Failure to specify the proper control level of hydrate alkalinity in high pressure industrial boilers has resulted, over the years, in a myriad of problems. In the 1980s, the authors encountered a high heat flux, moderately high pressure, “process-on-the-tube-side” heat recovery boiler in a west coast oil refinery that had experienced two failures in quick succession. Several different mechanisms had been proposed to explain the failures. Caustic gouging was strongly suspected in the case of the first failure. While reviewing the available literature during the process of formulating an internal chemical treatment program and control regime, the authors uncovered a particularly compelling previously published paper that appeared to have been largely forgotten by the industry. This work was used as the basis for specifying hydrate alkalinity control in this system, with great long term success.
INTRODUCTION
The deleterious effects of the accumulation of excessive hydroxide alkalinity concentrations in crevices, and under porous deposits in high temperature, heat flux steam generation equipment have been well documented(1)(2)(3). The Joint Thermal Research Committee of the ASME specifies the feed water and internal chemistry to be maintained in a wide variety of steam generation systems, operating under a very broad spectrum of operating conditions(4). Most conspicuously absent are specifications and guidelines for the levels of hydroxide alkalinity that can be carried under the various conditions encountered. This omission is intentional, and is based on a lack of consensus among the current and past members of the Industrial Sub-Committee on how to present and specify the proper levels of hydroxide to be carried under the operating circumstances under consideration.
Material published many years ago by ASME, under the auspices of the same ASME Committee(3) and later presented by Nalco Chemical Company(5) is worthy of re-evaluation, and might prove to be a suitable starting point to enable ASME to provide additional guidance on the specification of hydroxide alkalinity in modern steam generating systems. The authors used this material to solve a troublesome occurrence of caustic gouging in a west coast oil refinery 875 psig heat recovery boiler. The authors hope that this paper will stimulate interest in re-visiting this aspect of control, as well as a fresh look by ASME, including additional work if necessary, eventually leading to the establishment of more specific guidelines for the industry. This paper, then, is an effort to bring “new gold from an old mine”(6) to the attention of the industry, using one interesting industrial case history as a vehicle.
RESULTS
A major West Coast refinery operated a very high efficiency, high heat flux, 875 psig process tube side once through waste heat recovery steam generator for many years, in conjunction with the operation of a hydrogen plant. The operating specifics of this system are shown in Table 1. The system operated on properly deaerated feed water (Table 2), consisting primarily of condensate and supplemented as necessary with demineralized make-up water produced by the refinery central utilities division. The condensate was recovered from steam generated in this and in other lower pressure heat recovery boilers operating elsewhere in the operating division and returned from process exchangers and surface condensers located through out the refinery operating division.
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