Turboexpanders and Process Applications

Heinz P. Bloch, Claire Soares

• Publisher: Gulf Professional Publishing
• Year: 2001
• Print ISBN: 9780884155096
• eBook ISBN: 9780080519777
• Topic: Business
• Subtopic: Industrial Management
• Index: Business

Turboexpanders and Process Applications offers readers complete application criteria, functional parameters, and selection guidelines. This book is intended for the widest possible spectrum of engineering functions, including technical support, maintenance, operating, and managerial personnel in process plants, refineries, air liquefaction, natural gas separation, geothermal mining, and design contracting.The text distinguishes between cryogenic turboexpanders that are used to recover power from extremely cold gases, and hot gas expanders that accomplish the same objective with gases reaching temperatures in excess of 1000 degrees Fahrenheit. The authors have assembled in this book an optimum combination of process and mechanical technologies as they apply to turboexpanders. - A highly practical, well-illustrated, and up-to-date overview of turboexpander construction features - Appeals to a wide range of engineers

CHAPTER ONE

Why and How Turboexpanders Are Applied

Turboexpanders are expansion turbines, rotating machines similar to steam turbines. Commonly, the terms “expansion turbines” and “turboexpanders” specifically exclude steam turbines and combustion gas turbines. Turboexpanders (Figure 1-1) can also be characterized as modern rotating devices that convert the pressure energy of a gas or vapor stream into mechanical work as the gas or vapor expands through the turbine. If chilling the gas or vapor stream is the main objective, the mechanical work so produced is often considered a byproduct. If pressure reduction is the main objective, then heat recovery from the expanded gas is considered a beneficial byproduct.

Figure 1-1 Modern turboexpander installation. (Source: Atlas Copco.)

In each case, the primary objective of turboexpanders is to conserve energy. Contemporary turboexpanders do this either by recovering energy from cold gas (cryogenic type) or from hot gases at temperatures of over 1,000 degrees. Current commercial models exist in the power range of 75 kW to 25+ MW, so many applications are possible.

Changing market conditions, accentuated by growing environmental awareness on a global scale, are improving market receptivity for the turboexpander. Machinery manufacturers, quick to sense this market potential, have developed design features within their turboexpander ranges that offer user-friendly features, promoting ease of maintenance and operation, and aid design optimization.

TURBOEXPANDERS FOR ENERGY CONVERSION^*

Substantial energy can be recovered using low-grade waste heat, process gas, or waste gas pressure letdown.

Centrifugal (radial inflow) turboexpanders are well adapted to such energy conservation schemes and, with recent developments that have increased their reliability, are suitable for unattended service on a 24-hour, 7-day week operational basis. Some of the recent developments include better shaft seals, thrust bearing monitoring, and superior control devices.

Turboexpanders are well qualified to meet the requirements of energy conservation. Decades of development in turboexpander technology have resulted in highly efficient machines that can be applied in the profitable recovery of energy from waste heat sources and gas pressure letdown. Increasing demand and the progressive depletion of energy sources point to the need for conservation and for the recovery of energy from sources once thought unprofitable.

In the past, the use of the turboexpander as an energy recovery device was limited for a number of reasons:

• The return on capital investment did not justify a power recovery system unless more than several thousand horsepower was recovered.

• Finding a market for recovered power was difficult when there appeared no immediate use for it within the plant.

• Continuity and reliability of this energy source was required if it were used as “base load,” which required standby equipment, spares, and appropriate operator attention.

• Lack of confidence in new power recovery schemes that were not yet proven made both government and private industry reluctant to invest in these systems.

Recently, there has been a substantial shift in conditions and user attitudes. With increasing cost of power, the return on capital investment has vastly improved. A more favorable regulatory climate and changes in attitude of utility companies toward returning electricity to their grid have made novel power producing schemes practical and attractive.

High-efficiency expanders and their relatively short payback period made even smaller units economically attractive. These machines have demonstrated a high degree of reliability. Hundreds of units have been in continuous uninterrupted service for many years; this has removed the need for backup equipment and has demonstrated that unattended operation is entirely feasible.

What follows is a summary of turboexpander applications, an overview of what constitutes the present state-of-the-art, and the features incorporated in turboexpander design, which enable it to meet a host of power recovery requirements.

TURBOEXPANDER APPLICATIONS

For many years, turboexpanders have been used in cryogenic processing plants to provide low-temperature refrigeration. Power recovery has been of secondary importance. Expander efficiency determines the amount of refrigeration produced and, in gas process plants, the amount of product usually depends on the available refrigeration. Accordingly, there is a large premium on efficiency and, of course, on reliability.

The main market for turboexpanders has been in low-pressure air separation plants, expanding down from 5 bar, and in hydrocarbon processing plants, expanding natural gas from as high as 200 bar. The air separation expanders are roughly divided into two types. The first type ranges from a few horsepower up to 100 hp. Here, the expander power is too small to be economically recovered and is, therefore, absorbed by an oil brake or similar device. The second type ranges from 100 hp to over 2,000 hp, where the power is used to drive electric generators or process booster compressors.

Hydrocarbon gas expanders range in the order of 100 hp to 8,000 and more hp. The majority of these machines are usually designed for power recovery duty, with a process compressor directly driven by the expander. The gas is usually expanded from an inlet pressure in the 100 bar to 50 bar range, down to outlet pressures in the 50 to 15 bar range. This results in an expansion ratio of 2:1 to 4:1, a very suitable expansion for a single-stage expander. Typical efficiencies range from 84% to 86%.

There are numerous, large turboexpanders operating in the pressure range of 130–200 bar, most of them in well-head natural gas service.

Expanders are also used for the pu...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Chapter 1: Why and How Turboexpanders Are Applied
• Chapter 2: Turboexpander Fundamentals
• Chapter 3: Application of Cryogenic Turboexpanders
• Chapter 4: Application of Hot Gas Turboexpanders
• Chapter 5: Specifying and Purchasing Turboexpanders
• Chapter 6: Special Features and Controls
• Chapter 7: Turboexpander Protection and Upgrading
• Chapter 8: Specific Applications and Case Histories
• APPENDICES
• Index