Power Plant Chemistry Measurement Advancements Orp

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Power Plant ChemistryMeasurement Advancements:

Oxidation Reduction Potential Abst rac t

Measurements which show the potential for corrosion of a plant’s

metallurgy due to interaction with the system water have gained

in importance within the last two decades. Among the most

successful of these measurements are those which measure an

actual potential which is based upon the oxidation/reduction

reactions taking place. The measurement of Oxidation Reduction

Potential (ORP) has been used extensively in both nuclear and

fossil fuel power plants to show the potential for corrosion in asystem. Optimum ORP levels are dependent upon system

metallurgy and water chemistry treatment. The system reaction,

and thus the ORP response, will vary depending upon a number

of factors, such as oxygen, hydrogen and oxygen scavenger

concentrations. Comparison of the oxidation/reduction potential

at various points in the system is useful for observing total

system response to variations in chemical addition and system

transients which can lead to more corrosive conditions in the

water.

Corrosion PotentialWithin the last 15 to 20 years, extensive work has been done in

both the nuclear and fossil fuel power industries to determine the

response of system metallurgies to water chemistry conditions.

Perhaps the most successful type of measurement used for

determining the corrosivity of the water to the system metal has

been the measurement of corrosion potential. This potential has

been observed to be affected by a number of factors, including

the activity of the oxidizing species (oxygen, copper), activity of

the reducing species (oxygen scavenger, hydrogen), type of

oxide layers present, temperature, and flow rate. 1-11

This method for obtaining a measurement of the potential for

corrosion utilizes a reference electrode, typically a silver/silver

chloride (Ag/AgCl) half-cell, and a measuring electrode, typically

platinum or some form of metal which is similar or identical to the

system metallurgy (such as a steel alloy). The names given to

this particular measurement vary extensively – corrosion

potential, oxidation/reduction potential (ORP), electrochemical

potential (ECP), or redox (another name for ORP).

It is important to understand that these methods all measure the

oxidizing or reducing potential of the system with respect to the

water. However, each of the listed names above specify a

method of obtaining the oxidation/reduction potential in a slightly

different manner.

ORP, also called redox, utilizes a platinum measuring electrode

with a Ag/AgCl reference electrode to measure the potential.

Most fossil plants and the secondary side of some pressurizedwater reactor (PWR) nuclear plants make this measurement at

ambient temperature through a sampling system, similar to the

measurements of pH, dissolved oxygen, or conductivity. Other

nuclear plants, especially those with a boiling water reactor

(BWR), make this measurement either in-situ (directly in the

process) or in a high flow, at-temperature autoclave system

which provides a sample which is exactly representative of the

process water.

The ECP measurement typically utilizes a Ag/AgCl reference

electrode and the system metallurgy itself as the measuringelectrode. By doing this, the “actual” potential of the system itself

is determined. This measurement is utilized extensively within

the nuclear industry. BWR plants often use the ECP as a control

point for the addition of hydrogen, and as such have found that

this is a more accurate method for ensuring the correct dosage

into the system. 6,11 PWR plants often use the ECP on the

primary side to observe variations in the primary water during

startup, steady state, and shutdown conditions. 6

The measurement of corrosion potential utilizes a Ag/AgCl

reference electrode and a measurement electrode of the samemetallurgy as that of the system metal (usually some form of

steel alloy). In theory, utilizing a measuring electrode similar to

the system metal, rather than platinum, should produce a

potential more representative of the system metal itself.

However, many plants have found the platinum electrode to

actually be equally or even more responsive and sensitive to

system changes. 1,2,5,6,8



Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential 2

In addition, the platinum electrode has been observed to maintain

its responsiveness to ORP variations longer than the other

electrode types.1 This measurement is typically used most by

the nuclear industry, and is usually measured in conjunction with

either the ECP or ORP measurements.

The focus of this paper will be primarily on the ORP (redox)

measurement, although the results obtained from ECP and

corrosion potential are mentioned, since these are usually seen

to be qualitatively, if not quantitatively, very similar. This

discussion will not address the obvious issues of differences in

results due to various sampling schemes, such as an ambient

temperature sampling system, an in-situ installation, or a high

flow, at-temperature autoclave.

