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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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RECOMMENDED WATER/STEAM CHEMISTRY LIMITS FOR HRSG

There are a variety of HRSG configurations and operating pressures. For the purposes of this document two configurations have been considered: an industrial HRSG and a three-drum HRSG where the steam is used in a steam turbine to generate electrical power. These are shown in Figures 1-2. The chemical limits are always dictated by the steam purity requirements, the highest drum pressure on the HRSG, and, if applicable, by the highest pressure at the duct-fired condition.

Although the temperature of the turbine exhaust is much lower than the flue gas temperatures in an equivalent water-tube boiler, sudden heat flux changes and inefficient circulation in HRSGs, particularly during start-up, require a higher purity feedwater and a lower chemical treatment rate than would typically be associated with the respective operating pressures.

Depending on the configuration, the LP drum may be isolated from the higher-pressure drums and require its own chemistry or may be, in effect, a feedwater heater that feeds all the higher-pressure drums in the unit. In the latter case, only volatile chemicals can be used for treatment in the LP drum as water from the LP drum is often used as at- temperation water for superheat and reheat. The intermediate pressure (IP) drum often receives blowdown from the high-pressure (HP) drum, therefore the IP drum chemistry is to a certain extent set by the HP drum chemistry. The HP drum chemistry is determined by its operating pressure when operating at its highest capacity.

The term "drum water" is equivalent to "boiler water" for the purposes of this document.

Tables with suggested limits can be found at the end of this document. These limits do not contain any safety factor to provide reasonable response time for an excursion.

Recommendations regarding sampling and analysis can be found in the ASME "Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers" (CRTD-81) and "Steam and Water Sampling, Conditioning, and Analysis in the Power Cycle" (ASME PTC 19.1 1-2008).

Parameter

Normal

Comments

Specific conductivity at 25°C, [j,S/ cm

<0.1

Carbon dioxide will absorb into demineralized water upon standing and will increase the conductivity. High conductivity can be due to acid or caustic contamination

Silica as SiO?, ppb (ng/l)

<20

Once-through HRSC require <10 ppb

Sodium as Na, ppb (ng/l)

<5

TOC as C, ppb (ng/l)

<300

Table 7. Soft Water to Storage Tank (1)

Parameter

Normal

Comments

Unneutralized specific conductivity at 25°C, [iS/cm

<120% of incoming water conductivity

Total hardness as CaCOs, ppm (mg/l)

<0.2

Some on-line hardness analyzers measure calcium and not total hardness

(1) Water should be free of hydrogen su

fide

Parameter

Normal

Comment

Materials

Mixed

All Ferrous

Mixed metallurgy systems contain copper and iron alloys

Cation conductivity at25°C/ iS/ cm

<0.2

<0.2

Assumes no contribution from treatment chemicals

pH at 25°C

8.8-93

9.4-10.0

Higher pH values may be required to produce a pH of 9.4 in the LP drum

Unneutralized specific conductivity at 25°C, pxS/cm

Specific conductivity should be consistent with the pH (1)

Silica as Si02, ppb (pxg/l)

<10(2)

<10(2)

Limit shall be consistent with OEM purity requirements (2). Cation conductivity is the primary method for detecting inorganic contaminant inleakage but will not detect silica. Silica testing may be required for troubleshooting purposes.

Iron as Fe, ppb (pxg/l)

<10

<10

Copper as Cu, ppb (pxg/l)

<3

Not Applicable

Dissolved oxygen as 02, ppb (M-g/l)

<20

<20

Sodium as Na, ppb (pxg/l)

<5

<5

TOC as C, ppb (pxg/l)

<300

<300

Excludes contributions from treatment chemicals. Plant experience will dictate

  • (1) pH=log10(SC-0.3CC)+8.57; where SC is specific conductivity and CC is cation conductivity
  • (2) Refer to Table 4 or your specific OEM limits

Parameter

Normal

Comment

Materials

Mixed

All- ferrous

Mixed metallurgy systems contain copper and iron alloys

Cation conductivity at 25°C, ^S/cm

<0.4

<0.4

For demineralized makeup, assumes no contribution from treatment chemicals

Sodium as Na, ppb (pig/l)

<5

<5

For demineralized makeup

pH at 25°C

8.8-9.3

9.4-10.0

Unneutralized specific conductivity at 25°C, ^S/cm

Normal HRSG feedwater +5 ^S/cm (for soft water makeup)

Silica as Si02, ppb (pxg/l)

<10

(2)

<10 (2)

Limit shall be consistent with OEM purity requirements (2). Cation conductivity is the primary method for detecting inorganic contaminant inleakage but will not detect silica. Silica testing may be required for troubleshooting purposes.

