Knowledge Base

Holiday testing internal pipeline coating


High Voltage holiday testing of internal pipeline coating, types of brushes to use, inspection of pipeline coatings.



High voltage testing of internal pipeline coatings

Internal pipe diameters do vary depending on the pipes wall thickness, internal coating thickness may also play a part in the overall internal diameter. Brush design does not allow for a lot of give in the brush diameters. Variations in the internal diameter can play a large part so knowing the exact internal diameter is important to the size and the design.

PCWI manufactures porosity holiday detectors and a vast range of brushware to suit all testing applications.

HV detectors; DC and Pulsed DC

High voltage detectors pulse DC and DC direct current constant voltage.


Internal cone cup brushes

Internal cone cup brushes

Cone cup brushes these are used mainly on small bore tubes up to a hundred millimetres.


Small disc brushes

Disc brushes these are used on medium sized pipe up to three hundred millimetres.


Open disc brushes

Open disc brushes are used on larger bore tubes.


Large open internal disc brushes

Extremely large pipes use open brushes for the larger diameter pipe. These can be manufactured as split full circle brushes or we can manufacture internal half brushes.


Internal half brushes


Spiral round steel pipe
To test the coatings both internal and external this must be done with brass wire brushes. Coils and carbonised rubber neoprene probes will not work.

Note the air gaps when using coils and carbonised strips

Note the air gaps coils and carbonised strip probes should not be used.


Brass wire brush over weld

Brass wire probes fall into the gaps and voids so eliminating areas that would otherwise be skipped over and not get tested. The test capabilities of brass wire is far greater than that of coils and carbonised strips as those probes need more voltage to find the same floors. They also need more volts to bridge the air that’s created as they ride over the higher spots. Brass wire falls in to and around the welds much better than any other probe.


Earthing for HV DC testing

Before you start the test make sure you have the detector and pipes earthing correct. Also test the detectors voltage output at the brush with a crest meter. Earthing is critical so make sure that not only the detector is earth but also the pipe sub-straight earth the ground.


"Ovality" of some pipes

Large diameter pipe ovality
This does happen, long straight runs and bends. Pipes can vary in ovality. Brushware needs to be capable of varying with it.


High voltage leads for porosity detectors

High voltage leads
Carbon cored resistance leads, 7.5 mm to 8.5mm in diameter and up to 35 kV mainly for DC porosity detectors. Copper cored is used with pulse DC detectors. You can get a much reduced in size 5mm lead which can be used up to eight KV.

HV copper cored extension lead

High voltage extension leads need to be copper cored. These need to be attached to the DC and pulse probe handles. With DC units copper cord leads cannot be attached directly to the unit doing away with the probe handle. DC units need some resistance in the lead or damage to the unit will occur. To do away with the probe handle a special lead would need to be manufactured so please contact us.


High voltage lead connectors

High voltage lead connectors
As you can see here the handle end which is plastic and the brush end which is aluminium.

The high voltage copper cored extension lead needs to be longer than the pipe being tested. Generally for a six meter pipe you would need eight and a half meters.


Brushware travel
Push through with a rod. Pull through with a rope with the high voltage lead trailing pull through with the high voltage lead dependant on the brushes drag as you may pull out the lead from its connecting plug.

Pushing or pulling brushes through pipes

Single line pull of small internal disc brushes


The pull through on the rope with the disc brushes generally pull through from three points. This will keep the brush perpendicular to the pipe. Note the disc will need the extra support should you be pulling it through with the high voltage line. The extra disc four hundred millimetres in front of the brush gives the extra support to keep the brush upright.

Pulling cone cup brushes through a pipe

Cone brushes are a new style and these can be the push through or pull through with a rope or pull through with a high voltage lead.

They’re designed so as not to do damage the thinner coatings in smaller tubes and also easily lined up to pull through but they are non-returnable as with most brushes even the discs.


Using a flat brush in a rotating pipe

You can also use a flat brush in a rotating pipe. The draw-out speed needs to be calculated. Pipe diameter and rotation speed must be calculated not to exceed one meter in four seconds.

Cone cup brushes can only travel in one direction where no reverse available.

Disc brushes, a hundred and fifty millimetre disc brush is approximately a hundred and sixty five millimetres in diameter.


Brush flare on brass bristle brushes used in pipes

This flares the bristles out so pulling it back is very difficult.

A quarter turn while pulling may also allow it to reverse, although don’t rely on it. Once it’s pushed in push it all the way or pull it all the way through.


Getting a line through a pipe

So how do you get a line through a pipe?

Well you can throw a golf ball throw with a pull throw rope attached or you can use a garbage type bag or a shopping bag with a line attached to it and blow it through with the air.


Disc brush losing contact with a pipe

Be careful of disc brushes in pipe internals the weight of the push rod may cause the brush to sag, crush, causing gaps at the top. These areas would not be getting test voltages so maybe you need a brush with support wheels to hold it central in the pipe.

With all the environmental issues to contend with today. No one needs premature coating failure. Corroded or pitted steel that can be environmentally difficult and costly to repair. Leaking or ruptured tanks and pipelines can leave everlasting environmental disasters.

It’s a difficult world we live in today, maybe it’s time to do a rethink on the level of testing that you're doing and consider what's needed for the future.

Porosity Holiday Detector probes flat brushes, rolling pipe brushes internal and external pipe brushware special requirements made to order. Coils high voltage leads, earth leads, probe extensions and adaptors to suit all brands contact


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



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