E-Journal of Advanced Maintenance Vol. 11 No. 3 (2019) 112-117
Japan Society of Maintenology

Application of Ice Plug Technology to Nitric Acid Pipe Systems
Daiki OSANAI 1,*, Tomohiro ICHIKAWA 1, Ken SATO 1, Wataru NISHIYA 1, and Masaki
HOSOGOE 1
1

J-tech Co., Ltd., 1-108, Aza Iyasakadaira, Oaza Obuchi, Rokkasho-mura, Aomori 039-3212, Japan

ABSTRACTAt the maintenance of hydraulic pipe system during continuous operation in many industrial plants, an ice plug
technology for temporary partial isolation is generally applied. We have been developing ice plug technology inorder to apply to nitric acid pipe systems. As chemical reagents such as nitric acid possess volume shrinkage
characteristic in contrast to water, we implemented mock-up test in order to study specific freezing process ofnitric acid and application ability of the ice plug technology.
Consequently, we confirmed nitric acid transferred completely to white solid phase around -60 °C to -70 °C.
Furtehrmore, in mock-up test, approximate 40 °C of freezing point depression originated from freezing
concentration process of nitric acid was observed. It did not affect ice plug formation, it was possible to form ice
plug possessed sufficient pressure resistance performance with freezing completion time of estimated freezingprocess.
From the above results, we obtained the prospect that our technology is applied to nitric acid pipe systems.
KEYWORDS

ARTICLE INFORMATION

Ice plug technology, Nitric acid pipe system, Volume shrinkage
characteristic, Freeze concentration process, Freezing time

Article history:
Received 18th October 2018
Accepted 29th Octorber 20191.

Introduction

At the maintenance of hydraulic pipe system during continuous operation in many industrial plants,an ice plug technology for the temporary partial isolation with freezing a part of internal water isgenerally applied. On the other hand, many number of nitric acid pipe system with their various
concentrations are installed in the irradiated nuclear fuel reprocessing plant, so that we have beendeveloping the ice plug technology to apply to these nitric acid pipe systems.
Water possesses expanding characteristic when it is frozen, but chemical reagents such as nitric acidpossess shrinkage characteristic in reverse. In addition, they show freezing point depression originatedfrom freezing concentration process. Because of these unique characteristics, it is important toconfirm specific freezing process of nitric acid solution to realize safe and reliable ice plug application
to nitric acid pipe systems.
Here in, we report an ice plug application technology which we have studied based on the above
backgrounds, and ice plug formation process.

2.

Overview of ice plug technology

Ice plug technology, as shown in Fig. 1, is to make a blockage plug by freezing liquid in pipe
systems under operation. In order to implement maintenance work of valves which are difficult to beisolated mechanically, this technology is applied to a part of pipe systems which are under continuousoperation. The blockage plug is formed on upstream side from maintenance objects by forcibly
cooling liquid in upstream pipe from its outer surface with refrigerant such as liquid nitrogen.
―――――――――――――――* Daiki OSANAI, E-mail: daiki-osanai@j-tech66.co.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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D. Osanai, et al./Application of Ice Plug Technology to Nitric Acid Pipe Systems
Injecting liquid nitrogen, forming ice plug
Othersystems

Maintenance object
Ice box made by Styrofoam

Extracting fluid ; downstream of ice

Fig. 1. Schematic description of ice plug technology

3.

Nitric acid for test

Nitric acid passes through specific freezing process as compared with water. Moreover, it is known
that nitric acid possesses volume shrinkage characteristic. Sections 3.1 to 3.3 describe thesecharacteristics of nitric acid.
3.1.

Freezing point

Fig. 2. shows various crystal structures of nitric acid [1]. In order to simulate actual plant situation, 3
kinds of concentrations (3.0, 6.0, and 13.6 mol/L) are selected in this paper. This concentration range
covers the maximum concentration using in plant (13.6 mol/L) and the concentration (6.0 mol/L)
which shows the lowest freezing point.
3.2.

