Industrial Heating

42 MAY 2015 ■ IndustrialHeating.com

VACUUM/SURFACE

TREATING

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
Vibration Monitoring
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 pe-

Fig. 3. Monitoring condition for a condition-oriented maintenance strategy

Fig. 4. Vibration monitoring for a condition-based

maintenance strategy

Loss/EUR Delayed

exchange Zone D

Zone C

Zone B

Zone A

fl fx fy fu

Frequency

Vibration magnitude

Normal operation

Start

Start-

up

phase

Cycle measurements Damage begins Operating life

Premature

exchange

Best

exchange

Total

loss