Reducing Corrosion and Enabling Faster Start-ups Through Nitrogen Generation

The cycling nature of Salt Valley Generating Station created a series of problems related to operating and layup chemistry, corrosion, and layup procedures. General corrosion and oxygen pitting, which occurred in plant equipment during shutdowns, were traced to less-than-ideal layup practices and the initial system design, which allowed air-saturated makeup water to enter the HRSGs.

As a result there were increased elevated iron levels in the evaporator water during operation, which could possibly lead to deposition. This is undesirable due to underdeposit corrosion and loss of efficiency. Leaks, stress corrosion cracking (SCC), or corrosion fatigue (CF) could occur due to oxygen pits.

Due to air-saturated makeup, it also took a long time to realize the permissible main-steam cation conductivity (CC) limit during startups. Even after several hours of full-load operation, the condensate-pump discharge (CPD) CC limit was rarely achieved.

During operation, CPD dissolved oxygen (DO) levels were 2 to 10 times higher than recommended and were near saturation when makeup was delivered to the HRSGs during shutdown. The DO cannot be removed with a reducing agent (oxygen scavenger) due to concerns over single phase FAC.

Finally, capping a wet HRSG with nitrogen from industrial-type cylinders became cost prohibitive. A detailed improvement program was required to address:

  • General corrosion and oxygen pitting in the HRSGs, condenser hotwell, and steam-turbine LP blades
  • Improvement of startup-time by acquiring steam purity more rapidly
  • Improvement of cycle chemistry

Solution

A multi-pronged approach was used to address these problems over a few years. This approach followed three basic principles as illustrated in “Cycle Chemistry Guidelines for Shutdown, Layup and Startup of Combined Cycle Units with Heat Recovery Steam Generators,” published by EPRI in 2006. These principles include:

1. Maintain the same electrochemistry of HRSG water during operation and wet layups

2. Keep air away from water and/or prevent water stagnation during wet layups

3. Protect equipment (for example, the steam turbine) from moisture ingress during dry layups

The design and installation of equipment, modification of operating procedures, optimization of chemistry, and development of layup guidelines were the solutions that satisfied these principles.

The equipment installed included a recirculation system (RS) for each pressure section of the HRSG, nitrogen generator (NG, Figure 1), dehumidification system (DHS, Figure 2), and gas-transfer membrane skid (GTMS, Figure 3)

Nitrogen generator skid saves cost of rented, vendor supplied compressed N2 bottles.

Figure 1. Nitrogen generator skid saves cost of rented, vendor supplied compressed N2 bottles.

Desiccant wheel dehumidifier prevents pitting of LP steam turbine blades which could lead to CF or SCC.

Figure 2. Desiccant wheel dehumidifier prevents pitting of LP steam turbine blades which could lead to CF or SCC.

Gas-transfer membrane skid deoxygenates makeup water before it enters the steam cycle.

Figure 3. Gas-transfer membrane skid deoxygenates makeup water before it enters the steam cycle.

The RS system enables water to be pumped from the economizer section to the evaporator section of each pressure system. It features a baffle bypass in individual drums to ensure flow via the evaporator tubes (Principle 2, above).

99.9% of pure nitrogen is supplied by the NG at its design flow rate. This NG replaced the pressurized cylinders and paid for itself within a year. The HRSGs were capped using nitrogen while pressure decays between runs (wet layup), and it is used to purge and blanket the HRSGs when they are drained for extended layups (Principles 2 and 3).

DO and CO2 from the makeup water are removed by the GTMS, with DO being decreased from 8 ppm to less than 5 ppb (Principle 2). Using DHS, warm, dry air is circulated through the LP section of the steam turbine to the condenser hotwell for shutdowns longer than three days (Principle 3).

A relative humidity within the hotwell/turbine of less than 15% is maintained by the DHS. This reduces the chance of turbine-blade pitting and hotwell corrosion, which can lead to CF and SCC. During operation, AVT(O) is the feedwater chemistry where ammonia alone is added to maintain a pH of 10. In wet layups, this chemistry is maintained without adding any reducing agent (Principle 1).

Although it was SOP to break vacuum for an overnight shutdown, this was changed to maintain condenser vacuum overnight so that chemistry and startup time can be improved (Principle 2). A decision tree based on shutdown duration was ultimately developed that can guide plant operators to select between various system layup options.

Results

1. No oxygen pitting was detected during the recent HRSG inspection last spring, and there was a prevalence of a salmon-colored protective oxide characteristic of oxidizing treatments. Prior inspections before the upgrades revealed significant general corrosion and oxygen pitting (Figures 4-7).

2. The improvements saved approximately 60 minutes of startup time from wet layup. By keeping the HRSGs in a wet layup instead of dry, for long periods (months), because of the RS, NG and GTMS, saves six to eight hours from dispatch to full load. Depending on natural-gas prices, the startup cost is reduced by as much as $45,000.

Oxygen pitting in HP steam drum found during inspection in 2005.

Figure 4. Oxygen pitting in HP steam drum found during inspection in 2005.

“Old” pits, no longer active, have been repassivated.

Figure 5. “Old” pits, no longer active, have been repassivated.

LP baffle reveals protective oxide coating and no signs of FAC or oxygen pitting.

Figure 6. LP baffle reveals protective oxide coating and no signs of FAC or oxygen pitting.

Preferred salmon-colored protective oxidecoating also is evident in HP drum.

Figure 7. Preferred salmon-colored protective oxidecoating also is evident in HP drum.

 

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This information has been sourced, reviewed and adapted from materials provided by Nel Hydrogen.

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