Review of Once-Through Steam Generators

Abstract

This paper reviews the design concepts pertaining to the forced-circulation once-through steam generator. To aid the production engineer in evaluating different generators for application in different situations, this paper discusses elements of feedwater treatment, aspects of design, defines common terms the production engineer might not be familiar with, and describes several figures included for determination of heat requirements, pressure loss, generator thermal efficiency, heat losses, working pressures and tube spacing.IntroductionThe advent of high-pressure steam injection as a secondary recovery technique has created a requirement for a generator suitable for this application. The generally satisfactory and sometimes spectacular results achieved in several of the initial commercial steam flooding projects has created a rapid increase in the requirement for suitable generators. The result has been the appearance on the market of several competitive designs of thermal recovery generators. The production engineer is thus faced not only with the problem of understanding an unfamiliar piece of equipment, but evaluating the merits of several competitive designs.This paper is concerned only with reviewing the design concepts pertaining to the forced-circulation once-through generator. The high operating pressures used in this service dictate the use of a water tube design where the pressure is contained in relatively small tubes with resultant economy in manufacture and decrease in operating hazard. The once-through steam generator was specifically developed for thermal recovery applications and features a single pass of water through the generator coil and no separating drum. The units are generally designed to produce approximately 80 per cent quality steam, so that the weight ratio of water to steam at the outlet of the generator is about 1:4, which is a much lower ratio than in conventional boiler designs. This water-steam ratio is not sufficiently high to insure complete wetting of the tube walls in the outlet portion of the generator coil, therefore the maximum tube wall temperature must be determined using a steam rather than a water heat transfer film coefficient. Since the steam film coefficient will be considerably lower than that achieved with water at an equal mass velocity, the maximum tube wall temperature would be considerably higher than in a conventional boiler unless a lower radiant absorption rate is used. The method of calculating the tube wall temperature will be outlined later.The once-through generator was specifically developed for thermal flooding applications and provides features not available in conventional steam boilers. Among the important advantages are the following:1. It will handle feedwater with a relatively high percentage of solids, provided the solids have been converted to soluble form.2. It does not have a separating drum and is essentially only a pipe coil. Because of the small volume of water and/or steam contained in the coil and the lack of a drum, it does not conform to the classic definition of a boiler.3. It does not have level controls, low level cutouts, etc., as required in a conventional boiler installation, and does not require continuous blowdown and constant operator attendance. Feedwater treatment is an important consideration in the satisfactory performance of a once-through generator, and deserves special emphasis. Some of the main considerations are as follows.HardnessThe special nature of the thermal recovery heaters where 80 to 85 per cent of the feedwater is vaporized in a single pass through the heater makes it essential to have zero hardness in the feedwater at all times. We recommend the dual-ion-exchange system as providing the minimum hardness leakage in spite of difficult raw waters, with the maximum assurance of always providing this minimum hardness leakage. We believe an ion exchange system is a prerequisite for the successful operation of a wet steam injection system for secondary oil recovery.JPTP. 409ˆ