3. Forsthoffer's Rotating Equipment Handbooks: Compressors

By William E. Forsthoffer

Over recent years there have been substantial changes in those industries which are concerned with the design, purchase and use of special purpose (ie critical, high-revenue) rotating equipment. Key personnel have been the victims of early retirement or have moved to other industries: contractors and end-users have reduced their technical staff and consequently have to learn complex material 'from scratch'. As a result, many companies are finding that they are devoting unnecessary man hours to the discovery and explanation of basic principles, and having to explain these to clients who should already be aware of them. In addition, the lack of understanding by contractors and users of equipment characteristics and operating systems often results in a 'wrong fit' and a costly reliability problem.Forsthoffer's Rotating Equipment Handbooks: Compressors provides detailed coverage of characteristics, types, operation in a process system, (using the concept of required and produced gas head) performance relationships, selection, what determines the turbo compressor curve shape, surge/stall/stonewall, the effects of fouling, the design basis of journal and thrust bearings, balance drums, seals, critical speeds, control and protection guidelines, series and parallel operation, component condition monitoring, troubleshooting and many other aspects.Forsthoffer's Rotating Equipment Handbook: Compressors is the third title in the five volume set. The volumes are: 1. Fundamentals of Rotaing Equipment; 2. Pumps; 3. Compressors; 4. Auxiliary Systems; 5. Reliability Optimization through Component Condition Monitoring and Root Cause Analysis'.

* One of a five volume set which is the distillation of many years of on-site training by a well-known US Engineer who also operates in the Middle East.* A Practical book written in a succinct style and well illustrated throughout.

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 16, 2005
• ISBN: 9780080949345

William E. Forsthoffer

President of Forsthoffer and Associates USA. Bill has authored 6 successful books at Elsevier, including Machinery Best Practices in 2011. He has 60 years’ experience in the Rotating Machinery Industry as a rotating machinery designer, project leader and trouble-shooter and has visited over 500 Plants Globally. Bill has had the opportunity to be involved with all types of rotating machinery: pumps, compressors, gears, mixers, extruders, melt pumps, steam turbines gas turbines, centrifuges, spin dryers and their associated components (Rotors, Bearings, Seals and Support Systems). His involvement has consisted of total component and system centrifugal compressor design for De Laval (Siemens), specification writing for ExxonMobil, selection of all types of rotating equipment for all major vendors, design audits, shop testing, start-up and troubleshooting for all major gas processing chemical and refining companies world-wide.

Book preview

1

Rotating equipment overview

 Introduction

 Definition of rotating equipment

 Classifications of rotating equipment

 Site equipment examples

 Performance and mechanical design similarities

 The equipment ‘train’ or ‘unit’

 Important fundamentals

Introduction

Take a minute and list all the different types and kinds of rotating equipment you can think of. Even if you have not been involved with rotating equipment for a long time, when you consider the types of equipment that you come in contact with every day, your list will be sizeable. Imagine if we pursued our objectives by looking at each individual piece of equipment. You would never remember all the aspects and the book would be long and very boring. We will not attempt this approach. Rather, this section will divide all types of rotating equipment into four major classifications. The function of each individual classification will be defined. Throughout this book we will cover many types of rotating equipment.

One good thing to remember is to always ask yourself what the function of this particular type is, what does it do? We will find that many aspects covered in this book will have the same common function.

Our approach therefore, will be to observe the similarities in both performance and mechanical aspects of various types of equipment. We will see that many of these relationships apply regardless of the type of equipment that is considered.

Now think of any rotating equipment unit that you have come in contact with and review that unit considering the different components that comprise it. You will find that every unit of rotating equipment consists of a driven machine, driver, transmission device and is supported by auxiliary equipment. That is, each unit is made up of all the classifications of rotating equipment that are described in this section. This is an important fact to remember in troubleshooting equipment. In essence then, each unit is a system.

Definition of rotating equipment

Figure 1.1 presents a basic definition of rotating equipment.

Figure 1.1 Definition

As we shall see, there are different classifications of rotating equipment. Regardless of the classification, rotating equipment moves product. More properly stated:

Rotating equipment moves money!!

Stop the equipment and the source of revenue stops! This is a very important fact to remember. If you want management to approve your recommendation, you must be able to justify it economically! The form of any recommendation to management should be as shown in Figure 1.2.

Figure 1.2 A successful recommendation

If you proceed as shown in Figure 1.2 you will be able to obtain and maintain management support. Remember, you can learn a great deal in this book. However, if you cannot implement what you have learned the information is totally useless to the company. If you cannot obtain management support, you will never implement any action plan.

Classifications of rotating equipment

There are four (4) basic function classifications of rotating equipment. Refer to Figure 1.3 which defines the classifications of rotating equipment.

Figure 1.3 Classifications of rotating equipment

Figure 1.4 is a partial listing of some rotating equipment types grouped according to function.

Figure 1.4 Major types of rotating equipment

Site equipment examples

Following is an example of typical site rotating equipment.

Figures 1.5, 1.6, 1.7, 1.8 show examples of each rotating equipment classification.

Figure 1.5 High pressure centrifugal compressor (Courtesy of Dresser Rand)

Figure 1.6 Extraction – condensing steam turbine (Courtesy of MHI)

Figure 1.7 Multiple, convoluted diaphragm-spacer coupling (Courtesy of Zurn Industries)

Figure 1.8 Horizontal oil console arrangement (Courtesy of Oltechnique)

Performance and mechanical design similarities

During this book we will be examining many different types of rotating equipment. However, the task will be a lot easier if we begin our study by first focusing on the similarities of the equipment and then the specific differences.

