Close Attention to ACCs During Commissioning Pays Dividends.pdf

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COMBINED CYCLE JOURNAL, First Quarter 2004 41

STARTUP OPERATIONS

Close attentionto ACCs duringcommissioning paysdividends

By Lee Coll, PSC Commissioning Services Group

Nearly 90% of the powerplants orderedin the last few years have been gas-turbine-based peakers, cogenerationplants, and combined-cycle facilities.

Last has accounted for the lion’s share of thegenerating capacity. Many of these combined-cycle plants have been built or are under con-struction in the West to support the populationexplosion. Most are in water conservation areas.

A benefit of combined-cycle plants is that aboutonly one-third of their output is produced by thesteam (Rankine) cycle, so water consumption bydesign is substantially less than that for a largecoal-fired or nuclear station. Installing an air-cooledcondenser (ACC) instead of a surface condenser/wetcooling tower arrangement affords an opportunityto reduce water use to an absolute minimum.

ACCs have a rich European history. All of themajor suppliers have offshore roots: GEA PowerCooling Systems, San Diego; Balcke Duerr (now

owned by Marley Cooling Technologies Inc, Over-land Park, Kan); and Hamon, Somerville, NJ. Firstpowerplant in the US equipped with an ACC isthe 20-MW Simpson 1 unit installed in 1969 (seesidebar). Until the combined-cycle boom, only a fewrelatively small generating units deviated from thetraditional surface condenser.

Many of the combined-cycle units are merchantplants which means they operateonly when customers need powerand they can offer the best price.High reliability and high availabil-ity are paramount. Until recently,industry operating experience with ACCs was thin but it has improveddramatically in the last couple ofyears.

What operators have learned isthat ACCs have intrinsic charac-teristics that present challengesnot normally encountered duringplant startup. For example, theircarbon steel tubes are more suscep-

tible to atmospheric corrosion thanthe stainless, titanium, and copper/nickel alloys found in water-cooledsystems. Thus it is not uncommon to

find corrosion products in the downstream conden-sate and feedwater systems.

Potential problems: Localized waterside sites thatare not properly passivated may become active cor-rosion pits that eventually compromise the condens-er system. Construction debris and loose scale oftenresult in the fouling of condensate-pump strainers. Also, contamination from dust trapped in the systemduring fabrication and silica oxides from weldingconsumables are conducive to poor condensate qual-ity during the first few weeks of operation.

Successful cleaning

Two recently completed power projects (see tablefor descriptions) used a new process to clean the ACC during the steam-blow phase of commission-ing and thereby prevented downstream corrosionand contamination issues from occurring. In bothcases, the process converted atmospheric rust to aprotective film of protective magnetite on carbonsteel surfaces, eliminated particulate contamina-tion and construction debris, and rinsed silica con-tamination from the ACC and condensate piping.

These objectives were achieved in less time andwith less fuel and water than traditional methodswhile eliminating the steam plume and unaccept-able noise normally associated with commission-ing. Disposal of the condensate rinse water did notpresent a pollution problem.

Implementing the process involved:■ Routing the steam at the steam-turbine (ST)valves and bypasses via temporary piping to theST exhaust duct and then to the ACC.■ Directing steam flow through ACC “streets”via mechanical and process controls.■ Introducing an all-volatile chemical mixture to

convert the atmospheric rust to magnetite andpassivate the ACC.■ Balancing steam flow, quenching, and ACC oper-ation to optimize the conditions at which passiv-ation occurs and hydraulic flushing is conducted.The ability of the cleaning process to achieve the

desired cleanliness at the low loads associated withthe steam blowing activity afforded the advantage of

Details of combined-cycle projects commissioned

Plant 1 Plant 2

ACC manufacturer GEA Balcke Duerr

HRSG manufacturer Vogt Power Doosan

Plant arrangement Two on one Two on one

GT manufacturer GE GE

ST manufacturer Alstom Alstom

Steam duct diameter, in. 144 288

Design steam flow (max), 1000 lb/hr 1340 980

Steam-blow flow (max), 1000 lb/hr 450 315

Duration of steam blows, hr 100 85CFR achieved 1.36 1.34

Duration of passivation stage, hr 24 18

104ACC indd Sec2:41104ACC.indd Sec2:41 2/11/04 1:39:27 PM2/11/04 1:39:27 PM



42 COMBINED CYCLE JOURNAL, First Quarter 2004

STARTUP OPERATIONS

easily obtaining the reduced backpressure required.This condition is difficult to obtain if steam isbypassed to the ACC via the normal bypass valves.

