November 2006

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HP Water Management: Reduce risk of feedwater oxygen corrosion

The most critical asset for reliable boiler operation is the deaerator. Most plants do not routinely monitor or inspect these units because there are few early warning indications of poor performance...

Huchler, L. A., MarTech Systems, Inc.

The most critical asset for reliable boiler operation is the deaerator. Most plants do not routinely monitor or inspect these units because there are few early warning indications of poor performance. Failure to remove dissolved oxygen from the boiler feedwater (BFW) will result in corrosion that can perforate a feedwater pipe in several days. Reducing the risk of feedwater oxygen corrosion requires close monitoring of the deaerator performance and a contingency plan for emergencies.

As shown in Fig. 1, deaerators have two sections: mechanical-scrubbing and storage sections. The mechanical scrubbing section has two designs: spray or spray-tray. Feedwater enters the top of the scrubbing section where spray nozzles atomize water into small droplets. Low-pressure scrubbing steam flows upward, heating the feedwater and allowing mass transfer of dissolved oxygen between the water and steam. Oxygen has retrograde solubility in water, e.g., the higher the temperature, the lower the soluble concentration. Spray-tray deaerators use spray nozzles and horizontal slotted pans or trays and consume less steam than spray deaerators.

Fig 1
 Fig. 1    

Typical deaerator configuration.

Mechanical integrity. Even small misalignments in nozzle seals or trays will compromise the efficiency of the dissolved-oxygen removal. Typical mechanical problems in the scrubbing section include fatigued metal springs in nozzles, deteriorated spring seals or disrupted tray elements. Dynamic changes in scrubbing steam can rearrange trays. Rapid changes in feedwater pressure, or water hammer, can damage spray nozzles or, in severe situations, cause the deaerator to "rock" in its cradle, risking major structural damage. The chemical injection line typically has a quill that may plug with precipitated chemicals due to excessively high or low temperatures.

Routine monitoring. ASME guideline1 mandate effluent dissolved oxygen concentrations of less than 7 parts per billion (ppb) without adding chemical oxygen scavenger. Most plants add a chemical oxygen scavenger during dynamic changes in steam load and as the deaerator's performance deteriorates. As shown in Fig. 1, the most common chemical feedpoints are the downcomer between the mechanical-scrubbing and storage sections or directly into the storage section.

Plant personnel typically measure the concentration of the inorganic oxygen scavenger chemical (e.g., sulfite) in the boiler water and the concentration of the organic chemical oxygen scavengers in the feedwater. They seldom measure the dissolved oxygen concentration of the feedwater. By design, plant personnel feed excess sulfite to the system. The sulfite concentration can change due to increased demand, higher feedwater flowrates or changes in boiler blowdown rate. Consequently, changes in the sulfite concentration do not correlate to changes in deaerator performance nor do they guarantee conformance of dissolved oxygen specifications. There is no substitute for routine measurement of dissolved oxygen in the feedwater circuit.

The two most common methods to measure dissolved oxygen are: grab samples using colorimetric ampoules or online methods using an electrode-based technology. Both analytical methods require a continuously flowing sample. Fig. 1 shows that the optimal sampling location is downstream of the feedwater pump. The manual method uses a special glass ampoule that contains the reagents under vacuum. Plant personnel must immerse the ampoule into the continuously flowing sample and break the glass tip. The sample enters the ampoule and reacts, causing a color change that is proportional to the dissolved oxygen concentration. Chemetrics,2 the largest supplier of colorimetric ampoules, offers manual color comparators or automatic photometers.

Regarding online instruments that measure low concentration (ppb) of dissolved oxygen, oxygen in the continuously flowing sample diffuses through a semi-permeable membrane to the surface of an electrode.3 Current generated by the oxidation-reduction reaction correlates to the concentration of dissolved oxygen.

Performance testing. Plant personnel can maximize the reliability of a deaerator by conducting an annual performance audit. Determining the efficiency of a deaerator may require installing an online dissolved-oxygen analyzer. Plant personnel should calculate the feedwater residence time in the storage section of the deaerator to determine the approximate length of the test. In such testing, the operators can terminate feed of the oxygen scavenger chemical or change the feedpoint to a location downstream of the deaerator performance monitoring point (Fig. 1). Plant personnel should monitor the oxygen scavenger concentration and record the dissolved oxygen concentration. In all cases, the plant personnel should terminate the test and restore the oxygen scavenger chemical feed if the concentration exceeds ASME guidelines.

Contingency planning. Plant personnel can reduce the impact of a failed deaerator by creating contingency plans. A rental deaerator requires that the plant install a feedwater bypass to allow a quick connect during a failure. Plant personnel should proactively qualify rental equipment suppliers to ensure prompt response should they require a rental deaerator in an emergency. HP

LITERATURE CITED

1 Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers, CRTD – Vol. 34, ASME, New York.
2 CHEMetrics, Inc. www.chemetrics.com.
3 Suppliers of online dissolved oxygen analyzers: Orbisphere (distributed by Hach), www.orbisphere.com; Royce Technologies, www.roycetechnologies.com; Waltron LLC, www.waltronltd.com; Yokogowa, www.yokogawa.com.

Disclaimer: Identification of specific suppliers or instrumentation does not constitute an endorsement by the author or Hydrocarbon Processing. These references are not intended to constitute a comprehensive list of suppliers or commercially available instruments.

The author

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is president of MarTech Systems, Inc., an engineering consulting firm that provides technical services to optimize energy and water-related systems including steam, cooling and wastewater in refineries and petrochemical plants. She holds a BS degree in chemical engineering and is a licensed professional engineer. She can be reached at: huchler@martechsystems.com.

      

  

      

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