TTI Turboexpanders
Prepared By: Christian Giroux
Table of Contents Page(s)
Preface
1 page
Glossary
6 pages
I.
Description of Turboexpander Operation
1-14
Introduction
1
A . Expander
1-3
B. Compressor
3-7
C. Bearings and Lubrication
7-11
D . Thrust Balance
II.
11-14
Product Applications
14 - 1 7
A . Expander-Compressor
14-17
B. Motor-Driven Compressor
III.
IV.
17
TTI Frame Sizes
18
A. K-l
18
B. K-5
18
C. K - l 8
18
Advantages of TTI Machines A . Performance & Reliability B . Maintenance C . Cost
Bibliography
18 - 20 •
18-19 19-20 20
1 page
Appendices
25 pages A . Blade Angles
1A-3A
B . K-l & K-5
6 pages
C. K-l8
6 pages
D . TTI Manufactured Units & Systems
10 pages
-New Units
2 pages
-Redesigned Units
7 pages
Appendix B
K-l & K-5 1. Isometric View 2. Post-Boost Machine Cross Section 3. Pre-Boost Machine Cross Section 4. K-l Machine Outline 5. K-5 Machine Outline 6. Lube Oil Schematic 7. Machine Arrangement
Appendix C
K-18 1. Isometric View of Turboexpander 2. Isometric View of Skid 3. Post-Boost Machine Cross Section 4. Pre-Boost Machine Cross Section 5. Machine Outline 6. Lube Oil Schematic 7. Machine Arrangement
Appendix D
TTI Manufactured Units and Systems 1. New Units 2. Redesigned Units
Bibliography
Engel, Carl G., and Ward Rosen. 1983. Cryogenic Gas Plants. Petroleum Learning Programs, Ltd., Houston, TX. Munson, Bruce R., Donald F. Young, and Theodore H. Okiishi. 1990. Fundamentals of Fluid Mechanics, Third Edition. John Wiley and Sons, Inc., New York. Sawyer, John W., and David Japikse. 1985. Sawyer's Gas Turbine Engineering
Handbook,
Volume 1, Third Edition. Turbomachinery International Publications, Norwalk, Conn. Vance, John M. 1988. Rotordynamics of Turbomachinery.
John Wiley & Sons, Inc., New
York. Welch, Harry J. 1983. Transamerica Delaval Engineering Handbook, Fourth Edition. McGraw-Hill Book Co., New York. Wilson, David Gordon. 1984. The Design of High Efficiency Turbomachinery and Gas Turbines. MIT, Mass.
Preface The intent of this paper is to familiarize the nonprofessional with the most basic aspects of turboexpander technology. The glossary is provided to clarify the technical terms (italicized upon first use in the paper) that will be used in the paper. The first section of the paper discusses the fundamental components necessary to make a turboexpander,
defines
the
nomenclature
of
these
components,
and
provides
visualizations of the components (that will hopefully make identification of the components possible, when encountered).
Next, a brief overview is given of the
applications in which turboexpanders are commonly used, and the variations on the basic machine that are used for each particular application are discussed. The body of the paper concludes with an explanation of TTI's standard machines and the advantages of TTI machines.
The appendices include a technical discussion of wheel blade angle
design and sets of drawings (including cross sections, lube oil schematics, and layout outlines) for our standard machines. If this paper succeeds in its purpose, the reader will have a fundamental understanding of the function, the need, and the embodiment of turboexpander technology.
Glossary actuator: component of the nozzle control system that accepts an input control signal (generally pneumatic) and outputs a linear movement of a rod that is directly proportional to the strength of the input control signal. adjusting ring-, component of the nozzle control system that is rotated by the movement of the actuator; thus rotating the inlet vanes by means of a system of linkages such as slots or swing arms that are designed to permit nozzle pin movement along a path chosen by the designer. The path of nozzle pin movement is chosen such that the ensuing rotation of the vanes corresponds to a nearly linearly proportional relationship between actuator rod movement and nozzle area; therefore giving an easily predictable change in nozzle area based upon the actuator input control signal axial, direction that refers to a path parallel to the axis of rotation. In the case of the turboexpander, the axis of rotation passes through the center of both wheels, along the longest dimension of the shaft (see figure below).
Axial Movement
center section: also referred to as the rotating assembly, this is the part of the machine that is in between the expander and compressor housings.
The center section
contains the rotor, bearings supporting the rotor, the bearing housing containing the bearings and containing the oil that the bearings are immersed in, the shaft seals and wheel seals, and heat barrier wall (HBW: insulative layer separating hot oil in the bearing housing from cryogenic process gas). The center section is the most commonly replaced assembly in any given machine. If there is a failure that does not affect the expander housing, compressor housing, or any of the components contained therein, the center section can be replaced, and the machine can be set back into operation. Since it is wise when purchasing a new
Gl
unit to purchase a spare center section and have it at hand for reduced downtime, the center section is also referred to as the spare rotating assembly (SRA), channel, the path in an expander or compressor wheel, through which the gas passes. This path is the space in between the blades on the wheel. clearance: the distance from the outside of one part to the inside of another part when one part is fitted inside the other, i.e. the length of the gap between two parts in an assembly. cold section: the components inside the expander housing including, but not limited to, the nozzles and nozzle control system. This is referred to as the cold section since the expander side of the machine is usually part of a cryogenic process. energy, a mathematical concept that unifies many different domains of physics. Energy provides a relationship between speed, height in a gravitational field (or any accelerating reference frame), heat, pressure, light, sound, waves, electrical potential, magnetic potential, chemical bonds, nuclear bonds, etc. As energy is defined, matter is driven in the direction of decreasing energy, i.e. a force is created in the direction of lower energy.
