TURBOEXPANDERS:

Basic Design, Operation and Troubleshooting

Tadeh Avetian, Director of Engineering

L.A. Turbine

June 23, 2020

MGGPA Virtual Training Presentation

Turboexpander Applications

Natural Gas Processing

Liquefaction

Dew Point Control

Petrochemical

Ethylene Plants

Air Separation

Refrigeration

Power Generation

2

3

Cryogenic Extraction

of Natural Gas Liquids

The most common

hydrocarbon application

is the extraction of heavy

hydrocarbons or Natural

Gas Liquids which involve

a Turboexpander and

a low-temperature

distillation column

called a demethanizer.

Photo courtesy of

BCCK.

4

What is a Turboexpander?

Think of a car’s turbocharger

used for gas processing

In a car, compression is

important and the turbine drives

the compressor

In a gas processing plant,

expansion is important and the

compressor loads the turbine

• Load doesn’t have to be a

compressor. Options include

generators or oil brakes (dyno)

Photo courtesy of total911.com

Turbocharger

Turboexpander

Why is Expansion Needed?

Expansion of a gas (i.e. significant pressure drop) produces a refrigeration

effect (Joule-Thomson)

Refrigeration allows for the liquefaction of some gases in gaseous mixtures

(e.g., natural gas), allowing for separation of these gases in a gas/liquid

separator (e.g., demethanizer)

p 1 T1 p 2 T2

p1 > p2

T1 > T2

Lower pressure

and temperatureHigher pressure

and temperature

5

Expansion Methods

6

Joule Thomson (J-T) Valve or Throttling Valve• Constant enthalpy expansion across an adiabatic valve with no work output

Turboexpander • Adiabatic expansion with work output

7

Some Basic Thermodynamics

𝑤 =ሶ𝑊

ሶ𝑚= ℎ01 − ℎ02 = 𝑐𝑝𝑇1 +

𝑣12

2− 𝑐𝑝𝑇2 +

𝑣22

21st Law:

J-T Valve: 𝑤 = 0 𝑇2 = 𝑇1 −𝑣2

2

2𝑐𝑝

0

Turboexpander: 𝑤 ≠ 0 𝑇2 = 𝑇1 −𝑣2

2

2𝑐𝑝−

ሶ𝑊

ሶ𝑚𝑐𝑝

A turboexpander will produce lower temperatures given the same pressure drop, however

requires a load to absorb the power generated.

Typical Process Overview

8

Enthalpy

Pre

ssu

re

RETROGRADE

DEW POINT

NORMAL DEW

POINT

TEX INLET

High pressure, moderately cold gas flows into the expander suction of the

Turboexpander

The gas flows through the expander variable inlet nozzles (Inlet Guide Vanes or

IGVs) and then through the wheel, exhausting at a lower pressure and at a

substantially colder temperature

After the expander, the pressure, temperature and enthalpy decrease

Thermodynamics in a Turboexpander

9

Mechanical Center Section (MCS) Assembly

10

MCS

Mech. Center Section

• Center casing holding the

wheels, shaft, bearings,

and other parts

• Common practice to have

one spare MCS assembly

on-hand

• Long lead times for repair

and parts of this section

• Spare MCS can be

installed while mechanical

issues are addressed

• More time and cost-

effective

MCS Internal Components

11

Drive Bearing

Compressor Seal

Expander Wheel

Expander Back

Wheel Seal

Heat Barrier Wall

Shaft

Load Bearing

Compressor Wheel

Inlet Guide Vane (IGV)

12

IGV

It fits within the expander case as noted in the previous slide, but

the IGV can be accessed, removed and replaced, by removing the

Mechanical Center Section (MCS) as described on the next slide.

Nozzle

Segments

Fixed Ring and Pins

Adjusting Ring

And Nozzle Fixed Ring

Expander and Compressor Cases

13

Expander Case

Compressor Case

IGV Mechanism

14

Pressure Profile Through Expander Stage

15

Expander-Compressor Power Balance

16

The power produced by the expander must balance the power

consumed by the compressor plus losses (e.g. bearings, seals)

The resultant shaft speed is a consequence of the power balancing

according to the following relationship:

expander compressor bearing lossP P P= +

( )

( )

( )

,exp 0

,

i

p comp

comp

fct U C

fct

fct

=

=

=

Radial Inflow Expander Wheel Options

17

Closed Design Open Design

AMB vs Oil Bearings

18

Oil Film Bearing

(Fixed Geometry or Tilt-Pad) Active Magnetic Bearing Cartridge

Active Magnetic Bearing (AMB) – Basics

19

Proven API Compliant Technology (25+ years)

Active - (i.e. electromagnets)

Stator - stack of laminations with copper coil

windings (axial brg has solid core)

