Specifications

DESCRIPTION AND OPERATION Document No. DER01122
Effective Date 30 Aug 07 Page 5 of 6
Various inputs including main bearing temperatures, bowl speed, bowl and conveyor torques,
cabinet interior temperature, and vibration are transmitted to the PLC, which then responds with
corresponding outputs to govern centrifuge operation. Critical status information is displayed on
the control panel, as well as alarm and fault messages that signal the operator of any potential or
imminent malfunctions. Any excessive variation from a pre-set limit causes the PLC to
automatically shut down the centrifuge and display an explanatory message for the cause of the
shutdown. In addition, emergency shutdown may be performed by the operator at any time from
the front panel.
The feed pump is fully controlled by the PLC in either manual or automatic operation. Automatic
operation provides maximum throughput, as the optimum feed rate is assured by the PLC under
varying operating conditions. Feed is automatically increased until the pre-set torque limit is
reached by either the bowl or conveyor drive motor. If feed slurry properties change, the feed rate
is automatically adjusted to remain within the torque limit.
DRIVE MOTORS
The centrifuge is powered by two inverter-duty, three-phase, explosion-proof electric motors. The
150 HP bowl drive motor is connected to the bowl assembly by a sheave and series of drive belts,
while a 60 HP electric motor is directly coupled to the gearbox input shaft by a flexible coupling. A
protective guard covers the sheaves, drive belts, and conveyor drive flexible coupling for
personnel protection.
PURGE SYSTEM
A continuous flow of air from the purge system prevents the cabinet interior from accumulating
explosive vapors. When power is applied to the centrifuge, the cabinet purge system initiates a
14-minute rapid purge cycle at 16 CFM. This cycle consists of five complete air exchanges. When
the rapid purge cycle is completed, the purge system automatically closes a switch that supplies
power to the cabinet.
During centrifuge operation, the purge system maintains a continuous flow of air to provide
positive pressure within the control cabinet to prevent entry of flammable gases. Minimum
pressure and purge flow sensors provide an output signal to the PLC if the purge system is not
maintaining satisfactory pressure within the cabinet. For further details of purge system operation,
refer to the Expo Technologies documentation in Section 12 - Supplier Data.
VIBRATION SWITCH
The vibration switch (Figure 1122-5) is a safety device designed to protect personnel and
equipment by shutting down the centrifuge in case of excessive vibration. Normally, the switch
contacts are held closed by a magnetic latch. However, strong vibration or a shock of 2 Gs will
overcome the magnetic latch, causing the switch armature to break away from the normally
closed position, providing an input to the PLC. A reset button on the side of the switch must then
be manually pressed to close the contacts and re-engage the magnetic latch. The vibration trip
level is adjustable by means of a set point control, which adjusts the air gap between the magnet
and latch arm plate. Turning the screw counterclockwise reduces the vibration set point in terms
of G force needed to trip the armature. When the control is turned fully clockwise, the switch will
trip at the maximum rated vibration level.