As an example, a valley has lower
energy than the top of a hill, thus something at the top of a hill is forced in the direction of the valley. enthalpy,
a definition of energy used in thermodynamics, which is relevant to
turboexpanders. Enthalpy is the energy contained in a gas, liquid, or solid that is solely due to the heat contained (i.e. temperature) and the pressure at a given volume. Thus, the enthalpy is the heat and pressure energy of a substance. In turboexpanders, much of the enthalpy is due to pressure energy. Enthalpy change is calculated using gas composition, temperature in, pressure in, and pressure out. eye: the outlet of the expander wheel and inlet of the compressor wheel. So-called the eye since it is reminiscent of an eye (see shaded portion of figure below). ,-r'
v
1« v
.-J^T)^"—j
• j»
Eye of an Expander Wheel
-y
G2
flag: taper placed on the surface of thrust bearings to provide hydrodynamic lift. They are called flags because when the tapers have been machined into the bearing, they resemble flags (see figure below).
Journal Bearing
Thrust Flag
Thrust Bearing Face
force:
a mathematical concept used to quantify the ability to move matter.
More
precisely, force is the rate at which momentum is changed. This means that a force accelerates a mass, the smaller the mass, the greater the acceleration. Force can also be related to the deformation of materials, the stronger the force (in tension or compression), the greater the deformation of the material. hydrodynamic
lift, a lift similar to that which is created by the wings of an airplane.
Hydrodynamic lift is specific to liquids (as is the case in oil-lubricated bearings) rather than gas (as is the case with an airplane's wing). The lift is generated by the pressure created within the fluid when it is forced to squeeze into a passage that narrows. loading device: a component that absorbs energy from a source (in this case the turboexpander), thus imposing a force opposite the source's direction of motion. When the power output by the source equals the power absorbed by the loading device, the steady state operating speed is reached. If the source outputs more power than the loading device, the system in question accelerates. If the loading device consumes more power than the source can output, the system slows down. In the case of turboexpanders, there are many options for loading. Among the possible loading devices are compressors, generators, and heat generating devices (such as oil brakes). nozzles: in a general sense, a nozzle is a passage through which fluids (liquid or gas) pass. Nozzles generally vary in area, and are usually intended to accelerate a fluid. In the case of the variable nozzles used in turboexpanders, the nozzles are used to accelerate, direct, and vary the flow. The area variation accelerates the flow, the
G3
shape and arrangement of the vanes directs the flow into a swirl, and the ability to vary the nozzle area by simultaneously rotating the vanes allows for control of the flow rate through the nozzles (and therefore the turboexpander). pin: a small cylindrical rod that is press fitted into a hole and used as a connecting member in a system of linkages. In the case of the turboexpander nozzles, two pins are inserted into each vane; both of these pins are fitted tightly into the vane, so that the vane and pins are essentially one piece. One pin is placed in a hole that is loose enough to allow rotation, but is not free to move in any other way. The other pin either is connected to a swing arm (the swing arm being connected to the adjusting ring) or is placed in a slot (machined into the adjusting ring). In either case (swing arm or slot), the pin is guided in an arc which rotates the vane (see figure below).
Arc along which moving pin travel
Moving Pin
power, a rate of energy production or consumption. In other words, how fast energy is being generated or used. For example, a gallon of gasoline contains a certain amount of chemical energy. A process that takes one hour to burn that gallon of gasoline is a much lower power process than a process that can burn that gallon of gasoline in one minute.
Realize with this analogy that there is a flow rate
dependence on power. To burn one gallon of gasoline in one minute, the flow rate of gasoline into the fire must be one gallon per minute. This is true with the turboexpander process as well.
The power of the turboexpansion process is
dependent upon the enthalpy change per unit weight of the gas (analogous to the chemical energy in a given quantity of gasoline), and the flow rate of the gas.
G4
radial, the direction defined by lines directed towards or away from a central point (such as the center of a circle). For example, light rays emanating from a source travel in radial paths away from the source. In the case of the turboexpander. the radial direction is the direction of movement of the rotor that can be viewed by looking down the axis of rotation (see figure below).
<
^
Radial Movement
rotating assembly, see center section. rotor: the components of a rotating machine that rotate. In the case of an expandercompressor, the rotor is composed of the shaft, the expander wheel, the compressor wheel. Conversely, the stator is composed of the components of a rotating machine that remain stationary. subsonic: slower than the speed of sound. In reference to compressible fluid flows (such as gas flows), subsonic refers to the fluid speed being slower than the speed of sound in the fluid. This is important for predicting properties of flow behavior since the behavior of a fluid varies greatly dependent upon whether the fluid is traveling faster or slower than its own speed of sound. For example, a subsonic fluid accelerates when forced to pass through a passage that narrows; on the other hand, a supersonic (faster than the speed of sound) fluid decelerates when forced through a passage that narrows. tangent, the direction defined by a line that contacts a curve (such as a circle) at only one point. The tangential direction of movement is the direction of motion for any point on a rotating body (see figure below). As a note, the tangential direction is always perpendicular to the radial direction.
of Motion Curve <-
G5
thrust: a synonym for force. In turboexpander terminology, thrust refers to a force that drives the rotor in the axial direction. Thrust is caused by differences in pressure in front of and behind each wheel (expander and compressor), and also is caused by differences in total pressure on the expander and compressor wheels. total pressure-, pressure measured head-on into a moving flow. This includes the dynamic pressure, which results from the force the fluid exerts while being slowed down. In other words, total pressure is the pressure you would measure on the windshield of your car, whereas static pressure is the pressure you would measure inside the car with the windows open, and dynamic pressure is the difference between total and static pressures. vanes: also referred to as nozzle segments. These are the teardrop-shaped components that are arranged in a circle and sandwiched between annular (an annulus is a circle with a hole in the middle, like a washer) plates to make the nozzle passages.
