Machinery assessment and predictive maintenance prolong equipment life and improve safety.
This article is based on a new Industrial Press book, Is My Machine OK? A Field Guide to Assessing Process Machinery, by Robert X. Perez and Andrew P. Conkey. This guide helps plant supervisors, maintenance personnel and reliability professionals assess the risk of industrial process machine failures.
Why Do We Monitor Machines?
To survive, processing facilities must be profitable. To thrive, they must become increasingly profitable to sell products for lower prices than their competitors. To maintain a sustainable competitive edge, process owners must always watch for smarter ways to increase yields while reducing raw material costs, energy needs, maintenance costs, etc. This article will help people working in organizations who wish to thrive and become industry leaders.
Because maintenance costs represent a significant portion of an organization’s expenses, maintenance budgets are often scrutinized during budget reviews. Lowering maintenance costs can greatly affect the bottom line. However, blindly cutting maintenance efforts without weighing the effects can be costly to the bottom line, drastically affecting the site’s risk profile. Organizations must choose whether to maintain their process facilities proactively or reactively.
Rotating machinery represents a major source of expense to maintenance organizations because of its complexity and labor-intensive nature. Machinery maintenance budgets—seen as having “low hanging fruit” opportunities—are targeted for review. Modifications to machinery maintenance programs must be carefully evaluated and approved by machinery professionals. Poorly managed rotating machinery can devastate a process organization by adversely affecting process availability, safety and efficiency.
Figure 1. Monitoring system schematic
A powerful method called reliability centered maintenance (RCM) establishes a safe minimum level of maintenance, focusing key maintenance resources specifically toward mission critical equipment, such as process machinery. RCM is an engineering framework. It establishes a complete maintenance philosophy and organization. It analyzes the functions and potential failures for physical assets (for example, pumps, compressors or gas turbines). Its primary focus is preserving system functions rather than equipment. RCM’s promise is reduced maintenance costs and improved equipment availability. Key steps of RCM include:
- Identifying key machine functions
- Determining machine criticality
- Identifying functional failure modes and effects
- Identifying failure consequences
- Identifying how failures can be prevented and predicted
- Identifying the causes of failure
- Selecting maintenance tasks
Once an RCM analysis is complete, several principle risk management strategies are recommended, including:
- On-condition maintenance tasks, such as condition monitoring
- Scheduled restoration or replacement maintenance tasks, such as preventative maintenance
- Failure-finding maintenance tasks, such as checking a steam turbine overspeed trip system to ensure that it is functioning properly
- One-time changes to the system, such as hardware design or operations
- Run to failure
Approved risk management strategies are then judiciously folded into an integrated maintenance plan that provides an acceptable level of process reliability, with an acceptable level of risk, in an efficient, cost-effective manner. These scheduled maintenance plans usually include a predictive maintenance program definition, such as vibration collection and analysis, and time-based maintenance activities, such as oil and filter replacements.
RCM combines predictive maintenance (PdM) techniques with applicable and traditional preventive measures. These preventive measures include cleanings, inspections, lubricant, replacement parts and additions and adjustments. The goal of PdM, or condition-based maintenance, is to assess equipment condition by performing periodic inspections—vibration analysis, temperature monitoring, oil analysis, ultrasonic analysis—or by using continuous (online) equipment such as vibration or temperature sensors.
Figure 2. Complex dynamic waveform decomposed into sine wave components. A is the complex wave to be processed. B1, B2 and B3 are the fundamental sine wave components of the original complex wave.
According to PdM philosophy, performing maintenance at a scheduled time is more cost effective than running equipment until it loses performance capability, adversely affecting the process. This view contrasts with a time-based maintenance approach, where equipment gets maintained (overhauled or refurbished) at a prescribed time interval, even if it’s not warranted. Time-based maintenance is usually labor intensive and ineffective in identifying problems that develop between scheduled inspections. It is not cost-effective.
Figure 3. Typical heart monitor