ULTRAPURE WATER® July 2016 1

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TECH BRIEFINGCHANGING BOILER FEEDWATER CHEMISTRY AND ITS IMPACT ON

METAL TRANSPORT

By Steven A. Dunn (Process Performance Management)

BackgroundA large three unit (2,400 pounds per square inch gauge [psig] drum) coal-fired generating station had recently been experiencing very high iron removal levels during recent boiler chemical cleanings. The units have full-flow, deep-bed condensate polishers with copper-based metals in the condenser and the first three feedwater heaters. The costs of the cleanings increased rapidly in recent years, spurring efforts to make changes to reduce iron corrosion product formation and transport to the boiler.

It is well known in the industry that most of the metal transport to a boiler originates from the pre-boiler system. The cause of the recent increase in iron levels during boiler chemical cleanings is thought to be due to the changing metallurgy of the pre-boiler feedwater heaters. Like many other power plants, this station is replacing the original cop-per metal based feedwater heaters with iron metal-based feedwater heaters as the old heaters fail. The change in the type of metallurgy in the pre-boiler system requires a change in pre-boiler chemistry (if possible) to minimize iron corrosion and iron metal corrosion product trans-port to the boiler.

Copper-based metals in pre-boiler sys-tems normally require low oxygen (re-ducing conditions) for minimum copper metal corrosion and corrosion product transport and thus, the normal industry practice of feeding a reducing agent (oxygen scavenger like hydrazine) to the

pre-boiler system to maintain reducing conditions in the boiler feedwater. With iron-based metallurgy in the pre-boiler system, minimum iron corrosion and iron corrosion product transport is achieved through somewhat higher oxygen levels, creating an oxidizing condition in the boiler feedwater. In mixed-metallurgy systems (some copper-based and some iron-based feedwater heaters), a balance has to be achieved between oxidizing and reducing conditions to achieve minimum total metal corrosion product transport to the boiler.

Older power plants were generally built with copper-based feedwater heaters. In these plants, the high-pressure heaters tended to fail first and were generally replaced with iron-based heaters. Over a period of years, as the copper-based feedwater heaters failed, they were generally replaced down the line from the boiler with iron-based heaters, until only the very lowest pressure feedwater heaters and the condenser remained copper based. Because the remaining copper in the system remains in low-

est pressure and temperature part of the pre-boiler system, sometimes the oxygen in the system can be allowed to rise somewhat, reducing iron corrosion, without causing a significant increase in copper corrosion. The generation units at the subject station are mixed metal-lurgy with the copper remaining in the low-pressure/temperature parts of the pre-boiler systems.

Because of the metallurgy situation in the pre-boiler systems, it was thought that allowing the oxygen to rise somewhat in the boiler feedwater, creating oxidizing conditions, would greatly reduce the iron metal corrosion transport in the system. It was hoped that because the remaining copper in the system was in the low-pressure/temperature part of the pre-boiler system, that moving to oxidizing conditions in the boiler feedwater would not significantly increase copper corro-sion, and thus the total metal transport to the boiler would be reduced. To test the theory, a metal corrosion product transport study was commissioned to monitor the iron and copper levels at the

Figure 1. The general arrangement of the metal transport samplers behind the water sample panels (A), and interior of the metal sampler (B).

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Figure 2. Typically exposed sample filters (A), and column regeneration station (B).

Figure 3. Atomic absorption spectrophotometer (A), and automatic sampling carousel used with the spectrophotometer (B).

boiler economizer inlet and in the main steam to the turbine while the pre-boiler system was changed from reducing to oxidizing conditions. The main steam was monitored to ensure that increased amounts of oxygen did not cause sub-stantially increased amounts of copper to be transported from the boiler to the steam turbine.

Study DesignMetal oxide levels in pre-boiler systems are generally in the micrograms per liter (µg/L) (parts per billion [ppb]) levels. Because measuring metals at ppb levels accurately and consistently is nearly im-possible, composite samplers are often used to bring metal levels up into the milligrams per liter (mg/L) (parts per mil-lion [ppm]) range, where metals are much easier to measure. Composite samplers generally consist of a filter for collecting particulate metals and a resin column for collecting the dissolved metals.