Turbidity, NTU

<1

<1

Provides on-line trending for particulate excursions such as iron, but is not a replacement for specific analyses

Iron (3) as Fe, ppb (fxg/l)

<20

<20

Copper as Cu, ppb (ng/l)

<10

Not Applicable

TOC as C, ppb (fxg/l)

<300

<300

Excludes contributions from treatment chemicals

  • (1) The above values represent typical contaminant values and will vaiy by application. Ultimately, feedwater limits for individual parameters must be met. Condensate with high contaminant levels or high percentage condensate return may require some form of polishing to be acceptable as feedwater. Actual acceptable values must be determined on an individual site basis.
  • (2) Refer to Table 4 or your specific OEM limits
  • (3) Low pH of the return condensate increases iron contamination in the HRSG. Additional steps may be required to ensure an elevated condensate return pH.

Parameter

Mixed

All-ferrous

Comments

Dissolved oxygen (1) as 02, ppb (pig/l)

<10

2-10

Mixed system presumes oxygen scavenger is added.

ORP(1), mV

-100 to -350

0 to 50

Optimize for minimal iron and copper pickup. Values based on saturated KCI, Ag/AgCI-reference electrode at 25°C at the feed pump discharge

Total iron as Fe, ppb (pxg/l)

<10

<10

As low as possible

Total copper as Cu, ppb (fxg/l)

<3

<3

As low as possible

TOC as C, ppb (fxg/l)

<300

<300

pH at 25°C

8.8-9.3

9.6-10.1 (2)

Specific conductivity should be consistent with the pH (3)

Cation conductivity at 25°C, ^S/cm

<0.2

<0.2

Degassed cation conductivity may be used for diagnostic purposes.

Silica as Si02, ppb (pxg/l)

<10(4)

<10(4)

Limit shall be consistent with OEM purity requirements (4). Cation conductivity is the primary method for detecting inorganic contaminant inleakage but will not detect silica. Silica testing may be required for troubleshooting purposes.

  • (1) Critical in this area is the control of oxidation-reduction potential and pH. Excessive use of chemical oxygen scavengers can create an environment where flow accelerated corrosion can occur
  • (2) For units operating with AVT chemistry, the feedwater pH must be greater than 9.4 to obtain an LP drum pH >9.4 to minimize FAC
  • (3) pH=log10(SC-0.3CC)+8.57; where SC is specific conductivity and CC is cation conductivity
  • (4) Refer to Table 4 or your specific OEM limits

Drum Operating Pressure, psig (MPa)

psig 0-300 (MPa 0-2.07)

  • 301-450
  • (2.08-
  • 3.10)
  • 451-600
  • (3.11-
  • 4.14)
  • 601-750
  • (4.15-
  • 5.17)
  • 751-900
  • (5.18-
  • 6.21)

Comments

Dissolved oxygen ppm (mg/l)

< 0.007

< 0.007

<0.007

<0.007

<0.007

Total iron as Fe, ppm (mg/l)

<0.1

<0.05

<0.03

<0.025

<0.02

Total copper as Cu, ppm (mg/l)

<0.05

<0.025

<0.02

<0.02

<0.015

Total hardness as CaCOv ppm (mg/l)

<0.3

<0.3

<0.2

<0.2

<0.1

pH at 25°C

8.3-10.0

8.3-10.0

8.3-10.0

8.3-10.0

8.3-10.0

TOC as C, ppm (mg/l)

<1

<1

<0.5

<0.5

<0.5

Excludes added treatment chemicals. Carryover or foaming may require lower TOC values

Table 10a. Drum Water for Multi-Drum HRSG using Demineralized Water and Phosphate-based Treatment (1, 2)

Parameter

Drum pressure, psig (MPa)

Comments

<75

(0.52

MPa)

  • 76-300
  • (0.52-2.1

MPa)