In addition, the mV potentials in this paper will be reported in two

ways. The first method will be to show the units as milliVolts

(mV) followed by “SHE”, which stands for Standard Hydrogen

Electrode. This refers to a method of reporting the mV potential

with respect to a common standard, in this case the SHE. This

electrode is never used in industrial measurements, since it

requires a fixed partial pressure of hydrogen for its use. By

convention, its potential is usually defined as 0.00 V, regardless

of temperature. Since the Ag/AgCl is almost always used as the

reference electrode in the above measurements, a conversion

can be made which shows the actual potential measured (using

the Ag/AgCl) with respect to the SHE. The second method of

reporting used will not have any designation following the mV

value. This refers to a potential measured with respect to the Ag/AgCl electrode at sampling temperature, regardless of

whether the temperature was ambient or similar in temperature to

the process.

Oxidation-Reduction Potential

Whether the measurement is ORP, corrosion potential, or ECP,

the basic measurement principle is the same. Chemical

reactions which involve the transfer of electrons between

reactants are known as oxidation/reduction, or redox, reactions.

A species with lesser affinity for electrons in solution will lose

electrons, increasing its electrical charge. This species willbecome more positive in the oxidation reaction. This half-

reaction is termed the oxidation, because the initial reactions

studied during early investigations of oxidation/reduction involved

oxygen as the oxidizing species. A species with greater affinity

for electrons in solution will accept electrons, reducing its

electrical charge. This half-reaction is termed the reduction,

since the species becomes less positive. Equations 1 and 2

summarize the half reactions taking place.

Xm + pe - Xm-p (reduction -- gain of electrons) (1)

Yn Yn+q + q e- (oxidation -- loss of electrons) (2)

The X m represents the species which is reduced to X m-p by

gaining p electrons. The Y n represents the species which is

oxidized to Y n+q by losing q electrons. The species which gains

electrons is reduced, and acts as an oxidizing agent. The

species which loses electrons is oxidized, and acts as a reducing

agent.

In order for the whole oxidation-reduction reaction to occur, the

two half-reactions must occur simultaneously. If the example

above is simplified by assuming that both oxidation and reduction

reactions involve a single electron, Equation 3 can be written.

Xm + Y n Xm-1 + Y n+1 (standard redox equation) (3)

If a solution is strongly oxidizing, it has a deficiency of electrons

available and thus will attempt to acquire electrons. Likewise, if a

solution is strongly reducing, it has electrons available and will

attempt to give up electrons. The tendency for a solution to

donate or accept electrons can be sensed as an electrical

potential on an inert electrode.

If a solution is strongly oxidizing (electron accepting), it will

withdraw electrons from the measuring electrode’s metal surface,

creating a positive potential on the electrode surface. The ORP

will thus be more positive in solutions which have a higher activityof oxidizing reactants than reducing reactants. Likewise, in a

reducing solution, a more negative potential will exist since the

solution has electrons available which are donated to the surface

of the measuring electrode. The total potential measured is a

result of a multitude of redox reactions (some reversible and

some not), temperature variations, non-ideal electrode response,

and many other factors. It is therefore more useful to look at the

ORP reading from a comparison standpoint rather than a

theoretically accurate standpoint.

ORP in t he Power Plant Water/Steam Cycle

ORP can be used at various points within the water/steam cycle

to establish a baseline for determining the potential for corrosion

in the system. Figure 1 shows some of the common points

where ORP can be measured in a fossil-based plant. Sampling

points in a nuclear power plant would be similar. ORP testing at

various plants has shown the potential to vary most with changes

in dissolved oxygen and oxygen scavenger, since both affect the

oxidizing or reducing environment of the system.




Testing at BWR plants employing hydrogen water chemistry have

shown a direct relation between hydrogen and the corrosion

potential as well. It is important to note that the complexity of the

reactions associated with these parameters is such that a change

in any of the concentrations can potentially cause a variation in

the concentration of another.

