EMAT ultrasonic probes are set to dominate power station boiler wall thickness
surveys in the near future, due to their ability to produce accurate readings on
heavily scaled surfaces.

EMAT's produce ultrasound in a totally different way to conventional probes.   
The physics of how the probes work is rather complicated, but basically the
vibrating piezo electric crystal is replaced by an electromagnet, which
produces eddy currents, which in turn produce ultrasound in the material.    

Unlike conventional probes, EMAT's do not require a liquid couplant to transfer the ultrasound from the probe into the pipe material. Neither do they require a good contact surface.  In fact the probes will happily work at a small distance from the pipe surface. This allows the probe to function when the test surface is heavily corroded, or has a large oxide scale build up.
	EMAT probe - Click image to enlarge

A major cost benefit of this ability to work on poor surfaces is that expensive and
time consuming blast cleaning of the boiler wall is no longer necessary.

EMAT probes are also proving to be more accurate, as they are able to ignore
the scale thickness and calculate only the true metal thickness.

How can the ultrasonic wave get into the pipe without a couplant ?

Because the ultrasound wave is first created in the pipe, not the probe. Although
ultrasound cannot travel across air gaps, the electromagnetic waves and eddy
currents produced by the EMAT can.  

Why will EMAT probes become popular for boiler wall inspections ?

The major advantage of EMAT probes is that the tube surface now needs very
little preparation before testing.   With standard probes the surface often
requires shot blasting.   This is an expensive and time consuming procedure
and power companies will not be long in recognising the cost saving.   

Are EMAT's expensive ?

Costing under £300 the probes are extremely good value for money, offering
a longer working life than standards probes and of course, no expensive
couplant to buy, or probe reshoes.

Can I use the probes on any material ?

Unfortunately one of the major drawbacks with EMAT's is that physics of how
they produce sound waves, means that they will only work on carbon steel
that has been subjected to a temperature of around 350 to 400 deg C for a
considerable amount of time.   Specifically boiler tubes where the high
temperatures produce a coating of magnetic oxide on the tube surface.

Are they easy to use ?

Very, in fact we found them much easier to use than a conventional probes.  
Using an EMAT is rather like using a permanent magnet.   When the probe is
placed on the surface the magnetic force can be felt, holding the probe to the
pipe.

Rather than taking a series of spot checks, with an EMAT it is better to scan
the pipe in one go.   Used with a modern thickness gauge or UT set the EMAT
will automatically record the minimum thickness found during the scan.    

Why are they more accurate.

This is mainly down to the physics, which as previously stated are very
complicated to understand.   But, also providing this increased accuracy,
is the fact that EMAT's once calibrated, stay calibrated.   As the probe
face does not need to come into contact with the surface, it does not wear
(an adjustable wear ring keeps the probe clear of the surface).  Hence no
need to keep recalibrating for zero offset error.    Additional factors are no
errors due to varying contract pressure and couplant thickness.

Can you explain more about the physics of EMAT's

Basically there are two types of EMAT probes, one that works using the
Lorentz Force and the Magnetostrictive type.   One interesting sidenote is
that the probes produce shear waves, rather than compression.

 

Advertising Feature
EMAT Ultrasonic Transducers now available from GE Panametrics    Click here for more information

Reader Comments:

"The only problem I encountered was the large dead zone at the front end
meaning you had to read small thicknesses on the repeat echo. We found
that delaying off the timebase so it read 2 - 12 mm helped a lot."

John Constable

Want to know more about EMAT's ?    

Do you have a question or opinion about EMAT's ?

Have you used them before ?

Why not send in your views and questions by email ?  

Send your questions or comments to support@ndtcabin.com

EMAT image courtesey of GE Panametrics