by Ellen Donlin, Colfax Corp

A power company in the Atlanta, Ga., area wanted to replace its older, coal-fired electric generating units with gas turbine, combined cycle technology to reduce nitrogen oxide (NOx), sulfur dioxide (SO2), carbon dioxide (CO2) and other emissions. These large turbines typically use natural gas as the primary fuel source, with fuel oil as a backup in the event of an interruption in the natural gas supply.

The manufacturer selected to supply the turbines on this upgrade project previously had to manage installation of the secondary fuel oil system as an onsite construction project that proceeded in numerous stages. That approach required considerable time and associated expenses to specify, order, ship and install equipment, including pumps, motors, filters, bypass valves and piping, monitoring panels and other crucial components.

A better solution was needed, and a fluid-handling company with experience in the power generation industry manufactured a preassembled packaged module, designed as a single, integrated fuel oil system that could be shipped to the utility on a frame, ready for quick installation.

Secondary Fuel Source Considerations

The choice of secondary fuel used at power plants varies worldwide, based on availability and cost, but the most common options are naphtha, natural gas liquids, crude oil, bunker fuel oil and distillate (No. 2 fuel oil).

Each fuel has its own unique requirements-including treatment or cleaning/filtration-to minimize excessive erosion or corrosion to the hot gas parts of the turbine. If initial viscosity is high, fuel heating might be necessary to reduce viscosity to a manageable level.

For this project, Grade 2 distillate fuel oil (American Society for Testing and Materials [ASTM] standard) with a typical low viscosity range of 33 to 48 Saybolt Universal Seconds (SSUs) was chosen.

Regardless of fuel used, reliability is a key element in the selection of components for handling it, particularly in an application as vital as electric power supply.

Designing a Fuel Oil Pumping Solution

Many firms use advanced 3-D computer aided design (CAD) product engineering modeling software programs to draft systems and modules. The images provide a visual scope of supply, component verification and general arrangement layouts.

A fluid handling system manufacturer used such a program to demonstrate how an integrated system with fuel filtration, main fuel oil injection pump, preferred electric motor, instrumentation, piping and components would streamline secondary fuel delivery. The 3-D CAD renderings significantly enhanced communications with the turbine manufacturer throughout the project.

This approach also had the benefit of reducing the number of shop engineering change notices, which in turn reduced the total manufacturing cost. Another advantage was that modules could be designed to fit standard size intermodal freight shipping containers, which lowered logistics costs. (See Figures 1 and 2.)

Figure 1. 3-D modeling

Figure 2. Intermodal baseplate. The fuel oil system developed for the gas turbine manufacturer was installed on this 20 ft. base , which occupies less space than the equipment typically used by power generation plants and fits into a standard intermodal container for shipment.

Through empirical testing and analysis, three-screw pump scientists developed flow charts depicting pressures, flows and horsepower based on viscosity range, then selected the correct pump model to lower risk and provide maximum flow assurance. The filtration unit and pump were specified and sized accordingly to meet the turbine's fuel pressure/flow requirements for this low viscosity fuel, which is intended to be used with a high discharge pressure fuel injector.

Manufacturing and Assembly

A rotary, positive displacement, three-screw pump technology was chosen for the main fuel oil injection pump, based on years of proven success in distillate fuel oil applications worldwide. This technology works well in low viscosity applications, as well as at higher viscosities, such as some crude oils and all residual fuels. The application has a normal discharge of 100 bar (1,450 psig), with relief-valve setting at 120 bar (1,740 psig). Delivered flow of a simplex pump per turbine is approximately 360 gpm. (See Figure 3.)

Figure 3. Typical performance curve