The advantage of a portable system is that it can cost less to procure and install since there is no permanent wiring required. However, if a facility decides to hire an outside firm, even this option can be costly, running between $600 and $1,200 per day1 while still providing some level of predictive monitoring. The disadvantage of a portable system is that machine problems do not follow a schedule and there is a very real possibility that a machine can develop problems or even fail between the periodic assessments.
Permanently mounted sensor systems attempt to address the issues presented by portable systems, but they do so at a very high cost. Acquiring and installing a permanent system can run into the hundreds of thousands of dollars when you factor in the costs of the sensors, diagnostic machine and software, and the installation and maintenance of long wire runs that are necessary to power the sensors and collect the vibration data. These costs can dramatically affect the return on investment (ROI) of continuous machine vibration monitoring for predictive maintenance and put such systems beyond the financial reach of most companies.
While permanent machine monitoring has traditionally been performed using wired sensors, costs for wiring vibration sensors are high, ranging from $50 to $100 per foot2. Wire installation costs are a driving factor that limits the affordability of vibration monitoring. Wireless sensors address this cost issue. Additionally, wireless sensors offer to simplify sensor installation, reduce maintenance associated with wiring faults, permit new sensor locations that would not have otherwise been accessible with wired sensors, and offer greater flexibility with easy installation or removal, as required.
In summary, wireless sensors have the promise to make vibration monitoring practical for most companies.
However, with all of the upsides to wireless monitoring, it does not come without its drawbacks. To begin with, battery life has traditionally presented usability problems because battery replacement is an additional maintenance activity that can offset the savings provided by wireless sensors. Also, in the past, wireless sensors have been limited by the usable bandwidth available for transmitting vibration data. Either a limited amount of data could be sent over a narrow bandwidth or more data could be sent over a wider bandwidth, but battery life would greatly suffer.
With recent advances in wireless technologies, however, companies can now achieve better battery life, longer transmission distances and more robust data delivery. This is achieved by using a wireless protocol that is optimized to use short power-on times, brief on-air times and ultra-low power acknowledgments that minimize the time the sensor node is on while still transmitting a full dynamic vibration spectrum over the air on a frequent basis.
As an example of how wireless sensing technology can benefit industry, a large, privately owned company with a sizeable commercial refrigeration warehouse in Virginia purchased a wireless sensor system after experiencing inconsistent results in vibration data.
The warehouse consists of 13 total compressor units. Each unit has 15 monitoring points, varying between vertical, horizontal and axial locations.
Before investing in its wireless sensor system, the company paid a consulting service to visit its facility once a month to take one vibration data point per point on the compressor units. This totaled 195 (13 compressors, 15 points each) single data points per month for the cost of $1,000.00. This method yielded inconsistent data, resulting in unnecessary maintenance work done to the machinery.
On occasion, the vibration consultants highly recommended having equipment sent to the manufacturer to have maintenance work done. However, in several of those instances, the manufacturer said the equipment was within spec and working as it should. This happened on more than one occasion, costing the company time and money only to find out its machines were working properly.
Wireless sensors placed in the vertical and axial positions on a compressor at a commercial refrigeration warehouse
With its new wireless system, the company was able to easily install sensors and monitor the compressors for several days each month. The solution the company purchased consisted of software, a collection server, two primary receiver nodes and 15 total vibration sensor nodes (enough to cover one compressor).
This solution has enabled the company to collect vibration data per compressor unit for an average of 2.3 days per month (since the company moves its sensors between compressors every few days). This means the company can now monitor machine vibration trend lines of roughly 3,000 points per monitoring location per month compared (when monitoring once per minute) to the one data point per monitoring location per month when using the consultant.
The wireless monitoring system has paid for itself very quickly. Whereas the company was previously paying $12,000 per year, with its new system, the company only paid a one-time installation cost of $13,425 for the equipment and software. Furthermore, the company is able to monitor its machines for 56 running hours per month (compared with the two minutes per month using the consulting service), which is leading to better diagnostic information. Finally, the company’s cost per sample taken for assessing machine health is reduced from $5.13 per sample to less than a penny per sample.
In conclusion, while wireless vibration monitoring is not necessarily the right solution in all cases, as the example shows, the recent introduction of powerful and low-cost wireless vibration monitoring systems is appropriate for many situations.
References:
- United States Department of Energy. Operations and Maintenance Best Practices: A Guide to Achieving Operational Efficiency. Release 3.0, Aug. 2010 www1.eere.energy.gov/femp/pdfs/omguide_complete.pdf
- United States Department of Energy. Industrial Wireless Technology for the 21st Century. Dec. 2002. http://www1.eere.energy.gov/manufacturing/industries_technologies/sensors_automation/pdfs/wireless_technology.pdf