Dissolved Oxygen

Dissolved oxygen plays an important part of the corrosion

process on metal surfaces in power plants. Oxygen is an

oxidizing substance, so it will directly affect the ORP levels in the

system water. Control points of the oxygen concentration vary

among different water chemistry treatments, but can typically be

summed up in two methods. The first method attempts to

remove as much oxygen as possible (typically down to 5 ppb or

lower) through some form of mechanical and/or chemical means.

The second method maintains a higher level of oxygen (typically

50 to 200 ppb). The type of method chosen will determine theexpected potentials.

At one PWR nuclear plant, oxygen concentration was maintained

below 10 ppb in the main condensate. 6 Shutdown of two

condenser halves was done for cleaning purposes. This resulted

in an increase in condensate temperature which in turn caused

an increase in the oxygen content of the condensate. The initial

increase in oxygen was from about 10 ppb to as high as 50 ppb.

ORP measurements in the downcomer tube bundle were seen to

increase from -200 mV SHE to -100 mV SHE. ORP

measurements at the high pressure feed water heater were seento increase from around -50 mV SHE to as high as +100 mV

SHE. When the condenser was returned to normal operation,

the condensate oxygen concentration dropped back down to

about 20 ppb. The downcomer ORP dropped down to around -

150 mV SHE, and the high pressure feedwater heater ORP

dropped down to about +10 mV SHE. However, the measured

oxygen at the feedwater remained constant at 2 to 3 ppb, and the

measured feedwater hydrazine concentration was seen to remain

constant at 130 ppb. The ORP was thus much more sensitive to

system changes at the feedwater and downcomer area than the

oxygen or hydrazine measurements.

The relationship between ORP and oxygen concentration was

observed at one fossil fuel plant. 2 Oxygen excursions occurred

due to an air ingress, with the oxygen level spiking from 10 ppb

to around 1 ppm, and then leveling off to 150 ppb for about 4

hours. The feedwater ORP levels were seen to rise sharply two

and a half hours later. The ORP increased 200 mV within about

half an hour, and then slowly decreased over the next six hours.

Testing was done at this plant to observe the relationship

between higher oxygen levels and the ORP value. The ORP

value was relatively low (-300 mV SHE) during normal operation

of oxygen concentrations of less than 5 ppb. As the oxygen level

was increased, the ORP level quickly increased to almost +75

mV SHE by the time 25 ppb oxygen was present in the system.

At 100 ppb of oxygen, the ORP value had leveled off at around

+100 mV SHE. Further increases above 100 ppb showed little

increase in the ORP.

From the previous two examples it can be observed that oxygen

directly affects the ORP of a system, as would be expected since

oxygen by definition will increase the oxidizing activity of the

system. ORP will therefore become more positive at higher

levels of oxygen. Figure 2 shows a theoretical example of what

might be expected from ORP readings in the condensate when

oxygen is introduced into the condenser. Notice the direct

relationship between oxygen and ORP. Notice also that as

oxygen levels exceed 100 ppb, the ORP does not increase much

further. This graph does not represent actual data at a plant;

however, it does represent results of the oxygen vs. ORP

relationship that have been observed at numerous plants.

Oxygen Scavenger

Oxygen scavengers, such as hydrazine, serve a primary purpose

of chemically removing oxygen from the system. However, they

can also be useful for corrosion prevention since the presence of

most oxygen scavengers will create a more reducing

environment in the system. The results of testing performed at anumber of power plants, both nuclear and fossil based, show the

response of ORP to changing oxygen scavenger concentrations.

At one nuclear PWR plant, hydrazine transients were observed

when flow was lost from the injection pumps. 1,10 Hydrazine

injection took place in the feedwater system, immediately after

the point where the condensate demineralizers can be inserted

into the lineup. The highest levels of hydrazine were observed in

the feedwater system before there could be much reaction of

hydrazine with oxygen or other oxidizing contaminants, and

before there could be any appreciable thermal breakdown ofhydrazine. During this time, the ORP of the feedwater varied

only by 10 mV, while a large change was seen in the downcomer

portion of the steam generators (see Table 1). The effect that

hydrazine had on the ORP of the downcomer water was directly

related to the length of the excursion and the change in

hydrazine level. The relationship between more positive ORP

readings and lower hydrazine concentrations was obvious. Note

that the hydrazine levels are given for feedwater hydrazine.




