17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China

Acoustic Emission In-service Detection of Cryogenic Storage Tank Floors

Yanting XU
1
, Shoubao DING
1
, Fujun LIU
1
, Yadong WANG
1
,


Shaoping DONG
2
, Qingyang WU
2
and Jiele XU
1



1
Zhejiang Provincial Special Equipment Inspection Institution, Hangzhou, Zhejiang, China,
Phone: +86 571 86026412, Fax: +86 571 86026412; e-mail: yanting_xu@126.com
2
Sinopec Zhenhai Refining & Chemical Co. Ltd.Ningbo, Zhejiang, China


Abstract

This paper explores AE in-service testing of cryogenic storage tank floors of monolayer
or bi-layer (dual tanks) by means of recommended methods of sensor coupling and mounting.
For AE inspection of monolayer tanks, we tried to use water instead of ordinary couplant
such as vacuum grease, which is unsuitable to couple sensors because of its high sound
attenuation in cryogenic temperatures, and we got ideal results. As for cryogenic bi-layer
tanks, it is hard to fix sensors directly to the shell of inner tank to collect signals from the
floor, we found that sensors can be placed on protruded ends of braces connected to inner
tank shell. An example of AE in-service testing of a cryogenic bi-layer tank floors is given
and significative experience is achieved.

Keywords: AE in-service testing, cryogenic storage tank floors, coupling, bi-layer tanks

0 Introduction

AE online inspection of vertical atmospheric tanks can offer owners the priority sequence
of maintenance. Based on AE testing results, the safety levels of all tanks inspected can be
graded to A to E. In most cases, more than 50% of storage tanks are unnecessary to shutdown
for immediate maintenance, and these tanks can continue to run several years, only less than
20% of them need to shutdown for further examination and repair. AE testing of tanks can
offer considerable financial, safety and environmental benefits by providing information on
tank integrity without draining or incurring extensive down time, and generally only the areas
with problems need to be maintained, thereby minimizing costs. Otherwise, opening a tank
for inspection and repair introduces oxygen into the tank, the corrosion process starts all over
again when it is put back into service. Therefore, AE testing has many benefits such as
on-line testing, global monitoring, rapid inspection, and cost reduction. However,
applications of testing atmospheric tanks with AE are not so successful compared to AE
testing of pressure vessels because of the shorter history of AE application on tank floors,
relative immature mechanism or theory of acoustic emission sources, and other difficulties,
i.e. they generally have larger diameters (up to 100 meters and over), loading conditions are
limited and the floors are inaccessible, and false source locations and lost source locations are
easy to occur. Sensors with lower frequencies have to be adopted to acquire data from floors
to minimize the propagation attenuation because of longer sensor spacing, even if more
noises are easy to interfuse into useful signals, which make the interpretation of testing data
and the integrity assessment of tank floors more difficult. Furthermore, the selection of
effective sensor coupling modes is another problem of cryogenic storage tanks with
insulation in that ordinary couplants generally are unsuitable to couple sensors because of
their high sound attenuation in cryogenic temperatures. As for cryogenic bi-layer tanks, the
inner tank is inaccessible, so it is hard to fix sensors directly to the shell of inner tank to
collect signals from the floor. This paper will discuss these issues and give some
recommended practices. [1-4]

