Dissolved Oxygen Measurement In Fossil Power Plants
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Many analytical methods of monitoring the concentration of Dissolved Oxygen have been explored and improved upon over the years to help mitigate the effects of corrosion presented by dissolved oxygen throughout the Steam/Water Cycle, but empirical evidence shows that despite these technical advancements... applying the proper analytical technology based on the process conditions and following maintenance procedures is of utmost importance for maximizing Corrosion Prevention. By the end of this report, you’ll discover why dissolved oxygen materializes in your plants process, what to consider when selecting a technology and how to ELIMINATE User Error with a Hands Free AUTOMATED SOLUTION that monitors the concentration of Dissolved Oxygen with pinpoint accuracy!
THE HYDROLOGICAL CYCLE
The Hydrological Cycle of Earth is a lot like the process you’d find inside a power plant. As the sun heats up bodies of water, the water evaporates into the atmosphere and gathers to form clouds. When the water vapor in the clouds has cooled enough to where it condenses, it begins to rain. Because this falling rain is pure or neutral (pH7), it’s called ‘Hungry Rain’...due to its high affinity for Oxygen and Carbon Dioxide.
A Steam Boilers Hydrological Cycle is very similar to earth. When the water inside the boiler is heated to create steam, much like how the sun evaporates bodies of water, three destructive elements occur...
Free Oxygen is released from the boiler water
Carbon Dioxide that’s captured inside of solids is also released into the steam
Solids drop out into the boiler (Precipitate)
This Free Oxygen and Carbon Dioxide are then carried by the steam throughout different locations in the plant. As the steam cools, condensation occurs and produces ‘Hungry Condensate’, much like the hungry rain falling from the clouds. Being that gases are more soluble at lower temperatures, as this condensate cools, any free oxygen, carbon dioxide or other gases present become much easier to dissolve into the water. This ‘Hungry Condensate’ is a key reason why Dissolved Oxygen becomes present in the Steam-Water Cycle.
Why Is Dissolved Oxygen Such A BIG Concern!?!? ~ YOU
CORROSION
When excess Oxygen is dissolved into the condensate, corrosion begins forming pits or holes with rusty nodules called “tubercles” in the iron pipe. Not only that, but when Carbon Dioxide is absorbed into the condensate it forms Carbonic Acid, or in the Earth’s Hydrological Cycle it’s more commonly referred to as ‘Acid Rain’. This excess CO2 in the condensate line begins to eat away and erode a trench into the bottom of the iron pipe. This iron that’s been eaten away by oxygen and carbon dioxide corrosion is carried by the pumped condensate to the boiler where it becomes an insulating deposit (Boiler Scale) and causes a number of other issues such as...
Restrictive Heat Transfer
Boiler Tube Heat Stress
Wasted Fuel
Oxygen corrosion occurs when water temperatures cool below 212 Deg. F (at atmospheric pressure). The concentration of oxygen absorbed into the condensate is proportional to its temperature and pressure. For example...
70° F water at 0 psig can contain oxygen approximately equal to 8.6 ppm
150° F water at 0 psig can contain oxygen approximately equal to 4.3 ppm
212° F water at 0 psig can contain oxygen approximately equal to 0.0 ppm
In order to remove this oxygen to prevent corrosion, other equipment such as a deaerator and boiler feedwater heater are employed to bring the oxygen levels down. Ideally, at the economizer or boiler inlet, dissolved oxygen concentration should be at or near 0 ppb. However, instances have occured where Dissolved Oxygen analyzers, specified to accurately monitor the concentration down to 0.1ppb , have inaccurately presented readings of 0 ppb when in actuality there were levels of 2-3 ppb.
To the plant chemist and other personnel, if the readings are ideal then preventative measures are not taken to reduce oxygen levels...because no one knows they exist!
If oxygen corrosion were occurring within the boiler, during an internal inspection the inspector may notice “rusty, crusty looking knobby projection” (tubercles) scattered throughout the boiler and boiler piping. These tubercles are the result of, and additional protection for, oxidation corrosion. If allowed to remain, oxygen corrosion would continue until holes are formed resulting in a plant outage and $1,000,000+ lost daily! Even if the oxygen is removed from the water, corrosion will continue until the tubercle and the concentration of solids under it are removed. Due to the high risk of corrosion throughout the Steam-Water Cycle, Dissolved Oxygen Monitoring is seen as a necessary Core Parameter.
