Case Studies in Mechanical Engineering

Decision Making, Thermodynamics, Fluid Mechanics and Heat Transfer

Stuart Sabol

• Publisher: Wiley
• Year: 2016
• Print ISBN: 9781119119746
• eBook ISBN: 9781119119760
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Using a case study approach, this reference tests the reader's ability to apply engineering fundamentals to real-world examples and receive constructive feedback

Case Studies in Mechanical Engineering provides real life examples of the application of engineering fundamentals.  They relate to real equipment, real people and real decisions.  They influence careers, projects, companies, and governments.  The cases serve as supplements to fundamental courses in thermodynamics, fluid mechanics, heat transfer, instrumentation, economics, and statistics.  The author explains equipment and concepts to solve the problems and suggests relevant assignments to augment the cases.

Graduate engineers seeking to refresh their career, or acquire continuing education will find the studies challenging and rewarding.  Each case is designed to be accomplished in one week, earning up to 15 hours of continuing education credit.  Each case study provides methods to present an argument, work with clients, recommend action and develop new business. 

Key features:

• Highlights the economic consequences of engineering designs and decisions.
• Encourages problem solving skills.
• Application of fundamentals to life experiences.
• Ability to practice with real life examples.

Case Studies in Mechanical Engineering is a valuable reference for mechanical engineering practitioners working in thermodynamics, fluid mechanics, heat transfer and related areas.

Case 1
Steam Turbine Performance Degradation

A private investor-owned power company owns 15 GW of capacity including conventional fossil-fired generation and natural-gas fired combined cycle gas turbine power plants spread throughout the United States. The company competes in several unregulated power markets and takes seriously its ability to provide safe, reliable, low-cost power compared to its competitors while meeting all environmental permit requirements. Quarterly senior management reviews include reports on worker and contractor safety performance, the reliability and efficiency of the facilities, as well as any exceedances of environmental permits. The company spent time and resources establishing guidelines and procedures for regular performance monitoring at its generating facilities, including results analysis. These guidelines are routinely reinforced at every level of the organization with training for new recruits and refresher courses for midlevel management.

The performance-monitoring procedures and guidelines include techniques to analyze the test data based on industry guidelines, particularly ASME PTC Committee (2010) and technical papers from noted industry experts such as Cotton and Schofield (1970). For the company’s steam turbines, the condition of the various stages is related to changes in stage pressures at standard conditions knowing how the throttle flow to the machine has changed. The methods are based on the fact that, for a large multistage condensing turbine, all stages, except the first and last, operate with a constant pressure ratio (p₂/p₁.) This allows the general flow equation for flow through a converging-diverging nozzle for stages beyond the first stage to be simplified to equation (1.1)

(1.1)

where

• 
  , P and υ are the flow rate, absolute pressure and specific volume to the following stage;
• Φ is a constant flow function (area).

The flow function Φ includes unit conversions, constants of proportionality, the area of flow, and the coefficient of discharge for the nozzle and blade path. Except for unit conversions it has units of area.

A production engineer at one of the company’s coal-fired power plants with three 600 MW subcritical single reheat units has been monitoring the units’ performance according to company procedures. In just over 7 months since the last major overhaul one unit has lost 3.4% of its output, and the cycle heat rate has increased 0.6%. Using the guidelines, most of degradation in performance can be explained by changes in the flow-passing capability of the steam turbine and losses in the high-pressure (HP) turbine efficiency.

However, there are changes to characteristics that are not discussed in the corporate standards or the technical papers available in the office. In particular, the intermediate pressure (IP) turbine’s extraction temperature has risen noticeably from the expected value. Efforts to explain the symptoms as instrumentation issues have failed. Rather than dismiss or ignore the findings, you, the engineer, are determined to find the cause, its economic value, and to recommend a course of action to address the issue.

1.1 Steam Turbine Types

The variety and application of steam turbines is enormous. It includes the utility tandem compound unit pictured in Figure 1.1, mechanical drives for onshore or marine applications, combined-cycle and single Rankine-cycle units, super critical, single or double reheat units, and nuclear power-plant applications. One way to categorize the various models is by size. Very basically, smaller installations typically serve as variable speed mechanical drives for pumps and compressors. These may be as large as 50 to 75 MW and have inlet conditions up to 750 psi (5.2 MPa) and 700 °F (644 K). Many are located within chemical processing plants or refineries and exhaust into a lower pressure steam header that provides steam for heating, or to drive smaller steam turbines that may exhaust into a surface condenser. The larger varieties will be multistage units with an axial flow exhaust.

Figure 1.1 Alstom steam turbine.

Source: Reproduced by permission of Alstom.

Up to about 150 MW, steam turbines typically have an axial flow exhaust with throttle conditions as high as 1500 psi (10 MPa) and 900 °F to 1000 °F (755 K to 810 K). Figure 1.2 shows a drawing of a Siemens axial flow machine. Such turbines may be used in a chemical process plant and have a controlled extraction for process heat or other uses. This size is also common in combined cycle power plants with uncontrolled expansion to the condenser. Occasionally, an axial flow machine will have single reheat as part of the cycle. If it is a condensing cycle, the condenser can be placed on the same elevation as the turbine. Combined cycle units utilize waste heat from a gas turbine to generate steam; thus, steam-turbine extractions for regenerative heating are not employed in a combined cycle. A single Rankine cycle would employ uncontrolled extractions for feedwater heating.

Figure 1.2 Typical axial flow exhaust steam turbine.

Source: Reproduced by permission of Siemens Energy.

Above approximately 150 MW, the last stage blade (L-0) becomes too long to manufacture and operate reliably. The low pressure (LP) turbine becomes a dual flow design with steam entering the center section and steam traveling in opposing directio...

Table of contents

• Cover
• Title Page
• Table of Contents
• Foreword
• Preface
• Introduction
• Case 1: Steam Turbine Performance Degradation
• Case 2: Risk/Reward Evaluation
• Case 3: Gas Turbine Compressor Fouling
• Case 4: Flow Instrument Degradation, Use and Placement
• Case 5: Two-Phase Hydraulics
• Case 6: Reliability and Availability
• Case 7: Efficiency and Air Emissions
• Case 8: Low-Carbon Power Production1
• Case 9: Heat Exchangers and Drain Line Sizing
• Case 10: Optimized Maintenance
• Case 11: Project Engineering
• Case 12: In the Woodshop
• Appendix
• Glossary
• Index
• End User License Agreement