G6
I. Description of Turboexpander Operation The heart of the machine is the rotating assembly.
The rotor is made up of the
expander and compressor wheels, and the shaft connecting them. The rotor, the bearings supporting the shaft, the shaft seals, and the bearing housing are all included in the rotating assembly, despite the fact that the rotor is the only rotating component. These components together are often referred to as the center section or the rotating assembly. The expander wheel extracts energy, and the compressor adds energy to a stream of gas. The compressor gets the energy that it adds to the gas from the expander. The energy is transmitted from the expander to the compressor through a shaft connecting the two wheels. Since the shaft turns at high speeds, fluid-film bearings are used to support the forces exerted on the rotor, and constrain the motion of the rotor. Fluid-film bearings are immersed in oil. Shaft seals must separate the oil and process gas, otherwise oil will leak into the expander and compressor housings, and process gas will leak into the bearing housing (thus diluting the oil). Since heat is generated due to friction in the bearings, an oil cooling system is necessary when supporting the shaft using fluid-film bearings. Since the elements of the rotor are all connected, force felt by one element is felt by all elements.
This is a problem since the expander and compressor streams are
independent of each other; consequently, a sudden rise or drop in pressure and/or flow in either stream can thrust the rotor.
Since this motion can significantly damage the
machine, thrust balancing systems are an important component of turboexpander design. In the following essay, each of these elements is discussed.
Attention will be
focused on the physical form of the devices used to perform the tasks mentioned above. The explanations of machine operation are qualitative; to avoid getting bogged down in the technical design considerations. Although form is discussed and pictorially represented, the descriptions are of a general nature. Therefore, simplified concepts are presented, not actual devices. A. Expander The expander operation will be discussed along the path traveled by the gas through the cold section. Operation begins when the nozzles are opened and the pressure
1
drop accelerates the gas. The nozzles control the amount of flow and direct gas into the expander wheel channel. In the expander wheel channel, the gas pressure drops further, and the enthalpy lost by the gas is absorbed by the expander wheel. ("Enthalpy" is the energy contained in the fluid; enthalpy is determined by any two properties of the fluid, such as temperature and pressure.) (Note:
Enthalpy is often referred to as "head." Head refers to the height of a
water column in earth's gravity, the taller the column of water, the higher the head. Think of the concept as a bucket of water with a hole in the bottom. The more full the bucket is (the higher the head), the higher the pressure at the bottom, and the faster it will spout out of the hole. The faster the water drains, the more energy it has.)
Fig. la: Nozzles Closed
Fig. lb: Nozzles Open
The nozzle segments, a.k.a. inlet vanes, are circularly arranged around the expander wheel (see Figs, la&b); note that the arrangement of the nozzles promotes an inward swirling motion, i.e. vortex. In most cases, each vane is pivoted about a fixed pin. Rotation of each vane is controlled by a second pin, which is connected to an adjusting ring. To transfer motion from the adjusting ring to the moving pin, several designs are possible. The two most common methods of guiding the pin are a swing arm connecting the pin and adjusting ring, or slots for the pin on the adjusting ring. A pneumatically operated actuator (Texas Turbine uses Fisher® actuators) controls the motion of the adjusting ring.
Although the actuator could take many forms, including electrically
2
powered mechanisms (motors, solenoids), pneumatic actuation is generally used, a result of the often volatile nature of the gases being processed. The pressure sent to the actuator determines the position of the actuator rod; the actuator rod position determines the angle of the adjusting ring, and consequently how wide open the nozzles are, thus regulating the flow rate (in most cases from 0-120% of design flow). The enthalpy drop across the nozzles creates a swirl (vortex) that is moving faster than the outside rim of the expander wheel. Once the gas enters the wheel channel, it is decelerated from the speed of the swirl exiting the nozzle to approximately the speed of the wheel rim. This slowing of the gas initiates the rotation of the wheel, and is the first mechanism by which power is absorbed by the expander wheel. Since the wheel's exit is closer to the center of the wheel than the inlet, the gas must be spinning more slowly at the exit than at the inlet. (This is just like the fact that the center of a record spinning on a phonograph is moving more slowly than the outside rim.) In addition, the angle of the channel outlet is designed to oppose the vortex, slowing the swirl speed to zero, ejecting the gas straight out of the eye at operating speed. Slowing the swirl down in the channel by driving the gas toward the center of the wheel is the second mechanism by which power is absorbed.