Rotor – iron-based laminations (axial brg requires

solid ferrous material)

Current is supplied to stator by power amplifiers,

which induce controlled magnetic forces

Gap sensors monitor radial and axial position

RTDs for temperature monitoring

Auxiliary bearings for protection

Turboexpander Compressor Shaft for Magnetic Bearings

20

Radial/Axial

Gap Sensor

Sleeve

Aux Bearing

Landing

Sleeve

Thrust

Disk

Speed Sensor

Sleeve

Radial Bearing

Lamination

Sleeve

Magnetic Bearings Principle of Operation

Attractive Electromagnetic Suspension

21

Magnetic Bearings Principle of Operation

Control of Rotor Position

22

AMB vs Oil Bearings

General Advantages

23

Contact free → no friction or wear

Increased availability, essentially maintenance free

Lower bearing losses, especially for larger machines

Typically capable of higher shaft speeds → higher performance

Faster responding thrust balance system w/ actuated control valve

Smaller skid → smaller footprint

Faster start-up/commissioning

Remote connection for fault and troubleshooting diagnostics

AMB vs Oil Bearings

Advantages Without Lube Oil System

24

Easier to operate

Less maintenance concerns

No oil flush / cleanliness requirements

No oil carryover to process

No oil contamination/dilution concerns

No lube oil to warm up (can take hours in cold climates)

Increased Ambient Temperature Range

25

Turboexpander

Seal Gas System

Turboexpander

Controls/PLC

Oil Filters

Oil Cooler

Oil Reservoir

Oil Accumulator

Oil Pumps

Traditional Oil Bearing Turboexpander

Control Valves

26

Turboexpander

Seal Gas System

Turboexpander

Controls/PLC

AMB Controller

ARES AMB Turboexpander

27

Typical P&ID (IGV and Seal Gas Control)

28LATurbine.com

Typical P&ID (Process, Anti-Surge & ATB)

29LATurbine.com

Shaft Sealing – Typically Single Port Labyrinth

30

PROCESS

SEAL GAS

SUPPLY

15 psi ABOVE EBWP

Bearing

Shaft

Wheel

Heat Barrier Wall

Key

ATB Purpose

31

Controls the axial thrust of the expander/compressor rotor within safe load

limits, in either direction, on the thrust bearings

Keep rotating components of turboexpanders axially-centered

Best efficiency is achieved through consistent clearance between the

dynamic and static shroud line

Delivers increased reliability through non-contact between rotating and non-

rotating components

ATB Operation (Oil Bearings)

32

ATB system senses the hydrodynamic oil pressure of each bearing thrust-face which is

measured and connected to two opposite ends of a piston chamber

A piston/cylinder actuator adjusts a control valve that is connected between the

pressure port behind the compressor wheel and the suction of the compressor

In the event of thrust imbalance, the piston is actuated and either closes or opens the

valve to help balance thrust

Compressor WheelExpander Wheel

Bearing Bearing

Pback

Shaft

Compressor

Suction

Common Troubleshooting

33

High Thrust

Open anti-surge valve

Plugged expander wheel thrust holes

Wheel Seals

Excess seal gas flow to the compressor seal

Common Troubleshooting

34

High Bearing Temperature

Insufficient oil flow

Oil supply too hot

High vibration or thrust

High bearing current (AMB)

Insufficient cooling gas supply (AMB)

Common Troubleshooting

35

High Shaft Vibration

Excess rotor imbalance

Low oil viscosity

Elevated oil supply temperature

Aero induced vibration

Common Troubleshooting

36

Erratic Anti-Surge Valve Control

Process conditions vary from design parameters

Incorrect compressor flow transmitter settings

Anti-surge valve actuator not working properly

Compressor pressure transmitters not working properly

Incorrect Anti-surge controller parameters and/or tuning

Gas Processing & LNG Magazine Article

37

Fundamentals of turboexpander design and operation

May/June 2020

Authors: Tadeh Avetian & Luis Rodriguez, L.A. Turbine

Pgs. 31-36

https://laturbine.com/wp-content/uploads/2020/06/Fundamentals-

TBX-GPLNG.pdf

https://laturbine.com/wp-content/uploads/2020/06/Fundamentals-TBX-GPLNG.pdf

Questions?

38

L.A. Turbine

Tadeh Avetian, Director of Engineering

Email: TAvetian@LATurbine.com, Phone: +1 661 294 8290, x1210

Troy O'Steen, Director of Sales

Email: TOSteen@LATurbine.com, Phone: +1 661 755 0949

MCGPA

Bret Hunker, Bret.Hunker@enablemidstream.com

mailto:TAvetian@LATurbine.commailto:TOSteen@LATurbine.com

Thank You!