AIR IN-LEAKAGE
There are a number of locations throughout the steam/water cycle where subatmosperhic conditions can occur. If any leaks are present, this can cause Air Ingress which introduces Dissolved Oxygen into the process. Let’s take a look at 2 examples where Air Ingress commonly occurs...
The Condenser
In Fossil Power Plants, the condenser is a major component of the steam cycle with the primary purpose of condensing the exhaust steam from a steam turbine to obtain maximum efficiency. The condenser also converts the turbine exhaust steam into pure water (referred to as steam condensate or ‘Hungry Condensate’) so that it may be reused in the steam generator or boiler as boiler feed water.
During this process, air and non condensable gases carried by the steam must be removed from the condenser by a vacuum. However, sometimes these sub atmospheric locations, such as in the condenser, may contain leaks that present Air Ingress...resulting in reduced condenser vacuum, which in turn reduces the turbine backpressure and the efficiency of the overall cycle. This Air In-Leakage not only causes efficiency issues, but also increases the concentration of dissolved oxygen in the steam-water cycle resulting in added corrosion.
Pumps
When duty standby pumps are used to pump feed water, air ingress can occur through shaft seals within the pump. Typically, a Double Shaft Seal or Magnetically Driven Shaft would eliminate this concern.
What Steps Should Be Taken To ACCURATELY Monitor
The Concentration Of Dissolved Oxygen!?!? ~ YOU
In order to make an informed decision about which type of analyzer to use, there are certain technical considerations that should be taken. Such as what concentration levels are expected, what technologies are currently available along with their benefits and faults in your application, and most importantly... How to ELIMINATE User Error!!!
At the Condensate Pump Discharge, target values for conventional fossil and combined cycle plants are ≤10 ppb for dissolved oxygen. Maintaining within this range will improve treatment procedures and ensure that carbon dioxide contamination, which plays a part in cation conductivity, is minimal. Normally at start-up, Dissolved Oxygen concentration levels can read as high as 400 ppb, but levels during normal operations are typically as low as a few ppb.
Ultimately, you want to be able to accurately measure Dissolved Oxygen over the concentration range of 0-400 ppb while keeping high accuracies with readings as low as 0-10ppb. There are a number of different methods for monitoring dissolved oxygen, each with their own procedures and maintenance requirements, so the objective becomes finding an economical solution that provides real-time monitoring with minimal maintenance and high accuracy.
Analyzer Technical Considerations
There are 4 methods for monitoring dissolved oxygen I’m going to discuss, each with their own benefits... some more than others. Three of those methods, that remain relatively unchanged over the years, make use of a sensing probe consisting of two metal electrodes—an anode and a cathode—in contact with an internal electrolyte that is separated from the water sample by a semi-permeable membrane. Each of these 3 methods utilize different materials for the anode and cathode which create different electrical potentials in the internal electrolyte. Because all three methods measure the current or amps flowing between the cathode and anode, they may be termed amperometric methods.
AMPEROMETRIC METHODS
Galvanic
Galvanic Analyzers typically use lead for the anode material and a more noble material like silver, gold, or platinum for the cathode. The electropotential created between these two electrode materials produces a current equal to the oxygen concentration. Usually, a high density polyethylene membrane is preferred.
PROS
Galvanic type probes are not affected by dissolved hydrogen (H2) and are recommended for use when hydrogen is present
With a suitable electrolyte solution, the anode can remain protected upwards of 3 years
CONS
Deposits on the permeable membrane (e.g., iron oxides) will slow instrument response and can lead to a negative bias.
Flow Dependent
Diffusion rate of the membrane is typically fixed and proportional to the oxygen concentration. This rate is normalized with the initial calibration. If the membrane is fouled and the diffusion rate reduced, the readings will become erroneous.
Membrane tension and positioning can affect sample read out.
Galvanic type sensors that use an iodide or chloride salt containing electrolyte will consume electrolyte during operation. Periodic recalibration is necessary to adjust for this consumption and for other factors such as the electro-deposition of lead oxide (PbO) on the anode. Periodic electrolyte replacement is also required.
If DO sensor is used to monitor high (ppm) level sample concentrations the life of the sensor will be reduced. High sample concentrations increase the rate of the chemical reaction that takes place at the sensor membrane thus reducing the life of the sensor.
