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Home arrow Geography arrow Consensus on Operating Practices for Control of Water and Steam Chemistry in Combined Cycle and Cogeneration Power Plants: From the Center for Researc
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DERIVING CHEMISTRY OPERATING LIMITS

Specific HRSG configurations impose equally specific requirements for water and steam chemistry. The operating chemistry limits in this document are meant to provide a starting point from which each plant can develop its own plant-specific limits. Lower pressure HRSGs that provide process steam have very different requirements from high- pressure HRSGs that drive steam turbines for power generation. Selection of appropriate plant chemistry limits is greatly influenced by the presence of those pieces of equipment that are particularly sensitive to deposits and corrosion, the first and foremost being the steam turbine.

STEAM TURBINE

The steam turbine may be fed with steam from HP, IP and LP superheaters of one or more HRSGs and the purity of steam supplied to the turbine is one of the most critical parameters in the plant. The objective of the steam chemistry program is to prevent corrosion and deposits in the steam turbine. The presence of sodium hydroxide (caustic soda), sulfate, or chloride in the steam can cause turbine blade or rotor failures from stress corrosion cracking. Chlorides may also lead to pitting and corrosion fatigue.

The formation of deposits reduces turbine efficiency by creating turbulence and restricting flow and may become so severe that blades fail because of deposit-caused vibrations. Recent research shows that deposits and corrosion can occur over a broader range in the turbine than previously thought. Areas of high flow rate can become subcooled to the point that condensation begins on blades that otherwise would be above the condensation line. Flow stagnation can lead to areas of superheat in cooler sections of the turbine.

The steam chemistry is the fixed point in the cycle. Regardless of the operating pressure, source of the steam, attemperator water flow, number of drums or makeup system, the steam chemistry limits must be met to maintain the reliability of the turbine. For reference, limits from various turbine manufacturers are shown in Table 4. These limits apply to condensing steam turbines, not back-pressure turbines. The chemistry requirements of back-pressure turbines are less stringent.

Steam (HP, IP, LP )

Alstom

GE(1)

Mitsubishi

Fuji

Siemens

Reheat

No reheat

Specific conductivity at 25°C, pS/cm

3-11

NS

NS

NS

NS

NS

Cation conductivity at 25°C, pS/cm

< 0.2

<0.15

<0.25

< 0.3

<0.3

< 0.2

pH at 25°C

9.0-9.6 (2)

NS

NS

NS

9.0-9.3

NS

Silica as Si02, ppb (pg/kg)

<20

<10

<20

< 10

<10

<10

Total iron as Fe, ppb (pg/kg)

<20

NS

NS

< 20

<20

<20

Total copper as Cu, ppb (pg/kg)

<3

NS

NS

< 2

<3

<2

Sodium as Na, ppb (pg/kg)

<10

<3

< 6

< 5

<10 (3)

<5

Chloride as Cl, ppb (pg/kg)

NS

<3

< 6

< 5

NS

NS

Sulfate as S04, ppb (pg/kg)

NS

<3

< 6

NS

NS

NS

Oxygen as 02, ppb (pg/kg)

NS

NS

NS

< 10

<10

NS

TOC as C, ppb (pg/kg)

NS

<100

<100

NS

NS

NS

  • (1) Using phosphate treatment (greater than 2.6 Na:P04 molar ratio) or AVT
  • (2) pH 9.0 - 9.3 with copper-alloy condensers
  • (3) Na+K NS is not specified

The most critical of these parameters are sodium, silica, chloride and sulfate. Chloride and sulfate concentrations are typically measured indirectly by determining the cation conductivity of the steam. Cation conductivity is also influenced by organic and inorganic anions, such as acetate and carbonate. Unlike chloride and sulfate, carbonate species are volatile and can be removed by heating or purging the sample with an inert gas prior to measuring the cation conductivity. When so treated, the parameter is referred to as degassed cation conductivity. The general consensus is that degassed cation conductivity is valuable during commissioning and for troubleshooting. The most direct and accurate method for measuring chloride and sulfate is by ion chromatography.

 
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