A Microprocessor-Based Steam Generator Quality Controller

Abstract

Abstract The measurement and control of steam quality is a key factor in the operation of a steam stimulated enhanced recovery project. This paper is presented to discuss the development of paper is presented to discuss the development of a microcomputer based monitoring and control system which has proved effective in field applications. Introduction Steam quality is a term describing steam, defined as that weight percent of water which has been vaporized. Conventional oil field steam generators are designed to produce 80% quality steam, having 20% of the water by weight still in the liquid form. This proportion of liquid is required to prevent scale buildup in the tubes. The buildup acts as an insulator, causing overheating of the tubes and possible rupture under pressure. The liquid portion carries dissolved solids through the generator, preventing deposits from forming. If steam quality is too high, the dissolved solids do not stay in solution and deposit on tube walls. If steam quality is too low, a lesser amount of heat is transferred to the formation or the characteristics of the displacement are changed. Certainly many assumptions of project performance are erroneous if steam of a quality other than that assumed is provided. Current practice for measurement and control of steam quality is to occasionally check quality by manual methods and manually adjust water or firing rates to obtain the desired quality. Even less accurate and reliable is the method of assuming a steam generator heat transfer efficiency and setting in a fixed fuel/water ratio. Steam quality is currently measured by three methods; dissolved solids concentration, orifice meter, and heat/material balance. QUALITY MEASUREMENT METHODS Dissolved solids concentration is the most common method used. When the water enters a steam generator, it will contain salts in solution. As vaporization occurs, the same amount of salts will be contained in a reduced amount of liquid. Therefore, the steam quality is directly proportional to the increase in dissolved solids concentration. The measurement of dissolved solids concentration may be either titration for chlorides or measurement of electrical conductivity. Because it is the most convenient, conductivity measurement is the most widely used method. The orifice method has been used by many operators, and provides the best on-line indication of steam quality changes. it requires measurement of the inlet water flow, inlet water temperature, steam differential, and steam pressure. Quality may be calculated using the orifice equation, (1) Vhere: W = Mass flow, C = Orifice constant, h = Orifice differential, = Density at flowing pressure (assumed to be a function of dry steam density and steam quality i.e., =, X = Steam quality fraction. Although quite accurate, this is normally not a direct indication method, requiring significant calculations to obtain a reading.