Non-destructive testing (NDT) maintenance inspection of process plant – a current look

Introduction
Every car owner knows - that the longer you own your car the more it will cost you in running costs. As with all items of plant the longer you need them to perform the more maintenance is usually required to ensure they do.

This is certainly the case with Vessels, Storage Tanks and their associated piping systems which may have been built with a specific design life in mind. The design life now becomes less relevant than the “intended life” which could be extended to 40, 60, 80 or more years. This poses the question “How long can an item of plant be expected to last”. If all factors are right – good engineering design, built to specification, regular maintenance and routine inspection then the time period can be very long. This paper is intended to cover some specific aspects of Maintenance Inspection by various NDT methods.

• NDT techniques can be intrusive or non intrusive.
• NDT techniques may be carried out remotely without the requirement for scaffolding.
• Standard NDT techniques currently employed are Radiography, Ultrasonics, Magnetic Particle and Liquid Penetrant. To all these techniques needs to be added the most important consideration - visual inspection.
• A review of the Specialist NDT techniques includes several methods - Acoustic Emission, Pulsed Eddy Current, EMAT, Long Range UT, TOFD, Phased Array, Thermal Imaging, UT Robotic Scanners, Tank Floor Tester, ACFM, RVI, Heat Exchanger Inspection - IRIS/EC/Dinsearch/RFEC
• For this current NDT technology – how confident are we in the equipment and the competency of the inspector delivering it?
  
  
  
  
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NDT Techniques for Process Plant
Currently there are many techniques being applied some Standard NDT methods (i) Radiography utilising x and gamma rays for weld & material quality, profiles & product build up  (ii) Ultrasonics pulse echo for weld & material quality and wall loss through corrosion/erosion (iii) Surface Inspection methods Magnetic Particle & Liquid Penetrant for cracking and other surface breaking defects. In addition there are a variety of what could be classed as “Specialist NDT” methods. It is the “Specialist NDT” techniques being applied that I wanted to provide some practical background information.

These are:

• Acoustic Emission Testing to detect corrosion of the Tank Floor

Storage Tank assessment with Acoustic Emission (AE) has been developed as a quick and relatively inexpensive way to detect corrosion of tank floors.  It is used as a screening tool to aid in determining internal inspection needs.

The procedure is deceptively simple but requires great attention to detail. AE sensors are attached around the base of a tank as shown. The number of sensors depends on the tank diameter and varies from six on small tanks to 24 on large tanks (say 75m and above).  These are passive, listening devices operating in the 25 kHz to 70 kHz range. The aim is to detect acoustic emission signals from corrosion processes and scale fracture.

Very high sensitivity is needed to detect small signals from corrosion across a large diameter tank. This means that ‘quiet’ test conditions are vital. The tank must be blocked-in and left to settle after being filled to the test level. Any mixers, recirculation systems or heaters must be shut down for the test. Nearby construction, loading or maintenance activities may need to be halted for two to three hours.

Fig 1
Wind is the most common noise source, causing vibration, roof movement and impacts from particles and debris in extreme cases. Wind speed should be monitored continuously with an upper ring of sensors to help identify noise from the upper regions of the tank.

AE data is monitored and recorded for one to two hours in most cases.  More time is needed when testing is halted temporarily due to increased noise levels. In some cases, testing must be halted completely until the weather improves.

Data Evaluation Location plotting is then used to identify clusters of AE activity from the tank floor.

The general level of AE activity and any localised clusters are compared to the grading criteria.  These allow for such factors as the tank contents, any lining or coating, and the detection sensitivity. The outcome is then reported in terms of the AE grade (A-E).

An ‘A’ or ‘B’ grade tank would require no near term action whereas a ‘C’ grade would indicate a need for inspection or retesting in 3-5 years.  Tanks graded ‘D’ or ‘E’ have the highest probability of suffering significant corrosion damage and will normally be given the highest inspection priority.

