INDUSTRY NEWS
Cost-Effective Monitoring & Protection for Smaller Plant
Protecting smaller, yet critical items of rotating plant such as industrial fans and pumps has become increasingly important as the focus to improve efficiencies, reduce energy consumption and eliminate downtime has moved beyond protecting more expensive assets such as turbines and compressors. However, achieving the right balance of cost versus benefit in using more sophisticated and expensive systems for smaller machinery operating in less critical applications can be challenging, especially given the exceptionally tough circumstances facing many plant operators today.
SENSONICS have been working with industry for nearly 40 years, developing effective and affordable monitoring
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www.sosmagazine.biz March 2021
and protection devices including those for smaller items of plant where the need for low-cost sensors which offer direct connection via 4-20mA current loop, is more important than ever. However, matching this type of product to the application is not always straightforward so it’s important to outline the key points which need to be considered along with the problems that can occur.
For example, the majority of absolute vibration transmitters available today utilise a standard piezoelectric compression technique coupled with electronics performing a current sink in the circuit loop to the PLC/DCS. The advantage of this solution is that no other power is required for the sensor and a single cable pair can be used from the connecting system to the machine. However, whilst the sensor is simply mounted on the machine bearing case and connected to the current loop, there are pitfalls to be avoided. These include
the earthing regime, low frequency vibration and high acceleration noise, which can lead to measurement error and spurious machine shutdown in protection applications, resulting in expensive downtime.
Overcoming Earthing Problems Earthing and grounding issues are where the majority of problems in new installations occur, particularly in heavy industry where power usage is high and effective earthing regimes can be challenging.
The internal sensor arrangement relies on a very high impedance circuit to extract the charge from the ceramic sensor and with limited isolation to the external case pick-up from a noisy earth is unavoidable, appearing on the output as an unstable current reading. This is because at the connecting system end the transducer low point is usually connected through a terminating resistor to a different,
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