How Various Fuels Affect the Design and Operating Cost of Steam Generators
Authors: Hampton L.A. 
Published: March 2010

The costs of steam generators vary appreciably with the type of fuel. A costcomparison between oil-fired equipment and a gas-fired unit indicates that theoil-fired equipment, when installed, can cost as much as 25 percent more than acomparable gas-fired unit. The cost analysis also shows why the operating andmaintenance costs may run 22 per cent higher. Essential operating andmaintenance techniques are discussed to emphasize the necessity of maintaininghigh thermal efficiency, which tends to reduce the over-all operating andmaintenance costs.

INTRODUCTION

FUEL FOR THE STEAM GENERATOR represents one of the most significantoperating costs in stimulating the production of heavy oil; however, there area number of related costs associated with the firing of particular fuels thatare often inadvertently overlooked in the initial economic or feasibilitystudies. It is the purpose of this paper to point out some of these costs, which in turn may reflect a more realistic operating cost estimate of the steamgenerators. To illustrate where and how some of these costs occur, a comparisonhas been made between a gasfired generator and one that is fired with leasecrude. Included in this comparison is a description of the equipment andits function, along with the operating and maintenance techniques that aregenerally practiced to achieve the maximum level of performance.

STEAM GENERATORS

The once through generator (see Figure 1) was specifically)7 developed forthermal recovery applications and provides features not available inconventional ste:3tm boilers. Among the important advantages are thefollowing:It will handle feedwater with a relatively high percentage of solids, provided the solids have been converted to soluble form.It is essentially only a pipe coil and does not have a separating drum and, because of the small volume of water and/or steam contained in the coil and thelack of a drum, it does not conform to the classic definition of a boiler.It does not have level controls, low-level cutouts, etc.

STEAM GENERATOR DESIGN

The basic design of this type of generator does not change appreciablywhether it is gas-fired or oil-fired, except where additional components (see Figure 1) are necessary and essential in the firing of crude oil. Theseadditional facilities include a pilot gas system (usually LPG), atomizing aircompressor system, atomizing steam system, electric and hot-water-to-oil heatexchangers, electric and pneumatic controls, soot blowers, -t pump and heatset, an adequate fuel storage tank and all the necessary piping and materialsto hook up these facilities. Perhaps one distinct design difference would bethe "L"-type convection section, as shown in Figure 2. This design featureshorizontal tubes with longitudinal welded fins and is mounted in a position forparallel flow of flue gases rather than the cross-flow design of theconventional economizer sections. This "L"-type convection section wasprimarily developed for oilfired generators, burning poorer grades of crude oilor pitch, as this design is less susceptible to fouling.