A previous check of hydrazine levels had given downcomer

hydrazine levels of 54 and 79 ppb where feedwater was 100 ppb.

It was noted at one fossil plant that sample locations with higher

hydrazine residuals experienced more reducing (more negative)

potentials. 4,12 The hydrazine concentration was initially 40 ppb

and was reduced over a period of time to zero. The ORP was

seen to have a direct correlation with the level of hydrazine.

While the level of hydrazine was at 40 ppb, the ORP reading was

approximately -340 mV. When the hydrazine feed was stopped

completely, the ORP was seen to rise to about +80 mV, where it

leveled off. Large step changes of the hydrazine concentration

correlated to large step changes in the ORP. At another fossil

plant, the ORP levels at the economizer inlet were recorded

before and after hydrazine feed was cycled on and off. An

excellent correlation of ORP vs. hydrazine was observed, with

ORP readings of around -80 mV with hydrazine and +60 mV

without hydrazine. 4

An interesting effect of the oxygen scavenger and ORP

relationship was noted at a few nuclear PWR plants. (Note:

Although reported as ECP, both plants tested ORP as well and

reported qualitatively similar results. As such, the actual

potentials were somewhat different in magnitude, but the

response was basically the same.) In the secondary side of one

plant, the ECP values were observed in the final feedwater for

various concentrations of hydrazine residual. 8 The ECP was

approximately -100 mV SHE when there was no hydrazine

present in the water. As the hydrazine residual increased, theECP value dropped exponentially until it leveled off at

approximately -500 mV SHE at 100 ppb of hydrazine. Increasing

the hydrazine concentration beyond 100 ppb did not serve to

reduce the ECP any further. A similar test was performed in the

secondary side of another plant. 5 At two of the sample points

(condensate pump discharge and low pressure feedwater pump

discharge), a similar effect to that seen at the other plant was

observed. The condensate sample point’s ECP did not vary from

around +200 mV SHE until the hydrazine concentration was

increased to above 15 ppb. When the hydrazine concentration

was increased above 15 ppb, the ECP began droppingexponentially until it leveled off around -50 mV at 100 ppb of

hydrazine. At the feedwater sample point, the ECP dropped

linearly from 0 mV SHE at 0 ppb hydrazine down to -350 mV

SHE at 15 ppb hydrazine. When the concentration was

increased above 15 ppb, the ECP actually was observed to

increase slightly, leveling off at about -300 mV SHE at

approximately 100 ppb hydrazine.

It should be noted that an increase in ORP with increasing

hydrazine dosage is not normal, and could possibly indicate

something other than hydrazine affecting the ORP reading.

It is important to understand that when no oxygen scavenger is

present, the ORP value of the water is largely dependent upon

the oxygen concentration in the water. However, when the

oxygen concentration drops much below 5 ppb due to oxygen

scavenger addition, the ORP value becomes more of a function

of the oxygen scavenger concentration. Figure 3 shows a

theoretical example of what might be expected from ORP

readings in the final feedwater when hydrazine concentration is

varied in the feedwater. Notice the inverse relationship between

hydrazine and the ORP value. Notice also that as hydrazine

levels exceed 100 ppb, the ORP does not decrease much

further. This graph does not represent actual data at a plant;

however, it does represent results of the hydrazine vs. ORP

relationship that have been observed at numerous plants.