301-900 (2.1-6.2 MPa)

  • 901-1550
  • (6.2-10.7

MPa)

  • 1551-2400
  • (13.8-16.6

MPa)

For units that increase operating pressure when they are duct-fired, use limits associated with >1500 psig

pH at 25°C (3)

9.8-10.5

>9.0 and phosphate feed consistent with residual found in drum

The HRSG must come off line in an orderly manner as quickly as possible if the pH cannot be maintained above 8

Phosphate (4) as P04, ppm (mg/l)

10-20

5-20

2-15

2-10

0.2-3

See Section 5 for further discussion of sodiunrphosphate molar ratio

Minimum IMa:P04 molar ratio

2.8

Maximum free caustic (5) as NaOH, ppm (mg/l)

8

4

2

1

1 ppm maximum NaOH (as NaOH) conforms to phosphate continuum at all pressure levels.

Unneutralized specific conductivity at 25°C, ц-S/ cm

<140

<120

<75

<50

<30

Determine by operating experience.

Silica as Si02, ppm (mg/l)

Per silica curves shown in Figures 3-4(6)

  • (1) For cases where the LP drum does feed other drums, the chemistry of the LP drum should be the same as the feedwater chemistry. Recommendations assume that the LP drum is on a separate steam circuit from higher pressure drums i.e., LP drum does not feed the IP or HP drum. If LP drum feeds attemperation—no phosphate should be added.
  • (2) Phosphate treatment is not recommended for units operating above 2400 psig (16.6 MPa)
  • (3) The pH at 25°C of the drum should always be greater than 9.0 after the effect of ammonia is accounted for. See text for calculation.
  • (4) Phosphate residual should be adjusted to prevent hideout.
  • (5) Lower caustic levels may be required for units with severe concentrating mechanisms or mechanical carryover. Captive alkalinity requires Na:P04 molar ratios less than 3.0. See phosphate treatment discussion for exceptions to the recommended treatment ratio.
  • (6) Calculated from formulas presented by Beardwood, E. S., "Silica in Steam Generating Systems," Proceedings of the International Water Conference, 2008, Paper No. IWC-08-12.

Parameter, Units

LP Drum, if LP Drum Feeds Other Drums

HP and IP Drums, All Configurations and LP Drum that Does Not Feed Other Drums

Normal Values (Cation Conductivity <1 pS/cm)

Limits if IP and HP drum cation conductivity is >1

pS/cm

Drum pressure, psig (MPa)

All pressures, all drums

All pressures, all drums

<1800 psig (<12.4 MPa)

1 800-2400 psig (12.4-16.5 MPa)

Cation conductivity at 25°C, pS/cm

Approximately same as feedwater

<1

<5

<3

pH at 25°C (minimum)

Approximately same as feedwater (specific conductivity and pH can be lower due to ammonia loss in the steam. Minimum pH 9.4)

Consistent with drum pH and ammonia/amine feed

Consistent with ammonia/amine volatility. Usually 0.1 -0.2 pH units less than the feedwater pH, preferably at economizer outlet

Unneutralized specific conductivity at 25°C, pS/cm

Consistent with drum pH and ammonia/amine levels

Silica as Si02, ppb (fxg/l)

Approximately same as feedwater

Per silica curves shown in Figures 3-4 (2)

Per silica curves shown in Figures 3-4 (2)

Per silica curves shown in Figures 3-4 (2)

  • (1) For cases where the LP drum does feed other drums, the chemistry of the LP drum should be the same as the feedwater chemistry. The use of AVT on an isolated LP drum is NOT recommended. In systems containing copper alloys, the LP drum should not be the source of feedwater to the higher-pressure drums if the unit is operating on AVT. Consistent high-purity feedwater is critical when using this treatment. Condensate polishers are highly recommended.
  • (2) Calculated from formulas presented by Beardwood, E. S., "Silica in Steam Generating Systems," Proceedings of the International Water Conference, 2008, Paper No. IWC-08-12.