System preparation. During the steam blows,temporary piping is used to route steam to theexhaust duct at the steam bypass nozzles. Tempo-

rary diffusers replace the equipment provided fornormal operation to avoid damaging the fine perfo-rations with foreign material.

Resources to bypass the deaerator and conden-sate tank beneath the ACC dictated filtration andpumping equipment that matched the capacity of

A ir-cooled condensers (ACCs) are beingspecified for an increasing number of com-bined-cycle plants today. Lee Coll of PSC

Commissioning Services Group, Baton Rouge,

La, who wrote the accompanying article on pre-operational cleaning of ACCs, says that in thelast three years, the number of plants relying onair-cooled condensers has jumped nearly four-fold, from 4% of new plant orders to 15%.

The reasons are several:■ Many new plants are being built in the West,where water resources typically are strained byrapid population growth and limited supply.■ Thermal discharges to many natural water-ways are limited. In New York City, for example,KeySpan Corp was denied a permit to dischargeheat from its new Ravenswood combined-cyclefacility (see Powerplant Awards section in thisissue) into the East River and opted to install an ACC on the roof of the space-challenged facility.■ In some locales, a sensitive ecosystem mili-tates against installation of an evaporative cool-ing system. This category may be expanded toinclude fog-prone areas with high populationdensities where cooling-tower drift is conduciveto hazardous driving conditions.■ Combined-cycle plants have relatively small

steam systems—ones more suitable for ACCapplication than those characteristic of coal-firedand nuclear stations. ACCs have come a long way since the first unit

installed at a US electric generating plant wascommissioned at Black Hills Power Inc’s (RapidCity, SD) 20-MW Neal Simpson 1 in 1969. It is stilloperating today. Simpson 1 essentially served asa demonstration plant for Wyodak Generation Sta-tion, Gillette, Wyo, jointly owned by PacifiCorp,Portland, Ore (80%), and Black Hills (20%).

Wyodak was where ACCs came under themicroscope. Extrapolating from the 20-MW Simp-son 1 to the 330-MW Wyodak, which started up in1978, was perhaps “irrational exuberance.” Wyo-dak remains today the largest ACC in the nation, bya significant margin. For more than its first decadeof service, the plant was hampered by operatingproblems with the ACC—including load restrictionsattributed to poor performance, freezing (completewith cracked tubes, etc), and off-design efficiency.One by one the problems were solved and sincethe early 1990s the plant has performed well. Avail-

ability and capacity-factor numbers in the mid 90sreportedly are the norm today.

At least some engineers close to the project

think that many of the problems—such as thoserelated to the distribution of water flow within thetubes and to thermal expansion—were magni-fied by the quantum leap in size. They point to the

much better track record from initial operation of ACCs of similar design, but half the capacity orsmaller. At most new two-on-one combined-cycleplants, for example, the steam-turbine/genera-tor generally is rated about 150 MW and, whereinstalled, ACCs have performed well.

The entire industry benefited from the operat-ing experience at both Simpson 1 and Wyodak. Toillustrate: Designers learned from Simpson 1 that2 in. elliptical tubes are too small for successfulwintertime operation and 4 in. tubes are the normtoday, having gained experience at Wyodak. Suc-cessful freeze-protection strategies, variable-fre-quency drives for cooling fans and sophisticatedsystems to control their operation, and a flow-con-trol device to ensure proper distribution of waterthroughout the condenser are among the manycontributions these plants have made to the ACCknowledge base.

Industry “buzz” for years was that ACCs havefreezing problems. Interestingly, a little researchindicates that’s more the exception than the rule.For example, engineers at a 22-yr-old, two-on-one

combined-cycle facility in Alaska say they don’thave icing problems, not even when they lose agas turbine. All they do is back off on the fans (shutdown some, reduce the speed of others). Experi-ence of several others is similar.

At one plant in Massachusetts that has experi-enced a freeze-up, operating personnel were ableto thaw out tubes over a two-day period—whilekeeping the plant in operation. What they did wasto shut down fans, restrict air flow through the unitwith a temporary plywood structure, and operatethe steam turbine/generator at the highest back-pressure possible. The plant never tripped andthere was no permanent damage.

