Condition Based Maintenance

 

CONDITION BASED MAINTENANCE USING WIRELESS NETWORKS

 

Background


When recorded over a period of time, the pattern of vibrations produced by operating machinery
can be analyzed to detect changes that indicate that the equipment is due for maintenance and
repair. Detecting and predicting the possibility of equipment failure allows personnel to fix or
correct a problem before it occurs. This predictive maintenance strategy which saves resources
until needed, is referred to as "Condition-Based Maintenance" or CBM. The goal of CBM is to
perform maintenance only upon evidence of need.


CBM necessitates the use of vibration sensors that gather the measurement data. The recordings
are sometimes made manually with sensors and recording equipment carried from machine to
machine on a periodic basis; or the data is taken from sensors affixed to the machinery and wired
back to some collection point. It is in this process that the use of wireless techniques can
produce significant savings.


Introduction


This white paper describes the WiHART Systems utilization of  peer-to-peer wireless mesh networks for
monitoring the vibration in rotating machinery in an industrial environment. It can be used in
both discrete manufacturing (aircraft, auto manufacturing) and process industries such as
refineries and chemical plants.


The document briefly discusses the economic issues and makes a business case for vibration
monitoring. Following a brief overview of the basic concepts the paper will discuss wireless
mesh networks as applied to monitoring in the most cost effective manner (as opposed to wire based
implementation).

The ARC Advisory Group estimates that 5% of production in North America is lost to unplanned downtime. The focus of this white paper is the utilization of vibration measurement and wireless communication to monitor equipment condition in order to predict failure. The need for monitoring industrial machinery is well understood but the problem has been the cost of installation using standard wired techniques which are cost prohibitive. Due to the high cost of a conventional wired vibration vibration system, only a small percentage of critical or near-critical assets are monitored online. Online monitoring solutions can cost over $4,000 per point to instrument and deploy.

Recent studies indicate the the cost of implementing condition based maintenance using wireless techniques reduces costs up to 90% over wired techniques. Intel in a 2005 analysis compared the cost of three techniques that can be used for condition based maintenance using the vibration measurements.

There are two primary models for data acquisition in industrial environments today: manual “sneaker net” data collection and fully integrated online surveillance. The manual model utilizes hand-held instruments and remote-installed or hand-carried sensors to which instruments are connected. Data is captured locally in the hand-held device for transport back to a central repository for analysis.

Online surveillance utilizes sensors that are hard wired to a data acquisition unit which processes the data and delivers it across a wired network to a central repository. Both manual and online systems are in place across a variety of applications and industries; however, they are not always a good fit. Manual data collection is insufficient in many applications due to the potential for user error, the high cost to train and keep experts, and the manpower required for frequent data collection. Online systems allow more reliable and frequent data collection. However, the cost of purchasing and deploying the modules and the network and power infrastructure can be prohibitive. Online solutions are appropriate for equipment and systems with a potential cost impact greater than $250K. For the majority of equipment in a typical industrial deployment, an online system provides an insufficient return on investment. An industry cross section shows that online system penetration into the market is less than 10%, primarily due to cost. In the remaining 90% of the market, 20% use manual data collection, and most are not happy with the level of prediction and correlation they provide. Finding a solution to address this market and tap into the remaining 70% is the goal of WiHART Systems.

Business Case


In general, vibration in rotating machinery is not beneficial. It can cause excessive wear,
cracking, loosening of fasteners, excessive noise, fracture of solder joints in electrical machinery
and a host of other problems. In the case of aircraft, serious vibration can cause catastrophic
failure leading to loss of life. The goal of vibration monitoring is to detect vibration patterns
which will lead to failure.

 

In numerous studies maintenance typically represents 15 to 40 percent of the total cost of an
installation. In some cases over a 20 year lifetime the maintenance can exceed the entire cost of
the equipment.


Let us assume that a company does $50 million in annual sales with a gross margin of 60% (very
good). The cost of goods sold is 20 million so the gross profit is $30 million. If one assumes a
28% maintenance budget then the cost of maintenance is $14 million/year. If one can save 10%
on maintenance this translates to $1.4 million directly to the bottom line. One would have to
increase sales by almost 2.4 million to achieve this level of performance. Is this 10% savings
achievable? The answer to this is an emphatic yes. This can be achieved by the installation of a
Condition-Based Maintenance capability using wireless vibration monitoring to detect imminent
failure in rotating machinery. Figure 1 shows the distribution of costs for rotating machinery
maintenance in a petrochemical plant from 1973 to 1982. The actual dollar amounts are:


Pumps $22,600,000
Compressors $6,950,000
Blowers $2,230,000
Turbines $240,000
(From Charles Jackson “Practical Vibration Primer”)
Figure 1

 

 

In a 2005 analysis the cost of installation of wired vs wired installation using the same number of vibration sensors was $3,375,000 for wire and $262,500 for wireless.