Drum Operating Pressure

psig 0-300 (MPa) (0-2.07)

  • 301-450
  • (2.08-3.10)
  • 451-600
  • (3.11-4.14)
  • 601-750
  • (4.15-5.17)
  • 751-900
  • (5.18-6.21)

Silica as Si0„ ppm (mg/l)

<150

<90

<40

<30

<20

Total alkalinity (2) as CaCOv ppm (mg/l)

<700

<600

<500

<200

<150

Free OH alkalinity (1) as CaCOv ppm (mg/l)

Not Specified

Not Specified

Not Specified

Not Specified

Not Specified

Specific conductivity (3) at 25°C, pS/cm

5400-1100

4600-900

3800-800

1500-300

1200-200

Total dissolved solids in steam (4,5,6), ppm (mgd)

1.0-0.2

1.0-0.2

1.0-0.2

0.5-0.1

0.5-0.1

  • (1) Minimum hydroxide alkalinity concentrations in boilers below 900 psig (6.21 MPa) must be individually specified by a qualified water treatment consultant with regard to silica solubility and other components of internal treatment.
  • (2) Maximum total alkalinity consistent with acceptable steam purity. If necessary, should override conductivity as blowdown control parameter.
  • (3) Maximum values are often not achievable without exceeding maximum total alkalinity values, especially in boilers below 900 psig (6.21 MPa) with >20% makeup of water whose total alkalinity is >20% of total dissolved solids (TDS) naturally or after pretreatment by sodium cycle ion exchange softening. Actual permissible conductivity values to achieve any desired steam purity must be established for each case by careful steam purity measurements. Relationship between conductivity and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values.
  • (4) Achievable steam purity depends on many variables, including Evaporator water total alkalinity and specific conductivity as well as design of steam drum internals and operating conditions [Note (3)]. Since HRSCs require a high degree of steam purity for protection of the superheaters, more stringent steam purity requirements such as process steam restrictions on individual chemical species or restrictions more stringent than 0.1 ppm (mg/l) TDS turbine steam purity must be addressed specifically.
  • (5) As a general rule, the requirements for attemperation spray water quality are the same as those for steam purity. In some cases boiler feedwater is suitable; however, frequently additional purification is required. In all cases the spray water should be obtained from a source that is free of deposit forming and corrosive chemicals such as sodium hydroxide, sodium sulfite, sodium phosphate, iron, and copper. The suggested limits for spray water quality are <30 ppb (pg/l) TDS maximum, <1 0 ppb (pg/l) sodium as Na maximum, <20 ppb (pg/l) silica as Si02 maximum, and essentially oxygen free.
  • (6) TDS expressed as ppm (mg/l) in steam is proportional to TDS expressed as specific conductivity in the boiler water in each pressure range. For example, 5400 pS/cm in the boiler water is expected to produce 1 ppm (mg/l) TDS in the steam and 1100 pS/cm in the boiler water is expected to produce 0.2 ppm (mg/l) TDS in the steam for boilers operating at 0-300 psig.

Table 12. Example of Gas Turbine Injection Water for NOx reduction in combustion section (1)

Parameter

Normal

Unneutralized specific conductivity at 25oC, pS/cm

0.5

Cation conductivity at 25oC, pS/cm

<0.5

Silica as SiO2, ppb (pg/l)

<100

Na + K, ppb (pg/l)

<20

Calcium as Ca, ppb (pg/l)

400

Chloride as Cl, ppb (pg/l)

250

Fe + Cu, ppb (pg/l)

10

(1) Water must contain no suspended solids

Table 13. Example of Direct Injection Fogging Water (1)

Parameter

Normal

Comments

Cation conductivity, pS/cm

<0.25

If higher cation conductivity is experienced, degassed cation conductivity values can be used. Makeup tank specification must also be met.

Silica as SiO2, ppb (pg/l)

<20

Sodium as Na, ppb (pg/l)

<10

(1) Water must contain no suspended solids

Parameter

Normal

Calcium as CaCO3, ppm (mg/l)

50-100

Total alkalinity as CaCO3, ppm (mg/l)

50-100

Chloride as Cl, ppm (mg/l)

<50

Silica as SiO2, ppm (mg/l)

<25

Total iron as Fe, ppm (mg/l)

<0.2

Total petroleum hydrocarbons as hexane extractables, ppm (mg/l)

<2

Total dissolved solids, ppm (mg/l)

30-500

Suspended solids, ppm (mg/l)

<5

pH at 25oC

7.0-8.5

(1) Other fills may have different water requirements. See manufacturers' recommendations.

50 ?

 
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