The determination of the wall thickness of the tubes and
fittings in a heater is an essential part of inspection. Thinning deterioration
mechanisms can be identified and monitored through wall thickness measurements.
The two basic approaches used to determine the wall thickness of piping and
tubes are the following:
a. Nondestructive methods. These include
the following:
1. Measurement by means of ultrasonic, laser or
electromagnetic instruments.
2. Measurement of inside and outside diameter.
3. Measurement by means of radiation-type
instruments or radiography.
b. Destructive methods. One destructive
method is removal of a tube or tube section deep in convection banks and
inaccessible for direct measurement of the tube wall. However, with the
availability of internal ultrasonic based intelligent pigs to measure wall
thickness, the need for destructive examination is lessened.
Corrosion monitoring locations (CMLs) should
be selected to provide a means to determine the deterioration extent and to
determine the deterioration rate. This usually involves, as a minimum, placing CMLs
on all tube passes throughout the firebox. Particular attention should be made
to tubes where phase changes occur and where the highest tube metal
temperatures are expected. In addition, CMLs should be located on return bends
to assess their deterioration. Note that the required thickness for a fitting
may be different than that for the tube. For example, the inside radius of a
short-radius return bend will have a higher required thickness. Typically, the
number of tube thickness points is determined by criteria like a minimum of
three points per tube or readings every 5 ft – 6 ft (1.5 m – 1.8 m). Often
clean, corrosion-free services require fewer measurements, while high-corrosion
services require more measurement points. Although spot thickness readings can
identify general thinning, obtaining thickness in a circumferential band will
better identify any localized conditions like corrosion grooving. Thickness
measurements should be documented and monitored where bulging, sagging and
bowing is observed.
Thickness measurements should be recorded and
compared to historical readings in the same locations. These wall thicknesses
provide a record of the amount of thickness lost, the rate of loss, the
remaining corrosion allowance, the adequacy of the remaining thickness for the
operating conditions, and the expected rate of loss during the next operating
period.
Measuring and recording the thickness of tubes
and fittings when they are newly installed is considered important. If this is
not done, the first inspection period may not accurately reflect actual
corrosion rates. If the installed thickness of the tubes is not available at
the time of the first inspection, corrosion loss is usually determined on the
assumption that the wall thickness of the new tubes was exactly as specified on
the purchase order. This is not always true, and hence an error in the
calculation of corrosion rate may result.
The ultrasonic method for obtaining tube-wall
thickness is the most commonly used method. For most corrosion inspections,
straight-beam ultrasonic techniques are used. The sound is introduced
perpendicular to the entrance surface and reflects from the back surface, which
is usually more or less parallel to the entrance surface. Proper cleaning of
the external oxidation or compensating for the thickness of the oxide layer is
essential to properly assess metal loss rates. In many cases, cleaning the
oxide will be the only viable way to acquire ultrasonic thickness measurements from
the tube’s exterior surface. Application
of internal ultrasonic based intelligent pigs do not require removal of
external oxide layers when measuring base metal wall thickness.
Other methods to determine thinning of tubes
that are less accurate than spot ultrasonic or radiographic inspection include
local area scanning with electromagnetic acoustic transducer devices (EMAT) and
global-tube length inspection using guided ultrasonic wave devices. In the
first of these methods, the transducer compares a sample area of known
thickness with the same material properties as the tube being examined and then
either a hand-held or an automated crawler head is used to scan the tube areas
from the OD. If a thin area is detected, follow-up inspection using spot
ultrasonic or radiographic inspection is necessary. In the second method known
as guided wave, an acoustic wave is introduced into the pipe that travels
either axially or longitudinally along the tube. Defective areas as well as
welds send back signals to a receiver that are analyzed to determine if flaws
exist and at what length along the tube based on time and velocity, then
follow-up inspection using spot ultrasonic or radiographic methods is necessary to confirm whether or not flaws truly exist
at the identified locations. Although guided wave does not give thickness
measurements of flaws detected, it is valuable in evaluating lengths of tubes
where spot examination for localized corrosion would be prohibitive based on
the amount of measurements that would be required.
At plugged headers where access to the tube ID
is available, many types of calipers are available for measuring the inside diameters
of heater tubes, including the simple 36-in. (91.4 cm) mechanical scissors and
the 2-point pistol type, the cone or piston type, and the 4-12-point electric
type. A caliper equipped to measure several diameters around the circumference
of a tube is more likely than others to find the actual maximum inside
diameter. Ultrasonic (UT) based intelligent pigging which operates with the
ultrasonic transducers off of the surface (immersion method) can also be
utilized to acquire inside diameter of heater tubes. A large amount of data is acquired over the
full coil length with this method, enabling the entire coil to be modeled in a
3-dimensional color format, illustrating any damage patterns which may be
present.
It is general practice to caliper the inside
diameter of a tube at two locations: in the roll and in back of the roll. Since
an increase in internal diameter may not be uniform throughout the length of
the tube because of erosion, erratic corrosion, bulging, or mechanical damage
while cleaning, it is advisable to take several measurements to determine the
worst section of each tube. On heaters where the pattern of corrosion is
uniform and well established and mechanical damage is known not to exist,
measurements for approximately 36 in. (91.4 cm) into the tube may suffice. The
roll section of a tube in service should be calipered to locate the maximum
inside diameter at any point between the back edge of the tube flare, or the
end of the tube if there is no tube flare, and the rear face of the fitting or
edge of shoulder left in the tube by the rolling tool.
There are also laser-based profilometry
systems, which can provide high accuracy measurement along considerable lengths
of the tubes. These devices offer high accuracy but require clean and dry
conditions to provide consistent results. These systems are described in 9.6.
Thinning at the ends of rolled-in tubes is usually caused by erosion or
turbulence that results from change in the flow direction. This type of
thinning may also result from frequent rerolling of tubes to stop leakage and
will have the same general appearance as a tube with a slight bulge. The loss
of wall thickness is not uniform around the circumference. In this type of
deterioration, the most thinning usually occurs on the fireside of the tube.
This type of corrosion is generally accelerated on the fireside because of the
high metal temperature there. Eccentric corrosion may also be caused by
external scaling. It is often difficult to determine whether tubes have become
eccentric as a result of service, because the condition is not readily
detectable by visual inspection of the tube ends. An indication of eccentric
corrosion can sometimes be found by measuring several diameters at one
location. A reliable means of detection is to measure thickness with ultrasonic-,
laser-, or radiographic-type instruments, but these tools can only be used on
accessible tubes, usually the radiant tubes. Ultrasonic (UT) based intelligent
pigging can be applied to quickly detect and quantify eccentric corrosion
damage throughout the convection, cross-over and radiant tubes. Although this type of corrosion is more common
on radiant tubes, it has occurred on convection tubes, usually on those
adjacent to the refractory.
Each of the methods to determine wall
thickness—measuring the inside and outside diameters of tubes, measuring by
means of ultrasonic, laser or electromagnetic instruments, and measuring by
means of radiation-type instruments or radiography—can be used to check the
thickness of heater tubes.
Electromagnetic techniques cover a broad range
of applications including eddy currents, remote field eddy currents and
magnetic flux leakage. Each has its own benefits and limitations. Remote field
eddy current is commonly used on ferromagnetic tubes. It has benefits in that
it can measure wall thickness to within 5% of the nominal wall and will provide
indications of other defect mechanisms such as cracking. Most of these
techniques are applied using an electromagnetic sensor device that is drawn
through the ID of the tube which may require cutting the tube U-bends at the
ends to gain access.
No comments:
Post a Comment