INTRODUCTION
The purpose of this document is to identify the specifications required for eddy current inspection equipment to successfully inspect the broad range of tubes used in specific industries. This will be done by identifying the alloys and wall thicknesses of tubes common to specific industries and identifying the range of frequencies that might be used to inspect these tubes.
To accomplish this, it is only necessary to identify the lowest frequency and the highest frequency necessary; therefore, it will not be attempted in this document to identify every tube size and alloy in a specific industry, but rather just those requiring the highest and lowest inspection frequencies.
For a given tube alloy and wall thickness, the one skin depth frequency (1SDF) will be identified. (More information about the term one skin depth frequency is given in our Electrical Conductivity of Materials Manual, Report ect R8418-R1 and located on our web site at http://www.eddy-current.com/conduct.htm.) Tubes are typically inspected at this frequency as well as one half of this frequency and twice of this frequency. Using calculated minimum and maximum frequencies for most inspections in an industry will be safe in equipment selection; however, it may be wise to choose equipment that can go to one half of the calculated lowest frequency and to twice the calculated maximum frequency to guarantee good performance from the equipment at the frequencies used.
Tubes without fins, sometimes called prime surface tubes, are inspected with a differential bobbin coil. Frequencies of 1SDF and twice 1SDF are mixed to eliminate support plates. The mixer output is used to detect small volume defects, especially when they are close to support plates. An additional differential channel set at one half required frequency channels of 1SDF is used as a confirmation channel to distinguish between real pit type defects and other mundane anomalies such as roll stops and magnetic inclusions. A fourth channel is set to absolute mode for the detection of general thinning caused by erosion and corrosion. An eddy current instrument with four frequency channels is recommended.
MULTI-FREQUENCY GENERATION; SIMULTANEOUS INJECTION VERSUS TIME MULTIPLEXED
In a simultaneous injection multi-frequency eddy current instrument, four oscillators run simultaneously, generating the inspection frequencies for the probe. These are mixed together, amplified, and sent to the probe. The signal coming back from the probe is split into its individual frequency components before going through the detection system. This is similar to the concept of having multiple radio stations transmitting signals in the same area, but on different frequencies, and using receivers set to different frequencies to listen to the radio stations independently.
In a time multiplexed multi-frequency eddy current instrument, one oscillator generates several cycles of one frequency for output to the probe at a time, and then switches frequencies for a few cycles of the new frequency, and so on, for all four frequencies. One Manufacturer of time multiplexed multi-frequency eddy current instrument shows a diagram in which each frequency is sent out for about six cycles. If you were using inspection frequencies of 3 kHz, 6 kHz, and 12 kHz in differential mode plus an additional channel at 6 kHz in absolute mode, the time to generate these signals would be 2 milliseconds for the 3 kHz channel, 1 millisecond each for the two 6 kHz channels, plus half a millisecond for the 12 kHz channel, for a total combined time of 5.5 milliseconds. This would give a sample rate of just 200 Hz. This calculated 200 Hz sampling rate would result in inspecting at only one half a foot per second given that some Manufacturers of time multiplexed multi-frequency eddy current instruments have been know to recommend using a 400 Hz sample rate to inspect at 1 foot per second. We at Eddy Current Technology Incorporated have always believed in a sampling rate significantly higher, such as twice that rate.
Simultaneous injection multi-frequency eddy current instruments do not suffer from the sample rate problem of time multiplexed multi-frequency eddy current instruments; therefore, could inspect with inspection frequencies as low as 3 kHz with inspection speeds as high as 2 meters per second.
Simultaneous injection multi-frequency eddy current instruments are recommended for heat exchanger tube inspections.
Eddy Current Technology Incorporated built the world's first portable, multi-frequency eddy current instrument complete with a display and mixer in one package in 1980. This was a simultaneous injection multi-frequency eddy current instrument, and the first simultaneous multi-frequency eddy current instrument in which the inspection frequencies could be adjusted without physically changing the oscillator module. All multi-frequency eddy current instruments manufactured by Eddy Current Technology Incorporated have used simultaneous injection.
