Visual inspection vs mrt inspection
(Reading time 3 minutes)
In this article we will analyse two different inspection methods and their validity conditions:
- Visual Inspection
- MRT Inspection
HOW CAN A COMPETENT PERSON DECIDE WHICH IS THE BEST ASSESSMENT METHOD FOR EACH DEFECT?
The visual inspection can be defined as the first assessment method for a correct metallic wire rope evaluation.
The method consists in a deep look at the rope to search for deterioration modes and in reporting the results on a document.
In all the updated versions of ISO4309, the visual inspection has always been considered the main or the only inspection tool, even if in the last releases the MRT is included.
It must not be confused with a simple search for macro-defects, it is a very delicate operation and it should be performed by a competent person. A good technician must recognise different defect typologies such as broken wires, valley breaks, waviness etc. This kind of inspection can require hours, depending on the rope and inspection conditions.
The standard ISO 4309 states the assessment method for any type of defect that can occur on a rope, Table 1 lists the more common modes of deterioration and whether each can be readily quantified or needs to be subjectively assessed by the competent person.
HOW CAN A COMPETENT PERSON FIND THE DEFECTS ON THE ROPE?
Some defects are usually visible and easily recognisable with a visual inspection, for the hidden ones an MRT would be necessary.
1. Protrusion
The picture shows a Core protrusion, it can be caused by shock loading as well as improper installation.
Rotation-Resistant wire ropes must be installed on hoists with extra care and proper handling to prevent rope damage during installation.
By introducing twist or torque into the rope, core slippage may occur and it might protrude from the rope. This protrusion is an example of macro defect and it is enough to discard the rope.
2. External fatigue broken wires
Picture.2 - Fatigue broken wires
In this picture we can see an exterior damage of wires, whose square ends suggest it's due to fatigue.
Hoists wire ropes are subject to a lot of repetitive bending over sheaves, which causes the rope to develop cracks in its outer wires. The smaller the sheave (comparing to the wire rope diameter), the higher the bending fatigue.
Once the first broken wires appear, they create a domino effect and much more will appear shortly.
A deep visual analysis could detect these flaws, but grease, lack of light and inaccessibility often impede the inspection; in these cases MRT can easily identify them. In fact, although defects could either be visible or not, they definitely are there.
3. Valley breaks
Picture 3. - Valley breaks
A valley break is a special and quite difficult flaw to identify, because the breakpoint is usually located in the valley region between the strands. It is usually caused by the inter-strand nicking deriving from the forces between strands while the rope is under bending load, during spooling and when running over sheaves.
This phenomenon is very dangerous because the presence of valley breaks usually implies an internal deterioration.
Flexing the rope exposes hidden broken wires and makes you hear cracking sound from inside the wire rope. Of course bending the rope is not always possible, in this case a Magnetic Rope Test might be very useful to detect them.
Picture 4. - Bending a rope exposes hidden valley breaks
Be careful when inspecting a wire rope as these are probably the hardest type of wire break to spot!
4. Internal defects
In some types of wire ropes most of the defects are internal. In low-rotational ropes this happens very often and it is also described in the standard ISO4309.
The picture shows a low-rotating wire rope before and after the removal of the external layer of strands.
Picture 5. – Internal defects in a non-rotational wire rope
On the left side, the external layer has no defects, whilst on the right side the internal layer is full of broken wires. In this case visual inspection is not the correct method to evaluate the rope condition, an MRT is needed.
5. Internal corrosion
Internal corrosion is not so easy to find, it can be visually detected when a red powder is coming from the inter-strand zones of the rope. Unfortunately, this is not enough to evaluate the rope conditions because rust can also come from other parts of the plants, like sheaves and drums.
Watch out! This kind of defect can be critical in real applications, in case of suspected internal corrosion is better to perform an MRT.
Picture 6. – Internal corrosion
WHEN IS A MRT SUGGESTED?
The Magnetic Rope Test is suggested every time the Visual Inspection is not enough for a correct wire rope evaluation.
This picture shows a diagram corresponding to a magnetic rope test in a marine environment. The whole rope was covered with lubricant and it was very difficult to perform an in-depth visual inspection.
The visual inspector did not find any problems and the rope has been declared in good conditions but, when an MRT has been performed, it highlighted a different condition.
Picture 7. - MRT - Internal corrosion
The diagram reveals a very dangerous situation, there are some zones with a high peak concentration and this means that the rope is deeply corroded.
The rope has been immediately removed and a breaking test has been performed. The result shows that the residual load capacity was less than 1/3 of the nominal one (125kN vs. 400kN).
Picture 8. - Breaking load test
Picture 9. - MRT external broken wires
In this other picture the MRT diagram of a crane rope is shown. This plant is working 24/7 and the rope gets damaged very quickly; for this reason the device is permanently installed in order to ensure a continuous monitoring.
In this case it’s easy to find broken wires on the rope surface but it requires long plant stops. The MRT is useful to detect every defect on the rope, to monitor the situation and to schedule the plant stop and the rope replacement.
CONCLUSIONS
It is important to focus on the meaning of “visible”, which strongly depends on the inspection conditions, grease, speed, operator’s attention.
For external defect an MRT is not mandatory, but it is the only tool that can provide a quick and accurate assessment of whole rope conditions.
For internal defects, the situation changes. MRT is the only method reported in regulations (EN12927, ISO4309).
When it is not possible to perform a Magnetic Rope Test, the inspector must evaluate the rope conditions just basing his assessment on the visual inspection and his own experience.
To have a clear vision of all the different modes of deterioration and the relative assessment methods we suggest you to refer to ISO4309.
BIBLIOGRAPHY
[1] Technical standard ISO 4309:2017 / UNI ISO 4309:2019
[2] Look Inside the Rope: published by LEEA (Lifting Equipment Engineers Association)
https://leeaint.com/au/News-Look-inside-the-rope
[3] Wire Rope Forensics:
http://www.ropetechnology.com/bro_engl/wire_rope_forensics_a4.pdf
[“internal wire breaks”, pag. 27]
[4] V. Cacciatore, A. Canova, A. Vallan and B. Vusini, “Experience and technologies in NDT of ropes”. KEY ENGINEERING MATERIALS, (2007), vol. 347, pp. 627-632.
[5] A. Canova, B. Vusini, “Magnetic analysis of non-destructive testing detectors for ferromagnetic ropes”, COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, Vol. 27 No. 4, 2008, pp. 869-878.
[6] A. Canova, F. Degasperi, F. Ficili, M. Forzan, B. Vusini “Experimental and numerical characterisation of ferromagnetic ropes and non-destructive testing device”, Proc. of OIPEEC Conferebce 18th – 20th March 2009, Stuttgart, Germany, pp. 289-298.
[7] A. Canova, P. Frighi, P. Boschiazzo, B. Vusini, F. Ficili, “Controllo magneto-indutivo delle funi metalliche per ascensori”, Elevatori, marzo-aprile 2009, pp. 66-74.
[8] H.R. Weischedel, (1999), “Electromagnetic Wire Rope Inspection: Signal Generation, Filtering, and Computer-Aided Rope Evaluation”, The Nondestructive Testing of Rope. Krakow, Poland: (O.I.P.E.E.C.) International Organization for the Study of the Endurance of Wire Rope.
[9] Rope IQ – Manage wire rope with intelligence...
