BELT MISALIGNMENT - A dangerous and common cause of dust explosions
by Vaughn Entwistle, Features Editor, Milling and Grain
In the June edition of Milling and Grain we carried a special feature on the dangers of dust explosions and explored several strategies for reducing the risk of these serious incidents. So, it was with great interest that at the recent VIV Europe 2018. I discovered a relatively simple and cost-effective piece of technology for potentially preventing dust explosions on conveyors and bucket elevators.
Because bucket elevators account for 30-40 percent of all grain handling dust explosions, it"s easy to consider them as "dangerous" installations with a very real potential for the risk of explosion. One of the major culprits in these explosions is belt misalignment. A misaligned belt, rubbing against another part of the conveyer, can quickly generate a great deal of heat and friction, and might even shoot out sparks. When heat/sparks come together with a cloud of highly flammable grain dust, it often results in a powerful explosion.
However, research shows that these explosions and the resulting damage can easily be limited with various belt misalignment monitoring systems for horizontal and vertical transport.
To alert conveyor/bucket elevator users to belt misalignment, Dutch company Muller Beltex created Rub-Block. A typical Rub-Block system employs four temperature sensors—two mounted at either end of a belt system. Unlike competing sytems, Rub-Block detects belt misalignment based on raised temperatures generated by friction.
Rub-Block uses a PT100 temperature sensor to report the misalignment of a conveyor belt, elevator belt or HI-roller. The RB200DN can be used for elevators from 12 metres height. The advantage of the Rub-Block is that it emits a continuous standby signal to the PLC. If such a signal is absent, the reason for this must be investigated and, if required, the PT100 sensor must be replaced. As long as the Rub-Block emits a standby signal, it will also emit an emergency contact signal caused by its temperature rising, as soon as a belt touches it.
Off-track belts running into the side of conveyor equipment will cause friction, which may damage the belt, and might also generate heat and fire, leading to a dust explosion. The RB200DN is easy to install. Sensors are fitted on both sides of a conveyor or elevator belt. Once the belt comes off-track and rubs against the brass plate of the Rub-Block, it will create friction and the temperature of the Rub-Block brass plate will increase, triggering an alarm.
Bearing temperature sensor
Muller Beltex also makes a range of bearing temperature sensors and many feed mills and milling companies combine these products along with a speed switch to detect slip, thereby eliminating all possible ignition sources on a bucket elevator. The bearing temperature monitor can also detect bearing damage at an early stage. (Like the PT 100 sensor, all sensors used by Muller Beltex are ATEX and CSA approved).
60 percent of early malfunctioning bearings are caused by material fatigue as a consequence of continuous fluctuating pressures on the bearing"s surface and the use of incorrect lubricants. Bearing monitor sensors are able to detect damage at an early stage and prevent downtime and potential dust explosions.
The Adjustable PT100 Bearing Temperature sensor with grease nipple (type PT100V3C) and ½ NPT conduit entry has the advantage of being adjustable in terms of probe depth. This way the temperature reading is not just taken from the surface of the casing, but from the bearing ring itself.
In practice, this results in temperature differences of 20 percent to 40 percent compared to surface temperature measurements. This allows users to quickly intervene should they record a sudden temperature increase.
The PT100 emits a permanent standby signal. If such a signal is absent, the reason for this must be investigated and, if so required, the PT100 sensor must be replaced. As long as the PT100 emits a standby signal, it will also emit an emergency contact signal due by its temperature rising, as soon something will cause friction, which may cause damage to the machine, and may generate heat or even fire.
The sensor"s analogue signal can be connected direct to a PLC or, for increased security, to an independent monitoring system such as the GSI48 indicator; also, this analogue signal can be converted by a transmitter to a digital, profibus signal.
PT100 sensor: 4-core cable with a length of 3 metres. Probe (PT100) length 100mm, made of stainless steel. The probe has a diameter of 5mm and the brass house (adapter) has a G1/8 thread, which are used for all kinds of bearing houses.
The sensor"s body is made of brass to prevent corrosion. Monel is a nickel copper alloy, which is more resistant to atmospheric conditions, compared to stainless steel.
The measurement is performed on the bearing ring, creating an accurate measurement.
Maintenance (oil chart) match temperature measurements.
ATEX Classification version (Ex-i)
III 1/2 D Ex ia IIIC TX Da/Db, II 1/2 G Ex ia IIC T6..T1 Ga/Gb -40°C tot 185°C