Radial inflow

The flowrate of oil into the wellbore is also influenced by the reservoir properties of permeability (k) and reservoir thickness (h), by the oil properties viscosity (p) and formation volume factor (BJ and by any change in the resistance to flow near the wellbore which is represented by the dimensionless term called skin (S). For semisteady state f/owbehaviour (when the effect of the producing well is seen at all boundaries of the reservoir) the radial inflow for oil into a vertical wellbore is represented by the equation ... 

In the simplest case, for a pressure drawdown survey, the radial inflow equation indicates that the bottom hole flowing pressure is proportional to the logarithm of time. From the straight line plot ot pressure against the log (time), the reservoir permeability can be determined, and subsequently the total skin of the well. For a build-up survey, a similar plot (the so-called Horner plot) may be used to determine the same parameters, whose values act as an independent quality check on those derived from the drawdown survey. 

Turbines The two types of turbine geometries used in gas turbines are the axial-flow and the radial-inflow type. The axial-flow... 

Radial-Inflow Turbine The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed-flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. Radial-inflow turbines are only now beginning to be used because little was know about them heretofore. Axial turbines have enjoyed tremendous interest due to their low frontal area, making them suited to the aircraft industiy. However, the axial machine is much longer than the radial machine, making it unsuited for certain vehicular and helicopter applications. Radial turbines are used in turbochargers and in some types of expanders. 

If hquid droplets form as ihe gas is expanded in the turboexpander, one s first thought may be that a radial inflow design is the last diing to use, but the following explanation will show that this is the only design that can accomphsh expansion efficiently. 

Development The following discussion relates specifically to the use of what could be called radial-inflow, centrifugal-pump power-recovery turbines. It does not apply to the type of unit nurtured by the hydroelecti ic industry for the 1 ge-horsepower, large-flow, low- to medium-pressure differential area of hydraulic water turbines of the Felton or Francis runner type. There seems to have been little direct transfer of design concepts between these two fields the major manufacturers in the hydroelectric field have thus far made no effort to sell to the process industries, and the physical arrangement of their units, developed from the requirements of the hydroelectric field, is not suitable to most process-plant applications. 

Centrifugal (radial inflow) turboexpanders are well adapted to such energy conservation schemes and, with recent developments that have increased their reliability, are suitable for unattended service on a 24-hour, 7-day week operational basis. Some of the recent developments include better shaft seals, thrust bearing monitoring, and superior control devices. 

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. 

The efficiency of expansion turbines (partial admission axial, full admission axial, and radial inflow turbines) is a function of the following four basic parameters. 

For the preliminary estimate of the expected efficiency of expansion turbines, in most cases it is sufficient to neglect Reynolds number effects (Rg > 10 ) and use the efficiency and specific speed correlations shown in Figure 2-12 for partial admission axial impulse, reaction radial inflow and full admission impulse and reaction axial turbines. Due to the economic advantage of the radial turbine, die radial inflow turbine is die best selection when operating in die specific speed range 20 < Nj < 140, whereby die optimum efficiency will be achieved at N, = 80. 

Because high-pressure ratio requirements coincide in most cases with low specific speed designs, only partial admission axial or radial inflow turbines are seriously considered. 

Radial inflow turboexpanders are the best equipment for this application because they handle condensing streams, provide a high expansion ratio in a single stage, and are custom-built to accommodate the plant s process conditions. 

Most ethylene plants operate continuously with the expanders operating at or near design conditions. If necessary, due to their unique design characteristics, radial inflow turboexpanders can accommodate a wide range of process conditions without significant losses in thermal or mechanical efficiency. Expanders may be loaded with booster compressors, gear-coupled generators, dynamometers, or other in-plant mechanical equipment such as pumps. In ethylene plants, turboexpanders are typically used in eitlier post-boost or pre-boost applications. 

To reeap, a turboexpander is a radial inflow turbine. The expansion proeess is aeeomplished in two steps primary and seeondary expansion. Primary expansion oeeurs in the inlet guide vanes and seeondary expansion oeeurs in the radial wheel. The proeess is isentropie and thermally effieient with reeoverable eold energy. Turboexpanders used in dew point eontrol require the following ... 

Figure 4-37. Two radial inflow expanders mounted on a single gear box. (Source GHH-Borsig.)...

Figure 4-40. Two-stage radial inflow expander with variable inlet guide vanes (right). (Source Demag Delaval.)...

Power is generated by the pressurized gas expanding through an 11,000 rpm single-stage, radial-inflow turbine expander, which drives a synchronous generator. Exhaust gas from the expander is liquified by air-cooled condensers and is pumped back to the heat exchangers to repeat the cycle. 

Type of expander Single-stage, radial-inflow Single-stage, radial-mflow... 

With a lower temperature, the turbine is best used by allowing the baek-pressure to fall and thus obtain more power. In radial inflow turbines, the relative veloeity at the turbine inlet is small. Any ehanges are, therefore, far less signifieant than with high relative veloeity impulse wheels. Commonly, a turboexpander tolerates as mueh as a 30% ehange from its designed enthalpy. The effeet on effieieney was shown earlier in Figure 3-12. 

Dakin, R., Radial Inflow Turbines for Geothermal Application, Geothermal Resources Council, February 1986. 

Small Gas Turbines. These gas turbines are in the range from about 0.5 MW-2.5 MW. They often have eentrifugal eompressors and radial inflow turbines. Effieieneies in the simple eyele applieations vary from 15-25%. 

Miero-turbines are usually referred to units of less than 350 kW. These units are usually powered by either diesel fuel or natural gas. They utilize teehnology already developed. The miero-turbines ean be either axial flow or eentrifugal-radial inflow units. The initial eost, effieieney, and emissions will be the three most important eriterias in the design of these units. 

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. 

Small standby power turbines less than 2-MW. The smaller size of these turbines in many cases have centrifugal compressors driven by radial inflow turbines, the larger units in this range are usually axial... 

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