In sealed source applications, the radiation source does not make contact either with plant or the process material and the radiation in either the transmitted or the scattered mode is analysed to get information on the integrity of about the plant internals. In the case of process columns, the techniques provide information on problems like entrainment, presence of weeping, location and density characteristics of foaming, extent of vapour/liquid maldistribution in packed bed columns etc.

In tracer applications, a small amount of radioactive material in an appropriate physico-chemical form is added to a process stream and then the system's output response is analysed for either troubleshooting such as leaks, blockages, etc. or for information on process dynamics such as mixing, Residence Time Distribution, etc. Suitable mathematical models involving axial dispersion, multi-component mixing, Fourier analysis, etc. have been developed to analyse tracer data.

The paper reviews the authors' experience in gamma scanning and tracer applications for NDT of dynamic systems in chemical and petrochemical industries.

Gamma scanning is a non-invasive technique used for troubleshooting of process columns. The technique is also employed for debottlenecking studies of process involving multiphase flows and for predictive maintenance of column hardware. Some of the inspections carried out on different types of trays and packed bed columns are:

• Inspection of a 8.5 m diameter vacuum column in an oil refinery, to diagnose the cause of malfunctioning. The gamma scanning was able to detect the presence of entrainment and abnormal liquid/froth on few tray positions in the column. These results were very useful in correcting the cause for malfunctioning.

• Inspection of a 2.3 m diameter extraction column in an oil refinery to determine the cause of poor efficiency of extraction of benzene and toluene. The inspection could successfully demonstrate the interfaces between gas/raffinate which also agreed with liquid level indicator in the control room. The interface between hydrocarbon/solvent was also observed and was found to be displaced from the normal location.

• Inspection of a 1.3 m diameter tray type Ammonia separator column to detect the cause of reduced output. Gamma scanning results indicated that many trays had collapsed in the column. This was confirmed on later opening the column for remedial means.

  The characteristic residence time distribution obtained using radiotracer stimulus response technique is used for troubleshooting and process optimisation. Some of these studies include:

• RTD in an Aniline production reactor using Br-82 to investigate the cause for poor heat transfer from tube side to shell side of the reactor. Analysis of data indicated that 50% of the shell side volume was dead and was the cause for poor heat transfer.

• RTD study in a semi-kestner evaporator in a sugar factory to investigate the flow behaviour of sugar cane juice. Sodium-24 as NaCl in aqueous solution was used as tracer. The results indicated the MRT of 4 seconds at a flow rate of 100 MT/hr and mathematical modeling indicated that the system was behaving as a well mixed system.

• RTD study in a laboratory scale solid-liquid fluidised bed column using Tc - 99m as tracer to determine stimulus response for optimisation of design and operating conditions. The experiments indicated that with glass beads of 200-300 micronsize and water flow rate between 7-15 ml/s, the column behave like a plug flow system.