TTI Turboexpanders

Prepared By: Christian Giroux

Table of Contents Page(s )

Preface 1 page

Glossary 6 pages

I. Description of Turboexpander Operation 1 - 1 4

Introduction 1

A. Expander 1 - 3

B. Compressor 3 - 7

C. Bearings and Lubrication 7 - 1 1

D. Thrust Balance 11 -14

II. Product Applications 14 -17

A. Expander-Compressor 14-17

B. Motor-Driven Compressor 17

III. TTI Frame Sizes 18

A. K-l 18

B. K-5 18

C. K-l8 18

IV. Advantages of TTI Machines 18 - 20

A. Performance & Reliability 18-19

B . Maintenance 19-20

C. Cost 20

Bibliography 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

G l

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 Eye of an Expander Wheel j

-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).

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

Journal Bearing Thrust Flag

Thrust Bearing Face

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).

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.

Arc along which moving pin travel

Moving Pin

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).

rotating assembly, see center section.

rotor: the components of a rotating machine that rotate. In the case of an expander-

compressor, 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.

< ^ Radial Movement

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

Expander Out

M Compressor 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.

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

Pipe fills, creating turbulence. Flow slows down, increasing

Fig. 4a: Direct-to-Pipe Expander G u a r d Fig. 4b: Expander with Diffuser

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).

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.

Bearing Pad

Fig. 6a: Simple Journal Fig. 6b: Segmented Pads

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

Fil ter

Accumulator

M a i n Fil ter

A

j T C V 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.

Nozzle Seal Gas Bearing Seal Gas

Expander Wheel Bearing

Fig. 9: Typical Cross Section (Simplified)

Compressor Wheel

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-to-

metal 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.

Expander Thrust Pressure

(Plug rising opens port.)

Compressor Thrust Pressure

Comp. Wheel Back Pressure

Compressor Inlet Pressure

Expander Wheel 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

13

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

14

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

15

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

16

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.

17

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 -16 MMSCFD (million standard cubic feet per day). In post-boost

operation, the K-l can handle -11 MMSCFD. The peak power output of the K-l

expander is -220 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 -45 MMSCFD, in post-boost -30

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 -150 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, pre-

boost 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

19

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

20

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.

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

F l o w ex i t ing

Fig. 1A: Expander Blade Angles

34

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