Oct-2010

Monitoring and controlling corrosion in steam generation

At-temperature oxidation reduction potential (ORP) technology enables real-time measurement and control of corrosion potential in steam generation

Jerry Jones
Nalco Energy Services

Article Summary

Maintaining reliable and efficient operations in the steam generation process requires corrosion control in the preboiler and boiler water systems. Achieving a high level of control can often be difficult with traditional methods, but new state-of-the-art systems provide real-time monitor and control capabilities to proactively respond to system stresses and maintain tight control.

The most challenging part of the chemical treatment process is the monitoring and control of the water treatment chemistries and system blowdown. The basic problem with steam-generating systems is the fact that chemistry must be tightly managed. General chemistry testing and manual control adjustments have been used for decades, but it is the ability to monitor and automatically control that makes the difference between corrosion failures or a successful operation.

Excess and/or insufficient chemistry and blowdown will create corrosive and damaging environments in the preboiler and boiler. Chemical treatments, oxygen removal, pH and cycles management (blowdown control) are all involved in this process.

Monitoring and controlling corrosion is the primary water treatment objective in the boiler feed water (BFW). The areas of focus in this process are:
• Mechanical oxygen removal efficiency at the deaerator
• Chemical oxygen scavenging efficiency and scavenger level
• BFW pH monitoring and control
• Monitoring and controlling the BFW corrosion potential.

Conventional methods to monitor oxygen removal efficiency
Monitoring oxygen removal efficiency in the BFW process continues to be done using online dissolved oxygen analysers (best practice) and/or intermittent wet chemistry testing. Online analysis is capable of quantitatively measuring the dissolved oxygen levels at the <2 ppb levels desired, whereas wet chemistry testing is a qualitative representative of the dissolved oxygen level. Operations using wet chemistry monitoring methods will typically evaluate performance as <10 ppb dissolved O2.

Mechanical efficiency measurements are taken without chemical oxygen scavenger. Chemical scavenger efficiency is measured after the addition of scavenger in the piping between the deaerator storage and BFW pumps. Final oxygen monitoring is done after the BFW pumps to catch any oxygen leakage at the pumps.

Conventional methods to monitor and control chemical oxygen scavenging efficiency
The efficiency of the chemical oxygen scavenger (ppm scavenger/ppm dissolved oxygen) requires the ability to measure the mechanical dissolved oxygen level, dissolved oxygen level after chemical scavenging and the amount of chemical scavenger applied. The conventional method is to measure or estimate the chemical pumping rates (pump stroke and/or drawdown rate) and measure scavenger residuals with testing. Chemical scavenger adjustment (control) is conventionally done by manually adjusting chemical scavenger feed rates based on scavenger residual testing, dissolved oxygen or BFW flow rates.

Monitoring and controlling BFW corrosion potential
BFW corrosion potential has not been monitored in industrial plants until recently. A new at-temperature oxidation reduction potential (ORP) technology, Nalco Corrosion Stress Monitor (NCSM), now provides the ability to monitor BFW corrosion potential in real-time. This information enables chemical and operational changes that will maintain the pitting corrosion potential in a safe zone.

NCSM measures the at-temperature ORP of the BFW. The monitoring probe can be used to a maximum temperature of 400°F. The at-temperature ORP measurement is a direct measurement for the overall corrosion and pitting potential.

In any specific system, the NCSM measurement is used to determine if the system is operating at an mV level that is below the pitting corrosion threshold. Corrosion testing studies performed during development show how the ORP data is used to determine the pitting corrosion threshold.

Response to changes in dissolved oxygen
As base line oxygen levels increase, NCSM (mV) increases. Figure 1 shows NCSM tracking the changes in dissolved oxygen levels. These changes in dissolved oxygen can be attributed to variations in water flows and steam to the deaerator. Figure 2 shows NCSM’s response to a sudden spike in dissolved oxygen. (Note the recovery of the system’s NCSM corrosion potential. In this application, the spike was the result of a second boiler being brought on line.)

Response to changes in pH
Changes in pH alter the corrosion potential in the BFW, which will be reflected in NCSM’s mV level. The monitoring of pH is important, because it affects the rate of corrosion in the BFW circuit. Knowing the BFW’s pH is critical to evaluate how it impacts the BFW system’s corrosion potential.
The monitoring of pH (preferably online) is required when the system is being used for corrosion potential. The system measures cumulative inputs for corrosion potential (DO, pH, temperature and so on). The NCSM data is generally focused on dissolved oxygen removal, but its corrosion potential measurement is intended to be used as a core measurement to maintain the BFW system in the safe zone below the EPit levels.

Response to changes in temperature and oxygen scavenger dosage
The system’s redox measurement alters with a change in deaerator temperature. The majority of changes in deaerator/BFW temperature are generally associated with changes in make-up water flow rates and/or steam rates. These small variations in temperature are tracked by small changes in the dissolved oxygen (see Figure 3).