Knowledge Base

Understanding brushes and coils for high voltage porosity detectors - Video


This video provides you with some detail on the types of brushes and coils available, the pros and cons of the different types and how to use them.



Visit our Porosity product page for more information about our porosity detectors, brushware and accessories


Here's the video transcript for those who prefer to read

The instruments and support you need for your industry. Porosity Holiday Detector understanding, brushes and coils.


PCWI manufactures a vast range of brushware to suit these Holiday Detectors including coil.


This is the rolling pipeline brushes easy to open clip on the pipe and move along.


This is the range of external half brushes.


The coils are made to length suitable to most pipeline sizes. As you can see here pipeline coils, we make special lengths for certain different size pipes. We do not like to see them stretched on to far so specials are made to order.


We need to understand one thing. Once a pin hole has been tested and found it chars and carbonises. That means from the substrate you get a black char which goes to the top of the coating so it is very easy to find the second time and on subsequent passes. It is the initial flaw that is hard to find. Well, it depends on the voltage setting. If you have got low voltage settings it is going to be harder to find, but very easy to find the second time around. For the coils we have are made of square section wire and definitely not round wire.

You can already see here a coil that is stretched on a pipe can open up a lot and that can be very detrimental to the testing. If the pipeline coil is not tight enough, not enough stretch, you are going to have a sag between the bottom and the gap. It means this area is not going to be tested.

You can see here the coil has been stretched too far. It is a little bit like putting a 12 inch coil on a 15 inch pipe. This is the end result. You can see here there’s no gaps at the interface. This is the best way to have it but because the coil itself has weight that means it is going to have to support its own weight hence you are most likely going to have small gaps there which is not something that you really want.

This shows one against the other. You can see on one side you have no gaps and the other side it has been stretched and there are larger gaps. This is something that you really don’t want. As you can imagine you have a weld - although, this is a simulated situation just to show.

You can see the gap either side of the weld there. That area won’t be tested for the simple reason that the gap is too large. If you have higher enough voltage setting you are going to get some testing there but otherwise that area will remain untested. Even with the pipe fitting correctly you are going to have a problem in that area. The same as any lump or bump that is on the coating exterior where the coil rises up sharply. You can have circumferential welds, longitudinal welds, this is the situation that you are going to have when using coils.


You can see here the brushware falls into place much better on longitudinal welds. The contact area is very small on a coil. That is only there momentarily. It may only be 2mm and as it rolls along it actually increases over a bump so you want to be very careful as to how fast you move and how much gap you have got by pulling the coil up too tight.


The brush, as you can see the contact area is much, much larger. It is there for longer so you have more chance at finding a flaw with a brush then you do with a coil. The circumferential weld as you can see, as you rise up over it and whether it is a join the coating or an overlap, it actually lifts it up so you wind up with this gap that just does not get tested. Here once again showing a brush as it falls in, you know thousands of small needle like wires that are actually falling into the gap so it is getting tested multiple times. As the same on the longitudinal weld.

Now here we have some concept drawings. This is how I would like to see a coil used during testing. We move at the rate of 1 meter every 4 seconds. Using coils that have gaps and don’t sit all that well on the surface at times. The idea is that you push so far forward, you pull back and then you push further forward again and you continue doing this along the way with the possibility of even changing the position of the coil around the circumference of the pipe so as to make sure you fully test it. Once again showing the longitudinal weld and the circumferential weld you can see the bristles fall into place. It is much better testing.

This here is a small test that I have done many, many times. This has taken 12kv to bridge this gap. As you can see the wire is quite thin, it is about 150 micron. The spark leaves the end of the wire really easy rather than when you look at the spark coming off the coil which has taken 17kv to bridge the gap. You can see that it has taken a lot more voltage to leave the coil to go to the substrate. Considerably more, it is just worth keeping in mind.


Testing is always better with brushware and should be the default system.

This is just some of the brushware that we manufacture, specials etc.

Now as far as voltages go, may be you need to prove that you can find the sought of flaws that you are looking for. If you drill a hole for instance, and pass the coil over it or the brush over it and you continually increase the voltage until you find it, then you can also reduce the coating thickness and keep passing the probe over it until you actually burn through. I mean it depends really what you are looking for as to what you consider a flaw. I mean you can mechanically damage it and then run the coil over it and keep increasing the voltage until you ultimately find as a flaw. It is just something I think you should be looking at.

With the environmental issues today, no one really needs coating failure as you need to repair. Corroded or pitted steel or leaking ruptured pipelines are a disaster. It is a different world today than what it was a few years ago. May be it is time to do a little rethink on this porosity detector.

PCWI Porosity/Holiday Detector Probes will manufacture anything you require in this area. Earth leads, extensions, adaptors etc. Contact


Visit our Porosity product page for more information about our porosity detectors and accessories



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