MAINTENANCE
Electrode Cleaning
Frequency depends on Anode Size and Electrolyte solution
Refilling Pottasium Iodide or Potassium Chloride electrolyte requires less anode cleaning
Deposits on the Anode should be gently removed with a soft cloth
Membrane Replacement
Polarographic
Polarographic Analyzers typically use silver for the anode material and a more noble material like gold, or platinum for the cathode. Due to the low electropotential generated by the electrode materials, the polarographic analyzer needs a potential of roughly 0.5 Volts applied to the anode and cathode to produce a current high enough to produce a measurable signal. Usually, a fluoro-type membrane is preferred.
PROS
An auxiliary ring-shaped cathode (“guard ring electrode”) may be installed to surround the main centrally located disk-shaped cathode. The auxiliary cathode is to reduce residual oxygen after air calibrations and any interfering species present in the electrolyte due to anodic reactions in order to prevent an overestimation of dissolved oxygen concentration.
CONS
Since conventional fluoro-type membranes allow large volumes of oxygen through the membrane, the potential for depletion of oxygen near membrane surface requires a higher flow rate to assure oxygen levels are not depleted near the sensing membrane surface.
Fouling of the fluoro-type membrane surface leads to slower permeation and lower readings.
Flow Dependent
Dissolved silver ions from the anode may contribute to stray current from the anodic oxidation reaction
After electrode cleaning, the membrane typically needs replacing
Diffusion rate of the membrane is typically fixed and proportional to the oxygen concentration. This rate is normalized with the initial calibration. If the membrane is fouled and the diffusion rate reduced, the readings will become erroneous.
Deposition of Silver Chloride from the electrolyte solution onto the anode can change the calibration slope requiring periodic cleaning of the deposits from the electrode.
Consumes electrolyte
Applications with Hydrogen require compensation
MAINTENANCE
Electrode Cleaning
Frequency depends on Anode Size and Electrolyte solution
Refilling Pottasium Iodide or Potassium Chloride electrolyte requires less anode cleaning
Deposits on the Anode should be gently removed with a soft cloth
Membrane Replacement
Equilibrium
Equilibrium Analyzers use platinum for both the anode and cathode. Current flows through the anode to create an oxidation reaction that produces oxygen. The cathode reduces the oxygen content until equilibrium is formed where the partial pressure of oxygen is the same at both electrodes. The current needed to maintain the equilibrium is proportional to the oxygen concentration. Usually, a silicone membrane is preferred.
PROS
Equilibrium sensors are not typically cleaned or refurbished. The net reaction of oxygen being reduced at the cathode and oxygen being produced at the anode leads to no net consumption of oxygen or water in the overall reaction. Once they have reached their end of life they are replaced with a new sensor.
No electrolyte or oxidation of the anode results in less frequent calibrations
CONS
Oxygen diffuses out of the sample and into the semi-permeable membrane only until equilibrium is reached. Therefore, the sample must only be supplied to the membrane at a flow rate sufficient to allow for representative sampling of the process being monitored.
Fouling of the membrane surface leads to slower permeation and lower readings.
Turbulent Flows cause membrane vibration and convective transport of dissolved oxygen
Not recommended for applications with Hydrogen
MAINTENANCE
Electrode Replacement
Once exhausted, replace with new electrode
Membrane Replacement
What Is ULTIMATELY Required Of
THESE 3 TECHNOLOGIES...
Without appropriate consideration to all of the factors below, dissolved oxygen measurements via Galvanic, Polarographic or Equilibrium may be erroneous...
Appropriate membranes must be utilized
Electrodes are to be kept clean
Appropriate temperature and pressure compensations is applied
Flow rate sensitivity is understood
Interferences are minimized
Response times are optimized
Calibration and calibration checks are performed as required.
THE ‘HANDS FREE’ SOLUTION
Luminescence
The O2 measurement is made based on measuring the luminescence of a layer that is sensitive to oxygen. The luminescence changes according to the partial oxygen pressure. The quantity of dissolved oxygen gas in the liquid is calculated with the aid of the measured partial oxygen pressure and the temperature.
The oxygen sensor optically measures the liquid’s O2 content based on the luminescence measurement principle, where an oxygen-sensitive layer is exposed to blue light. As a result, molecules in the oxygen-sensitive layer are excited. In the absence of oxygen, the molecules light up red. In the presence of oxygen, the oxygen molecules collide with the molecules in the oxygen-sensitive layer. The molecules that collide with oxygen no longer light up. Through this process, a link is created between the oxygen concentration and both the light intensity and the speed at which the light intensity is reduced. The light intensity reduces when the oxygen concentration is higher, whilst the light intensity reduces at a faster speed. The oxygen content is calculated using the time difference between the exposure to the blue light and the molecules lighting up (phase shift) and the product temperature.