A highly accurate flowmeter is normally included for recording the amount of water that passes through the sampler while taking samples. The exposed filters are acid digested and the columns are regenerated, with the resulting solutions brought up to 1 L so that the metals concentration in the resulting solutions will be in mg/L (ppm). The analysis results are then di-vided by the liters through the sampler to determine the actual concentration of metals in the sampled streams. The Electric Power Research Institute (EPRI) has set recommended levels for iron and copper in boiler systems at levels that are expected to prevent problems with boiler deposits, these levels are 5 ppb for iron and 2 ppb for copper.

TestingThe metal transport testing of the first unit (Unit 1) started on October 13, 2015. There were two metal transport samplers installed on Unit 1 with one sampler installed on Unit 2 as a control in case there were unexpected changes in boiler makeup water. The samplers were installed on the economizer inlet sample lines on Unit 1 and Unit 2 with the third sampler installed on the Unit 1 main steam sample line. Figure 1 (A) shows the general arrangement of the metal transport samplers behind the

water sample panels, while the left photo in Figure 1 (B) pictures the interior of the metal sampler.

Figure 2 (A) shows typically exposed sample filters, while Figure 2 (B) is a pic-ture of the column regeneration station.

The atomic absorption spectrophotom-eter pictured in Figure 3 (A) was used for sample analysis. Figure 3 (B) shows the automatic sampling carousel used with the spectrophotometer.

Unit 1 ResultsThe Unit 1 boiler feedwater system was allowed to start converting to oxidiz-ing conditions on November 14, 2015. Figures 4 and 5 provide testing results. Note the dividing line between reducing and oxidizing conditions.

It can be observed that the copper increased somewhat when the unit was switched to oxidizing conditions, but the later copper levels were well below the

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conditions in the Unit 1 boiler feedwater also has the added benefits of eliminating the oxygen scavenger chemical feed. This eliminates a chemical cost, removes a hazardous chemical from the plant site, and ends or simplifies chemical feed system maintenance. Also, the increased oxygen greatly reduces the potential for flow assisted corrosion (FAC), increasing the safety at the plant.

Because of the positive results on Unit 1, the samplers were moved to Unit 2 and then to Unit 3 for metal transport testing in these units. Very similar re-sults to Unit 1 were achieved on Unit 2 and Unit 3 when they were switched to oxidizing conditions in the boiler feed-water systems.

The testing proved that in some cases, having some copper in the lower tem-perature/pressure end of the feedwater system does not preclude the benefits of having oxidizing conditions in the boiler feedwater for reducing iron transport, chemical costs and the potential for FAC failures.

Author Steven A. Dunn is a senior chemistry consultant for Process Performance Manage-ment of Denver, CO. He has more than 31 years

of experience in managing coal-fired power plant boiler, pre-boiler, and cool-ing water system chemistries. He gradu-ated from Peru State College (Peru, NE) in 1980 with a BS in physical science.

Key words: BOILERS, COPPER, CORROSION, IRON, MAINTENANCE, METAL OPERATIONS, TRANSPORT, POWER, TUBING

Figure 5. Unit 1 main steam total metals.

Figure 4. Unit 1 economizer inlet total metals.

2-ppb EPRI guideline. As expected, there was a large reduction in iron transport. The forth particulate iron result under reducing conditions gave a value that was unbelievably low compared to the other data and was dropped from the data set.

Figure 5 provides data for the Unit 1 main steam samples. Note that the cop-per levels were essentially unchanged and remained well below the 2 ppb EPRI guideline for copper. The iron

may have dropped under oxidizing conditions because of lower iron in the boiler feedwater.

SummaryAs can be seen from Figures 4 and 5, the iron transport in the Unit 1 boiler feed-water system has been cut in about half without a substantial increase in copper transport in either the boiler feedwater or the main steam. Moving to oxidizing