New research and techniques of World Maintenance

New research

Quite apart from greater expectations, new research is changing many of our most basic beliefs about age and failure. In particular, it is apparent that there is less and less connection between the operating age of most assets and how likely they are to fail.

Figure 1.2 shows how the earliest view of failure was simply that as things got older, they were more likely to fail. A growing awareness of 'infant mortality' led to widespread Second Generation belief in the "bathtub" curve.

However, Third Generation research has revealed that not one or two but six failure patterns actually occur in practice. This is discussed in detail later, but it too is having a profound effect on maintenance.

New techniques

There has been explosive growth in new maintenance concepts and techniques. Hundreds have been developed over the past fifteen years, and more are emerging every week.

Figure 1.3 shows how the classical emphasis on overhauls and administrative systems has grown to include many new developments in a number of different fields.

The new developments include:

decision support tools, such as hazard studies, failure modes and effects analyses and expert systems
new maintenance techniques, such as condition monitoring
designing equipment with a much greater emphasis on reliability and maintainability
a major shift in organizational thinking towards participation, team-working and flexibility.

A major challenge facing maintenance people nowadays is not only to learn what these techniques are, but to decide which are worthwhile and which are not in their own organizations. If we make the right choices, it is possible to improve asset performance and at the same time contain and even reduce the cost of maintenance. If we make the wrong choices, new problems are created while existing problems only get worse.

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DRY GAS SEALS: How to Implement Proactive Monitoring

BY MICHAEL FORSTHOFFER. In the last 10+ years there has been a significant increase in the technology and instrumentation used on dry gas seal systems to monitor the condition of the seals. Most notable has been the call to monitor secondary seal condition – there have been significant failures of the secondary seal without any issue with the primary seal. The majority of these failures have been due to oil migration across the separation seals. As the oil is not drained out, it accumulates around the secondary seal, fills the grooves, and causes the secondary seal faces to contact. There are various approaches to monitoring secondary seal condition. Probably the most beneficial is utilizing a back pressure control valve in the primary vent (Figure). Figure: Using a back pressure control valve in the primary vent as part of the monitoring of secondary seal condition in a dry gas seal. This sets the primary vent at a pressure below the primary seal gas supply pressure, but high en...

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How to clean superalloy parts? brazing technique removes oxides in deep, narrow cracks

Investment cast parts used in modern gas turbines are made of expensive superalloys that can withstand extreme thermal, mechanical and chemical loads experienced by hot gas path components. Parts with hundreds of thousands of service hours, however, become severely oxidized. To improve efficiency and reduce the risk of unscheduled outages, these parts must either be periodically refurbished using a brazing repair process or replaced. To facilitate brazing repair, all oxidation, sulfidation and hot corrosion must be removed from surfaces, cooling passages and deep, narrow cracks. Oxide scale typically forms on the mating faces of cracks that occur in hot gas path areas. These cracks become packed full of scale. It is the goal of the service shop to repair these components by filling the cracks with a braze alloy. Unfortunately, braze alloy cannot flow into cracks filled with oxide scale. Figure 1: By varying the pressure between positive, negative, and atmospheric levels, the Dynamic FI...

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Understanding the Types of Maintenance and Choosing the Right Strategy

This article provides an overview of the nine main types of maintenance, ranging from preventive to corrective maintenance and everything in between. Although specific terminology may vary, it's important to ensure that everyone is talking about the same thing and that the underlying principles are clear. Whether you refer to it as condition-based maintenance or predictive maintenance, what matters most is that we can have a meaningful conversation about when and how to use it. As such, this article also explores the appropriate applications of condition-based maintenance. Many people have questions about the different types of maintenance, so this quick overview provides a helpful starting point: There are nine distinct types of maintenance, which can be classified into two main categories: Preventive Maintenance and Corrective Maintenance. Preventive Maintenance is performed proactively before any equipment failure occurs. This category encompasses: Time-Based Maintenance (TBM)...

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Failures in babbit bearings

There are literally dozens of ways bearings can fail. Some of the more common include: • Babbitt fatigue • Babbitt wiping due to rotor to stator contact • Babbitt flow due to high operating temperatures • Foreign particle damage • Varnish build-up • Electrostatic discharge damage (frosting) • Electromagnetic discharge damage (Spark tracks) • Oil “burn” or additive plating due to high temperatures • Loss of bond between babbitt and base metal • Chemical attack • Pivot wear in tilting pad bearings • Unloaded pad flutter • Cavitation damage This is taken from a paper, Babbitted bearing health assessment" by John Whalen of John Crane, Thomas Hess of Rotating Machinery Group, Jim Allen of Nova Chemicals Corporation and Jack Craighton of Schneider Electric. Babbitt fatigue Babbitt fatigue is caused by dynamic loads on the babbitt surface. Typically in bearings of this type, the dynamic loads are caused by vibration and result in peak film pressure fluct...

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Grounding brush discharge monitoring

In recognition of the possibility of static charge build up in condensing steam turbines, API 612 (2005) specifies that grounding brushes be installed. The electrical flow to ground through these brushes be monitored and useful information can be extracted. This article carries excerpts from the paper, “Babbitted bearing health assessment” by John K Whalen of John Crane, Thomas D Hess of Chestnut Run, Jim Allen of Nova Chemicals and Jack Craighton of Schneider Electric. Grounding brushes take current from the rotor to ground so that a charge does not build up on the rotor to the point where it discharges to ground though the best path possible – which is usually the closest point between the rotor and stator which is usually (hopefully) the point of minimum film thickness in a bearing. Typically this point of minimum film thickness is found in the active thrust bearing (as will be shown later). Shaft grounding brushes serve two purposes. The brushes are able to transmit modest amo...

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