Non-disruptive & on-line inspection

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Gamma-ray scanning fortroubleshooting, optimisation andpredictive maintenance ofdistillation columnsThis paper describes Non-disruptive and on-line inspection technique to diagnose process malfunctionsand pinpoint internal damage within distillation columns in petroleum refineries and chemical plants

Non-disruptive & on-line inspection

D istillation columns are considered as one of themost critical components in petroleum refiner-ies, gas processing installations and chemicalplants. Plant performance depends to a large

extent on the ability of these columns to function asintended. Defective columns may lead to serious con-sequences to the plant operation and hence the qualityof the product. Thus, when a distillation column expe-riences irregularities, it is urgently necessary to findout exactly what is happening inside the column. Anykind of problem can result in a large financial lossespecially when it involves off-spec product, lost ofproduction or unexpected shut down of the plant. Theearlier the problem is identified and corrected, thelesser will be the loss and the costto rectify it. In most cases, trouble-shooting to pinpoint the cause ofthe problem in columns is con-ducted on-line. It seems that this isthe only way column performancescan be observed. Inspection whilethe column is in operation also ena-bles continuous monitoring andassessment of the component.

Gamma-ray scanning, often re-ferred to as “column scanning”, isa convenient, cost-effective, fast,efficient and non-intrusive tech-nique to examine inner details ofa distillation column, while it is inoperation. Column scanning al-lows an engineer to study tray or

packing hydraulics inside a column at any set of on-line condition. It provides essential data to optimisethe performance of columns, extend column run times,track the performance-deteriorating effects of foulingand solids deposition, and identify maintenance re-quirements well in advance of scheduled turnarounds.This on-line knowledge can reduce repair downtimesignificantly.

By analysing the scanning results, a number ofcommon malfunctions in trayed or packed columnscan readily be determined, some of which are sum-marised in the following table.

In Malaysia and the surrounding region, the examina-tion of process vessels (columns, separators, reactors,

Gamma scanning can identify a number of common malfunctions in columns.

Mechanical • Displaced or damaged trays, demister pads and packingproblems • Corrosion resulting in partial tray damage

• Missing, collapsed or buckled trays or manways• Out-of-place liquid or vapour distributors• Level control problems on chimney trays or base liquid level

Rate related • Entrainment (slight, moderate, severe, jet flooding)problems • Weeping or dumping trays

• Dry or flooded trays due to loading conditions• Unequal liquid levels on trays and in parting boxes, troughs

and collectorsProcess • Foaming on trays or in reboilers, condensers and accumulatorsrelated • Maldistribution of vapour and liquid in packingproblems • Liquid hold-up due to plugging and fouling

• Superheated or subcooled feed or reflux

HYDROCARBON ASIA, JAN/FEB 2005 63

catch-pots etc.) in petroleum refineries, gas processinginstallations and chemicals plants using this technol-ogy is routine. Scans are frequently carried out on aspot-check basis to investigate column malfunctions. Inaddition, many process columns are scanned on a regularbasis to provide early warning of deterioration in theirperformance. The examinations are carried out by highlyexperienced and well-trained engineers and technolo-gists from the Plant Assessment Technology (PAT) Group

at the Malaysian Institute for Nuclear Technology Re-search (MINT), Bangi, Selangor. Having being involved inthe column scanning technology since 1980, PAT’s tech-nical staffs have performed hundreds of scans on varioustypes of process columns across the country and aroundthe region.

Principle of measurementColumn scanning is a procedure whereby a process

column is non-disruptively examined by moving asealed radioactive source emitting gamma-ray in con-junction with a radiation detector along the exterior ofthe interposed column. Throughout the period of in-vest igation, the radioactive material remainspermanently encapsulated within a special source hous-ing and makes no contact either with the column orwith the process material. A source holder with anappropriate collimator is used to direct the radiationbeam to the column. Interaction of the radiation withthe medium of interest will produce changes in theintensity of the beam that can be correlated to theproperty of the medium.

The technique is based on the fundamental relation-ship:

where I is the intensity of a beam of gamma-ray of initialintensity, Io, having traversed distance x in a material ofdensity, r. The mass absorption coefficient, m, is constantfor a given gamma-ray energy and material composition.