Freeze concentration process

Once nitric acid starts to freezing, it passes through freeze concentration process, which leads tofreezing point depression and hindrance of crystal growing rate [2]. This is caused by crystallizationof the water including nitric acid solution. The viscosity of remained solution increases greatly anddiffusion characteristic decreases. In addition, this phenomenon is significant in higher concentration
solution.
3.3.

Volume change rate

Table 1 shows volume change rate of both water and nitric acid when they are frozen. We canunderstand that the volume shrinkage of nitric acid increases with the increase in its concentration.

4.

Beaker test

As preparation of mock-up test simulated actual scale application, we implemented beaker test inorder to study the effect that characteristics described in Section 3 affect ice plug feasibility.
The purpose of this test is to study 3 items according to freeze concentration process on each nitricacid concentration, that is, (1) Whether freezing point depression is or not, (2) Crystal formation
process, and (3) Freezing time until ice plug forming completely.
3.0 mol/L

HNO3・H2O(L)

13.6 mol/L

Table 1. Volume change rate of both water and nitric acidWater
H2O(S)

HNO3・3H2O(S)HNO3・H2O(S)6.0 mol/L

Nitric acid (HNO3)

Concentration (mol/L)

－3.0

6.0

13.6

Volume change rate (%)+9.0－1.0－6.0－15.0

HNO3(S)

Fig. 2. Solid-liquid phase equilibrium diagram
for a binary system composed of Nitric acid and Water113

D. Osanai, et al./Application of Ice Plug Technology to Nitric Acid Pipe Systems

4.1.

Test method(1) 1200 ml nitric acid is poured into 2L beaker and resistance temperature detector is putted incenter of beaker. This detector is set by clamp so as its tip is at half of liquid level height.
(2) This beaker is inserted into Dewar vessel which is filled with liquid nitrogen. Moreover, thisbeaker is set so as liquid level height of liquid nitrogen is higher than one of nitric acid solution.
Liquid nitrogen is added whenever necessary in order to keep up its liquid level height.
4.2.

Results and Discussion(1)

As shown in Fig. 3, temperature reduction of each nitric acid is suppressed at freezing point
described in Reference [1] (3.0 mol/L : -10°C, 6.0 mol/L : -42°C, and 13.6 mol/L : -23°C) andtemperature shows a constant value, which means each nitric acid reaches solid-liquid phaseequilibrium. That is to say, we could not find freezing temperature depression originated fromfreeze concentration process.
(2) While observing white crystal formation process of 13.6 mol/L nitric acid, as shown in Fig. 4(a) – (d), we confirmed that ice crystal formation started from surface of beaker after 5 minutes
from liquid nitrogen pouring into (Fig. 4(a)). As time passes, ice crystal grew into white crystal
toward center of beaker.
Ice crystal just after formation is translucent sherbet state, but when temperature of beaker
central reaches -60°C to -70°C, and after 55 minutes from cooling start when phase transition
can be completed (Fig. 4 (d)), translucent crystal changed to white crystal. We confirmed white
crystal possessed functionable hardness as ice plug.
(3) According to other report [2], hindrance of nucleus growing rate by freeze concentration
process is significant on higher concentration solution. In this test, as nitric acid concentration
was higher, it was necessary long time to reach -60°C to -70°C.
From the results of beaker test, we confirmed ice plug formation of 13.6 mol/L nitric acidcould be completed by securing 55 minutes of freezing time after pouring liquid nitrogen.

5.

Mock-up test

Fig. 5 shows mock-up test equipment simulated actual scale application. We implemented mock-up
test by using this equipment. The purpose of this test is to confirm: (1) freezing process of 13.6 mol/L
nitric acid in plant process pipe systems and (2) pressure resistance performance of formed ice plug.

(a) 5 min(b) 20 min

Temperature distribution
measurement point

White crystal

Translucent crystal(Sherbet state)(c) 40 min(d) 55 min

Fig. 4. White crystal formation process (13.6 mol/L)

Fig. 3. Results of Beaker test114

D. Osanai, et al./Application of Ice Plug Technology to Nitric Acid Pipe Systems
Resistance temperature detector(B)Pressurization line

(D)

Nozzle(D)Measuring point(A)Pipe for application(C)Ice box

Upstream

Cross section of pipe

Downstream

Fig. 5. Mock-up test equipment

5.1.