• Pulsed Eddy Current

The RTD-INCOTEST is a Pulsed Eddy Current system for estimating material thickness through insulation and other protective coatings. The system was developed by RTD and is used by MBI under license. Low frequency (pulsed) eddy currents are generated in the surface of the material. Below the surface, weaker eddy currents occur, the decay of which is a function of the thickness of the material contained within the magnetic field. When the eddy currents reach the opposite face (back wall) their behaviour changes.

Fig2
By measuring the strength of the eddy currents and the time taken before the deviation occurs at the back wall, the thickness can be calculated. The calculation gives the average thickness of the material within the enclosed magnetic field. The initial benefit of the system was that thickness of material could be estimated without the need to remove insulation. The system is well suited to detect wall losses of the Shell or Roof of an Insulated Tank. The system has also been widely used to assess Sphere Legs without the need to remove any part of the fire proof supports.

Other applications include flow accelerated corrosion in pipe work, corrosion through coatings, concrete, marine growth and corrosion on heavily corroded areas without the need for preparation required by more conventional methods.

• Time of Flight Diffraction ( TOFD )

Fig3 Fundamental geometry, two probe approach. 
In the TOFD technique the transmitting and receiving probes are located equidistant over the
weld centre and scanned parallel with the welds, normally a single pass is sufficient to attain the required inspection coverage.  A transmitting probe emits a short burst of energy into a material and propagates an angular beam, if the sound is obstructed by a defect, some of the energy is diffracted at its edges and travels towards the receiving probe. The signals are recorded and graphically displayed in a grey scale form and analysed using software algorithms.

Fig 4 Weld root corrosion in a Storage Tank

The data is collected using a simple scanning frame or scanner with optical encoders for positional information. The data can be enhanced by special software routines and is
                                                                       

• Thermal Imaging

Fig 5 Product level in an Oil Storage Tank

Thermography or Thermal Imaging is a technique that is used to remotely gather thermal information from any object or area and convert it into a visual image. Thermal imaging systems can check these minute changes in temperature in an accurate, non-invasive manner to see if they fall within correct operating parameters. In the process industry the application of Thermal Imaging and analysis is limitless. It can be very effective for detecting the insulation efficiency in Vessels, Tanks and pipework whilst on-line and any repair can be highlighted for the next planned outage.
It could also be used to quickly detect the level of product in a un-insulated Tank (see below). Electrical surveys, fuses, junction boxes etc all increase in temperature when under excessive load or otherwise not operating efficiently. Product flow and blockages, Rotating and other mechanical equipment can be checked.

• Ultrasonic Robotic Scanners

Fig 6
For the majority of applications Ultrasonic scanning is performed using a multi –purpose magnetic wheeled scanner. The scanner head can be configured to suit the surface profile of the item under inspection. The equipment performs a “Rasta” scan across the test surface, employing compression probes which are focussed to suit the application. Coverage of up to 6m2 per hour is possible. The step, sweep and part length can be adjusted to achieve the inspection resolution required.

Shear wave transducers can be added to the probe set-up to collect additional information about the discontinuity orientation. For applications, which require a more job specific scanner the Company have access to additional systems.

The data collected is presented as a C scan image with accompanying B and D scan views. In addition to these colour maps, the statistical data accessible through the P scan software can be used to monitor growth of corrosion or erosion or hydrogen damage following repeat inspections.

Fig 6 & 7
P-Scan processed data image, different colours indict corrosion depth. The image to
the right is an actual cast of the defect shown.

• Tank Floor Tester

Fig 8TFT deployed in a bulk storage tank
The original tank floor tester was developed to carry out rapid inspection of bulk storage tank floors to detect corrosion pitting in ferro-magnetic steel plates. Handscan/pipescan was developed to inspect pipe, from 3” diameter and above, and plate materials using the same principles.

When an electromagnetic field is induced into a material the magnetic flux generated tends to remain within that material. However, if a defect is present in the area of the magnetic field a portion of the magnetic flux is forced into the air surrounding the defect, creating a flux leakage field. “Hall Effect” transducers can detect the presence of the flux leakage field.