The main challenge plant managers seem toface with ACCs is that summer power generation islimited, particularly in hot, humid areas. One PM inthe Northeast told CCJ that while his facility’s win-tertime rating was 330 MW, the plant could be lim-ited to 280 MW on a particularly difficult day. Thereare cooling-system enhancements can be added toboost summer output—for example, a wet helpertower.

But the cost of such enhancements, where pos-sible, must be weighed against the potential reve-nue gain and the option of adding peaking turbines.

Air-cooled condensers: Why they’re hot




COMBINED CYCLE JOURNAL, First Quarter 2004 43

STARTUP OPERATIONS

the plant’s condensate system (diagram above).Portable vacuum pumps were used to initiate vacu-um and minimize backpressure during steam blow-ing and operation of the ACC.

Operation of the ACC fans also is required duringthe cleaning process. Bear in mind that the routingof steam through specific streets can be controlledby duct valves, but not all systems have the largevalves required for this purpose. The same objec-tive can be achieved by operating selected fans onindividual cells. The ability to route most of thesteam through a particular street and through spe-cific cells allows hotter steam to penetrate furtheralong the steam path, thereby maintaining thehigh temperature needed to achieve the desired

metal passivation.Process details:■ Prepare gas turbine (GT), heat-recovery steamgenerator (HRSG), and ACC for operation afterreviewing process steps in detail.■ Conduct green-rotor runs on the first GT,directing any steam produced to the exhaustduct and into the ACC. Supply feedwater fromthe demineralized-water storage tank.■ Pull a vacuum on the ACC. Collect conden-sate formed and route it to a temporary storagetank.■ During steam-blow operations, collect conden-sate from the ACC, filter for gross debris, and pol-ish for recycle. Introduce the volatile passivationchemicals via the quench water to the exhaustduct, making sure to monitor the condensate forpH and concentration of reducing agent.■ Cool specific rows, or “streets,” to selectivelydistribute steam through the ACC. Fan opera-tion assures sufficient hydraulic loading of eachportion of the steam circuit to efficiently flushand passivate each cell.

■ Monitor the process by sampling condensatefor conductivity and total suspended solids (forexample, filter return condensate through Mil-lipore discs). Watch carefully for the change in

condensate color. Initially it should be a dirtyred, gradually converting to black, and then to auniform gray appearance. Last indicates a pas-sive magnetite film.Results. At both of the projects, by the time the

combined steam blow of both HRSGs was complet-ed, quality of the condensate returning from the ACC was sufficient to allow recycle of untreatedcondensate to the condensate tank without adverseimpact on the HRSGs and condensate pumps. Bothplant chemists monitored silica and conductivity

levels and found that they exceeded expectations.Some minor considerations specific to ACC

design, HRSG size, and piping design resulted inslight procedural changes between the two plants.For example, control of steam flow was enhancedon the first project because the exhaust-duct mani-fold was valved. The second project offered chal-lenges until fan operation was refined. Note thatthe first project took longer to passivate because its ACC was larger than the one at the second site.

Other benefits. During HRSG and ACC com-missioning, the correct operation of several plantauxiliary systems was verified earlier than is nor-mally done. Other activities required as a part ofstartup operations were advantageously scheduledand completed during the steam blow and ACC pas-sivation, including these:■ Hogging ejector operation was verified becausethey were used—in addition to the temporaryvacuum pumps—to remove air from the systemduring ACC passivation.■ Burnout of the HRSG fireside, which is requiredbefore SCR catalyst loading, was achieved dur-

ing the low-load operating period to achieve sys-tem cleaning.■ Valve-related problems were identified andcorrected during the steam blows. CCJ

Quenchwaterinjection

Condensatesampling

Targetinsertionspool

High-pressure steam

Intermediate-pressure and hot-reheat steam

Low-pressure steam

HRSG

Condensate

Air-cooled condenser

Duct

Deaerator

Polishingfilters

Temporarycondensate pumps

Vacuumpump

Cold reheat

ST

Dewateringbox

Temporary system arrangement to ensure thor-ough pre-operational cleaning of an air-cooled con-denser is relatively simple to install. Process allowscleaning in less time and with less fuel and waterthan traditional methods


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