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Phase 2SEGAS SERVICES Training Program Turbo Expander Page 1 of 13

SEG S Ser vices Train ing Cent r e

Dam ietta LN G Plant

Turbo Expanders

Contents

Introduction

Types of Turbo Expanders

Radial Flow Impulse Style Turbo Expander

Auxiliary Equipment

Start-up, Running and Shutdown Checks


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INTRODUCTION

A turbo expander can be referred to as a gas expander or turbo expander. It is used in industryto harness excess energy from an industrial process. Since its first application in the early

1960's turbo expander design has evolved and improved. Today hydrocarbon process

designers use turbo expanders for almost all hydrocarbon liquid rejection and hydrocarbon

dew point control, ethane recovery, and liquefied natural gas production. Turbo expanders are

very efficient for energy recovery from large volume gas streams (natural gas or waste gas).

This energy can be used to produce electricity, drive compressors or provide power

transmission to a variety of equipment (refer to figure #1). In a refinery process hot

combustion gases from a catalytic cracker can produce 20000 to 200, scfm of combustion

gas. These gas products can be 20 psig and greater at temperatures up to 1400F. In some

cases these gases are vented to atmosphere, wasting valuable energy. A turbo expander could

recover 20000 hp from the greater of such a flow and produce electricity worth I milliondollars per year.

In a plant operation using cryogenics to cool and condense gases below -SOP, a

turboexpander is used to achieve extremely cold temperatures by letting the gas expand and

absorb energy. A turboexpander lowers gas temperatures by extracting energy from the gas

expansion process. This reduces gas temperatures lower than can be achieved by throttle

values .The energy imparted on the turbine wheel can be used to power other equipment. The

turboexpander has the ability to act as a refrigerator in the separation and liquefaction of

gases.

Figure 1 Turbo expander and axial flow compressor


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TYPES OF TURBO EXPANDERS

There are two main types of turbo expanders:axial flow and radial flow designs. Axialflow turbines operate under the same principals as a steam turbine. They could be single stage

or multistage with impulse or reaction blading or a combination of the two. Turbines of this

type are power recovery turbines. They are used where flow rates, inlet temperatures or total

energy drops are high. Radial flow turbo expanders are normally single stage with

combination impulse reaction blades. The rotors can resemble a centrifugal semi open

compressor impellor. Radial flow turbo expanders are used primarily for low temp service.

The 9O-degree inflow radial turbine has found the widest acceptance. (Refer to figure # 2 &

3)

Figure 2


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Figure 3 Typical Turbo expander cross-section


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Radial Flow Turbo Expander

A turbo expander is a machine that continuously expands a gas from a greater pressure to alower pressure while causing the gas to do work. The turbo expander cools the gas very

significantly and the process produces shaft power. The working components of a turbo

expander perform the same functions as a turbine stage. The working principle of a reaction

design turbine (turbo expander) is as simple as whirling sprinkler heads or a water wheel

driving a mill. There are two basic types of turbine blade designs: the impulse and reaction

type.

In the impulse style of turbine complete gas expansion occurs in the nozzle (refer to figure #

4 & 5). The gas passes through one or more rotating buckets thereby reducing initial velocity.

The number of buckets depends upon the desired efficiency and power requirement. Turbines

smaller than 300hp are usually single stage machines. If these machines must extract highhorsepower two or more impellors are used. Large turbines usually use a combination of

impulse and reaction blade design.

Many applications use the turbo expander to drive a generator or they can be matched to a

centrifugal or axial flow compressor. It can also be used in the cryogenics industries for

refrigerating gases to very cold temperatures. (Refer to Figure # 6)

Figure 4 Radial flow impulse style turbo expander


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Radial flow ( 90 degree inflow )Turbo expander ( impulse type ofbalding )

Nozzle inlet vanes

Gas flows in thisdirection into turboexpander turbine

Figure 5 Turbo Expander


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The Helium gas turbo expander is used in a cooling process. The cold high-pressure helium

gas passes through a turbine which itself drives a compressor in a secondary helium loop. The

gas therefore does work in the primary loop and is cooled still further. The energy extractedis transferred to the secondary loop as heat, and finally removed in a water-cooled heat

exchanger. The speed of the turbine is regulated by means of a throttle valve, which controls

the flow rate in the secondary loop, which is also called the brake circuit.

The outstanding features of these machines are their long life, smooth running, compactness

and high reliability. They are essentially maintenance-free. The turbines are remarkable in

that they run at very high speeds (100,000 to 150,000 rpm) but do not need any active control

system. They use dynamic gas bearings, which are hermetically sealed against contact with

the atmosphere, and the process gas itself acts as a bearing lubricant. As the shaft revolves,

Figure 6 Helium Gas Turbo Expander


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special flow elements (pivoted segments, spiral grooves) create a pressure gradient, which

centres the shaft both radially and axially and prevents any direct metal-to-metal contact.