Owners of bulk storage tanks, vessels and piping systems are faced with the prospect of examining large surface areas to detect the presence of localised underside or internal corrosion. Inspection of such large areas using conventional methods, such as ultrasonic testing, would be prohibitively expensive. The Tank Floor Tester and Handscan/Pipescan systems have been designed to provide a rapid scanning tool to inspect large areas in order to identify indications of material loss due to corrosion.






Fig 9 Pipescan deployed on connecting pipe

Once located the area can be closely scrutinised, using ultrasonic or other techniques, to accurately depth and size material loss. The equipment sensitivity is dependant on the thickness of the material, typically 30% wall loss in 6.4mm thick material, 40% wall loss in 12.5mm material and 60% wall loss in 19mm thick material can de identified.

• Long Range UT

Fig 10 Teletest applied to buried pipeline
Ultrasonic thickness checks for metal loss due to corrosion or erosion are highly localised, in that they only measure the thickness of the area under the transducer itself. To survey a large area requires many measurements and access too much of the surface of the component being examined. Where access is difficult or costly a detailed survey becomes unattractive economically with the result that often a limited sampling only is carried out. Similar restrictions also apply to other methods of measuring wall thickness such as radiography.

Fig 11 Schematic arrangement of the system


Long range ultrasonic methods overcome this restriction by using guided ultrasonic waves. These are of much lower frequency than that used for normal ultrasonic tests. They have the property that they can travel many metres with minimal attenuation and therefore offer the potential of testing large areas from a single test point.

                                                                                 Fig11







                




     

• Electro-Magnetic Acoustic Transducers ( EMAT )

Fig 12  
The system is designed for rapid assessment of corrosion in piping, including normally inaccessible areas under pipe supports.

Electromagnetic Acoustic Transducers induce ultrasonic waves in metals without the need for an acoustic coupling medium.

Fig 13

Although emat probes sit on the pipe surface, they are “dry coupled” and relatively unaffected by coatings and light surface rusting. The system operates by transmitting Lamb waves around the pipe circumference. The probes are held in a fixed orientation and carried along the pipe by a motor driven scanner with an encoder to track its position. Ultrasonic (thickness) data is recorded and displayed by a computer for evaluation on the spot and off-line review.

Access for Inspection

Fig 15 Rope access for UT inspection of insulated pipework
One other consideration for the NDT systems described previously the need for access to the Inspection location without requiring scaffold. This can be achieved by either using a magnetic wheeled crawler for transport or by utilising Rope Access techniques. The majority of the systems described previously can be applied without the expense of scaffolding.

The newer NDT techniques how confident are we in the equipment and the competency of the NDT Inspector to deliver it?

As technology moves ever more quickly forward there are NDT systems appearing all the time. Some have been covered in this article for the Maintenance Inspection of Process Plant but some have not. There are no third party qualifications for some of the methods described and you are very much reliant on the company supplying you with the service.

It is important to ensure the company carrying out the NDT has been accredited by UKAS to BS EN ISO/IEC 17020 “General criteria for the operation of various types of bodies performing inspection” and is able to provide technical procedures for all NDT methods employed.

In the UK some of the Specialist NDT methods are beginning to be covered and TWI offer a CSWIP qualification in Long Range UT, PCN offer Level 2 exams for TOFD & Phased Array. In addition the HSE have sponsored some recent work in looking at several NDT methods including recently PANI 3 which was looking at how we can get the best out of the Industry’s Ultrasonic operators and also the assessment of Tank Floor Testing with the results available on the HSE website found at www.hse.gov.uk/research/rrhtm/rr481.htm.

In conclusion to get the best chance of success for your maintenance inspection needs employ an accredited inspection company providing qualified inspectors working to approved procedures with a large tool box of calibrated NDT equipment both standard and specialist.

Author - Alan Hipkiss