As an example, we have chosen to first present pumps then compressors as topics. As was just discussed, both pumps and compressors are driven types of equipment and move product. Regardless of the product phase or state, their functions are identical. Refer to Figure 1.9 which compares dynamic pump and compressor performance.

Figure 1.9 Pumps and compressors

The same comments can be made concerning mechanical components. Refer back to Figures 1.5 and 1.6 and ask are the functions of the casings, internal seals, shaft end seals and bearings the same? Absolutely! A bearing performs the same function whether it is in a pump, compressor, gearbox, etc.

Figure 1.10 shows how both performance and mechanical functions are similar regardless of the classification or type of equipment.

Figure 1.10 Classifications of rotating equipment

As we proceed through this book, we will discover that positive displacement or dynamic performance principles will be the same regardless of the type of equipment (pump, compressor or turbine). Also, the mechanical principles presented for bearings, seals, etc. will apply whether the component is in a pump, turbine, gear, etc.

The equipment ‘train’ or ‘unit’

As stated, the objective is to learn the functions of equipment and major components so that they can be effectively condition-monitored to maximize site safety and reliability. Having defined the four (4) classifications of equipment, how many of these classifications are present in a pump unit or compressor train? … All four (4)!!

Regardless of the type of unit or train, a driven, driver, transmission(s) and auxiliary system(s) must always be present. When you are asked to inspect G-301 or K-101 you are actually inspecting G-301 pump unit or K-101 compressor train. Failure to recognize this fact will severely limit your troubleshooting scope and ability.

As an example, a call from the unit shift manager may state that G-301 discharge pressure is zero – what are possible causes? A few are:

 Process change

 Pump wear

 Coupling failure

 Pump shaft failure

 Driver shaft failure

 Pump or driver shaft seizure (no oil)

 Pump seal failure

 Process valve closed

 Steam inlet valve closed (if driver is a steam turbine)

Do you get the point! … The entire unit or train, all four machinery classifications must always be considered in rotating equipment design, revamps and troubleshooting.

Important fundamentals

Before discussing specific facts concerning all the rotating equipment on site, some important fundamentals need to be presented. The environment or surroundings for any piece of rotating equipment play an important part in determining the availability of that particular item (Refer to Figure 1.11).

Figure 1.11 The rotating equipment environment

This figure shows that the rotating equipment environment is the process unit in which the equipment is installed. The surroundings of the equipment will be defined early in the project. Proper design of process conditions, piping and foundations, selection of other components (drivers, transmission devices and auxiliaries) and proper specification of ambient conditions all must be considered. If any of these items are not taken into account, the end user of the equipment will be faced with a history of an unreliable process and will pay dearly in terms of lost product revenue.

It is important to understand that the life span of rotating equipment is extremely long compared to the specification, design and installation phase. Refer to Figure 1.12. A typical installation will have a specification design and installation phase of only approximately 10% of the total life of the process unit. Improper specification, design or installation will significantly impact the maintenance requirements, maintenance cost and availability of a particular piece of machinery.

Figure 1.12 The life span of rotating equipment

The objectives of the end user are shown in Figure 1.13.

Figure 1.13 The objectives

In order to maximize the profit, a piece of machinery must have maximum reliability, maximum product throughput and minimum operating cost (maximum efficiency). In order to achieve these objectives, the end user must play a significant part in the project during the specification and design phase and not only after the installation of the equipment in the field. Effective field maintenance starts with the specification phase of a project. Inadequate specifications in terms of equipment operating conditions, mechanical design, instrumentation and the location of the instrumentations will reduce equipment availability.

The definitions of reliability and availability are shown in Figure 1.14.

Figure 1.14 Definitions: Reliability and availability

Reliability does not take into account planned down time. Availability considers planned downtime (turnarounds, etc.). Both values are stated in percentage. Typical equipment availabilities when properly specified are 97% +.

Availability directly affects the product revenue. Product revenue is the value obtained from the product produced in one day. For refineries, the loss of revenue can exceed a million U.S. Dollars a day. This will occur if a critical piece of equipment (unspared) is shut down. Remember, even though the unit may be down for a short period, the time necessary to bring the process back within specification may be significantly longer. It is very valuable to obtain the product revenue figure for the unit in which you are working. As will be shown, this value can significantly influence management in terms of decision making.

It is important to understand that upset conditions with any piece of rotating equipment can occur in a very short period of time. In addition, the requirements for reliable operation (a minimum of three (3) years continuous run) require enormous amounts of equipment revolutions all on a very thin film of lubrication oil. Failure to maintain this film is one of the major reasons for reduced equipment availability. Figure 1.15 presents a typical fact sheet concerning a turbo-compressor.

Figure 1.15 Interesting facts

As an exercise you may want to determine how far this shaft would travel in one year if its diameter were four inches. There are 5,280 feet in one mile and one revolution around the earth equals 25,000 miles.

2

Compressor types and applications

 Introduction

 Positive displacement compressors

 Dynamic compressors

Introduction

In this chapter we will overview compressor types and their typical applications. The two basic classifications of compressors are positive displacement and dynamic compressors.

Positive displacement compressors are constant volume, variable energy (head) machines that are not affected by gas characteristics.

Dynamic compressors are variable volume, constant energy (head) machines that are significantly affected by gas characteristics.

The type of compressor that will be used for a specific application therefore depends on the flow rate and pressure required and the characteristic of the gas to be compressed.

In general, dynamic compressors are the first choice since their maintenance requirements are the lowest. The next choice are rotary type positive displacement compressors since they do not contain valves and are gas pulsation free. The last choice are reciprocating compressors since they are the highest maintenance compressor type and produce gas pulsations. However, the final selection depends upon the application requirements as discussed