1 AE in-service Testing of Cryogenic Storage Tank Floors of Monolayer

Generally, based on tank diameter and impurity sediment of medium, a quantity of small
holes with 200-250mm diameter and 0.3-1.0m apart from tank floor plate can be opened in
the insulation of tank cylinder for sensor mounting. The testing procedures of them are same
to that of tanks in normal temperatures. In cryogenic temperatures, one problem is if sensors
may produce electronic noises, another is how to choose a suitable couplant with lower
attention. We think that sensors with pre-amplifier integrated should not be used because
more self-excited electronic noises may be induced.
To find a simple and valid sensor coupling technique, we tested the coupling effectiveness
of vacuum grease (as a common couplant) and water/ice in cryogenic temperatures, and made
a comparison with that of vacuum grease in normal temperature. The testing was conducted
by means of sensors R15-AST, R3I-AST, and Acoustic Emission Station of PAC, USA. Using
pencil break of 0.5mm, HB in a place of 100mm apart form sensors as the simulation
source and considering the means of responding amplitudes of 10 simulated signals, We
found that the average amplitude response of water/ice as the couplant in cryogenic
temperature is nearly the same as that of using vacuum grease in normal temperature, but
when using vacuum grease instead of water/ice in cryogenic temperatures, the effectiveness
will be deteriorated and more than 10dB of the mean amplitude response will be reduced.
Therefore, we think that using water/ice as the couplant in cryogenic temperature is a simple
economical and effective practice in AE testing of tanks. Before testing, a couple of drops of
water can be dropped on the sensor surface and then put it on the polished shell wall for a few
seconds until a thin layer of ice is formed between sensor surface and tank wall , a special
clamp can be used for further fixation.
Waveguides can be used to AE testing of cryogenic storage tanks, but in most cases they
can not be adopted because of easy cracking when welded them to tank walls in service
conditions, which will severely damage tanks, unless they are welded to tank walls during
their construction or former shutdowns.

2 AE In-service Testing of Cryogenic Storage Tank Floors of bi-layer

A cryogenic bi-layer tank consists of an inner tank for storing medium and an outer tank
for insulation. Generally, granular insulation and some inert gas such as nitrogen are filled in
the space between the shells of inner tank and outer tank, a rigid insulation is located between
two floors. The inner tank is inaccessible and there is scarcely any metallic connection
between two shells, so it is hard to mount sensors directly to the shell of inner tank to collect
signals from the floor. By viewing the construction drawings of this kind of tanks and
observing them in field, we found that mounting sensors on protruded ends of braces from
inner tank shell may be a good and the only choice for AE in-service testing of a cryogenic
bi-layer tank floors. Obviously, longer braces and their longer distances from brace-shell
connections to inner tank floor are disadvantageous for signal propagation, but there is no
other choice.

3 A Example of AE In-service Testing of A Cryogenic bi-layer Tank Floor

We test the floor of an ammonia tank (22m diameter, 8000m
3
) in service with 12 AE
R30I-AST sensorsAcoustic Emission Station, PAC, USAmounted on protruded ends of
braces from inner tank shell. A brace is 2770 mm long and the distance from brace-shell
connection to inner tank floor is 1310 mm, as is shown in figure 1.
To verify the validity of signal transmitting, we hit valves in inlet and outlet pipes with a
metallic rod, and to calibrate the sensitivity of sensor coupling, we conducted pencil break
tests. The fluid level in the tank was increased to 15m for inspection, which is the highest
level can be reached. The test results are shown as in figure 2 and figure 3, which indicated
that this practice is feasible.





Sensor
Outer tank bottom plate
Inter tank bottom plate
Inter tank shell plate
Outer tank shell plate
23100
22000
800
450
1310
300
22800
2
7
7
0


2
6
0
0


Concrete foundation
Figure 1. Structure and sensor-mounting places of a bilayer tank



4 Conclusions
As an economical and convenient couplant used in AE testing of cryogenic tanks,
water/ice is a good choice when waveguides can not be adopted because of welding difficulty
or cost consideration. AE testing of cryogenic bi-layer tanks by means of braces is acceptable
and can offer us indicative results.


5 Reference
[1] Physical Acoustics Group, Acoustic Emission Testing of pressure systems and tanks. Insight: Brochure
PAL, 1996.
[2] Van de Loo P J, Hermann B. How reliable is acoustic emission (AE) tank testing? The quantified
results of an AE user group correlation study. Insight: Proceedings of 7th ECNDT, Copenhagen,
1998.
[3] Sokolkin, A V, Ievlev I Yu, Cholakh S O. Use of acoustic emission in testing bottoms of welded vertical
tanks for oil and oil derivatives. Insight: Russian Journal of Nondestructive Testing, Vol.38(12), 2002,
p902-908.
[4] Yanting Xu, Yadong Wang, Fujun Liu, et al. Two different applications of AE technology on detecting
the leakage on storage tank floor. Insight: Chinese Journal of Nondestructive Testing, Vol.29(9), 2007.




Figure 3. Three-dimensional source
locations of a bilayer tank
Figure 2. Planar source locations
of a bilayer tank