This relationship shows that an in-crease in material density will reducethe radiation signal, or conversely adecrease in material density will re-sult to an increase in the level ofradiation intensity transmitted to thedetector.

Radioactive sources used for distil-lation column investigations shouldbe capable of penetrating the wallthickness of the column and the me-dium of interest. For this reason,high energy gamma-ray from Cobalt-60 or Caesium-137 is usuallyemployed. It is worth to note that thestrength of the sources used for thisactivity is extremely small such thatthe scan poses insignificant radiationhazard to plant personnel. For thesake of comparison, the level of ra-diation intensity for column scanningactivity is approximately between 1/10000th to 1/1000th of the gamma-

rays intensities used in examining welds.

Inspection procedureColumn scanning is generally performed without any

pre-preparation of the column. All that is required isaccess to the uppermost platform. No insulation needs tobe removed, and the scan does not interfere with theoperation of the column. In practice, most columns haveconvenient platforms or walkways from which sourceand detector can be suspended and it is rare indeed thatadditional scaffolding or staging is required.

In performing measurements on a column, a source ofgamma-ray is placed on one side of the column whilst asensitive gamma-ray detector is placed on the opposite side.The source and detector are synchronously lowered downthe exterior of the column in small increments while trans-mitted radiation-intensity measurements are being recordedvia a portable nucleonic counting device. The system isthen interfaced to a powerful portable computer to facili-tate data storage and analysis. To ensure that the sourceand the detector are maintained in the same horizontalplane, especially in windy conditions, guide ropes are

Gamma-ray absorption scans of a trayed column and a packed column.

Non-disruptive & on-line inspection

64 HYDROCARBON ASIA, JAN/FEB 2005 Visit our website at: http://www.safan.com

normally used. Scans can be made across the diameter ofthe column. Or they can take place along a chord, toidentify problems within a particular quadrant.

As the source and detector scan through a normalvapour space in a process column, a transmissionpeak is observed and when the “scan-line” intersects astructural material, such as a tray with aerated liquid,an absorption peak is observed. The end result of the

scan is a density profile of the inside of the processvessel. For distillation column, correlating the changesin density seen in the scan profile with column inter-nals and what is known from column service recordsyields an accurate picture of the unit’s performanceand physical condition.

Distinguishing features of the technology• Capable of non-disruptive and non-invasive investi-

gation.

• Scans are performed on-line; hence, normal opera-tion of the column is not disrupted. They are carriedout completely external to the process.

• Full length scanning capabilities.

• No preparation of columns (for additional scaffold-ing or staging) and components dismantling isrequired. Only requires access from readily avail-able platforms or walkways.

• Insulation or lagging need not be removed.

• Performs non-contact measurement, therefore, isnot affected by high temperature, high pressure,corrosiveness etc.

• Utilises small amount of “sealed radioactive source”with in-built radiological safety features to complywith national and international legislative require-ments.

• Applicable to all column sizes.

• Uses computer-aided data acquisition, retrieval andanalysis system enabling fast, accurate and reliableresults. Data collected and stored by computer can beused immediately as well as for future comparisons.

Recommended scanning programmeIn keeping a systematic and a complete record of

column operating conditions, it is highly recommendedthat a regular programme of column scanning shouldbe implemented as follows:

• Troubleshooting Scan TM or Diagnostic Scan TM :Diagnose operational, mechanical and processrelated problems. Gain empirical confirmation ofmodels and simulations. Locate bottlenecksthrough real-time analysis.

• Blank Scan TM or Dry Scan TM: Scanning of internalstructures of a column before operation or duringshutdown.

• Reference Scan TM : Record normal operatingprofile after start-up or during commissioning forbaselining. Verifies the system design andoptimise system performance.

• Performance Scan TM : Evaluate existing column

Gamma-ray scanning for non-disruptive and on-lineinspection technique to diagnose process malfunctions andpinpoint internal damage within distillation columns inpetroleum refineries and chemical plants.

HYDROCARBON ASIA, JAN/FEB 2005 65

performance and developproject engineering. Study trayor packing hydraulics at anyset of on-line conditions.Investigate the effectiveness ofantifoam addition.