Fig. 6. Schematic description of measurement areaof temperature distribution in pipe

Setting test condition(A) Pipe for application
As the 40 A-sized pipe which is one of SUS304L pipe is mainly used as nitric acid pipe
systems in actual plant, this pipe is selected for the mock-up test.
(B) Pressurization line
To confirm pressure resistance performance of the formed ice plug, a nitrogen gas cylinder isconnected with the application pipe, and this system is pressurized from its upper stream side.
The test pressure is set on 0.9 MPa by assuming 1.5 times of the pump discharge pressure ofchemical reagent pipe system in the actual plant.
(C) Ice box
As we are aiming to simple application of ice plug technology, an ice box (W:294 × D:240 ×
H:420 × t:34 mm) made by expanded polystyrene is used because of its easy molding. As theaim of this test is ice plug formation which possesses 5 times length of the internal diameter ofthe pipe (40 mm) [3], the ice box with liquid nitrogen immersion area of over 200 mm heightis used. Moreover, the dimensions of ice plug is changeable by taking account of usability onsite.
(D) Measuring point of temperature distribution inside pipe
As the nozzle is attached with center of ice box as shown in Fig. 6, resistance temperaturedetector enables us to measure temperature distribution in pipe.
5.2.

Confirmation test for temperature distribution

In order to study the freezing process of nitric acid in pipe, ice plug technology is applied to test
equipment shown in Fig. 5, and also temperature distribution in pipe is measured at measuring point
shown in Fig. 6.
13.6 mol/L nitric acid was used for confirmation of temperature distribution, because it showedsignificant hindrance of crystal growing rate originated from freeze concentration process and thehighest volume change rate in freezing process.
5.2.1.

Test method(1) The mock-up test equipment is filled with nitric acid and subsequently an ice box is set on
application area,(2) Ice plug technology is performed after this ice box is filled with liquid nitrogen,(3) Temperature of nitric acid in the pipe is measured from liquid nitrogen filling to ice plugformation completion with resistance temperature detector.
5.2.2.

Results and discussion(1) Compared with the freezing point (-23°C) of 13.6 mol/L nitric acid described in Reference [1],when temperature of nitric acid in pipe reached -60°C, it reached solid-liquid phase equilibrium
state at A in Fig. 7. As a result, the freezing point depression of about 40 °C was observed.
(2) Slight temperature rise was observed at B in Fig. 7. Phase transition from liquid phase to solid
phase caused this latent heat release.

115

D. Osanai, et al./Application of Ice Plug Technology to Nitric Acid Pipe Systems(3) Subsequently, at C in Fig. 7, the temperature of the nitric acid in pipe was decreased sharply
from around -70°C and it reached the liquid nitrogen temperature (-196°C) and became theconstant temperature. This phenomenon shows that nitric acid transfers to white crystal
completely around -70°C. And after transition, the temperature decrease steeply due to heatrelease.
Based on results of beaker and mock-up test, we estimate freezing process of nitric acid in the pipe
used in this paper. Table 2 shows estimated freezing process of 13.6 mol/L nitric acid.
This estimated freezing process takes account of the result that translucent crystal of nitric acidchanges to white crystal around -70°C in beaker test. In addition, it takes account of the freezing point
depression originated from freeze concentration process which was observed in mock-up test. We
considers that in like manner of beaker test, ice crystal formation starts from pipe wall and it grows
toward center of pipe as time passes.
Moreover, 60 minutes was set as the freezing completion time in estimated freezing process. It takes
account of the freezing time in mock-up test when nitric acid reaches temperature of liquid nitrogenand becomes completely stable crystal.
5.3.

Pressure test for formed ice plug

The function of the ice plug depends on mainly contact state (adhesion / friction force) between
internal surface of pipe and ice plug [4]. In general, nitric acid solution shows shrinkage
characteristics during freezing and shrinkage change rate is greatly in relation to concentration changeof solution. So, we concerned about the decrease of adhesive force. Therefore, in order to confirm
pressure resistance performance of each ice plug formed from 3 concentrations, pressure test wasimplemented under the condition shown in Table 3.
5.3.1.

