A Waltron Case Study: Trouble Shooting Stator Coolant Chemistry Issues to Avoid Potential Failures

Background – Cooling the Stator

Large steam turbine generating units have water-cooled generator stators which use this system (See Fig 1) to remove excess heat from the generator as it converts rotational force into electrical energy for transmission to the electrical grid. Each water cooled stator is comprised of hollow conductors through which the stator cooling water can flow to remove excess heat from the generator. Older generators have conductors compromised of copper alloys while newer units utilize stainless steel. For copper conductors, corrosion/oxidation of the copper presents a significant risk when corrosion in one location leads to unwanted oxide deposits in the conductors and restrictions of cooling water flow. This reduction in cooling water flow, or “plugging” can result in overheating of the stator and the catastrophic failure of the generator from overheating. This risk has been eliminated in more modern generation units with a material change to stainless steel conductors that are essentially corrosion resistant.

However, a very large number of copper containing units remain in service. So stator coolant chemistry and the minimization of unwanted copper corrosion products is a critical issue for industry.

An excellent guide to stator coolant chemistry has been developed by the International Association for the Properties of Water and Steam (IAPWS) and this, and other water/steam chemistry guidelines, can be downloaded for free from the IAPWS website at – https://iapws.org/documents/techguide.

The goal of stator coolant chemistry is the formation of a stable copper oxide (passivation) on the internal water contacted surfaces. The first step in this process is the use of demineralized water in the stator coolant system and its constant purification via an inline mixed bed ion exchanger.

The next step in copper corrosion protection is technically very interesting with two stable copper oxides being possible under low and high dissolved oxygen environments.

Under low dissolved oxygen conditions, less than 20 mg/L, cuprous oxide (Cu₂O) is the stable, dominate oxide.

Under high dissolved oxygen conditions, greater than 2000 mg/L cupric oxide (CuO) is the stable dominate oxide.

The stability of both these oxides is further enhanced by having an alkaline pH environment.

There are four possible copper stator chemistry programs that are utilized by generator suppliers:

1. Low oxygen, neutral pH
2. Low oxygen, alkaline pH
3. High oxygen, neutral pH
4. High oxygen, alkaline pH

While each of these operating zones result in a low corrosion environment for copper stators, transitioning between these operating zones forces the oxides to change state leading to:

1. The unwanted release of copper oxides from the surface,
2. Their transport and redeposition within the stator
3. The blocking of the conductor flow paths.

This results in cooling water flow restrictions and the increased risk of overheating and generator failures.

The specific stator coolant chemistry program is selected by the generator manufacturer, and it should not be deviated from during operation.

Trouble Brewing Down Under

At a 250 MW coal-fired plant in the Southern Hemisphere, commissioned in the early 1980’s, the copper containing stator was designed for a low-oxygen, neutral pH cooling regime. The system included an open air head tank designed to establish a closed loop once the stator coolant system is filled up and degasses. However, ongoing operational issues required the utility to drain and refill the stator multiple times. This resulted in an increase in the stator dissolved oxygen levels and increased corrosion and formation and transport of cupric oxide (CuO). Within a few weeks, they discovered a buildup of CuO that was blocking the stator coolant filters. Within a few weeks, they discovered a buildup of CuO that was blocking the stator coolant filters resulting in frequent change outs. Additionally, a supply issue with the difficult-to-source, specialized filters created the potential need to shut the unit down while they waited for more filters to be sent from the supplier.

Due to the age of the plant, it had no online dissolved oxygen analyzers on the stator coolant system. When the plant was commissioned, then industry-standard amperometric (Clark cell) dissolved oxygen sensors were unreliable in such applications. Entrained, low levels of hydrogen gas in the stator coolant water is able to diffuse through the sensor’s membrane and interferes with the electrochemical reaction, preventing the analyzer from being able to provide a reading.

As a result, the plant was operating with no visibility into a critical water chemistry parameter impacting the stator coolants system. It was apparent, due to the filter blockages, that a major stator coolant chemistry issues was present that, if not addressed, could result in the very real possibility of a generator overheating event and the potential for a catastrophic generator failure.

Diagnosing the Problem

To get answers, they turned to an international water/steam chemistry expert who is based near the plant. He is familiar with the plant and postulated that the low-oxygen chemistry had destabilized due to the repeated drain and refills. Instead of cuprous oxide prevention, excessive cupric oxide had been created and was transporting around the stator coolant system. But the cupric oxide was not creating a protective layer that can be formed in a high oxygen regime. Instead, just enough CuO was created to flake off and clog the filters. To confirm, the plant needed rapid, accurate, and reliable dissolved oxygen measurement.

The consultant turned to the Waltron 9165 Luminescent Dissolved Oxygen (LDO) Analyzer. The LDO analyzer was rapidly commissioned and installed overnight. Unlike older galvanic probes, Waltron’s luminescent technology eliminates membrane and electrolyte issues, delivering fast, drift-free measurements, even in hydrogen-rich environments. As suspected, the dissolved oxygen was concentration was in the dangerous “in-between” range of ~100 to 2000 ppb due to air ingress. Furthermore, the ongoing filter blockages required repeated filter changes allowing additional oxygen to enter the stator coolant system further exacerbating the problem.

Corrective Action

With real-time data in hand, operators immediately began an offline nitrogen sparging process to strip out excess oxygen via the entrained gas removal system. The Waltron LDO Analyzer provided instant feedback and on-screen trending reports enabled staff to adjust sparge flow rates confidently and efficiently. Within hours the unit was returned to its low oxygen, neutral pH design basis for the stator coolant and its chemistry stabilized. This now stable situation has allowed for additional trouble shooting to determine if stator coolant chemical cleaning to remove unwanted copper oxides is required and what the current risk to the conductor bars are from any oxide deposition in these locations.

The Keys to Success

For plants of any age, luminescent oxygen online analysis provides vigilant monitoring and pro-active mitigation where galvanic sensors fail. The Waltron difference for this plant:

• Simplicity + Accuracy – easy installation, immediate trustworthy results.
• Reliability – even under harsh conditions it was immune to hydrogen interference.
• Confidence for operators – actionable data, clear local interface, trending at a glance

Waltron didn’t just provide an instrument — it provided the assurance to keep power flowing safely.