Dissolved Oxygen
Monitoring of oxygen is necessary for the following reasons: • Copper alloy corrosion in condensate and feedwater systems is controlled by ORP. The valueof ORP is influenced by the levels of oxygen and reducing agent. • Oxygen reacts synergistically with chloride, resulting in hydrogen damage boiler tube failures.
• In all-ferrous feed-water systems, very low levels of oxygen (<1 ppb) in conjunction with high levels of reducing agent (hydrazine or alternative) result in very strong reducing conditions.
Under such conditions, the feed-water reacts with the ferrous alloys in the feedwater system to generate corrosion products. These corrosion products are subsequently transported to the boiler, which leads to a) increased chemical cleaning requirements, and b)increased risk of boiler water-wall tube failures by under-deposit corrosion.
• Flow-accelerated corrosion of carbon steel components is accelerated by over-feeding reducing agents and stopped by eliminating them and/or adding oxygen at concentrations above 20 ppb in high purity feed-water. Overfeeding of reducing agents is, therefore, not recommended and oxygen in high purity water is beneficial in reducing generation of irono-xides.
• Many plants with all-ferrous feed-water systems operate without the use of reducing agents,and have significantly reduced iron transport. Dissolved Oxygen Target Value in Economizer Inlet
This dissolved oxygen target provides an indication of the oxygen control for reducing, oxidizing and oxygenated feed-water, and is thus important for controlling feed-water material corrosion in all-ferrous and mixed-metallurgy feed-water systems.
Dissolved Oxygen Target Value in Drum Boiler Water This dissolved oxygen target is provided as an indication of potential boiler water-wall corrosion.Its level is determined by the oxygen at the economizer inlet and the boiler recirculation ratio.
Dissolved Oxygen Target Value at De-aerator Outlet monitoring of oxygen at the de-aerator outlet serves as a check for the effectiveness of de-aeration.
Dissolved Oxygen Target Value at Condensate Pump Discharge. This new dissolved oxygen target value (10 ppb) is provided to minimize total corrosion in mixed-metallurgy low pressure feed-water heaters and to provide an indication of air in-leakage into the cycle for all feed-water systems.
Silica
Monitoring of silica is necessary because of the following reasons:
• precipitation of silica forms silicate deposits on the turbine that are not soluble in water and are very difficult to remove;
• silicate deposits cause losses in turbine capacity and efficiency; and
• significant amounts of silica can enter the condensate/feedwater undetected in the nonreactive colloidal form through the makeup, causing boiler water and steam target values to be exceeded in spite of apparently good-quality makeup.
Silica Target Value in Steam Based on the solubility of silica in superheated steam, a limit of 10 ppb was chosen for units with reheat. Compliance with this target value should limit deposition of silica in the turbine
Silica Target Value in Boiler Water Vaporous carryover is the major contributor to silica in steam as illustrated by the partitioning predictions in Figure 3-13. Also Figure 3-3 shows the distribution ratio, or concentration in thesteam to concentration in the boiler water, of silica as a function of pressure.
High boiler waterpH reduces silica carryover. This effect is insignificant for the pH range (9.0–9.6) used in high-pressure boilers as shown in Figure 3-13 and, for that reason, is not considered in theseguidelines.
Operation at higher boiler water pH values will cause these guidelines to be slightlyconservative. The limits for silica are based on the need to limit the amount of silica carried overinto the steam and is based on partitioning data and the thermodynamic code.3-35
Silica Target Value in Condensate and Makeup Water
The target value of 10 ppb silica in condensate is necessary for these reasons:
•
to achieve recommended steam purity under normal operation without relying on boilerblowdown, leaving the full blowdown capability available to control transients and maintainthe chemistry within guidelines at all times, and
•
to allow maximum flexibility during excursions.The target value for makeup water is the same as that for the condensate: however, this targetvalue should be modified if necessary to be consistent with the capabilities of the makeuptreating equipment design
To keep the hydrogen inside the generator, various places in the generator are required to seal against hydrogen leakage to atmosphere. Picture beside is from Siemens Generator that is very easy to understanding how seal oil works.
The seal oil it self actually a portion of the lube oil, diverted from the lube oil system. It is then fed to a separate system of its own with pump, motor, hydrogen detraining or vacuum degassing equipment, and controls to regulate the pressure and flow.
The seal oil pressure at hydrogen seals is maintained generally about 15 psi above the hydrogen pressure to stop hydrogen from leaking past the seals.
Once of the critical component of the seal oil system is the hydrogen degasifying plant. The most common method of removing entrained hydrogen and other gases is to vacuum treat the seal oil before supplying it to the seals. This is generally done in the main seal oil supply tank. As the oil is pulled into the storage tank under vacuum through a spray nozzle, the seal is broken up into the fine spray. This allows the removal of dissolved gases. In addition there is often re-circulate oil back to the tank through a series of spray nozzles for continuous gas removal.
After passing through he generator shaft seals, the oil goes through the detraining sections before it returns to the bearing oil drain. As a safety feature there is often a dc motor driven emergency seal oil pump provided. This motor will start automatically on loss of oil pressure from the main seal oil pump. This is to ensure that the generator can be shut down without risk to personnel or the equipment.
There are two type of generator seal oil system, scavenging type and vacuum type.
Water and Air Contamination in Seal Oil
Water contamination in seal oil can vaporize when it comes in contact with the spinning turbine shaft. This water
vapor can enter the generator shell, and cause a reduction in the purity of the hydrogen gas.
Lower dew point readings are consequently noted. Also, entrained air can be released from the oil and into the hydrogen, again reducing hydrogen purity.
Decreased hydrogen purity results in increased windage loss which reduces the amount of usable energy available to produce electricity. This can result in lost revenue to the utility!
To minimize these losses, the concentration of water and entrained air in the oil should be kept well below the
saturation level. Since the seal oil is drawn from the main turbine lube oil reservoir, measures must be taken to
control water and air contamination there.
It is recommended that a Pall Vacuum Dehydration Purifier be used on the main turbine lube oil reservoir to remove 100% of the free water and air, and up to 80% of the dissolved water and air present in the oil.
This will result in improved performance and reliability of both the hydrogen seal oil and main turbine lube systems
Click here for Generator Auxiliary System.
Removing oxygen or uncondensable gas inside power station water steam cycle can by injecting hydrazine (N2H4) and by mechanically use de-aerator tank. Since hydrazine said toxic chemical, de-aerator tank performance should be maximalized.
There are two steps of work for de-aerator tank work for removing oxygen and uncondensable gas as descibe below.
Spray Scrubbing Deaerators
Perform best at stable high load conditions.
Perform best when approaching water stream is approximately 50 0 F less than the feedwater temp. (Higher temperatures reduce the amount of steam addition and can lead to increased venting needs).
Rely on excellent spray nozzle operation to break the water up into fine droplets.
Step one is the prime element is self adjusting spray valve that allows incoming water, which is to be de-aerated to discharge as a thin walled, hollow cone spray.
Most of the dissolved oxygen and free carbon dioxide have been remove at this point
Step Two is water from collection basin flows down the vertical down comer and into the scrubber section where it comes in contact with upcoming steam.
Through carefully sized orifices, the steam and water violently mix, heating and removing the remaining gases from water
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