Since every action has an equal and opposite reaction, the
deceleration of the gas's rotational speed is proportional to the acceleration of the rotor. The power absorbed by the wheel is what drives the compressor, but is not the only goal of the machine. The machine is used to refrigerate gases. The refrigeration is a natural, unavoidable result of the gas expansion and the drop in enthalpy. This means that a nozzle alone could refrigerate the gas, but any enthalpy lost by the gas would be lost forever. Using an expander connected to a loading device, on the other hand, allows recovery of most of the energy lost by the gas's enthalpy drop, and results in more refrigeration than a simple nozzle expansion. B. Compressor While the expander is recovering energy from the expanding gas, the compressor acts as a loading device, putting that energy to use. The compressor's operation is very much the opposite of the expander's. For example, the expander is moved by gas, and the compressor moves gas. The gas enters the rim of the expander wheel, and leaves from
the eye of the wheel (along the axis of rotation); the compressor's flows are the reverse (see Fig. 2). The gas's enthalpy is increased across the compressor by an amount less than (equal to, if not for inefficiency) the enthalpy drop across the expander. The resulting increase in enthalpy corresponds to an increase in temperature and pressure. Expander In
M Compressor Out
Expander Out
Compressor In
Fig. 2: Flow Directions
The compressor plays an important role, in that it is loading the expander. Any resistance to the expander's rotation is a load. The load is important for a couple of reasons. First, the operating speed of the machine is a result of balance between forces produced by the expander and compressor. If the machine is turning slower than design speed (with all other variables at design), the expander will generate more force than the compressor, and the machine will accelerate. If the machine is turning faster than design speed (with all other variables at design), the compressor will use more power than the expander can extract, and the system will slow down.
Second, if there is no power
conversion device, such as the compressor, there is no power recovered.
Take, for
example, the two extremes of loading. At maximum load, the expander wheel is locked in place; therefore, it generates a maximum amount of force because it cannot turn. (The larger the drop in enthalpy, the greater the force exerted.) If the device does not turn, no power can be recovered. The other extreme is an expander wheel free to spin, with no load at all. In this case, the expander is spinning at maximum speed for a certain drop in
4
enthalpy.
(The larger the drop in enthalpy, the faster it can go.) No load means no
loading device, and thus no means for power recovery. The expander and compressor wheels have many similarities, but are usually distinguishable from each other. An expander could be used as a compressor, or vice versa, simply by rotating in the opposite direction. This is not done because if the same wheel were used to both expand and compress, efficiencies would in general be low. Figures 3a&b show typical expander and compressor wheels, respectively. Note that the direction of rotation of the compressor is opposite that of the expander.
This is to
represent being placed on opposite ends of a shaft, facing away from each other, and to illustrate how the channel curvatures differ from one another.
Fig. 3a: Typical Expander Wheel
Fig. 3b: Typical Compressor Wheel
Both of the wheels' flows are similar at the rim and the eye. At design speed, the inlets (expander rim & compressor eye) of both are designed to "catch" gas with no shock. This means that the gas does not hit the blade at the inlet, changing speed rapidly; it instead follows the path of the channel, entering the wheel smoothly and efficiently. Furthermore, the expander has swirling gas entering the rim and, at design speed, gas coming straight out of the eye; alternately, the compressor has gas coming straight into the eye, and gas swirling out of the rim.
Despite similarities in the flow, the main
difference is that flow is being "caught" by the expander, and "thrown" out of the compressor. Expander wheels "catch" the gas more smoothly and eject the gas straight out of the eye when the channels are radial or C-shaped. Compressor wheel "throw" the
5
gas out of the rim efficiently with radial or backswept channels. For further explanation in determination of the blade angles, see Appendix A. In concluding the discussion of expander and compressor operation, it is important to note the role played by diffusers. The diffuser is placed at the outlet of the expander and the compressor to allow the exiting gas to slow down smoothly. If the gas were to exit directly into a pipe without a smooth transition, turbulence would occur. In almost all cases, turbulence leads to inefficiency. If the expander eye led directly to an open pipe, the gas would try to fill the pipe, creating a rapid increase in static pressure due to the decrease in velocity (see Fig. 4a). (As a subsonic fluid travels from a smaller area to a larger area, its velocity decreases and its static pressure, which is measured perpendicular to the direction of flow, increases.) In the case that the expander has a well-designed conical diffuser, the gas slows down and gains static pressure gradually (see Fig. 4b). In the case of gradual deceleration, there is less heat generation, and lower energy losses. In addition, the static pressure at the inlet of the diffuser is lower than the static pressure at the inlet of the pipe in the direct-to-pipe configuration.
A lower static pressure at the outlet of the expander means a greater
overall pressure drop, and therefore a greater enthalpy drop, which means it is altogether a higher efficiency device. Pipe fills, creating turbulence. Flow slows down, increasing
Fig. 4a: Direct-to-Pipe Expander
Guard
Fig. 4b: Expander with Diffuser
The compressor diffuser can take several forms; but at Texas Turbine, a simple vaneless diffuser is used. Other options include volutes (see Fig. 5a), or vaned diffusers (see Fig. 5 b).
(Note: The vaned diffuser is the reverse of the inlet nozzles to the
6
expander. Though, in most cases, the vanes on a diffuser are fixed, i.e. do not move. Variable vane diffusers lead to complexity that is unwarranted by the resulting increase in efficiency.) The vaneless diffuser is chosen since it is simple, and performs efficiently over a wide range of operating conditions.
On the contrary, volutes and fixed vane
diffusers operate efficiently only close to their design point. The vaneless diffuser (see Fig. 5c, Fig. 9) is made up of two annular plates. The plates are separated by a distance roughly the width of the compressor outlet. As the gas travels radially outward, diffuser area increases; therefore, the gas slows down and gains pressure. The intent of the diffuser is to recover pressure by slowing gas down without causing turbulence, which can lead to stall (reversed flows). Stall, which in itself leads to inefficiency, can also lead to the dangerous condition of surge. An increased likelihood of stall occurs when the angle of gas entering is too shallow (measured from the tangent to the wheel rim), and when the outer radius is too much larger than the inner radius.
C. Bearings and Lubrication Having described the expander and compressor, it is now necessary to discuss the nature of the bearings, which support the shaft. The bearings are a very important part of the design.