 

Why Do We Monitor Machine Vibration


The overall goal for Condition-Based Maintenance is to monitor the health of a rotating machine.
There are other parameters besides vibration which are useful in determining machine health.
One can measure temperature, flow, and pressure or even do oil analysis to predict eventual
failure. The best technique is to use vibration because it is the overall indicator of mechanical
conditions and the earliest detector of developing defects. Vibration can detect the following
mechanical conditions:


Out of balance
Misalignment (Bent Shaft)
Damaged roller element bearings
Damaged or worn gears
Mechanical looseness
Noise
Cracking


Basic Concepts of Vibration Measurement


The standard technique for measuring vibration is to use a piezoelectric accelerometer which is
attached to the rotating device at the appropriate point to measure the vibration. An
accelerometer is a sensor that produces an electrical signal that is proportional to the acceleration
of the vibrating component to which the accelerometer is attached. The acceleration parameter is
a measure of how fast the velocity is changing (see Figure 2). Normally the acceleration input
data is converted to a velocity waveform (see Figure 3).(For those mathematically inclined, acceleration is the first derivative of velocity)

.
Spectrum Analysis


If you examine the waveforms in Figures 2 and 3 which are based on the direct output of an
accelerometer you will see that they seem to be complex waveforms. A French mathematician
named Jean Baptiste Fourier (1768–1830) made a unique discovery involving complex
waveforms. Fourier discovered that all complex waveforms such as the ones derived from
vibration sensors can be broken down into a series of sine waves. The sine waves have different
amplitudes and frequencies. In other words, complex waveforms are actually a superposition of
much simpler sine waves. By using techniques initially developed by Fourier one can find the
constituent frequencies that make up the complex waveform.
As computers became more powerful and wide spread it has become feasible to apply Fourier
methods using a mathematical algorithm called a Fast Fourier Transform (FFT) to change a timeseries
waveform into a frequency spectrum (see Figure 4).

 

By examining the frequency spectrum of the vibration signal one can examine the health of a
vibrating machine. This is because certain frequency spectra may indicate abnormal operation or
anomalies in the normal operation of rotating machine.
It is often the case that since wire-based sensors are too expensive, due to the cost of installing
wires from the sensor point to a control room, a portable vibration analyzer is used. As
described earlier, maintenance personal periodically take the portable analyzer to the rotating
machine to make vibration measurements. The analyzer has a powerful computer and does the
FFT to determine the vibration spectrum as shown in Figure 4. As we will see, by using
wireless vibration sensing the need for periodic visits by maintenance personnel is not required

 

 

 

 

WiHART Systems combines advanced wireless communication with MEMS technology to drastically reduce costs and improve performance.

 

Condition Based Maintenance Requirements

 

Smart Sensors , Lower Cost , High Performance

 

The traditional technique for measuring vibration is the utilization of piezoelectric sensors as accelerometers. Piezoelectric devices require external amplification and are connected to the local electronics by a wire. In the standard wired system the vibration information is then transferred by wire (4-20 ma current loop, etc) for storage and analysis. The cost of the wired system very often exceeds the cost of the actual monitoring devices. Due to these cost limitations the number of monitoring points have been severely limited.

 

In most cases where the cost of wired vibration monitoring is prohibitive the normal procedure is to have technicians monitor the rotating equipment by walking about the facility and taking measurements with hand held vibration monitoring devices once a week to once a month. In many cases the monitor points are difficult to approach such as fan motors on 70 foot tower.

 

Wireless versus Wire

 

As outlined above vibration measurements using traditional wire based techniques does not permit a cost effective comprehensive monitoring system. In the case of wireless sensors there is absolutely no need to install a wired infrastructure. Every device is essentially portable and battery powered. If local monitoring is required for a short period on a device the wireless sensor can be placed on the rotating machine and monitoring is initiated immediately. The wireless mesh technology provides for a complete network in any refinery, factory, petrochemical plant or any facility with rotating machinery.

 

The “Game Changer” for Wireless Vibration Measurement

 

WiVIB1 (Micro Sensor Interface Module (uSIM))

 

The WiVIB1 changes the paradigm for the vibration measurement. It is truly a “game changing” technology. The WiVIB1 is a small module about ~1 square inch. It is the size of a postage stamp and is an extremely low power device. It does not use a traditional piezoelectric sensors and instead uses a MEMS accelerometer as the vibration sensor. This sensor equals or exceeds the specifications of standard piezoelectric devices in almost all areas. It is totally digital and requires no external amplifiers, analog to digital converter or signal conditioning. It is a completely solid state device which translates into low cost and reliability.

 

The WiVIB1 is a smart sensor with a small but powerful RISC processor that is capable of performing local signal processing such as FFTs (Fast Fourier Transforms) to calculate a frequency spectrum locally. This reduces the amount of data that has to be transmitted back to the central control room.

 

The WiVIB1 is so small that they can be carried in your pocket and placed directly on the rotating machine, etc.

 

The WiVIB1 will provide the lower cost, lowest power consumption and the highest performance of any wireless vibration sensor on the market today.

 

Implementation locations for WiVIB1:

 

Gearboxes

Bearings

Pump Impellers

Cooling Tower Fans

Compressors

Motors

Conveyors

 

Benefits of wireless vibration monitoring:

 

Increase uptime:

Improve equipment availability by preventing unplanned shutdowns and equipment failure with more frequent monitoring.

Reduce maintenance costs:

Spot damages early, schedule maintenance and prevent minor damage from progressing into complex and costly major damages.

Better planning: With ample warning time for repairs,maintenance can be scheduled more efficiently and spareparts can be available when needed to simplify parts inventory management.

Manage offsite assets: Monitor difficult to reach and distant assets such as tank farm pumps, cooling tower fans or pumps and compressors that are widely dispersed across the plant. Use wireless technology to quickly set up a solution anywhere in the facility.

Monitor assets during startup, after repairs and other milestones:

Reduce the commissioning risk for newly deployed or repaired assets by closely monitoring their start times

 

 

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