSG W t Ch i t C t l td


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All-Volatile Treatment, contd.

AVT has lost a lot of popularity as it tends toncrease carryover o c or e an su a e, w c

then deposit on the LP turbine blades. In addition,amines and organic oxygen scavengers can break,

potentially corrosive to turbine blades and after-boiler components.

as proven o e muc e er a m n m z ngfeedwater system and boiler corrosion.

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AVT and Ammonical Corrosion

In systems where ammonia is used, ammonical

and the pH is over 8.3.

Copper, its alloys, and other non-ferrous metals area ac e an severe amage resu s ue o eformation of a stable cupric ammonium complex ion.

- 3 2 3 4

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Heat Recovery Steam Generators (Hrsg) and Treatment Chemistries