Bearings support loads, restrict the motion of the device, and play an
important role in preventing shaft vibrations.
Lubrication has been included in this
section because it is essential to the bearings' effective operation. There are four types of fluid film bearings used by TTI. There are simple journal bearings, tilting (or segmented) pad radial bearings, tapered land thrust bearings, and tilting (or segmented) pad thrust bearings. The choice of radial bearing does not limit the
7
choice of thrust bearing or vice versa. In other words, if one of the two types of radial bearing is chosen, either of the types of thrust bearing can still be chosen. The choice of bearing is a question of simplicity (and therefore cost) versus performance. The difficulty involved in making the simple journal is deciding upon proper clearances, and assuring that sufficient lubrication will be provided. Journal bearings create more lift when they have less clearance, but heat up more; furthermore, with too tight of a clearance, very little oil will flow through the journal, and hydrodynamic support is lost. A simple journal is circular, with a very small amount of shaft clearance (see Fig. 6a). A tilting pad radial bearing has multiple pads (typically 3-5) that rock on pivots. TTI actually makes segmented pad bearings, whose pads (typically 3) are not pivoted, and are therefore free to move within a small range of motion (see Fig 6b).
Bearing Pad
Fig. 6a: Simple Journal
Fig. 6b: Segmented Pads
Tapered land thrust bearings are simply circular plates with tapered flags that rise in the direction of rotation. These flags face the shaft's large diameter center section (see Fig. 7). The tapers on the thrust bearings are very slight, but must be sufficient to create hydrodynamic
lift at operating speeds.
The tapered land thrust bearing is usually
machined out of the same piece of metal as the journal bearing, so that the journal and thrust bearings are actually one solid piece (see Fig. 9). In the case of segmented pad thrust bearings, the side faces of the pads can be used to support thrust loads, thus the pads supporting the radial and thrust loads are one and the same.
8
Thrust loads
Radial loads
at each end
by seals Fig. 7: Shaft Diagram
The simple journals and tapered thrust bearings are used on the larger units, mainly because the larger units turn slowly enough to use the simpler device. The small, high-speed units require segmented pads, which dampen vibrations and maintain rotor stability more effectively. Although little is understood about the physics of hydrodynamic bearings, the working machines that use them have proven their performance. The theories used to describe the forces created by the bearings are impractical, and though some computer software simulates bearing performance, an experienced engineer is truly needed to design effective bearings. Though the theory of fluid film bearings is rather incomplete, there are general statements that will always be true.
For example, high viscosity
(Viscosity is the internal friction, realized as thickness; thus, alcohol has low viscosity, and honey is highly viscous.), high speeds, or small bearing clearance all generate more lift (good) and more heat (bad); therefore, deciding upon the proper geometry and lubrication is a matter of careful consideration and compromise. Since the bearings depend on hydrodynamic forces for normal operation, constant lubrication is necessary. The bearings need oil to lift and support the shaft, but the oil's own internal friction is the reason for heat being generated. The flow of oil, however, is also responsible for carrying heat away from the bearings.
This heat is not only
undesirable because of the loss in efficiency, but also because of the bulk and complexity the necessary oil cooling system adds to the machine. A simplified lube oil schematic is shown in Figure 8.
The oil starts in the
reservoir and is pressurized by the main oil pump. Two oil pumps are shown, but only one is used at a time. The second oil pump is an auxiliary pump. The oil is then directed
9
Filter
Accumulator
Main Filter A
j
TCV
Cooler —
>
Fig. 8: Simplified Lube Oil Schematic
either through the cooler or around the cooler by the temperature control valve (TCV). The oil cooler is normally an air-cooled heat exchanger, but in some cases, a water-cooled shell and tube type heat exchanger is used instead. The oil is then purified by the main oil filter. An accumulator is attached to the line in between the main and guard filters. The accumulator gathers oil in case of a sudden shutdown, in which case the accumulator empties, providing lubrication to the bearings as the system coasts down. The oil enters the bearing housing after passing through the guard filter. A small amount of the oil moving towards the bearing housing is diverted to the thrust balancer as a thrust control signal (This will be further explained in the next section.). This lube oil setup is highly simplified; therefore, it does not include any valves (designated by a bowtie shape in a lube oil schematic), check valves (which allow flow in only one direction, designated by a "Z" shape in a lube oil schematic), pressure control valves, safety relief valves, or gauges. Seal Gas
Bearing
Seal Gas
Nozzle
Expander Wheel
Bearing
Compressor Wheel
Fig. 9: Typical Cross Section (Simplified)
10
Knowing that the bearings are flooded with oil, and given that the bearings are in between the expander and compressor streams, the question of how to seal the bearing housing must arise. This is a difficult task, considering the fact that a seal will have to be maintained despite the fact that the rotor is moving and the housing is not. This problem is dealt with using labyrinth seals buffered by seal gas (see Fig. 9). Labyrinth seals are very common seals found in all types of turbomachinery. A labyrinth seal is a series of teeth that fit tightly around the perimeter being sealed. The seal provided by the labyrinth is sometimes further supported by a small amount of buffer gas (seal gas) fed into the labyrinth. D. Thrust Balance Thrust balance is a necessity in any working turboexpander. Most thrust bearings alone do not satisfy the need for thrust control. It is therefore necessary to balance the thrust when fluctuations in flow or pressure exert varying axial forces on the rotating assembly. To provide thrust balance, the expander wheels have holes drilled in their channels.