PROS
No electrolyte or membrane cartridges to replace
No oxygen consumption
Unaffected by flow
Long life
Low Maintenance (Calibrations every 1-2 years)
ZERO Interferences
CONS
N/A
MAINTENANCE
Calibrate Every 1-2 Years
Replace the Sensor Spot every 2-3 Calibrations
How Can We Make This Analyzer
A HANDS-FREE AUTOMATED Solution For YOU!?!?
Economy Power & Instrument’s specialty is to be a "Solutions Provider" that will assist you in the instrumentation selection, specification, and application process. As such, we’ve paired our knowledge and expertise to provide efficient start-up, training, calibrations and service work to complete all of your applications needs.
Although we offer all of these solutions ourselves, there is one solution to monitoring Dissolved Oxygen Concentration that handles almost all of the work ENTIRELY on it’s own...
Presenting...
Want a Copy Of The 9065 Data Sheet!?!?
FEATURES:
DUAL STREAM MEASUREMENT
With our versatile electronics, Waltron has become the FIRST to provide TRUE Dual Stream Measurement by pairing TWO LDO Sensors to ONE Transmitter! This unique feature provides a Dual Line Display along with Dual 4-20mA Outputs that ultimately SAVE THOUSANDS in upfront costs!
CALIBRATION / EXTENDED SENSOR SPOT LIFE
The die in the luminescent layer (Sensor Spot) has a lifetime which we can measure in Light Pulses. After 1 MILLION Light Pulses, a calibration of this spot is required. After 2-3 Calibrations, close to 5 years, the spot finally needs replacing!
To extend the life of this sensor spot in AUTO Mode the sample time will be automatically adjusted based on the deviations of consecutive measurement readings.
By default, the sample time will be 2 seconds. If, after this time, the measurement value deviates by more than 10%, the sample time will remain 2 seconds between sensor readings. If the measurement value deviates less than 5%, the sample time will be doubled up to a maximum of 60 seconds between sensor readings. This extends the life of the sensor spot while providing fast response times.
EXAMPLE:
1. If the deviation is > 10% between consecutive readings:
The sample time is changed to 60 => 30 => 15 => 8 => 4 => 2 seconds, respectively.
OR
2. If the deviation is < 5% between consecutive readings:
The sample time is changed to 2 => 4 => 8 => 15 => 30 => 60 seconds,
respectively.
ZERO INTERFERENCES
Unlike other measurements that experience interferences in their internal electrolyte solution and clogging of their permeable membranes, the Luminescent Measurement Technology remains independent of any interferences allowing for installation into any application within the plant cycle.
LOCAL/REMOTE STANDY OPERATION
During a plant shutdown or if no water is present in the sample stream, you no longer need to shut down the analyzer. Whether using the local push button display or by using RS-232 or Profibus communications, you can place the analyzer in ‘STANDBY’ which stops the light pulses and extends the life of the sensor spot.
ALARM ACKNOWLEDGE & RESET
With a General Alarm, Temperature Alarm and (2) Concentration Alarms the 9065 Analyzer will display Flashing Alarm Indication, allows user acknowledgement, or if left untouched will automatically reset the alarm once within acceptable ranges.
HIGH ACCURACY
Boasting an accuracy of +/- (1ppb + 2% of the measured value), Waltron’s 9065 Analyzer can give you pinpoint accuracy ranging from 0.10 to 2000 ppb.
ECONOMICAL
At a fraction of the cost of the competition with our Single-Stream Automation coming in under $9k and our Dual Stream Measurement being less than $15k, there is no better solution that Waltron’s 9065 LDO Analyzer!!!
For Budgetary Pricing, to Schedule A Meeting, Request Technical Information/Manuals, or To Try One Of Our Demo Units...
CONTACT Brandon Jamison
Specialized Accounts
Economy Power & Instrument, Inc.
10616 Summit
Lenexa, KS 66215
P:(913) 469-1111
F:(913) 469-1054
E: Jamisonepi@aol.com
Thank you for taking the time out of your busy day to review this Lunch & Learn, and don’t hesitate to contact me for further assistance!
Regards,
Brandon Jamison
Brandon Jamison
Specialized Accounts
Economy Power & Instrument, Inc.
References:
Fossil Plant Cycle Chemistry Instrumentation and Control: State of Knowledge Assessment. EPRI, Palo Alto, CA: 2007. 1012209. EPRI
Boiler/Feedwater Guidelines NB-410 – Revision 2 7/12