FOSSIL-FIRED ELECTRIC POWER GENERATION
In the past, most main steam condensers were made from aluminum brass, and to a lesser degree, 90/10 or 70/30 copper nickel. Outside diameters were typically 1 inch (smaller in older lower power units) with a .049 wall. One half of 1SDF for .049 aluminum brass is 6 kHz. This would be one of the lowest frequencies that would be used in an Electric Power Plant.
Increasingly, the condenser tubes are being made of titanium with a wall thickness of .020 to .028. Two times the 1SDF for .020 titanium would be 1.1 MHz. This is probably the highest frequency that would commonly be used in an Electric Power Plant.
Low pressure feed water heaters typically use aluminum brass or 90/10 copper nickel with wall thicknesses no thicker than in condensers; therefore, lower frequencies will not be required for low pressure feed water heaters.
High pressure feed water heaters are typically made out of a stainless steel or other high alloy steel. Although stainless steel has a higher electrical resistivity than titanium, the tube wall thickness is typically .049 or greater, resulting in using inspection frequencies below that used for thin wall titanium condenser tubes.
CHEMICAL PLANTS, INCLUDING PETROCHEMICAL AND FERTILIZER PLANTS
Chemical Plants tend to use carbon steels (ferrous), aluminum brass, high alloy steels (non-ferrous), and titanium as alloys for heat exchanger tubes.
As Chemical Plants have a lot of heat exchangers with carbon steel tubes, their needs will include a Near Field TM or remote field eddy current testing capability and referably both.
When aluminum brass is used in heat exchangers in Chemical Plants, their wall thicknesses tend to be significantly greater than wall thicknesses in Electric Power Plants due to the corrosive nature of the chemicals involved. A common wall thickness for an aluminum brass tube in a heat exchanger in a Chemical Plant would be 0.083 inches for which the 1SDF is 4 kHz and the one half 1SDF would be 2 kHz. This is the lowest inspection frequency generally required by Chemical Plants. Due to the sampling rate limitations at low frequencies like this with a time multiplexed multi-frequency eddy current instrument, this type of instrument would not be suitable for inspecting these tubes due to the probe speed limitation.
It is also true that stainless steel and high alloy steels found in heat exchangers in Chemical Plants tend to have thick walls. The thinnest wall found in a heat exchanger tube with a high alloy steel in a Chemical Plant would be 0.049 inches. For inconel, this requires a 1SDF of 150 kHz and the two times 1SDF would be 200 kHz. All multi-frequency eddy current instruments on the market today for heat exchanger tube inspection would meet this requirement of being able to inspect at 200 kHz.
AIR CONDITIONING HEAT EXCHANGERS
Most heat exchanger tubes found in air conditioning heat exchangers are a copper alloy tube with an integral fin. In most cases, the fins are skipped at support plate areas. These are referred to as skip finned tubes. These tubes are also occasionally made of 90/10 copper nickel, and in rare circumstances, titanium. Some heat exchangers in the air conditioning industry do not have fins and air conditioning Inspectors tend to refer to these as "prime surface tubes". The wall thickness in the skipped zone of a copper tube can be as thick as .049 inches, giving a 1SDF of 3 kHz and a one half 1SDF of 1.5 kHz. Due to the sampling rate limitations at low frequencies like this with a time multiplexed multi-frequency eddy current instrument, this type of instrument would not be suitable for inspecting these tubes due to the probe speed limitation.
The highest frequency requirements for the air conditioning industry would be in a finned titanium tube where the thickness under the fin could be as low as .032 inches, giving a 1SDF of 200 kHz and a two times 1SDF of 400 kHz. All multi-frequency eddy current instruments in production today intended for the heat exchanger tube inspection market meet this requirement.
In air conditioning heat exchangers with finned tubes, support plates are at a greater distance from the probe coils than in non-finned tubes. This, and also the fact that lower frequencies are used in the case of copper tubes, results in a relatively small support plate signal. As a result, these tubes are often inspected without using two frequencies in differential mode to mix out support plate signals.
Finned tubes in the air conditioning industry are often inspected with a combination cross axis differential probe which is sensitive to general thinning, eliminating the need for a separate absolute channel.
For the reasons stated in the above two paragraphs, air conditioning heat exchanger tube inspections is often carried out with a two frequency, two channel eddy current instrument, although an Inspection Company working in the air conditioning industry should have available four channel equipment to inspect those heat exchangers that have prime surface tubes.