The hole location is chosen so that under design conditions the average
pressure on the front of the wheel is equal to the pressure on the back of the wheel. To separate the front and back of the wheel, labyrinth seals are placed around the outside rim of the wheel (See Fig. 9). Since the flow does not truly have a steady state, the pressure at the hole in the expander wheel is not constant. This means that the back of the wheel sometimes has more force on it than the front and vice versa.
To partially correct for this, a thrust
balance mechanism is employed. The compressor wheel is used to control the thrust. On the compressor wheel, there are no holes, and the rim is sealed with a labyrinth seal. There are no holes on the compressor wheel because the pressure behind the wheel is regulated by the thrust balancing system, rather than a drilled hole. On standard thrust balancing systems, thrust is controlled by the pressure at the thrust bearing faces (see Fig. 10). The standard method of thrust balancing involves drilling a hole on the thrust bearing face to sense pressure.
The pressure from each
bearing pushes on opposite sides of a plug in a cylinder. The plug moves in the direction
11
of the lower pressure, either opening or closing a valve.
The valve connects the
compressor inlet stream and the back of the compressor wheel. When the valve is open, the back of the compressor wheel is at the compressor inlet pressure (the lowest pressure in the compressor stream); therefore, thrust on the compressor wheel is in the direction from the compressor to the expander. When the valve is closed, the pressure on the back of the wheel is slightly lower than the compressor discharge pressure, which means that the thrust is now in the direction from the expander to the compressor. This method ceases to balance properly if the hole that senses the pressure on the thrust bearing face is damaged. It is common for a momentary thrust to cause metal-tometal contact between shaft and thrust bearing. If this happens, the soft bronze that the bearing is made of can smear, covering the hole that senses pressure.
If the hole is
blocked, it no longer senses pressure, and the unit continues to thrust in the direction of the damaged bearing, causing further damage. (Plug rising opens port.) Expander Thrust Pressure Compressor Thrust Pressure Comp. Wheel Back Pressure
Expander Wheel
Compressor Inlet Pressure
Compressor Wheel Fig. 10: Pressure Regulated Thrust Balance
Texas Turbine's trade secret thrust balancing system uses changes in oil flow, rather than pressure, to sense and respond to thrust (see Fig. 11). The flow regulated thrust balancer splits the oil into two flows, one to the expander bearing and the other to the compressor bearing.
Before the flows reach the bearing, they are forced to pass
through small orifices (0-1). Before passing through the orifice, the flows have the same
12
pressure. When fluid passes though an orifice, it loses pressure; the greater the flow, the higher the pressure loss across the orifice.
This means that the bearing that has the
smaller clearance (the rotating assembly is thrusting towards it) will have the smaller flow and therefore the smaller pressure drop. If the flow has a smaller pressure drop, it has higher pressure.
Having higher pressure, the smaller of the two flows will push the
amplifier in the direction to cut off flow through the amplifier for the higher flow. The higher flow being cutoff removes the pressure from its side of the hydraulic cylinder plug. The plug therefore moves in the direction towards the cut off flow. The second set of orifices (0-2) is used to create enough resistance so that most of the flow passes through the bearing housing, and only a small amount passes through the thrust balancer. The flow regulated thrust balancer has noticeable advantages over the pressure-regulated system. For one, the flow-regulated system does not depend on an easily damaged hole that senses pressure.
It is also very sensitive.
A small displacement of the rotating
assembly will cause a faster response than that of pressure regulated systems, and thrust imbalance will be corrected before it becomes a problem.
Expander Wheel
Compressor Wheel Fig. 11: Flow Regulated Thrust Balance
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The previous discussion of thrust balance does not apply to the TTI K-l and K-5. To begin, both wheels are drilled, not just the expander. The wheels are not drilled at only one radius, as most expander wheels are. The wheels are drilled through in several locations. The labyrinth seals are not on the rim, either. Instead, the seals are concentric circles that contact the back of the wheel. This means that the back of the wheels are flat, not contoured, as are the wheels on most units. The seals separate each row of holes in the wheel. By separating each row of holes, the pressure on the front and back of the wheel are approximately the same at each radius.
This system does not completely
eliminate thrust, so segmented pad thrust bearings are incorporated to support thrust loads and dampen vibrations.
II. Product Applications A. Expander-Compressor The expander-compressor is the most common product application.
The
expander-compressor is most often used in gas plants. The expander is used to refrigerate natural gas for separation by liquefaction.
The compressor is used to increase the
pressure and density of process gas. Different gas processing plants have various setups, but for the purpose of our machine, we are concerned only with whether the gas plant is arranged so that the expander-compressor is in either pre-boost or post-boost operation. Pre-boost operation means that the gas stream passes through the compressor before the expander.
This arrangement creates a large thrust toward the expander
(especially at startup) because pressures are higher on the compressor side than on the expander side. In a typical pre-boost arrangement (see Fig. 12), process gas is dehydrated (to prevent freeze ups due to hydrate formation), then compressed.
After being
compressed, the gas is cooled in a gas/gas heat exchanger. The cooled gas/liquid mixture then passes through a cold separator. The cold separator allows the liquid in the stream to drop out and flow to the demethanizer. The expander then further cools the gas exiting the cold separator. The gas/liquid stream is then sent to the demethanizer, where the liquid that formed in the expander falls out and the gas (mostly methane) leaves the top of
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the demethanizer.
Liquid leaves the bottom of the demethanizer, and a fractionation
system separates the components (the liquid is usually composed of ethane and heavier hydrocarbons). 60-90% of the ethane from the gas stream entering the plant is typically removed (when ethane is recovered).
