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Porosity Holiday testing, you need to know this - Video

 

Porosity Holiday Testing, what you may not know, and you need to know

 

 

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

 

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

PCWI Porosity / Holiday Detectors. The instruments and support you need for your industry.

Porosity / Holiday testing information. The things you won't know, should know, then may be better off not knowing.

PCWI manufactures a vast range of brushware to suit holiday / porosity detectors including wet sponge.

PCWI manufactures a vast range of brushware to suit holiday / porosity detectors including wet sponge

 

Low voltage wet sponge detectors

Shall I explain this unit?

A low voltage wet sponge tester

This is a low voltage wet sponge test you would create the sort of flaw you want to find. A low voltage wet sponge test would only find flaw number one. That would also be dependent on the flaws diameter, coating thickness and the flaws capability of being wetted out.

A low voltage wet sponge tester will only find flaw number 1

If you're satisfied with that result then the low voltage wet sponge test may be suitable for your project. That is, providing you can get back to revisit a few years later to evaluate the coating system and / or to carry out repairs. This test does not guarantee a flaw free coating system. It depends on what a flaw is.

 

A low voltage wet sponge tester

 

I think maybe you should look at the small video we've done on low voltage wet sponge testing, its use and its history before I need go any further.

Low voltage wet sponge testing

 

High voltage detectors.

PCWI's high voltage detectors. DC and Pulsed DC

 

Pulsed DC, DC direct current constant current unit.

 

Laboratory controlled humidity test at 33 and 75%.

Length of spark discharge test at varying humidity’s.

Laboratory controlled humidity test

We've set up a small test jig enclosed in a chamber so we can control the humidity. We will have a look at the 10kv high voltage discharge.

Humidity test in lab

 

The air gap has to be started wide and then reduced until the spark breaks over. During these tests the ionised air needs to be purged as it's easier to pass a voltage through it.

That's why it's easier to find a flaw on the second pass. But that's not the only reason.

Pin hole before testing and after testing

Pin hole before and after testing

The pin hole after testing is charred and carbonised. When carbonised is more easily found on subsequent passes. 

Controlled laboratory spark gap test. 33 and 75% humidity

Controlled air flow / exchange air flow. Voltage 10kv DC. Discharge point sharp as in brass bristle. Earthing plate flat.

  • Spark gap jump 33% humidity 8.4 millimetres
  • Spark gap jump 75% humidity 9.3milimetres

So does this finding mean that you increase the test voltage at lower humidity’s by 10% or so?

Comparison spark gap lengths in air between brass wire bristle brush, coils, square section wire and carbonised rubber strip probes.

Coil, brush and carbon rubber test setup

 

This is the set up and it was carried out at 50% humidity. The applied voltage 10kv. Air gap reduced til arc over.

  • Brass wire bristle brush 8.8 millimetres 1.14 Kv per millimetre
  • Coils square section wire, 5.1 millimetres. 1.97 kilovolts per millimetre
  • Carbonised rubber strip 3.3 millimetres. 3.33kv per millimetre

The gap that is created by a longitudinal weld, this air gap needs to be bridged before you even get to the coating.

coil gap on longitudinal weld

 

Here you have a circumferential weld. You can see the air gap once again needs to be bridged.

coil on a circumferential weld

 

When testing a pipe's longitudinal circumferential weld and spiral welds, take into account that result. Anything that lifts the coil into the air by 2 millimetres would need an extra 4kv to test that area. Even worse when using a rubber probe.

A brass bristle brush lays in the gaps much, much better than a coil or a rubber strip probe.

Brass wire brush on a longitudinal weld

 

Tested a 4.3 millimetre thick PVC sheet with 1 millimetre holes drilled in it. A new hole drilled for every test increased the voltage with every pass until the floor was found.

  • Brass wire bristle brush at 45 degree angle 10kV
  • Coils square section wire 13.1kV
  • Carbonised rubber strip at 45 degree angle 13.6kV
 
Compare probe types Brass wire brush, coils & carbonised rubber
 

To find a hole in the test pieces consistently, that is every time you would need to add 10% additional voltage. Note that is a hole to the bare steel substrate with no dielectric strength on the coating.

You may notice that the voltage to find the flaw is a lot higher than the voltage required to bridge the gap in free air. The coil free air test was 1.97kv/mm multiplied by 4.3mm test material is 8.5kv to go through the hole. In the test material instead it took 13.1kv.

It took one and a half times more voltage to find the hole.

You may notice that the brass bristle brush found the hole at 10kv. That is twice, x2, as much as its free to air test. Although it found it at 3.1kv less than the coil and 3.6kv less than the rubber probe.

Tested 0.5 millimetre thick PVC sheet with 1 millimetre holes drilled in it. New hole drilled for every test. Increased the voltage with every pass until the flaw was found.

  • Brass wire bristle brush at 45 degree angle 1.25kV
  • Coils square section wire 2.5kV
  • Carbonised rubber strip at 45 degree angle 2.2kV

To find the hole in the test pieces consistently, that is every time, you would need to add 10% additional voltage.

Note: That is a hole to the bare substrate with no dielectric stress on the coating.

So do these tests indicate that you need to use a brass bristle brush on all the irregularities or just use brass brushware on all surfaces? Now you have seen why I say a brass bristle brush is the best probe to use. Everything else is just a short cut.

On the 500 micron coating found the hole at 1.2kv using the brass bristle brush. It took twice as much voltage 2.5kv to find the hole with a coil.

Tested the 500 micron film with 30kv and could not make a burn through. This leads me to the following. The formula of 125 volts per mil (thou) using a brass wire brush would only find a flaw through to the substrate. Or should I say maybe finding a flaw exposing the substrate. Changing to a coil or rubber probe you would not find it.

 

Coating dielectric strength test

Coating dielectric test

Pass the brush over the surface, keep increasing the voltage until it burns through.

Depending on the coating type and thickness you may find that you do not have enough voltage available to burn through.

You need to establish what the true breakdown voltage of the specified coating is to be comfortable with what test voltage that you are applying to the coating system.

 

Set test voltages

You need to apply enough test pressure on the coating in the way of a percentage of its total dielectric strength as a test that would find all flaws in the coating.

Coating flaws

A spark leaves a sharp edge better than off a flat surface. Look out for probes that lay flat parallel to the surface they need more volts to leave, like rubber probes they also have high resistance. Probes that have gaps in them, gaps increase the need for the spark to travel further.

 

Coil condition

Checking coil condition

Check all coils for correct length, misalignment, steps up at the ends, gaps, rust and coating contamination.

 

Brush condition

Brush condition

 

You need to keep them in better condition than what these here are.

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 very costly to repair. Leaking or ruptured tanks and pipelines can leave everlasting environmental disasters.

It's a different world we live in today. Maybe it's time to do a rethink on the level of testing that's needed for the future.

Test to ensure coating longevity

 

PCWI 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, adaptors to suit all brands. Contact sales@pcwi.com.au.

 

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

 

 

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