Hydrogen Water Chemistr y

In BWR nuclear plants, hydrogen is often added to the feedwater

to avoid stress corrosion cracking of stainless steel piping. It has

been shown that the intergranular stress corrosion cracking

(IGSCC) of sensitized stainless steel can be minimized by

maintaining the ECP below the critical potential, typically -230

mV SHE in high purity water. 6,11,13 Hydrogen addition to the

feedwater decreases the amount of oxidizing species in the

reactor water by recombination in the downcomer, and thus

reduces the production of oxidizing species from the radiolysis ofwater in the core. 11,13 The addition of hydrogen to the feedwater

creates a more reducing potential (more negative) due to

decrease of oxidizing species which results from the increased

hydrogen concentration. The potential of BWR water with

“normal” hydrogen water chemistry has been observed to be

typically in the range of +50 to +200 mV SHE. 6,11 Only at

increased dosages of hydrogen will the potential be seen to drop

down to well below the -230 mV range. 6,11,14 It has been

observed that the potential can only be accurately measured in-

situ for proper hydrogen dosage control. 11

Location

Typical power plant water has different properties, such as pH,

oxygen / oxygen scavenger concentrations, or temperature,

depending upon the location in the system. Because of the

varying characteristics of the water, as well as different

metallurgy used throughout the system, the ORP response can

change significantly from one point to another.




A change in chemistry in one portion of the system may cause no

change in the ORP at one point and drastically alter the readings

at another point. This has been observed at many plants.

Although the majority of the ORP measurements have been

made in the feedwater or economizer, other points in the process

are also important to monitor for better characterization of the

overall system response to varying changes, such as load

increases, leaks, or excursions.

At one nuclear plant utilizing a Pressurized Water Reactor

(PWR), readings were taken at various points on two redundant

systems on the secondary water. 1,10 These points included the

Feedwater, Downcomer, Hotleg, Coldleg, and Demineralizer

Influent. An example of varying system responses can be seen

by examining Table 1. Testing of ORP response of the system to

hydrazine excursions revealed an interesting system response

between the feedwater and the downcomer portions. As can be

seen from Table 1, feedwater hydrazine levels dropped

considerably during excursions. The ORP response at the

downcomer sample point was significant, typically rising by as

much as 100 mV during each excursion. However, the effect of

loss of hydrazine flow on ORP was minimal in the feedwater

system. ORP readings differed by no more than about 10 mV

each time.

Another PWR-based nuclear plant measured the ORP response

at the outlet of the condensate pumps (point #1), the outlet of the

feedwater pumps (point #2), and the outlet of the high pressure

heaters (point #3).5

During steady state conditions, the potentialafter the condensate pumps was typically -150 to -100 mV SHE.

The potential after the feedwater pumps was typically -350 to -

200 mV SHE. The potential at the outlet of the high pressure

heaters was typically -550 mV SHE.

At the same plant, some transient conditions were introduced to

observe the effects on the potential at the three sampling points. 5

The first transient was characterized by an increase in the

condensate oxygen concentration (typically 6 ppb) and a

decrease in the feedwater hydrazine concentration (typically 70

to 100 ppb). This effect was caused by leakage of oxygen intothe condenser or the steam itself. The redox potential at point #1

was observed to increase by about 20-100 mV, with the larger

increases seen with larger increases in oxygen concentration and

larger decreases in hydrazine concentration. Point #2 was

observed to increase in potential by about 10-50 mV,

characterized by the same situation as point #1. The third point

had no measurable increase in potential for any of the above

transient conditions.

The second transient was characterized by an inleakage of

oxygen from systems connected to the feedwater line after the

condensate pumps (and after the first point of ORP monitoring). 5

A decrease in hydrazine concentration was also noticed during

this type of transient. Each measuring point had a significant

increase in potential during the time of the transient, with the

second point showing the most noticeable increase. The

increase at points 1 and 3 were about the same. The same

effect was noticed at another plant. 8 In both plants, the dissolved

oxygen monitor for the feedwater showed no appreciable

increases, while the potential measurements were seen to

increase at multiple points.