Almost all of the heavier hydrocarbons such as
propane, butane, etc., are removed from the gas entering the plant.). The gas from the top of the demethanizer is still very cold, and is therefore used to cool the inlet gas in the gas/gas exchanger. This mostly methane gas, now warmer, after having passed through the heat exchanger, is then sent to the sales gas pipeline.
In the post-boost setup (see Fig. 13), the gas passes through the expander first and is compressed just before entering the sales gas line. As in the pre-boost case, the gas is dehydrated first. The gas then passes through a heat exchanger and is cooled by gas exiting the demethanizer. The liquid that forms from cooling is then separated from the gas by the cold separator. Once again, the liquid goes to the demethanizer, and the gas is
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further cooled in the expander. The gas/liquid stream from the expander travels to the demethanizer, where the liquid falls out, and mostly methane leaves the top. The cold gas exiting the demethanizer is used to cool the inlet stream, and is then compressed to the pressure of the sales gas pipeline.
Fig. 13: Simplified Post-Boost Plant Arrangement
The post boost arrangement is the more common of the two, but the problems incurred by the pre-boost setup require unique designs.
If the turboexpander is not
specially designed for pre-boost operation, a higher risk of failure is faced. As mentioned before, thrust is the main problem with pre-boost operation. The thrust is caused by the fact that the compressor's inlet pressure is higher than the expander's outlet pressure. This causes an unavoidable thrust toward the expander in conventional designs. Since the thrust balancing system only controls the pressure on the front and back of the wheel, the hub (the center of the wheel, where the shaft is attached) is the only place where the thrust cannot be balanced. Texas Turbine, however, employs a trade secret pressure equalizer
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seal that all but eliminates the thrust problem (see Fig. 14). The system works by drilling a hole through the axis of the shaft.
To disallow the compressor stream's gas from
flowing through the hole and into the expander outlet, a cap sealed by a labyrinth is placed around the compressor hub. This makes the pressure on the compressor hub as low as the pressure on the expander hub, thus eliminating the pre-boost thrust problem.
B. Motor Driven Compressor Texas Turbine has also produced a motor driven compressor (TTI Job 8153). The motor driven compressor is simply a compressor wheel powered by an electric motor rather than an expander. Since 50 or 60 Hz electric motors rotate at a significantly slower speed than centrifugal compressors, a gearbox is used to reach the compressor design speed (in this case 22,000 RPM). Since TTI is not primarily a compressor manufacturer, this machine was more expensive than a high production volume machine manufactured by a company that .makes compressors exclusively. The main reason the customer chose TTI for this job rather than more common compressor manufacturers was the low temperature application in which the machine operates.
TTI is highly experienced in producing rotating
machinery for cryogenic applications, and was therefore able to do, with confidence, what well-known compressor manufacturers were not willing to do.
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III. TTI Frame Sizes A. K-l The K-l (see Appendix B) is the smallest of the TTI units. In pre-boost operation, the flows can be - 1 6 MMSCFD (million standard cubic feet per day). operation, the K-l can handle - 1 1 MMSCFD.
In post-boost
The peak power output of the K-l
expander is - 2 2 0 HP. B. K-5 The K-5 (see Appendix B) is very similar in design to the K - l , but is a larger frame. In pre-boost operation, the K-5 can handle - 4 5 MMSCFD, in post-boost - 3 0 MMSCFD. The peak power output of the K-5 expander is -850 HP. C. K-l 8 The K-l8 (see Appendix C) is TTI's large frame turboexpander.
In pre-boost
operation, the K-l8 can handle -240 MMSCFD, in post-boost - 1 5 0 MMSCFD.
The
peak power output of the K-l 8 expander is -3500 HP.
IV. TTI Advantages TTI produces top quality products, provides innovative solutions to difficult engineering problems, and gives excellent customer service both before and after installation of our machines.
Besides offering designs not available from our
competitors, TTI also possesses in-depth knowledge and experience with all of our competitors' designs, meaning we know the ins and outs (and therefore strengths and weaknesses) of their machines. All this is supplied at highly competitive prices that are only possible given our efficiency and long-term industry experience. A. Performance & Reliability TTI machines equal or better the performance of any competitor's machine in the same operating conditions. The competition at times claims impossible efficiencies that make their machine seem to perform better on paper than it does in reality. In truth, the efficiencies of all turboexpanders are limited by the current technology and the laws of
18 /
physics. Realistically, TTI machines can compete with any other turboexpander on the market. TTI does have a great advantage over its competitors in the field of reliability. Competitor's machines are more likely to experience thrust control failure than any other problem. TTI has a record of zero thrust failures on its machines. This is due to the thrust control mechanisms employed by TTI. Among these thrust control mechanisms employed exclusively by TTI are automatic internal thrust balancing and segmented pad bearings on the K-l and K-5. On the K-18, the flow regulated thrust balancer controls thrust better than any pressure regulated thrust balancer. On all of TTI's frame sizes, preboost thrust is prevented by the drilled though shaft and equalizer seal.
TTI's latest
innovation is the reverse-feed segmented pad bearing. The reverse-feed segmented pads dampen radial vibrations and support loads better than any bearing used by TTI or its competitors. In early tests, it was believed that the vibration probe was damaged, because the vibration readings were lower than ever expected.
Since no machine is perfect,
failures do occur. TTI machines, however, have yet to experience a thrust failure, thus making them the most reliable in the industry. Texas Turbine machines all parts from billet. That is to say that no parts made by TTI are cast, leading to stronger components that are more dependable. The housings are also machined from billet, giving them not only better material properties, but better flow properties as well. The grooves left by machining give smoother channels in the direction of flow, as opposed to the oddly shaped, rough surfaces in the flow passages of cast housings. B. Maintenance TTI machines are relatively easy to maintain.