Test method

After ice plug application in same manner described in Section 5.2.1, the mock-up test equipment ispressurized by nitrogen gas from its pressurization line for 15 minutes in order to check presence orabsence of liquid leakage resulting from collapse or dissolution of formed plug.
Table 2. Considered freezing process of nitric acid (13.6 mol/L)Time(min)Temperature
(°C)

Start
1.0

20

10－10

15－60

25－70

60－190

PipeFig. 7. Result of mock-up test (13.6 mol/L nitric acid)
Table 3. Pressure test conditions for formed ice plugs
Concentration of HNO3

3.0, 6.0, and 13.6 mol/L

Atmosphere temperature

30 °C ± 2 °C

Freezing time

60 min.

Pressure

0.9 MPa, 15min. ** According to“Chosou no taiatsu siken oyobi kimitsu siken” issued by METI.

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D. Osanai, et al./Application of Ice Plug Technology to Nitric Acid Pipe Systems
Table 4. Application and operation condition of ice plug application to nitric acid pipe systems
Atmosphere temperature

30 °C ± 2 °C

Pipe system

Horizontal pipe(～40A Sch20s SUS304L)

Refrigerant

Liquid nitrogen(LN2)Freezing time

60 min.

Ice box *W:294×D:240×H:420×t:34 mmPressure≦ 0.9 MPa

Operation Condition

Application Condition*: W: Length of liquid nitrogen immersion area, D : Pipe inner diameter
5.3.2.

Results and discussions

As results of the mock-up test, 13.6 mol /L nitric acid shows the highest volume change rate, and theice plug formed under freezing time (60 minutes) possessed pressure resistant against test pressure.
This ice plug possessed sufficient pressure resistance, so it is considered that volume change rate ofnitric acid solution does not affect contact state with internal pipe wall during ice plug formation
process. Therefore, similar pressure resistance performance will be obtained in ice plugs formed form
3.0 and 6.0 mol/L nitric acid. Table 4 shows application and operation condition of the ice plug
technology which is obtained from the mentioned test results and enables us to apply it to nitric acidpipe systems.

6.

Conclusions and future efforts

We obtained a prospect that the ice plug technology is possible to apply 40 A-sized nitric acid pipe
system installed in the irradiated nuclear fuel reprocessing plant by using liquid nitrogen (-196°C) as
refrigerant with 60 minutes of freezing time. The results in this paper are as follows:(1) In the beaker test, although nitric acid solution reaches solid-liquid phase equilibrium state at
freezing point described in Reference, it is necessary to cool it up to -60°C to -70°C until nitricacid solution transfers white crystal completely.
(2) In the mock-up test, although freezing point depression of nitric acid originated from freeze
concentration process was observed, but any effect on plug formation did not be observed.
(3) Ice plug formation can be completed with 60 minutes of the freezing time from estimatedfreezing process. It was confirmed that ice plug possessed the hardness standing up to testpressure and continuous liquid blockage function.
(4) As it is confirmed that any effect of volume shrinkage characteristic of nitric acid do not affectthe ice plug function, it is possible to obtain the ice plug which possesses sufficient pressure
resistance.
As future efforts, we will expand application scope of ice plug technology. To achieve this purpose,
we are going to apply this technology to pipe systems with other diameters, and also study freezing
process and pressure resistance performance of ice plug formed from chemical reagents other thannitric acid used in the plant (sodium hydroxide etc.).

References[1] Uchida, S.; Tsuda, N.;, Shousan, Sangyou tosyo, 1954, pp. 436. (Japanese)[2] M.Akyurt ; G.Zaki ; B.Habeebullah ,“Freezing phenomena in ice-water systems”, Energy Conversion andManagement , 43 (2002) 1773-1789.
[3] JFE Steel Corporation, Haikann touketsu kouhou, JP 2011-153673A, 2011-08-11 (Japanese).
[4] Yoshikawa, K.;, Oogata puranto ni okeru koori no riyou gizyutsu, Seppyo, 1987, 49, (4), pp.217-220.
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