Sensors and Controls for Bucket Elevator Safetyand Predictive/Preventative Maintenance

A. BELT BUCKET ELEVATORS

1. BELT SLIP:

A bucket elevator belt can slip on the head pulley due to:

Inadequate belt tension between head and boot pulleys

Belt wear and stretch Worn head pulley lagging Moisture between the belt and head pulley Material plug conditions Changing operating temperatures

This can lead to: Reduced belt life Dropped belts Fires Explosions (when elevating certain dusty

materials such as corn, sugar, coal, plastics, wood, certain metals)

As such, the belt speed must be monitored on every belt bucket elevator

When using a constant speed drive, belt slip at the head pulley can be monitored using a single sensor installed at the boot. If the belt slips at the head, the speed of the boot pulley and shaft will reduce proportionally. The sensor at the boot will detect this as a reduction in speed from normal and can provide warning and shutdown signals. When using a variable speed drive, two sensors are required. One sensor is installed on the head shaft, and one sensor installed on the boot shaft. The signals from these sensors are used to determine belt slip from the proportional speed. Additionally, belt slip/under-speed information can be a valuable tool for predictive or preventative maintenance.

Typical belt slip (under-speed) sensor mounted on the boot shaft of a bucket elevator:

2. BEARING TEMPERATURE:

Shaft bearings can fail due to: End of service life Incorrect or inadequate bearing lubrication Contamination from material dust

This can lead to: Catastrophic elevator failure Fires Explosions (when elevating certain dusty materials such as corn, sugar, coal, plastics, wood, certain

metals)

As such, all pulley shaft bearing temperatures (head, boot, Knee/bend) must be monitored on every belt bucket elevator.

Typical sensors are adjustable depth, positively mounted grease through continuous temperature sensors incorporating NTC Thermistors or RTD’s. When the bearing temperature exceeds a pre-set value, warning and shutdown signals are provided. Additionally, bearing temperature information can be a valuable tool for predictive or preventative maintenance.

3. BELT MISALIGNMENT:

A bucket elevator belt can misalign near the head or boot due to: Inadequate belt tension between head and boot pulleys Worn head pulley lagging Moisture between the belt and head pulley Material plug conditions Head and or boot pulley alignment issues Out of plumb shaft Poorly spliced belt Uneven loading of buckets Pulleys walking on shafts Worn our inadequate pulley crown Material build up on pulley surface

This can lead to: Reduced belt life Damage to elevator casing Damage to belt edge Dropped belts Damage to elevator buckets Fires Explosions (when elevating certain dusty materials such

as corn, sugar, coal, plastics, wood, certain metals)

As such, belt misalignment must be monitored on every belt bucket elevator.

Belt misalignment can be monitored by installing contact style sensors on each side of the head and boot pulleys, in line with the belt.

Typical sensors are force activated, temperature activated or high powered inductive. When the edge of the belt contacts the sensor, warning and shutdown signals are provided. Additionally, belt misalignment information can be a valuable tool for predictive or preventative maintenance.

4. PLUG CONDITION:

Plug conditions can occur due to: Inadequate discharge chute size Buckets thrown into and blocking the discharge Tramp material blocking the discharge Excessive feed rate at the inlet

This can lead to: Catastrophic elevator failure Belt slip Belt misalignment Broken buckets Fires

Explosions (when elevating certain dusty materials such as corn, sugar, coal, plastics, wood, certain metals)

As such, sensors to detect plug conditions should be installed in the area near the head discharge and/or in the area just above the inlet at the boot.

Typical sensors include flush probe or diaphragm style which detect material pushing against the sensor under a plug condition, and providing a signal to shut down the elevator.

B. CHAIN BUCKET ELEVATORS

Similar sensors to those used on belt bucket elevators are used to monitor chain bucket elevators. Shaft Speed sensors monitor for a stop condition. This is sometimes via a boot or head shaft mounted speed sensor or via a high powered inductive sensor detecting the chain or bucket movement. Bearing sensors monitor bearing temperature, and plug chute sensors monitor for blocked discharge conditions. Belt Misalignment sensors are not applicable.

WARNING -

Whenever using any type of bucket elevator to move materials that could create an explosive dust hazard condition, the sensors MUST be approved for the hazardous area where they are to be installed. Class II Division 1, Groups E, F, G

Additional considerations and precautions should be taken if the area in or around the bucket elevator is a gas hazard area.

Installation should be in accordance with the National Electrical Code for hazardous area locations.