Condition-Based Maintenance Strategy for Dry Vacuum Pumps in Furnaces
This article was originally published May 6, 2015.
Dry-compression vacuum pumps represent a perfect fit in supporting medium to harsh conditions in vacuum heat-treating applications by offering low operating costs and high reliability.
A condition-based maintenance strategy for vacuum pumps used in vacuum furnaces is enabled by a vibration monitoring system. This allows company maintenance departments to proactively participate with efficient and predictive allocation of resources because they will now focus more on planned downtimes instead of troubleshooting to recover from unplanned downtimes.
Vacuum in Heat-Treatment Applications
Vacuum furnaces are utilized for a broad application range these days.
One driver for vacuum furnaces is the increasing demand coming from the aerospace and automotive industries, where modern components need to fulfill standards that can only be reached by vacuum heat treatment. Unexpected furnace downtime creates high costs because each production step is just-in-time.
Various heat-treatment applications create different challenges for the utilized vacuum systems. One could cluster the applications roughly in three different groups.
• Standard applications (e.g., tempering, annealing, hardening): Media entering the vacuum system are mainly air and humidity. Only the smallest amounts of other vapors or particles can be expected.
• Medium applications (e.g., brazing, soldering, nitriding): Furnace outgassing contains aggressive vapors such as flux agents and ammonia. Significant amounts of vapors with potential to condense inside the pump can be expected.
• Harsh applications (e.g., sintering, MIM, carburizing): The furnace outgassing contains particles and vapors that tend to crack and build layers inside the vacuum pumps.
While all types of vacuum pumps typically perform well in standard applications, it is important to select the right vacuum-pump system for medium and harsh applications to have the ability to handle the specific outgassing. The risk of unexpected furnace downtime can be reduced by selecting the right pump type supported by an efficient and reliable maintenance strategy.
Typically, vacuum systems are combinations consisting of fore-vacuum pumps (rotary piston or vane-style) and roots-type vacuum blowers (rotary-lobe boosters). While roots blowers are mostly unaffected by furnace outgassing, higher attention must be given to the fore-vacuum pumps. These pumps have to compress to atmosphere, therefore, they face the risk of potential condensation inside the compression room. The main differentiator to adjust and optimize a vacuum system for specific applications is the fore-vacuum pump.
Dry pumps have the general advantage of an oil-free gas path. They lack oily surfaces inside the compression room, have relatively warm operating temperatures and excel, therefore, in handling condensable vapors and particles. Today’s standard in industrial markets are screw-type pumps with a variable-pitch rotor design, which widely replaced older technologies such as multi-stage roots and claw-type dry pumps because of their higher robustness.
Two screw-pump principles are promoted.
Simply Supported Rotor Design
The rotor is supported with bearings on both ends (Fig. 1). This range of pumps includes the most modern pumps on the market today, which excel with high compactness and low power consumption and noise level.
Such pumps are the best choice for medium applications. Acidic flux vapors extracted during brazing processes can quickly degrade pump oil, while dry pumps can simply transfer and pump them out without any increased need for maintenance or service.
Cantilevered Rotor Design
The supporting bearings are on one side of the rotor only, demanding a more stable shaft design and relatively long construction (Fig. 2). The main advantage is the lack of sensitive shaft seals and bearings on the vacuum side.
These pumps have proved their value for even the dirtiest applications. Internally cooled rotors ensure a moderately warm compression room, still avoiding condensation but reducing the cracking tendency of temperature-sensitive vapors. Buildup of layers inside the compression room is minimized. The cantilevered design enables a user to manually clean the compression room easily.
These pumps are the best choice for harsh furnace applications such as carburizing or sintering, where hydrocarbon vapors enter the pumps. The pumps tend to build up layers, requiring the related maintenance demand of periodic cleaning.
Vacuum pumps are valuable assets of a production company and require strategies for protection and maintenance.
The event-oriented maintenance strategy is of a very reactive nature. Any maintenance activity will be triggered by failures that already occurred. This requires sufficient well-trained resources to recover the system in time during a 365/24/7 production mode. In addition, an adequate number of spare pumps for potential defective parts must be readily available in inventory. The recovery time of an unscheduled downtime is typically higher than a scheduled downtime, and the related costs correspond too.
The time-oriented maintenance strategy proactively initiates maintenance work on a fixed time interval, independent of real pump conditions. This reduces risk on unscheduled failures and downtime caused by missed maintenance, but it can lead to premature exchange of pumps or components. Considering the full production life cycle, this tends to result in a higher cost of ownership. Compared with the event-oriented maintenance strategy, this offers advantages in better allocation of maintenance resources and reduced risk to unplanned downtime.