Locational testing during changes of hydrazine dosage was also

performed at this plant. 5 The hydrazine was dosed from after the

condensate pump, at a location immediately after the first

sampling point. An immediate change was noticed in the second

sampling point, as the potential dropped linearly from

approximately 0 mV SHE down to -350 mV SHE as the hydrazine

concentration was increased from 0 to 15 ppb. As the hydrazine

concentration was continually increased to as high as 150 ppb,

the potential slowly increased from -350 mV SHE to about -300

mV SHE. The third sampling point remained at approximately -

400 mV throughout the range of 0 to 150 ppb hydrazine. The

first sampling began at approximately +200 mV SHE and

remained constant until about 15 ppb of hydrazine had been

introduced. At that dosage point, the potential began to drop in a

slow, exponential fashion to a final potential of around -50 mV

SHE at 150 ppb of hydrazine.

Another nuclear plant utilizing a Boiling Water Reactor (BWR)

experienced high potentials (typically between 0 to +200 mV

SHE) during normal hydrogen water chemistry conditions. 11

These potentials did not vary at different points in the system.

However, upon addition of hydrogen at what was considered to

be normal rates for hydrogen water chemistry, the potentials first

decreased at sample points distant from the core, while the

potentials near to the core remained relatively high. The

corrosion potential was thus seen to be considerably different at

various points. As hydrogen dosage was increased to muchhigher than the normal rate, the entire system eventually dropped

to a very low potential (typically less than -300 mV SHE), with

little variance among sample points, although some higher

corrosion potentials were still seen in the core.

An interesting phenomenon was observed in measuring different

sample points at one fossil fuel-based plant that converted from

All-Volatile Treatment to Oxygenated Treatment (OT).




During AVT treatment, ORP readings were taken at the Hotwell (-

60 mV), Final Feedwater (-100 mV), Boiler Water (-80 mV) and

Main Steam (-80 mV). Readings differed by more than 100 mV

at various sample points in the plant, and it was observed that

the sample locations of more negative potentials had higher

hydrazine residuals. 4 Upon switching to OT, the readings were

about +100 to +120 mV at each point. 12 The resulting common

readings were expected, since the water chemistry would be

similar throughout the system.

It is obvious from the results seen in the previous examples that

the multiple location measurements can play an important part in

determining the overall cause and effect of various transients and

variations to water chemistry in the system. It is expected that

load variations on the system could also generate results that

vary from point to point. Regardless of the cause for the

variation, multiple points are useful for sensing overall system

response to process changes and upsets.

ORP And Corrosi on

ORP has a strong relation to corrosion within a system, since the

reactions associated with corrosion are oxidation/reduction

based. Any oxidants or reductants present in the system are

directly linked to corrosion production in a system. Optimal

concentration limits of oxidants and reductants vary depending

upon the type of metallurgy and the type of water chemistry

treatment being done at a plant. Figure 4 shows an example of

the optimal control ranges for ORP at the feedwater sampling

point in both nuclear and fossil plants for various water chemistrytreatments.

The trend in past years at most plants has been to remove as

much of the dissolved oxygen as possible, which almost always

requires addition of an oxygen scavenger such as hydrazine.

This can have a detrimental effect in all-ferrous plants, since

removing oxygen by oxygen scavenger will lead to a strongly

reducing environment (-300 mV or lower), which has actually

been seen to increase the erosion/corrosion of iron-based

materials as well as increasing the transported feedwater

corrosion products.3,4,9

It has been observed that high levels offlow-accelerated corrosion (FAC) occur in all-ferrous plants when

the ORP is less than -300 mV due to either an oxygen level of

less than 1 ppb or oxygen scavenger level of greater than 20

ppb, or both. 9

The primary oxidation reaction on steel surfaces is the oxidation

and dissolution of iron:

Fe Fe 2+ + 2e - (5)

The two primary reduction reactions on steel surfaces are the

reduction of hydrogen (Equation 6) and the reduction of oxygen

(Equation 7):

2H + + 2e - H2 (6)

O2 + 2H 2O + 4e - 4OH - (7)