TTI machines have modular
designs that make changing spare rotating assemblies very easy. In the case of the K-l and K-5, the SRA (spare rotating assembly) is a plug design that can be easily changed by one man (see Appendix B, Isometric View). The K-18 is a simple and robust design consisting of three main sections: 1) the expander case, 2) the SRA, and 3) the compressor case (see Appendix C, Isometric
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View). The spare rotating assembly, though not as small as that of the K-l or K-5, is still rather compact and relatively easy to change. The skids supporting the lube oil system and the turboexpander are very clean, organized, spacious designs. This makes changing and maintaining components of the lube oil system a far easier task in the field. In addition, standard TTI skids meet or exceed API standards with a very small list of exceptions.
Examples of how TTI
standard skids exceed API standards are: 1) the presence of dual accumulators 2) both accumulators are heat traced to prevent slow response in cold weather 3) differential pressure indication on the oil guard filter 4) oil flow meter (for seal gas and oil) 5) block and bypass around the seal gas pressure differential control valve and flow meters (for easy replacement/inspection without shutdown) C. Cost TTI produces the best machines in the industry at the most competitive prices. TTI units are economically priced, but still consist of top-quality engineering, materials, and manufacturing methods. The main factors affecting the cost of a machine are: 1) housing size, determined by flow rate and pressure 2) power output, determined by flow rate, pressure ratio, inlet temperature, and gas composition 3) odd process conditions, i.e. high pressure ratios, high pressures, high head, cryogenic compressor conditions, high molecular weight compositions, volatile gases requiring special materials and/or seals, etc. 4) required standards, i.e. API, customer specified 5) hazardous area classification greater than Class 1, Group D, Division 2
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Appendix A Blade Angles Blade angles are an important part of efficient turbomachinery.
Well-chosen
blade angles recover more power, prevent shocks, and increase overall efficiency. Since the expander and compressor flows differ, the blade angles determined differ. Flow exiting
Fig. 1A: Expander Blade Angles
The elements necessary for determining proper expander blade angles are shown in Figure 1 A. The vector labeled "flow exiting the nozzle" represents the actual velocity of gas entering the wheel rim. (Shown with a phantom line to point out that it has been moved to form the triangle; otherwise, the vector's arrowhead would share the same point as the other two vectors, at the wheel tip.) The vector labeled "flow entering the channel" is the flow exiting the nozzle, as viewed from the wheel tip. The "flow entering the channel" vector is therefore the "flow exiting the nozzle" with the "wheel tip speed" subtracted. Subtracting the wheel tip speed leaves a steeper angle, as measured from the tangent to the wheel. This angle is the necessary blade inlet angle. The "flow entering perpendicular to the rim" is the component of velocity that determines the amount of flow
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entering the wheel; when multiplied by the area of the inlet (wheel rim), volumetric flow rate is determined. At the eye, the outlet flow is ejected perpendicular to the plane. Exiting the eye of the expander without a swirl improves efficiency by improving diffuser efficiency. This is accomplished by making the "flow exiting tangent to the eye"' as fast as the "blade speed at the eye". For a given velocity of gas exiting the channel, the blade angle can be set so that the speed of the flow leaving the channel tangent to the eye is equal to the blade speed at that radius. Recall that points closer to the center are traveling slower than points farther away, meaning that the blade angle is not the same across the entire eye. In fact, the blade angle gets steeper (as measured from the plane tangent to the eye) closer to the center, since the blade speed gets smaller and the "outlet flow" speed is assumed constant (the outlet flow speed is equal to the flow rate exiting divided by the area of the eye).
Determining compressor blade angles depends on velocity triangles as well, but the flows differ, as seen in Figure 2A. The inlet of the compressor is essentially identical to the outlet of the expander, except the flows are all reversed. The rim is where the compressor is truly dissimilar from the expander.
2A
For a given flow rate, speed, and wheel diameter, a compressor gives its highest increase in enthalpy when the "'flow exiting the channel" is perpendicular to the rim, and the blade angle is therefore 90°. This is a radial bladed compressor. A radial bladed compressor delivers more boost (increase in head) at its operating point, but has a very narrow range of operation. (As a note: Forward leaning compressor wheels are possible, though TTI does not use them. A forward leaning compressor wheel is essentially an expander running in reverse. This type of wheel actually gives the highest head increase for a given speed, but has a very narrow range of operation and low efficiency.) When "flow exiting the channel" is ejected at a shallow angle, the "'outlet flow" leaves at a steeper angle. (The "flow exiting perpendicular to the rim" is assumed a constant determined by the volumetric flow rate exiting divided by the area of the outlet.) This is a back-leaning wheel. A back-leaning wheel needs to be bigger to achieve the same increase in enthalpy, but has a much wider range of efficient operation than a radial wheel. The back-leaning wheel is better suited to real flows than the radial wheel. The back-leaning wheel is less likely to create separation and therefore turbulence, which causes losses in efficiency. In addition, back-leaning designs tend to surge at lower flows than do radial wheels. Surge is a phenomenon that can cause extensive damage to any compressor. Surge occurs when the flow rate gets far enough below the design flow to no longer create a positive increase in head, this results in back flow until the compressor can once again give positive head, and the process repeats cyclically. In some cases, this simply results in noise and lowered efficiencies, in other cases surge can lead to complete failure of the machine. Blade angles are not the only factor controlling surge conditions, but a well-designed back-leaning wheel can lower the surge point significantly.
3A
