Machinery Failure Analysis Handbook

Sustain Your Operations and Maximize Uptime

Luiz Octavio Amaral Affonso

• Publisher: Gulf Publishing Company
• Year: 2013
• ISBN:
  9780127999821
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Understanding why and how failures occur is critical to failure prevention, because even the slightest breakdown can lead to catastrophic loss of life and asset as well as widespread pollution. This book helps anyone involved with machinery reliability, whether in the design of new plants or the maintenance and operation of existing ones, to understand why process equipment fails and thereby prevent similar failures.

PART I

Introduction to Failure Analysis

1

Fundamental Causes of Failures

This chapter discusses the basic concepts of root cause and failure. Some examples of failures related to design, fabrication, assembly, operation, and maintenance are included.

Failure occurs when the component or equipment no longer can perform its intended function safely. The function of the component is the primary reason why it was installed on the machine.

Premature failure happens when the defect occurs within the design life of the component. Design life is a design criterion understood statistically, not deterministically, which means that the expected dispersion of the component’s useful life should be taken into account. Design life usually is related to certain specific types of failure modes, anything else is considered abnormal. For example, the classical end of life failure mode for an antifriction bearing is surface fatigue. Therefore, if this bearing fails due to surface fatigue after a long enough period of time, we can say that it has reached the end of its useful life. Any other failure mode indicates an abnormality, no matter how long the period before failure. This concept does not apply to components designed for infinite life, for example, pump shafts.

The root cause of a failure is the fundamental reason that made the failure possible. Multiple root causes are more likely when dealing with machinery failures. The selection of the root causes of a certain event is done with an eye on the usefulness of the selected root cause. If we think of the relationship among all the events observed at a certain piece of machinery before failure, we will observe that some are closer in time to the failure than others and some of them play a more important role. An effective failure analysis selects the root causes that most effectively avoid repetition of the failure and in which some action is feasible. In addition to that, root cause description should be as detailed as possible, as it is not very useful to say that the root cause of a failure has been a maintenance error, for example; not very much can be done with this information. Rather, we should say that the root cause of a failure was the installation of an antifriction bearing due to the impacts produced by the use of inadequate tools and lack of training. Now, we know what has to be done to avoid repetition of the failure.

The various types of root causes can be classified in several ways. The categories that follow are arbitrary and intended to serve academic purposes only, creating a framework that will help understand the issue.

1.1 Design Failures

Design failures are born on the drawing board, when the machine designer determines a fillet radius or the specification of an antifriction bearing. Such failures can be avoided only through redesign of the failed machine or component. Great care should be exercised before deciding that the cause of a failure has been a design deficiency. Some examples follow:

1. Notches create stress concentrations that may be the origin of a fatigue crack. They can be easily avoided most times. Notches are found in shaft shoulders and threads, for example. Another example can be seen in Figure 1.1, where a fatigue crack originated at the point of stress concentration created by the reinforcement on the fan blade.

FIGURE 1.1 Fatigue crack initiated at a stress concentration spot of a fan blade.

2. Inadequate design criterion can cause unforeseen demands to damage the machine or component. In such a situation, one finds that the machine simply has not been designed to handle the unexpected condition, be it a contaminant that makes a fluid corrosive or some kind of vibration. Special purpose machines are prone to this type of problem, as they are designed for a specific service and it is not very easy to test them under real working conditions. Figure 1.2 shows an example of an unexpected process condition that led to the failure of a reciprocating compressor. In this case, the unforeseen condition was the ability of the gas to polymerize and create hard deposits inside the compressor. These hard deposits damaged the valves and the stem sealing, resulting in very low reliability of the machine.

FIGURE 1.2 Reciprocating compressor piston with hard deposits created by the polymerization of the gas.

3. Design modifications can result in unexpected failures if the modification is not done carefully. For example, pumps used to be designed to use packing for shaft sealing. When mechanical seals began to be used widely, many pumps were retrofitted. At that time, it was found that the vibration levels went up and reliability went down, as a result of the loss of the shaft support formerly provided by the packing.

1.2 Material Selection Deficiencies

A material selection–related failure can also be thought of as a design-related failure, as the materials of construction are selected during the design of the machine. This type of failure can be avoided through careful selection of the machine part materials. Figure 1.3 illustrates two reciprocating compressor oil scrapper rings that were damaged by high temperature. The high temperature was the result of excessive friction between the rings and the stem, as the rings were too hard. Changing the ring material to a softer rubber solved the problem.

FIGURE 1.3 Reciprocating compressor oil scrapper rings damaged due to high temperature.

Some contributing factors include

1. Structural materials normally are selected for mechanical strength. High strength may be followed by less ductility or less corrosion resistance. Failure may arise due to these or other characteristics of the material. The machine designer should consider these possibilities, and a compromise may be necessary when more than one possible failure mechanism is present.

2. Unexpected failure modes may force a change in material; a classic example is the brittle fractures of the liberty ships, which forced designers to consider other properties of the construction material.

1.3 Material Imperfections

Imperfection in the construction materials may also be the origin of machinery failures. Internal and external defects that reduce the resistance of the component are possible sources of cracks or localized corrosion, for example.

Such defects are intimately connected to the processing of the raw material during fabrication. Some classic examples include

1. Cast components: inclusions, voids, cold shots, and pores.

2. Forgings: contraction and bends.

3. Laminated parts: double lamination and lamellar decohesion.

The design of the components should take these possible defects into account, and quality control inspection should be specified accordingly.

1.4 Manufacturing Defects

Manufacturing defects occur during the processing of the raw materials used to fabricate the machine components. Although it may not be easily distinguishable from the previous type of failure, the recognition of the source of a failure is always very important for the prevention of future failures.

Some examples include

1. Cold forming introduces huge residual stresses that, if not relieved, may be the source of a fatigue fracture, if the part is cyclically loaded.

2. Machining operations can create notches that act as stress concentrators; part number inscriptions by indentation or eletroerosion also may be a source of cracks, if done in highly stressed areas of the component.

3. Heat treatment can crea...