Built-in permanent magnets prevent mechanical contact between the turbine rotor and itsbearings when it is stationary, thus making for easy starting of the turbine. (Refer to figure #

6)

AUXILIARY EQUIPMENT:

All types of expanders will use a means of sealing the process gas from the lube oil or vice

versa. The means, which this accomplished, is with a labyrinth style seal (non contact), a

carbon ring seal (packing), a mechanical contact seal or a liquid film seal. A liquid film seal

uses metallic sealing rings and uses a liquid buffer in the clearance area to provide a seal.

Lube oil systems

All turbo expanders and their driven equipment will have a lubricating system to provide

lubrication to the bearings of the rotating shafts. This system will include a reservoir to

contain the oil, mechanical pumps to supply oil throughout the system, oil coolers to maintain

proper lubricating oil temperatures. The system will also include safety relief valves to

prevent system over pressurization and regulators to maintain proper pressures. The oil

temperatures will be controlled by thermostatic valves, which direct the oil to the coolers as

required. The systems must have strainers in the oil tank supply line and filters close to the

bearings being supplied with oil to filter out any dirt. The oil pumps in the systems may be

mechanically driven off of the rotating shafts or they be electrically driven. Some systems

might use a mechanical pump as the main oil pump and an electric pump of the same

capacity as a backup should the main unit fail.. This type of system might also use thiselectric pump to supply oil in a pre-lube cycle and a post-lube cycle for the drive and driven

equipment. This will ensure that the bearings are supplied with cool, clean oil prior to start-up

and also upon shutdown. Timers and a specific control system will provide the proper inputs

for the pre and post-lube cycles. These lubrication systems will always have a mean to

monitor temperatures (gauges or electronic temperature pickups) as well as monitor oil

pressures that the bearings are receiving. These temperatures and pressures may also be

linked to a shutdown system, to protect all rotating parts and bearings. The oil coolers may be

an, air to oil cooler that uses a fan to cool the oil in a radiator style heat exchanger. The other

styles available may be water to oil style that uses cooling water from the plant to be

circulated within a heat exchanger.

(Refer to figure # 7)


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Seal oil system (Buffer liquid system)

On some types of systems the process gas that is being used to drive the expander may be

flammable or corrosive. If this flammable gas were to mix with the lubricating oils in the

bearing cavities it may present a dangerous condition throughout the lubricating system,

when the gases become entrained into the oil. Should the gases be corrosive it will shorten

the life of the oil and all components in contact with the oil. A similar situation could also

exist if the driven unit is a gas compressor.

In either situation the manufacturer may choose to provide a sealing flow of oil (supplied to

the seals at a pressure 20 psi higher than suction pressure) into a cavity between two seals.

These seals will separate the process gas from the bearing lubricating oil. A buffer gas supplyis used on the pressure side (process gas) to prevent any seal oil from flowing into the

compressor and being lost. The seal oil/buffer gas mix is drained by gravity back to a trap

that contains both a separator and a coalescer. The mixture is separated, the oil returns to the

oil tank and the buffer gas returns to the suction port of the compressor or expander.

On some applications an external supply of buffer gas may be introduced into the seal cavity.

This gas must be supplied at a pressure of at least 50 psi higher than system inlet or suction

pressure and is regulated by control equipment. The amount of buffer gas depends on

compressor style and suction/inlet gas conditions. (Refer to figure# 8,9 & 10)

Figure 7 Typical Lubricating Oil Systems


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Figure 8 Sealing Arrangements for Turbo Expander


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Figure 9 Sealing Arrangements for a Compressor Bearing Housing Using A BufferGas For Sealing


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START-UP, RUNNING AND SHUTDOWN CHECKS.

All the specifics of the start up, running and shut down procedures for turbo expanders anddriven equipment will be covered by the manufactures standard operating procedures.

Each manufacture will outline all the operating limits of temperatures, pressures, vibration

levels (if installed), speeds etc, of the equipment for the operations personnel. On start-ups

some machines may use a pre lube cycle that may for a set period of time prior to rolling the

turbine. This cycle may include the heating of the oil in the reservoir with the use of an

emersion heater to a specified temperature. When all conditions are met the expander may be

operated. If the expander uses hot process gas it will have throttling valves to allow gas entry

into the turbine. The speed of the turbine will be regulated by a governor control that uses a

shaft mounted speed-monitoring device. This will provide accurate turbine speed monitoring

and input to the throttle controls. Safety shutdowns will include, bearing oil temperatures,bearing supply oil pressures, rotating shaft vibration levels, inlet gas temperatures to the

turbine. These shutdowns will protect the drive and driven equipment from damage should

the machine operate outside of the manufactures design limits. Some machines will have

automatic shutdowns others will have to be monitored by operations and adjusted as required.

The post lube system for the lube system also runs on a set timed interval. This will ensure

that all excess heat will be removed from the bearing areas to prevent damage to the critical

components.

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