Calex has designed a temperature monitoring and alarming system for a leading flour milling company to help prevent fires and explosions. Grain passes through rollers that grind it into flour. The roller separation is finely adjusted; if they are too close together, excessive heat is generated by friction, which can cause the roller temperature to become critically high. The flour dust can then ignite, causing a fire or a devastating explosion. As well as the safety risk, this can lead to costly downtime.
Calex has worked closely with this customer to specify a system of 120 PyroBus infrared temperature sensors monitoring 60 machines in the milling room. The pyrometers are connected via an RS485 network to a PC running CalexSCADA software for temperature indication, alarming and data logging.
The system was selected for the customer's safety programme as part of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR).
Two PyroBus sensors (model PB21) are aimed at the milling surface of one roller in each machine. Monitoring both ends of the roller helps ensure a hazardous temperature at only one end is properly caught.
The sensors are connected in four groups, in a daisy-chain arrangement using junction boxes. Each group has its own power supply and is connected to an isolated RS485 repeater, so a power fault or surge in one group will not affect the rest of the network. All four groups are connected to a PC via an RS485/RS232 converter.
Four alarm relay output modules are provided for each group, to trigger alarms for warning and critical temperatures. One relay output is triggered by any alarm; this may be connected to a control room beacon and/or a text-message notification system.
The CalexSCADA software displays the measured temperature and alarm state of all 120 sensors at once. The user can view detailed information and temperature history graphs for each pair of sensors. The software also logs measured temperature, sensor temperature and alarm state at 5-minute intervals to the PC hard disk.
As the sensors may be positioned very close to the roller surface, the wide 2:1 field of view of the lowest-cost model PB21 gives good results.
To help keep the lens clean and ensure maximum accuracy, an air purge collar is used with each sensor. This customer has designed a special mounting to fit the internal thread at the opening of the air purge.