11/21/2022 | Conductivity | 8 MINUTE READ

The Importance of Cation Conductivity in Utility Plants

When operating a utility plant or industrial facility of any kind, numerous measurements can be performed to determine the quality of a solution. When looking specifically at the utility industry, a cation conductivity measurement allows plants to identify small problems that can then be remedied by plant personnel. Catching and fixing these issues early on lowers the risk of them turning into serious complications.

In most cases, cation conductivity is used for boiler water treatment in utility facilities. The method of taking cation conductivity measurements is the same as the strong acid cation column in a demineralization water treatment plant. The water sample is passed through a cation column that’s filled with a cation resin. This process is also referred to as acid conductivity. The following provides you with everything you need to know about cation conductivity and its use in utility plants.

cation conductivity
water molecule model, Science or medical background, 3d illustration.

What is Cation Conductivity?

Cation conductivity is among the best methods for monitoring contaminants in a water sample or stream. In high-pressure systems, cation conductivity measurements allow operators to monitor the return condensate for lower levels of contamination with anions like HCO3-, SO4-, and Cl-. Identifying the conductivity of feedwater, condensate, and makeup can be done by taking the solution through a column that consists of a strong acid resin. While there are numerous applications where this measurement can be used, it’s particularly useful in power plants that generate steam.

Why is Cation Conductivity Important?

Cation resins are able to get rid of any positively charged ions in the water, which are known as cations. The cations are then replaced with H+ ions. When a water sample has already been filtered and is practically free of contaminants, cations will be present in very low levels.

In the event that sodium chloride (NaCl) is found in a sample, the Na+ ion will be removed from the water when the sample is passed through the cation column. The Cl- ion will also react with the H+ ion to create hydrochloric acid that comes with a higher conductivity.

When measuring a water sample for cations or anions, keep in mind that these substances are always present in combination with one another. In this scenario, the sodium is the cation, while the chloride is the anion. This measurement is usually taken once the water has been treated to remove cations. It will also indirectly measure the concentration of anions in the water, the primary of which include sulphate and chloride in a steam sample.

In a utility plant, cation conductivity boosts the anion that’s present in the water and allows plant operators to determine the current steam purity. The majority of steam turbine manufacturers state that cation conductivity readings should come in at less than 0.3 µs/cm.

When high conductivity is noticed in a water sample, it’s usually the result of such issues as:

  • An exhausted cation column
  • High levels of anion contamination currently present in the water
  • High levels of organic decomposition products or total organic carbon
  • Overfeed of oxygen and amine scavenger chemicals

The types of organic decomposition products that might be present in this water include formate and acetate. If feed water chemistry in a utility plant is properly maintained and controlled, you should be able to maintain cation conductivity at levels below 0.5 µs/cm.

conductivity

How Does Cation Conductivity Work?

Cation conductivity measuring is a straightforward process that allows you to determine the level of contaminants in a sample of water. Ions are typically present in a stream because various compounds dissolve in the water. The high process temperatures that usually occur will hasten this process. Keep in mind that identifying any other contaminants in your system will require the use of additional measurement types.

The ions that are present in the water can be positively charged or negatively charged. Positively charged ions are known as cations and include substances like copper, hydrogen, iron, manganese, and sodium. The negatively charged ions are known as anions and include everything from sulfates to chlorides.

Anions are particularly harmful in a utility plant since they are the most common sources of corrosion. The cation conductivity process allows the positively charged ions to be separated directly from the process stream via a cation resin column.

An example of this process is when plant operators want to identify such contaminants as salt that could be present in the water because of a serious condenser tube leak. This salt dissolves almost instantly in the water, which leads to the creation of Cl- and NA+.

The cation resin captures the sodium ions that are positively charged, which displaces a hydrogen ion that bonds with the chloride ion to create HCL. Once HCL is created, the solution’s conductivity measurement will increase, which shows the plant operators that a contaminant is in the water.

