Last updated: 06.12.2018

Contents Issue 01 (2018)

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Chloride Contamination of the Water/Steam Cycle in Power Plants: Part V. Evidence for Chlorinated Compound Vapor Ingress Even after Condenser Re-tubing and Tubesheet Coating
Emmanuel K. Quagraine

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This paper builds on earlier hypotheses that at the power plant under discussion chlorinated compounds with significant vapor pressures can ingress in gaseous form into the condenser shell through weak seals and/or porous de-alloyed brass tubesheet at tube-to-tubesheet joints and are converted into chloride in the water/steam circuit. Aqueous seepage from the cooling water (CW) is also implicated, but is minor. Dezincification is the main corrosion mechanism.

The issue was addressed by tubesheet hole repairs with titanium epoxy and plastic epoxy application on all tubesheet faces. Yet failures linked with chlorine species attacks became obvious soon after such repairs, showing variations in the boiler chloride to sodium ratio. More sustained chloride cycling in the boiler to levels before the condenser repairs was observed only after an episode which led to spikes in the condensate extraction pump (CEP) dissolved oxygen (DO), CEP sodium, CEP conductivity after cation exchange (CACE), and steam sodium, and to increasing of the differential oxidation reduction potential at the CEP and deaerator outlets. Merely ~ 4.3 % of the chloride ingress from the CW system was estimated to be due to water leakage; the remainder was attributed to vapor ingress of chlorinated compounds. Inspection of the condenser waterboxes and the shell confirmed deterioration of epoxy cladding and tube-to-tubesheet joints.

The current paper provides further evidence from the period in which breaches may have occurred to the epoxy coating to support the concept that it is gaseous chlorine compounds and not necessarily water from the recirculating CW which is responsible for the chloride contamination of the water/steam cycle.

PowerPlant Chemistry 2018, 20(1), 4–22
Carbohydrazide vs Hydrazine: A Comparative Study
Mohammed Mahmoodur Rahman, Saad Abdullah Al-Sulami, and Fahad A. Almauili

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Hydrazine has been extensively used by the Saline Water Conversion Corporation (SWCC) in high-pressure boilers as an effective oxygen scavenger for the last several decades. However, due to its toxicity there have been serious thoughts of replacing it with a safer and more effective alternative.

Carbohydrazide, which is marketed under different trade names, was believed to be a good alternative to hydrazine that provides all of the additional benefits desired of an alternative oxygen scavenger of being safe to handle but without the deleterious impact on the cycle chemistry.

Trial tests with carbohydrazide on one of Al-Jubail Power Plant's boilers provided evidence that it is a good alternative to hydrazine. After two weeks of optimization, it was found that maintaining residual hydrazine in the range of 30–40 µg · kg–1 in feedwater (economizer inlet) was an appropriate method of controlling the dose rate of carbohydrazide and hence provided the optimum conditions for passivating the boiler. Accordingly, a dosing rate of 0.7 mg · kg–1 of carbohydrazide was found satisfactory for running the boiler smoothly.

This paper is a summary of the initial trials performed 12 years ago and serves as an introduction to a second article which will be published later this year in this journal. During the past 12 years, SWCC has been using carbohydrazide in all of its 8 plants. SWCC has done some studies with different brands and with 6–12 % carbohydrazide used in the steam cycle as well as during lay-up – this experience will be presented in the next paper.

PowerPlant Chemistry 2018, 20(1), 34–49
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