The reduction of hydrogen is seen to be favored at more

reducing potentials while the reduction of oxygen is favored at

higher (more oxidizing) potentials. 15 The optimal ORP control

point appears to be somewhat dependent upon the pH of the

plant water. 2,7 It has been shown that providing a less reducing

environment in all-ferrous plants at the recommended pH levels

(typically 8-10 pH depending upon the specific water chemistry

treatment) will minimize corrosion product generation. 3,4,7,9 This

can be done by lowering oxygen scavenger levels or eliminating

the oxygen scavenger levels and adding oxygen. This process

converts the magnetite (Fe 3O 4) to ferric oxide hydrate (FeOOH),

which has a much lower solubility. 9

At one plant, the ORP increased from -340 mV to +100 mV when

the level of hydrazine was reduced from 40 ppb to zero. The

total iron decreased from 14 ppb to about 5 ppb.4

At anotherplant, which has an all-ferrous system, hydrazine feed was

discontinued to reduce iron transport. The ORP was observed to

increase from -125 mV to -50 mV when the hydrazine dosage

was dropped from 20 ppb to 0 ppb. The feedwater pH of 9.2 to

9.6 remained the same during the test period. Iron transport

through the feedwater cycle did decrease from an average of

about 3 ppb to less than 1 ppb within an 8 month time period

following the change in treatment. 10

Some plants with all-ferrous metallurgy have begun switching to

an Oxygenated Treatment (OT) in which a small concentration ofdissolved oxygen, typically 30-150 ppb, is maintained in order to

minimize corrosion. Conversion to OT will correspond to an

oxidizing environment typically on the order of +100 mV or

more. 3,4 This has been shown to eliminate flow accelerated

corrosion by forming a protective oxide layer on the material

surface. 9




One plant experienced an increase of 500 mV during the

transition from reducing to oxidizing operating conditions, during

which the corrosion rates dropped to two to three times lower

than those experienced during reducing conditions.2 This highly

oxidizing environment is expected since no oxygen scavenger is

present and higher concentrations of dissolved oxygen are

present, both of which raise the ORP. All-ferrous plants

switching to OT and thus more positive ORP levels have

experienced significant drops in corrosion products.

Systems with mixed metallurgy have been found to have minimal

corrosion occurring when a more reducing (more negative)

potential is present. 3,16 While understanding of copper alloy

corrosion is not yet adequate, a relationship with ORP has been

established. Cuprous oxides (Cu 2O) and cupric oxides (CuO)

can form on copper base alloys. Formation of cuprous oxide

provides a protective barrier adjacent to the metal surface.

Cupric oxide can form by oxidation of the cuprous ions. Cuprous

oxide formation is thus preferred at lower reducing potentials,

while a more oxidizing environment will support the growth of the

cupric oxide. Reducing regimes are thus preferred in mixed

metallurgy environments. Even when dissolved oxygen levels

are kept below the 7 ppb limits typical of mechanical deaeration,

an oxygen scavenger such as hydrazine, which lowers the

reducing potential, should be maintained, or serious copper

deposition problems can occur. 3

This effect was observed at one plant which has a copper nickel

condenser and all-ferrous feedwater heaters.10

Hydrazineinjection had been terminated in an attempt to reduce the

potential for erosion/corrosion and iron transport through the

boiler cycle. A slow loss in turbine efficiency was observed after

the change. It was theorized that oxygen in the boiler water

converted copper in boiler deposits allowing it to volatize and

deposit on the first stages on the HP turbine. Hydrazine

injection was then re-established with a goal on maintaining a

more reducing environment in the feedwater and boiler water.

It has been observed at many plants that optimization of the

hydrazine level in copper-based systems can be done with theORP measurement in order to prevent copper attack by

excessive hydrazine levels.6 ORP monitoring has been used

while chemically cleaning boilers to ensure the proper

environment to dissolve iron oxide, insure passivation of clean

surfaces, and prevent precipitation of copper. Monitoring has

also been used in ammoniated EDTA cleanings to prevent

corrosion following the iron removal stage. 11

Conclusions

ORP has been seen to be useful in determining the response of

the system metallurgy to the water chemistry. The ORP has

been observed to be affected most by the activity of oxidizing

(oxygen) and reducing (oxygen scavenger, hydrogen) species in

the water. The ORP of a system has been observed to be

directly related to various types of corrosion, such as flowaccelerated corrosion (FAC) in all-ferrous plants, intergranular

stress corrosion cracking (IGSCC) in BWR nuclear plants, or

cupric oxide formation in mixed metallurgy plants. The optimum

ORP control point has been seen to vary somewhat from plant to

plant, largely depending upon type of water treatment,

concentration of oxidizing and reducing species, type of

metallurgy, and location of the measurement.

