Machinery protection in a box

Source: PlantServices.com

Feb 08, 2006

An increasing number of plants are leveraging automation systems for condition monitoring by taking vibration readings the same way they take pressure or temperature readings. While sensors and transmitters can provide this trending information to the automation system, there are many applications where a local protection device, or switch, is needed directly on the equipment.

Many plants are using simple mechanical devices some call “earthquake switches” to protect the equipment. The alarm setting on the mechanical switch is often crude, involving an adjustment of a spring-loaded lever in the switch while the machine is operating. “But you have to set them so they don’t go off under any normal operating conditions, and because they’re mechanical devices, you need to check the settings at some time interval, which seldom happens,” says Jim Ephraim, business development manager, Hardy Instruments (www.hardyinstruments.com). “So these switches usually don’t alarm until the equipment has suffered significant damage.”

Hardy’s new HI 5800 HS vibration switch offers a more effective alternative in the form of an electronic, low-cost, self-contained monitoring system. A setpoint adjustment dial and LED alarm indication allow it to be adjusted in much the same way as a conventional switch by turning a screw until the LED goes off, but the margin is quantitative and stable. “It’s a piezoelectric crystal sensor and signal conditioner in a box,” says Ephraim. “You can dial in a setpoint from 0.1 in. to 1.5 in. per second [IPS], then forget it. You can be confident that it will alarm when needed.”

A time delay can be adjusted for as long as 10 seconds to prevent nuisance alarms from transient vibrations, and an integral relay allows alarm and failsafe operation with both normally-open and normally-closed contacts. Alarms can be reset remotely, the switch can be bypassed for start-ups, and the system is rated Class I, Div. 1 for hazardous area installations.

An optional 4-20 mA analog output provides the capability to trend vibration levels for predictive maintenance. The analog signal is directly proportional to the total energy level in the 6 Hz to 500 Hz range and can be connected to a local indicator, equipment controls or a local or remote PLC or DCS.

“For example, you could mount it on a gearbox, wire it to a PLC and set the local alarm for 0.3 IPS,” says Ephraim. “If the control room sees the vibration rise to 0.2 IPS, they can call the vibration expert, who can use a spectrum analyzer to home in on the cause.”

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