Water Quality Meter

Measurements
One of the initial difficulties in making pure water measurements such as conductivity, pH, dissolved oxygen and sodium ion is preserving the integrity of the sample. This is especially difficult in power plants, where different sample points vary widely in their respective pressure, temperatures, and flows. Because of its low ionic content, pure water will quickly dissolve traces of contaminants from sample lines, flow chambers, containers and even the atmosphere. It is necessary to rinse new or unused sample lines a surprisingly long period of time before a representative sample can be obtained.

A constant threat of contamination comes from the atmosphere which contains oxygen, O2, as well as carbon dioxide, CO2. Oxygen from the air which leaks into the system can cause a perceived high dissolved oxygen reading. Carbon dioxide ionizes in the water to form a weak solution of carbonic acid. Carbon dioxide can cause errors in both pH and conductivity readings. For this reason, all pure water measurements should be made on closed, flowing samples which are free of leaks.


The flow rate of steam and boiler samples should be high enough so that any iron oxide particles or deionizer resins do not become caught in sample lines or flow chambers. The exchange of ionic species with accumulated particles in the sample lines or the electrode flow chamber can cause errors which are often very difficult to troubleshoot. This problem can be minimized by utilizing low volume flow chambers and small sample line diameters which enhance flow velocity.










Conductivity / Resistivity / TDS
Conductivity is the ability of a water sample to carry electrical current. In water, current is carried only by ionic materials - typically mineral contaminants which dissolve into positive and negative ions. Conductivity measurements remain the first line of defense in determining upsets, unacceptable contamination and other corrosive and depositing conditions which may exist. The high reliability, sensitivity and relatively low cost of conductivity instrumentation makes it the primary parameter of any good monitoring program. Many applications are measured in units of resistivity, the inverse of conductivity. Other applications require the measurement of total dissolved solids (TDS), which is related to conductivity by a factor dependent upon the level and type of impurities.

In power plant water applications, conductivity measures contaminants consisting mostly of mineral salts, although carbon dioxide from the air, organic acids from treatment amine decomposition, and other acids or bases are not uncommon. Conductivity is a non-specific measurement in that it responds to the concentration of any conductive material dissolved in the water. It cannot distinguish between materials present, whether they are treatment chemicals or contaminants. However, two types of conductivity measurements - specific and cation - can be made in order to obtain a more accurate determination of the level of contaminants vs. chemicals in a plant.


Specific Conductivity
Direct conductivity measurement of a water or condensed steam sample includes response to treatment chemicals such as ammonia or amines, corrosive mineral contaminants and carbon dioxide. By itself, this specific conductivity cannot distinguish among them because all of the ions will contribute to the overall conductivity of a sample. However, under normal operating conditions, treatment chemicals have the highest ionic concentration and dominate the response. Specific conductivity is therefore used along with pH as a reliable indicator of treatment chemical levels.


Cation Conductivity
Specific conductivity can detect only large amounts of corrosive contaminants, since the conductivity of the treatment chemicals serves to mask out lower levels typically present. To improve sensitivity to these corrosive contaminants, a water sample is passed through a cation exchanger, where two mechanisms are used to increase sensitivity to contaminants. In the first mechanism, the cation exchanger retains ammonia and amines on the cation resin in the cartridge, effectively removing their large background contribution to conductivity. The second mechanism consists of mineral salts being retained in the exchanger and replaced by acid which actually boosts the conductivity, increasing measurement sensitivity. The overall effect of the cation exchanger is thus to reduce the chemical contribution to conductivity and amplify the contaminant conductivity. Because of this, cation conductivity continues to be the most useful measurement for corrosive contaminant detection.


pH
pH is the measurement of the free acidity or alkalinity of a solution; in this case, the solution is water. The measurement of pH is critical to prevent corrosion processes from occurring. The second leading cause of boiler failure can be attributed to corrosion. However, pH measurement in high purity water can be extremely difficult. Pure water has a high resistance and a high vulnerability to contamination, and often possesses extremely high temperatures in the steam/ water cycle, so pH is often a very challenging measurement which can easily be measured improperly.

It has been argued that pH should not be measured in pure water since a conductivity measurement is simpler and assures high purity. If water treatment systems always produced pure water there would be no need for pH measurement, but treatment systems are never perfect. Conductivity cannot distinguish among contaminants, and therefore pH can be used in conjunction with conductivity to distinguish between contaminants which may lend more to a more acidic or basic pH level. pH has thus proven to be a very useful measurement in diagnosing system problems, such as a condensate leak in the condenser. The level of pH-adjusting ammonia or amine also requires pH measurement in addition to conductivity measurement to assure proper contamination detection.


Dissolved Oxygen (D.O.)
The measure of the amount of dissolved oxygen gas in the water is used to monitor performance of de-aerators, control chemical injection and to detect air leakage into vulnerable parts of the feedwater and condensate system. Oxygen corrosion and the associated corrosion products represent a great expense to power plant water components. Oxygen pitting is often seen in economizers during operation, while superheaters and reheaters are especially susceptible during standby conditions. All carbon steel components in a system are vulnerable to this type of attack. Copper alloy corrosion in condensate and feedwater systems is a function of oxygen. Oxygen can cause corrosion fatigue of boiler tubes as well as turbine disks and blades.

Totally eliminating oxygen from the water is virtually impossible. Potential sources of oxygen ingress include leaking turbine/condenser expansion joints, low pressure heater flanges and connections, turbine explosion diaphragms, leaking pipe joints, etc. Because the oxygen will always find a way into the system, the oxygen level is constantly monitored and controlled. This is typically done by means of a mechanical de-aerator and/or chemical reaction. Regardless of the type of treatment used, the dissolved oxygen level is always controlled to the parts-per-billion range. Continuous measurement of dissolved oxygen at several points is critical to the long term reliability of critical components, especially the boiler.


Sodium Ion
On-line measurement of sodium ion concentration is typically much more sensitive than conductivity in determining upsets, unacceptable contamination and other corrosive and depositing conditions. Sodium is a pervasive contaminant and can be applied to boiler systems to detect condenser leaks, boiler carryover, evaporator entrainment, break-through in cation exchangers and condensate polishers, and sodium content in water supplies and other processes. Sodium ion is often the choice measurement over conductivity in monitoring cation exchangers, where the effluent is a highly conductive dilute acid and sodium is the first ion to break through the de-ionizer bed. It is also used for monitoring treated boiler water and condensate containing a pH-adjuster or oxygen scavenger which can add to the background conductivity.

Source :http://www.lesman.com/unleashd/catalog/analytical/70-82-58-56.pdf

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