To find the right timing for exchange of defective parts, a condition-oriented maintenance strategy is required (Fig. 3). Indicators of increasing wear must be monitored continuously. Any trend indicated would be analyzed, and potential maintenance would be triggered only if conditions predict an increased risk for unscheduled downtime. Preliminary lead time from first indication of trend to maintenance is typically sufficient to allocate resources, order spares just in time to keep inventory low and combine as many maintenance activities as efficiently possible within a planned furnace downtime.
Unscheduled Downtime Cost Types
The target of a condition-based maintenance strategy (CBM) is to reduce unscheduled downtime and related costs, which can be described in different categories:
• Loss of production typically has the largest impact on calculation of cost of unscheduled downtime. This includes the typical hourly output as a loss per hour of downtime. If a failure occurs during production, there is a risk of losing a batch of valuable parts.
• Recovery cost considers two components. First is the related costs to keep manpower resources available to recover a system at any time. Second is the inventory costs to stock the right quantity and types of spares and exchange pumps.
• Secondary damage on equipment as a result of missed maintenance impacts the costs of unscheduled downtime. While a standard overhaul of a vacuum pump can be initiated in time and at clear costs, overhaul of a crashed pump might be significantly higher.
Implementation of a Condition-Based Maintenance Strategy
Bearings are key components of any rotating equipment. Any forces generated on rotating parts are a source of vibrations. Such forces could be created by misalignment, friction, missing stiffness, electric problems or imbalance coming from parts or growing layers on the pump rotors or particle intake.
Measuring vibration of the monitored housing gives us a better understanding of the aforementioned forces. Those vibrations can be measured by acceleration sensors that convert mechanical vibration into an electrical signal. Every individual root cause of vibration leads to a specific signal pattern that can be indicated and interpreted. Vibration monitoring and analysis consist of two main tasks:
1. Quantification of vibration and trend display
2. Inference of vibration root cause and deduction of required maintenance efforts and/or corrective actions for future prevention
Quantification and Alarming
According to ISO 10816, strength of vibration will be classified in four zones (A to D). Between the pump-speed window of fx to fy, a single measuring point is enough to evaluate the pump condition (Fig. 4).
• Zone A characterizes the new pump.
• Zone B indicates a safe mode for permanent operation.
• Zone C marks limitation in continuous use. Operation time has limitations.
• Zone D shows critical pump conditions. An unscheduled shutdown can happen any time.
A smart vibration sensor is able to perform autonomous monitoring tasks and provide warnings and alarms. Thresholds will be defined by pump parameters and system-specific coefficients.
Vibration sensors have to be configured with specific pump characteristics as number and rolling frequencies of bearings and rotation speed. The monitoring allows a very detailed indication of damages in the inner or outer ring of the bearing, the cage or the rolling elements.
Furthermore, determining factors from the application or the furnace must be considered. Any vibration coming from the pump’s surroundings will also impact the pump vibration and must be recognized and monitored as an external vibration. Such external vibrations should be measured during the installation period to take related impact into account when defining customized warning and alarm thresholds. Intermittent input of dust and particles can lead to application- and production-specific single events that must be separated in the monitoring task.
The overall view includes pump-specific parameters as well as external characteristics coming from the furnace and the application. The targeted result is to maximize zone B for safe mode in permanent operation. Predicted change from zone B to zone C is indicated by the defined warning level. Alarm level is the equivalent indicator that reports high risk of unscheduled downtime and trend development to zone D.
Understanding the Root Causes of Vibration
For integration of vibration monitoring into a total productive maintenance concept, it is mandatory not only to react to alarms but to understand the root cause behind them. Any failure or wear offers potential to anticipate future optimization. Detailed vibration analysis performed by experts enables the separation of normal wear from external causes.
The vibration sensor provides a time signal that is translated into a frequency spectrum by FFT (Fast-Fourier-Transformation). Analyzing both time signal and spectrum allows the detection of typical signs of wear by separating bearing-specific patterns.
The general pump condition can be evaluated from the trend analysis, which is a summary of individual monitoring tasks. A trend chart combines development of bearing characteristics and external vibrations coming from the environment since start-up or appearing later within the production period. Unexpected impacts during operation are also displayed. This can be a one-time event like a significant dust intake, which is shown as a peak within a generally stable chart. Intermittent impacts are the equivalent peaks if they can be indicated as periodic peaks.
Vibration monitoring systems for dry-compression pumps in heat-treatment applications offer huge potential to maximize equipment uptime and reduce risk for cost-intensive unscheduled downtime. Compared to the potential downtime cost, condition-based maintenance offers attractive return on investment (ROI) and supports a continuous-improvement process for maintenance management and system engineering.
For more information: Contact Mario Vitale, senior manager – marketing and sales support, Oerlikon Leybold Vacuum USA, Inc., 5700 Mellon Road, Export, PA 15632; tel: 724-325-6565; fax: 724-325-3577; e-mail: firstname.lastname@example.org; web: www.oerlikon.com/leyboldvacuum