Condition Monitoring of the operation of plant or machinery inevitably results in the emission of heat, either from energy conversion processes, for example combustion or electrical heating, or from the action of forces such as friction, torsion or mechanical stresses. The energy released is conducted to the surface where it is lost principally by radiation and convection.
The item of plant or process material acquires a surface temperature pattern which is characteristic of the process concerned. Any change in the process usually results in a change to the quantity or location of energy released with consequent modification to the surface temperature pattern.
All surfaces emit radiation at a rate which is related to temperature and usually falls within the infrared region of the spectrum. The radiation is invisible to the human eye but can be visualised using a thermal imaging system which presents the distribution of radiated heat as a visible picture. This property can be beneficially used for maintenance, process monitoring or on-line testing.
A portable thermal imager can be a very powerful tool in the hands of a maintenance engineer because it allows a rapid visual observation of plant to reveal anomalous temperature patterns which may signify the existence of a fault.
Thermography is a non-contact technique and enables observations to be made whilst the plant is in operation and under load.
The user can also make his inspection at a safe distance and it is often possible to carry it out with safety guards and fences still in position. Carry it out with safety guards and fences still in position.
Regular thermographic inspections can be used for predictive maintenance by the recording in trends in the surface temperature pattern. An initial set of measurements made when the plant is in good condition are used as a base-line with which subsequent observations are compared.
The surface temperature pattern is usually very sensitive to changes in the operation of the plant and may allow faults to be recognised before they become serious, allowing remedial measures to be taken before a breakdown occurs.
Modern thermal imaging systems have the facility to make accurate temperature measurements which can be presented either directly as an actual temperature value or as a difference from a reference level.
The magnitude of the temperature change between inspections can then be used as an indicator of the severity of the fault condition to help the engineer to decide the urgency of repair work. A classic example of the use of thermography in maintenance is the inspection of high voltage transmission lines and associated equipment.
Access to this type of equipment is often difficult owing to its size and safety considerations. The plant may be spread over a large area. A thermal imaging system presents a rapid and safe means of inspection.
On a smaller scale problems in low voltage distribution can be identified as shown here. A poor connection on one phase of the circuit was causing overheating in the vicinity of the fuse links. The problem can be readily detected by thermographic inspection and repaired before severe damage occurs.
Mechanical plant such as motors, fans, drives and couplings are also conveniently inspected by thermography. Shown here is a thermal image of an electric motor driving a pump through a shaft and coupling. The temperatures of various points on the motor, coupling and pump are displayed relative to the background temperature.
It is evident that the temperature in the region of the motor and pump bearings and the coupling are low indicating that the drive system is functioning satisfactorily.