The Science Behind Eddy Current and Remote Field Testing

Posted: 2017-3-3

By: Christopher Van Name, Gary Fischer, James Kocher

Source: Conco Services Corporation

http://www.conc...nger-tubing.pdf

With the increasing demand on the world’s power grids, now more then ever it is important to keep power plant condensers, feedwater heaters and balance of plant heat exchangers running at peak efficiency. While it is well known that keeping these units clean is important for maximizing power output, so too is monitoring each unit’s tube integrity and taking corrective action to prevent tube failure. The best way to monitor a unit’s tube integrity, detect patterns of tube wear and damage, and determine the specific wear and damage to a particular tube is through Non-Destructive Testing. Depending on the tube material, the best Non- Destructive Testing method to employ would be either Eddy Current Testing, Remote Field Testing or other variations of these electromagnetic techniques.

This paper discusses the science behind Eddy Current and Remote Field Testing, how they differ and which one to select depending on the situation. It will look at the construction of the probes and how they work. It will explain the difference between use of a single frequency or multiple frequencies and the advantages of multi-frequency testing. The paper will also identify the necessary procedure for a successful Nondestructive Test, including the types of tubes that can be tested and tube preparation.

INTRODUCTION

The modern Eddy Current Testing industry owes its existence in a very real sense to Michael Faraday, (1791 to 1867). This brilliant scientist’s discovery of and experiments into electromagnetic induction laid the foundation for the many Electromagnetic Testing techniques in use today. Though there has been much advancement in test instrument technology, computer software and test coil design, the basis of the electromagnetic techniques still rely heavily on the experiments performed by Faraday in the mid 1800’s.

Both Eddy Current Testing (ECT) and Remote Field Testing (RFT), also known as Remote Field Eddy Current Testing, use the principles of electromagnetic induction to detect defects in condenser and heat exchanger tubes. In both ECT and RFT probes, an alternating current flows through a wire coil or coils, generating an alternating magnetic field around the probe. When the probe is inserted into a metal tube, a circular flow of electrons will begin to move through the metal, generating its own magnetic field.

This circular flow of electrons is the eddy current. As the probe moves through the tube, the magnetic field generated by the eddy current will interact with the coil’s magnetic field. Defects in the tube wall, such as pitting or cracking, and changes in wall thickness will interrupt or alter the amplitude and pattern of the eddy current, changing its magnetic field. This change in the magnetic field then affects the coil by varying its electrical impedance, which is monitored by the test instrument. By plotting the changes in the impedance amplitude and phase angle on a monitor, a trained operator can compare the pattern displayed on the monitor to patterns of known test samples to determine the condition of the tube being inspected.

PROBE CONSTRUCTION

All ECT and RFT Probes consist of a coil or coils wrapped around a structure to form the coil. It is the manner in which these elements are designed and their interaction with each other that will determine how the eddy currents are induced and how flaws are detected in the test material. Most coils are built on a non conductive body (air core) but many can be constructed using ferrite cores and conductive shielding to help shape the eddy current field for special applications.

THREE BASIC COIL TYPES

According to James Cox, author of "Nondestructive Testing, Eddy Current: Classroom Training Handbook", there are three basic coil types:

1. Probe coil – Also referred to as a pancake coil, it is designed to test the surface of materials and can be applied to plates, welds or even tubing when fixed to a special device that spins the coil ("spinning probe technique") inside a bolt hole or a tube. When a probe coil is fixed to this type of spinning device it is commonly referred to as a motorized rotating pancake coil (MRPC) in the heat exchanger industry. This probe type can provide some very detailed information but is time consuming and expensive to operate.

2. Bobbin Coil – This coil type allows for the inspection of installed heat exchanger tubing from the inside diameter (ID) surface. The bobbin coil interrogates the entire circumference of the tube as it is drawn through the tube. This type of coil is the most widely used and is considered the "workhorse" of the tube testing industry.

3. Encircling coil – Also known as Feedthrough Coils, this coil type allows the inspection of round objects such as tubes, wires and rods from the outside diameter (OD) surface. Much like the bobbin coil, the encircling coil also interrogates the entire circumference of the material as it is passed over the material or the material is fed through the coil. This type of coil is used mostly in production monitoring activities.

Read the full paper at ConcoSystems.com