ORP has been observed to be equally or more sensitive to

system transients as traditional measurements of hydrazine and

oxygen concentration. It is expected that ORP will become astandard measurement at multiple points throughout a plant’s

water/steam cycle, and will be used along with measurements of

pH, oxygen and oxygen scavenger concentration in order to

optimize the water chemistry.




Figure 1: ORP Sampling Poin ts in a Fossil-based Power Plant

Figure 2: ORP vs. Oxygen Comparison for Condenser Oxygen Ingress*

* Note: This graph does not contain actual data from plant results. It represents an example of typical results that have been observed in a number ofplants

Blowdown

ChemicalFeed

Condenser

H-P TurbineI-P Turbine

Boiler

H-P Heaters L-P Heaters

Deaerator

ORP

ORP

ORP

ORPChemical

Feed

Economizer

CondensatePolishers

SaturatedSteam

Cooling Water Drum

Super

Reheater

heater

Hotwell

ORP

ORP

0

20

40

60

80

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Time, hours

O x y g e n ,

p p

b

-200

-150

-100

-50

0

50

100

150

200

O R P

, m

V S H E

Oxygen, ppb

ORP, Condensate




Figure 3: ORP vs. Hydrazine Comparison for Changes in Hydrazine Dosage *

* Note: This graph does not contain actual data from plant results. It represents an example of typical results that have been observed

in a number of plants.

Figure 4: Typical Oxidation/Reductio n Potentials

Range 1: Pressurized Water Reactor Nuclear Plant, Primary Side (mV, SHE)

Range 2: Boiling Water Reactor Nuclear Plant, High Hydrogen Addition (mV, SHE)

Range 3: Boiling Water Reactor Nuclear Plant, Normal Water Chemistry (mV, SHE)

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Time, hours

H y

d r a z

i n e ,

p p

b

-400

-350

-300

-250

-200

-150

-100

-50

0

50

100

O R P

, m

V S H E

Hydrazine, ppb

ORP, mV SHE

-800 mV -600 mV -400 mV -200 mV 0 mV 100 mV 200 mV

Range 4Range 5

Range 6Range 1

Range 2 Range 3




More Information

For more information on ORP, visit

www.honeywellprocess.com , or contact your

Honeywell account manager.

Honeywell Process SolutionsHoneywell1250 West Sam Houston Parkway SouthHouston, TX 77042

Honeywell House, Arlington Business ParkBracknell, Berkshire, England RG12 1EB UK

Shanghai City Centre, 100 Junyi RoadShanghai, China 20051

www.honeywellprocess.com

Range 4: (a) Pressurize Water Reactor Nuclear Plant, Secondary Circuit (mV, SHE)

(b) Mixed Metallurgy Fossil Fuel Plant, Oxygen Scavenger Addition (mV, Ag/AgCl)

Range 5: All-Ferrous Fossil Fuel Plant, No Oxygen Scavenger Addition (mV, Ag/AgCl)

Range 6: All-Ferrous Fossil Fuel Plant, Oxygen Addition (mV, Ag/AgCl)

Feedwater Hydrazine, ppb Downcomer ORP, mV

Date Time Before During Duration Before During

10/2/94 5:00 110 20 2 Hours -100 +10

11/3/94 15:30 115 30 20 Minutes -60 +22

11/4/94 3:00 110 20 60 Minutes -58 +86

11/9/94 13:30 140 40 40 Minutes -58 +64

11/13/94 21:00 110 50 20 Minutes -80 +10Table 1: Downcomer ORP Excursi ons Durin g Hydrazine Transients

SO-13-18-ENGJanuary 2013© 2013 Honeywell International Inc.

http://www.honeywellprocess.com/






Documents

Power Plant Chemistry Measurement Advancements Orp