Fundamentals of Turboexpanders “Basic Theory …

1 Fundamentals of Turboexpanders Basic Theory and design Edited Date: September 16, 2015 Presented By: Mr. James Simms Simms Machinery International, Inc. 2357 A Street Santa Maria, CA 93455 About the Author James (Jim) Simms has been involved with the design , manufacture, and service of Turboexpanders and other Cryogenic Rotating Machinery since 1969. He worked in the Engineering Department of various Turboexpander manufacturing companies until he founded Simms Machinery International, Inc. in 1988. The company primarily focuses on LNG Boil-off Gas Compressors aboard LNG Tankers (Ships).

2 In 1994 he, along with others, founded Gas Technology turbomachinery Services, Inc. to service Turboexpanders , primarily used in Gas Processing Plants. Introduction/Description The term "Turboexpander", Figure 1, is normally used to define an Expander/Compressor machine as a single unit. It consists of two (2) primary components; the Radial Inflow Expansion Turbine and a Centrifugal (Booster) Compressor combined as an assembly. Its Wheels are connected on a single Shaft. The Expansion Turbine is the power unit and the Compressor is the driven unit.

3 In a Gas Processing Plant, the purpose of the Turboexpander is to efficiently perform two (2) distinctly different, but complimentary, functions in a single machine. The primary function is to efficiently generate refrigeration in the process gas stream. This is done by the Expansion Turbine end efficiently extracting the potential heat energy from the gas stream, causing it to cool dramatically. This extracted energy is converted to mechanical energy to rotate the Shaft to the Booster Compressor end of the Turboexpander, which partially recompresses the residue gas stream.

4 The Turboexpander operates according to the thermodynamic and aerodynamic laws of physics. When designed properly, the Turboexpander can yield very high efficiencies at the " design Point" and reasonable efficiencies at other, or "Off- design ", Points. Application The typical Turboexpander process installation is shown in Figure 2, Simplified Process Schematic. High pressure, moderately cold gas flows into the Expander section of the Turboexpander. The gas flows through the Expander Variable Inlet Nozzles (Guide Vanes) and then through the Wheel, exhausting at a lower pressure and at a substantially colder temperature.

5 Gas flows from the Expander to the Demethanizer, where condensate is removed. It should be noted that the Expander Nozzles are used to control the gas flow rate in order to maintain the pressure in the Demethanizer. The Residue Gas from the Demethanizer Tower flows through the Feed Gas Heat Exchanger and then to the Booster Compressor end of the Turboexpander. The efficiency of the Booster Compressor is very important, as it can improve the expansion process for more refrigeration as well as more efficiently use the power extracted by the Expander.

6 While the engineering process to properly design a high Figure 1 Turboexpander (Expander/Compressor) Assembly Cross-Section Drawing 2 efficiency Turboexpander is very complex, requiring computerized analytical tools, the basic initial sizing process can be simplified by using certain basic equations and assumptions. Preliminary Size Calculations The Turboexpander operation is best described as a dynamic system which responds to the Process Stream variations. To start the initial sizing design process, a fixed set of Process Stream parameters, or " design Point", must be established.

7 This " design Point" is normally set by the plant process engineer's system analysis in the case of new plants. In the case of a Turboexpander re- design in an existing plant, the actual operating condition will dictate the new " design Point". The parameters required to size the Turboexpander are: Gas Composition Flow Rate Inlet Pressure Inlet Temperature Normally, the Expander Outlet Pressure is determined by the performance of the Booster Compressor's efficiency through a complex iterative analysis. For this simplified sizing exercise, we will assume the value of the Expander Outlet Pressure.

8 Energy Extraction Where does the energy come from? With reference to Figure 3, which shows the typical Expansion Process Graph, we see the process in terms of change of energy, or h, with units of btu/lb, both in the Ideal Process (Isentropic, or 100% efficient) and in the Actual Process, or real terms. These have been designated h's and ho, respectively. The ratio of these is the definition of the Isentropic efficiency, e, of the Expander ( , e = ho / h's ). For example, if ho = 34 btu/lb, and h's = 40 btu/lb, then: e = 34 / 40 =.

9 85 or 85% With the h's and e values known, we then only need the gas mass flow rate, w, to calculate the Horsepower developed by the Expander. Since the mass flow rate is normally given by the process engineer, we now have enough information to calculate the Horsepower with the following formula: HPExpander = (778 / 550) x h's x w x e 778 / 550 = (This is the constant value to change btu units into Horsepower terms) We will assume the mass flow rate, w = 20 lbs/second. So, if h's = 40 btu/lb, w = 20 lbs/second, and e =.

10 85, then: HPExpander = x 40 x 20 x .85 = Figure 2 Typical Turboexpander Process Schematic (Simplified) Figure 3 Expansion Process 3 Horsepower Balance The HP developed by the Expander must be absorbed in order to prevent over-speeding. The Bearings and the Compressor absorb this power to create a balance. The Horsepower Balance formula is: HPExpander = HPCompressor + HPBearings Example Let's assume that the Bearings consume a total of 30 Horsepower, which is subtracted from the Expander Horsepower. If we rearrange the formula above, then the available Horsepower to the Compressor is: HPCompressor = HPExpander - HPBearings = - 30 = Horsepower How Does the Booster Compressor Use Horsepower to Create Pressure?