8/13/2019 You dont have to like chemistry, but you have to respect it

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You dont have to like chemistry, but you have to

respect itPosted onSeptember 9, 2013byTeam CCJ

Cycle chemistry demands particularly close attention for units

experiencing weekday starts and weekend (or longer) shutdowns.Attendees at the last meeting of theAustralasian HRSG Users Groupwere polled regarding their two-shifting experience (shutdowns of four to

six hours) and what return-to-service/standby chemistry issues theywere facing (if any). One user said his 175-MW plant was designed forbase-load service but had been two-shifting/peak-chasing the last fiveyears. The 2 x 1 station has only one HRSG and it is of the once-through

design. Thus its optimal cycle chemistry is different from that used indrum-type boilers. However, this plant also was dealing with chemistryissues and had suffered stress corrosion cracking in the steamer.

One of the worlds leading powerplant chemists reminded the group ofthe two basic rules for two-shifting:

1. Dont allow the redox potential to change. In case youve forgottenwhat this term means, redox, or oxidation reduction potential (ORP),

is a measure of the nature and strength of an aqueous environment inmillivolts (positive when oxidizing, negative when reducing).

2. If using an oxidizing cycle, dont change pHthis to minimize theamount of corrosion-product transport. Its all about the interfacial

science, he said. You want to maintain the oxide in a stable condition.

Another chemist suggested holding vacuum between the operating

cycles if possible and to run makeup water through a membranedegasser to minimize its oxygen content. The first chemist added that

users should not worry about CO2 ingress.

AVT(O).An attendee wanting a better understanding of how all-volatiletreatment (oxidizing) works asked: Do you just run with no reducing

agent and rely on air ingress (which may be very minimal) or should youhave oxygen injection? One of the top chemists responded. It does not

matter, he said; the original concept for AVT(O) was to make it different

compared to AVT(R, reducing)specifically, eliminate the use of adestructive reducing agent. However, experience with AVT(O) has shownthat very low oxygen levelsthat is, less than 5 ppb at the condensate

pump dischargeis not optimal.

Ideally, the consultant continued, you want about 10 ppb at that point in

the system, from air or oxygen. He also suggested that users monitorsystem health by checking drum oxide color (red is good) and ensuring

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8/13/2019 You dont have to like chemistry, but you have to respect it

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that all surfaces touched by water are passivatedthis to obtain a

hematite dominated film.

Another chemist in the room said it was his experience that some plants

admit air on the vacuum side of the condensate pump, others inject

oxygen manually or via automatic control. He suggested that some valvetweakers may pose risks if manual adjustment of oxygen content isemployed. An automatic injection system is better, the chemist said.

There was considerable give-and-take on the subject. One user offeredthat in switching coal-fired units from AVT(R) to oxygenated treatment in

the early 1990s, plant personnel found there was insufficient oxidizingpower with AVT(O) and air was added. No issues.

An OEM representative remarked that for units with an LP-drumfeedwater tank system, CO2 ingress will give high cation conductivity(CACE, for conductivity after cation exchange) in LP steam and that thiscauses confusion as to whether carryover is occurring or not. The

chemists responded. One said CO2 is not an issue; you have to

understand what is causing the elevated CACE. Another suggestedtesting carryover using ion chromatography (IC) to get at the root cause

of the problem. But if you do this, he said, be sure you know if youresampling saturated or superheated steam.

A user then commented that OEMs give chemistry specifications forsteam, but IC analysis conducted to determine the reason for high

readings of cation conductivity at his plant showed no issues with sulfateor chloride so they continued to operate. Another user in the audience

held up the caution flag regarding operation outside of OEM limits. Arecent steam-turbine failure, he said, illustrated the importance of

documenting such operational decisions. You need to put in an officialrecord the justification for not running within OEM limits in case a legal

issue develops in the future.

Mention was made of ongoing work by theInternational Association for

the Properties of Water and Steam(IAPWS) that would soon provide

users validated guidance on issues discussed during the foregoingexchange. Stay tuned for developments.

