Turboexpander sizes

As an indieation of turboexpander sizes, for eaeh million eubie feet per day of liquid produet, this flash gas turboexpander would be sized at 6 hp the main turboexpander at 99 hp the main 1,000 psia eompressor at 626 hp (ineluding the stage driven by the turboexpander), and the flash gas eompressor at 88 hp. The feed gas is at 200 psi and eontains a substantial amount of energy above the 14.7 psi level. Otherwise, it would require 183 hp to eompress the feed gas. 

The reservoir may be either pressurized or atmospherie. It must have suffieient eapaeity to eontain all oil during drain-baek or shutdown. It must be equipped with an oil level indieator, a low-level alarm switeh, safety relief valve, a pump for oil makeup during operation, drain valve, heater, mist eliminator, strainers, and required valves. Expander reservoirs must be designed and eonstrueted in aeeordanee with applieable ASME eodes. Reservoir retention time is typieally between 5-18 min depending on turboexpander size and manufaeturer s sizing eriteria. This is an area where the owner/purehaser should ask for the manufaeturer s assistanee. 

Size, rotating speed, and efficiency correlate well with the available isentropic head, the volumetric flow at discharge, and the expansion ratio across the turboexpander. The head and the volumetric flow and rotating speed are correlated by the specific speed. Figure 29-49 shows the efficiency at various specific speeds for various sizes of rotor. This figure presumes the expansion ratio to be less than 4 1. Above 4 1, certain supersonic losses come into the picture and there is an additional correction on efficiency, as shown in Fig. 29-50. 

Dust-laden streams can also cause operational problems. A turboexpander that can efficiently process condensing streams (gas with fog droplets suspended) can usually handle a stream with suspended solid particles, as long as the particle size does not exceed 2-3 p. The newer designs reduce erosion of expander back rotor seals by disposing of... 

Turboexpanders eurrently in operation range in size from about 1 hp to above 10,000 hp. In the small sizes, the problems are miniaturization, Reynolds Number effeets, heat transfer, seal, and meehanieal problems, and often inelude bearing and eritieal speed eoneerns. In intermediate sizes, these problems beeome less signifieant, but bearing rubbing speeds and vibration beeome inereasingly important. 

Presently, designs for radial inflow turboexpanders in sizes up to 70 MW are available for use in geothermal power plants. Following are some of the most important features that make turboexpanders ideal for the reeovery of power from the vast available resourees of pressurized gas streams. 

Reciprocating expansion engines have been used since the early twentieth century and are still used to some extent, especially for volumetric flows below 10 ft /min. Reciprocating machines often suffer from high maintenance, excessive size, valve problems, and tlie fact that liquid will damage the valves. For these reasons they have largely been replaced by turboexpanders, even down to sizes around 1 hp. 

Since the early 1980s, major producers of turboexpanders have been able to supply these machines with bearings upgraded to fully magnetic, oil-free design. Initially, typical mid-size machines were described by the following parameters ... 

This type of liquefier is used for small to medium sized plants and mixed-product liquefiers. For large-capacity plants and where power is more costly, further optimization is used to design more complex liquefiers. Complex liquefiers utilize more equipment, such as additional packaged refrigeration units and turboexpanders, to achieve even higher efficiencies. The basic principle, however, is the same as illustrated and described for the simple liquefier shown here [3]. 

Properties predicted by these correlations determine not only the economic feasibility of gas processing installations they also dictate the design (size) of its major components. These components are usually the compressor, turboexpander, heat exchangers, external refrigeration (if any), and demethanizer. A typical arrangement for a cryogenic gas plant of these components is shown in Figure 1. 

Enthalpv/Entropy Correlations. The most expensive item in a turboexpander plant is the compressor which is designed from enthalpy/ entropy calculations. For a fixed horsepower the results are almost the same for Property-75, Peng-Robinson, GPA K H Soave, GPA K H Lee, GPA K H Starling-Han BWR, and GPA-k method. The largest variation predicted is in the discharge temperature (maximum differences of 6.8° and 5.4°F) which would affect the discharge cooler sizes (assuming 120°F) by 3%. 

Flow and horsepower sizes vary over a wide range. Turboexpanders with 4- to 6-in-diameter turbine wheels are typical. Ibrboexpanders have been built with wheel diameters over 17 in and as small as in. Miniature turboexpanders with wheel diameters below lin have a rotative speed above 100,000 rpm when only gas-lubricated bearings make a successfiil design possible. 

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