Pulp & Paper
Chlorinator Towers
Chlorinator towers are very often constructed from FRP
materials making inspection very difficult. The tower is tested by emptying
the tower for a period of 12 hours prior to the test. The sensors are then
mounted on the outside of the tower and it is monitored while being filled
up with a suitable liquid, in stages from 0 to 50 and up to the 100% of
capacity.
This method is able to not only determine the location of the problem but
able to identify the type of activity detected i.e. Debonding, fiber
breaking, matrix cracking etc.
Batch Digesters
Most batch digesters are constructed with a
carbon steel base and a stainless steel internal layer. Several digesters
have been reported to have inter-layer cracking that could propagate into
the parent material. Conventional NDT requires the costly removal of all
the insulation. However, AET only requires that a 4 inch diameter hole be
cut in the insulation, allowing the sensors to be mounted on the vessel.
The vessel is then subjected to a pressure increase over the normal working
pressure.
Recovery Boiler
Floors
The material of construction varies from a plain carbon steel, with or
without studs, to a composite stainless steel layer on a carbon steel
substrate. In the case of plain carbon steel construction, the main problem
appears to be water-side cracking. A recent research project was completed
to determine AET’s ability to detect and asses the critical size of water
side cracking. UT and destructive tests where used to confirm the AET
results, with a very good correlation.
A similar study was conducted on composite tubes. AET signature recognition
software was able to determine whether the activity was from cracking in the
carbon steel substrate or in the stainless steel layer, the foremost being
of most importance.
The technique used in both cases, is to mount the AE sensors on the
underside of the boiler floor, just prior to a hydro test. Access is gained
via the vestibule and the boiler floor. Other areas of the boiler can be
tested at the same time with the use of additional sensors.
Suction Rolls
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The early detection of internal cracking in suction rolls
has long been of concern in the paper industry. Presently, engineers
had to use very inefficient NDT methods such as PT. This requires the
roll to be removed from service, stripped of its inside equipment and
thoroughly cleaned and degreased before applying the penetrant. Using
the AET method, the roll can either be tested in the machine within a
two hour window, or tested outside the machine without any impact to
production. |
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If the test is conducted in the machine, the felt tension is increased by
10% above the normal working tension. If the test is conducted outside of
the machine, then a weight is applied to the center of the roll using a
crane or a fork lift.
A special technical committee to establish a standard procedure for this
test has been formed by TAPPI , membership includes Stan Botten of Quality
Services Laboratories, Inc.
Drying Cans
The use of acoustic emission to test drying cans has been in use for several
years. Each drying can is considered as a pressure vessel and is subject to
the normal pressure vessel codes and standards. One of the accepted methods
of testing a drying can is to pressurize the can using the steam pressure
and increasing the pressure by 10% above the maximum pressure it has seen in
the last six months. Recent studies have indicated that an alternative
technique is to use the cooling stresses as a means of detecting crack like
defects. This technique has been very success in testing high temperature
piping in power plant application.
Reel Spools
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The failure of a reel spool during the
winding process can cause serious delays and possible damage to the
paper machine. The only way to inspect the shafts on the reel spool to
date has been to remove the reel from service, strip the bearings and
housing, and then conduct MT and/or UT tests on the exposed area. The
use of the AET method has now been applied successfully to over 200 reel
spools, with all suspect reel spools being confirmed by either visual or
MT methods. |
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The test is conducted at a suitable
location, usually at the end of the reeling machine where an area is
selected so that anchor bolts can be fixed into the floor, allowing the
placement of the filled reel spool on a pedestal. Then a load is applied on
the center of the reel spool. The testing time is usually 45 minutes per
reel spool. Depending on the rate that the spools are supplied.
Oxygen Reactors
AET is being successfully used to monitor,
on a regular basis, any growth or deterioration in the structural integrity
of the vessel. Sensors are mounted on the reactor monitored during an
increase in operating pressure from 80% to 110% for an on line test. Or
offline during a hydro test with the pressure lowered to 50% and then
increased in stages to 110% of the highest pressure recorded in the last six
months of operations. This test can be completed in two days depending on
the accessibility to the sensor locations. Laboratory tests where conducted
to determine the type of activity that can be expected from the materials of
construction and the presence of SCC activity. A paper describing this work
was authored by Dave Bennett and published in the TAPPI journal. Copies are
available on request.
Contact Information
For further information about the applications described in
this document, or any other application you may require, contact:
Quality Services Laboratories, Inc.
by telephone
at 713-473-6111 or
Fax 713-473-6161
or
by E-mail at
sbotten@mistrasgroup.com |