Chemical cleaningis a subject about which relatively little has beenwritten regarding HRSGs. An attendee asked: Whats the best chemicalfor cleaning the steam/water side of HRSGsinorganic acid (such as HCLand HF) or organic acid (citric or formic)? Are boil-out, flushing, and

steam/air blowing alternatives?

There was no clarification as to whether the question pertained to a pre-

commissioning chemical clean or one required by poor operating

http://www.iapws.org/http://www.iapws.org/http://www.iapws.org/http://www.iapws.org/http://www.iapws.org/http://www.iapws.org/

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practices over the years. Most in the group offering an opinion assumed

the former, judging from their comments. An OEM engineer said thechoice of cleaning solvent often depends on environmental disposal

options and cost, adding that a degrease stage might not be sonecessary today. That almost certainly would be the case for a unit that

had a service history.

A user said it was necessary to do a full clean rather than just a

degrease, which most respondents apparently agreed with. Another OEMengineer said decisions on chemical cleaning normally are made by the

EPC contractor. That likely would be the case in the US, but notnecessarily the way things are done elsewhere.

A chemist with deep knowledge said solvent selection is site- andproject-specific, the former impacting accessibility of chemicals and

waste disposal. Experience in Australia and New Zealand, he said, has

been with citric acid, HF, and HCl. Of great importance, the chemistadded, is a detailed specification that defines the scope (Is the full steam

path included?), various steps in the processsuch as flushing, ironremoval stage, passivation stage, flush, and storageand the metrics for

determining when the unit is considered clean.

The discussion dug deeper with an OEM engineer mentioning that

superheater cleaning often is avoided in the US because of chromiumdisposal concerns. Regarding the waste stream from the cleaning of

evaporator panels made of steel containing small amounts of chromium,he was unaware of any issues. Yet another HRSG supplier was said to

have identified no issues associated with the cleaning of T91superheaters.

A second chemist joined the information exchange noting there may berisks associated with cleaning the superheaters of future plants because

of unknowns associated with the effects of solvents and inhibitors onadvanced materials.

While the conversation seemed to support the notion that less might be

more with regard to chemical cleaning, a naysayer took the floor citingunder-deposit corrosion failures in the preheater sections of one unit that

was not cleaned after construction 10 years earlier. One of the two mostvocal chemists said its not unusual to find heavy deposits in preheaters

and he didnt see that as a general problemunless the deposits weresuper heavy, as no boiling should occur in the preheater. This is unlike

the condition in an evaporator, where concentration mechanisms areconducive to under-deposit corrosion with heavy tube deposits and poor

water chemistry.

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The subject of air blows versus steam was the next topic. A user said he

preferred steam because of the poor performance experienced with airblows. An OEM reported good experience with air blows, with no

operational issues identified after commissioning. There are manydifferent variations of steam blows, a consultant noted: continuous blow,

intermittent blasts, low pressure, high velocity, etc. However, no advicewas offered on what was most effective and under what conditions.

When planning for boiler cleaning, the group was told, be sure to haveplenty of makeup water available in temporary tankage onsite. Supply

often is the limiting factor for steam blows.

To sample tubes, or not?Practical questions and insightful discussion

are a hallmark of AHUG. What are people doing in regards to HPevaporator tubes: sample or borescope inspection? Very difficult to

justify engineering to allow for tube sampling was the first reply, from a

user. Another owner/operator said his company has taken HP evaporatortube samples from all units in its fleet. First time was in 2010 after a

base-load plant had operated for 10 years. Samples were taken high upand to the sides; duct burners are in the center of the tube bundle.

A consultant said discussion on the subject at another user groupmeeting suggested that the heaviest deposits might not be found high up

in the tube. Another user apparently agreed. It sampled two tubes in oneunit, the first tube center and top, second tube sample center and side.

No issues were identified.

Yet another owner experienced several evaporator-tube failures and its

consultant reported lots of variability in tube condition. It was movingtowards use of a borescope to identify areas of concern. A chemist

questioned this looming decision. He considered it risky, saying youtypically find during a proper analysis that which you see bears no

relationship to the results from a scanning electron microscopetheinformation needed to determine when to clean a boiler.