• Scheduled Scan TM or PeriodicScan TM : Routine scans shouldbe conducted quarterly, half-yearly, annually or at apre-determined period forpredictive maintenance andlife assessment programme.Periodically monitor theprogress of fouling, pluggingand corrosion. Identify mainte-nance requirements well inadvance of scheduled turna-rounds.

• Prior Shutdown Scan TM :Checking the internal condi-tion of a column prior to ashutdown. Schedule parts andmaterials to be on-hand priorto a turnaround.

National and Internationalrecognition

Gamma scanning has been suc-cessfully used to investigate varioustypes of trayed and packed col-umns in petroleum refineries, gasprocessing installations and chemi-cal plants in Malaysia and nearbyregion. The successful utilisationof this technology has been recog-nised both nationally andinternationally. MINT, in co-opera-tion with the International AtomicEnergy Agency (IAEA) in Vienna,have organised several trainingcourses to promote this technol-ogy, to scientists and engineers fromRegional Co-operative Agreement(RCA) member countries, includ-ing Australia, China, Bangladesh,India, Indonesia, Japan, the Repub-lic of Korea, Malaysia, Mongolia,Myanmar, New Zealand, Pakistan,the Philippines, Sri Lanka, Thailandand Vietnam. The courses andworkshops have also been attendedby a number of participants and

Hydrocarbon Asia thanks DrJaafar Abdullah for contributingthis paper.

D r J a a f a r i sthe Manager andPrincipal ResearchScientist of Plant As-sessment Technology

Group, Industrial Technology Di-vision, Malaysian Institute forNuclear Technology Research(MINT), Bangi, Selangor. He hasbeen involved in a wide varietyof research and development us-ing radioisotope technology forindustrial plant assessment since1980. As the pioneer of gammascanning technology in Malaysiaand the surrounding region, hisexperience in using the technol-ogy for investigating andtroubleshooting various types ofprocess columns in petroleum re-fineries and chemical plants isextensive. He is recognised bythe International Atomic EnergyAgency (IAEA) as an expert andconsultant in the field of indus-trial applications of radioisotopes.He received his B.Sc in Physics,M.Sc in Nuclear Technology fromUniversity of Surrey, UK and Ph.Din Materials Engineering from Uni-versity of Wales Swansea, UK.He has published substantialamount of scientific papers, con-sultation documents, technicalreports and articles. He has wona number of awards both nation-ally and internationally for hisoutstanding innovations in thefield of radioisotope technology.He is a member of several profes-sional organisations and societies,which include MNS, MSNT, IPM,IMM and MARS. He was the chair-man of MSNT technical andtraining committee.

observers from European countries,the Middle East and the Africancontinent.

Radiological safetyColumn scanning uses a “sealed

radioactive source”, which emitsgamma radiation. The strength ofthe source is very minimal (between1,000 to 10,000 times less activity)compared to those mostly used inindustrial radiography. The sourceholder is designed with in-built ra-diological safety features to ensurethat exposures to the inspectionteam and plant personnel or thepublic do not exceed permissible lim-its recommended by the InternationalCommission on Radiological Protec-tion (ICRP) and the IAEA, as well asthe regulatory guidelines of theAtomic Energy Licensing Board(AELB) of Malaysia.

Concluding remarksThe application of radioisotope

techniques, in particular, gamma-rayscanning, can help improve the effi-ciency of a distillation column or itcan pinpoint the reason for a declinein performance, thus yielding largesavings through avoidance of loss ofproduction and wasted maintenanceeffort. Column scanning is only asmall subset of the many applica-tions of radioisotope technologyencountered in practice. The tech-nique is straightforward, quick,sensitive, unambiguous and safe.Realising the importance of the tech-nology, considerable efforts havebeen undertaken by MINT towardspromoting and expanding its usagein Malaysia and around the world.To ensure effective use of this tech-nique, MINT encourages andwelcomes collaboration and partici-pation from the petroleum andchemical industries. But more im-portant, and what is expected, is thereadiness of the industries to acceptand adopt the technology. Enquiry Number 01/02-02 HA