Why is Cation Conductivity Significant in High Pressure Boilers?

Cation conductivity is able to prevent certain contaminants from creating serious problems in utility plants. If sodium chlorine gets into a turbine, it may eventually lead to the development of a deposit on the blades.

If the turbine is eventually shut down without having dry-air protection, the oxygen in the surrounding air will combine with the moisture and sodium chloride deposit to facilitate an extremely corrosive reaction with the chloride ion. Pits will then form on the surfaces of the turbine blades. In time, a complete malfunction may occur.

A high-pressure boiler is a system with a pounds-per-square-inch reading of at least 1,450. If a high-pressure boiler experiences sodium chloride contamination, large amounts of corrosion can occur along the boiler tubes.

This issue can lead to severe hydrogen embrittlement failures along the water walls. By performing cation conductivity, you may be able to identify this issue early on, which gives you the opportunity to increase your plant’s clean water and reduce the amount of downtime that takes place.

high pressure boiler

What are Cation Resin Columns?

Cation resin columns are designed for the exchange of cations for ions, which occurs as a result of using negatively charged beads. These beads will capture any positively charged ions when the solution passes through the column. All positively charged ions will bind to the functional groups found on the plastic beads, which ensures that a hydrogen ion is effectively replaced.

The hydrogen ions that are released during this process will create a highly acidic solution. Cation resin columns tend to be sensitive to contaminants and are able to detect low concentrations of them. The right residence time, volume of resin, and flow can be ensured with the ideal length-to-diameter ratio of the plastic column.

The majority of cation exchange resins are outfitted with a basic color indicator that can change depending on the situation. For instance, the color will change when the resin has been exhausted and can no longer exchange ions. Different manufacturers will use different colors during these processes. When the resin has yet to be used, the color indicator will most likely appear as a dark red or dark purple. Once the resin has been exhausted, it will change over to a light red or light brown.

This color change occurs consistently until all resin has been exhausted. If all of the resin that’s found in a column has altered in color, the resin won’t be able to remove cations from the solution, which is why it should be replaced with regenerated resin when this occurs.

Cation and Anion Conductivity

To understand the difference between cation and anion conductivity, consider a glass of water with sodium chloride (NaCl) and calcium carbonate (CaCO3). These substances are broken down into ions of Na+, Cl-, Ca+, and CO3-. When this water is passed through a cation resin column with a hydrogen form resin, the solution will consist of H+ and Cl- as well as H+ and CO3-. On the other hand, the same water being passed through an anion resin column of hydroxide form resin will result in two separate ion combinations of H+ and OH-.

What Applications are Cation Resin Columns Used With?

The applications that can be performed with the use of cation resin columns include:

  • Identifying iron that’s caused by corrosion
  • Detecting sodium levels because of cooling system leaks
  • Detecting different mineral salts and acid contaminants
  • Identifying compounds that have been introduced to the steam from the boiler
  • Noticing leaks that cause contaminants to be introduced to the process

What Challenges Do Utility Plants Often Face With Cation Conductivity?

There are times when plants use cation exchange cartridges with a poor design or overly large size because of the belief that this extends the system’s service life. In reality, it can lead to bad resin kinetics, inaccurate measurements, leaching issues, and low exchange rates.

Along with making sure that the right ion exchange column and resin are used, the sample’s temperature will also need to be monitored. An incorrect temperature can result in bad cation conductivity measurements. In most cases, samples that are too hot can provide operators with poor readings. It’s possible to mitigate these problems by regularly maintaining the system, controlling the temperature and sample flow, and using a premium resin. Make sure that all temperatures are validated.

Utility plants rely on cation conductivity to keep corrosion at bay and make sure that the entire system is in good working condition. When you perform this process, make sure that you have a conductivity sensor by your side to take readings before and after the solution passes through the cation resin columns.

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Posted by Dominic O'Donnell on November 21, 2022

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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