AN EXPERIMENTAL LEAK TEST FACILITY FOR SUBCOOLED …. Mech...structures such as pipe whip...

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AN EXPERIMENTAL LEAK TEST FACILITY FOR SUBCOOLED WA TER LEAKAGES

S GHOSH1 & S. K. SAHA2 1Department of Mechanical Engineering, Future Institute of Engineering & Management, Sonarpur, Kolkata,

West Bengal, India 2Department of Mechanical Engineering, Jadavpur University, Kolkata, West Bengal, India

ABSTRACT

This paper demonstrates the detail of an experimental leak test facility to simulate loss of coolant accident

(LOCA) probabilities in primary heat transport pipelines of modern pressurized water reactors (PWRs). The facility was

erected at Jadavpur University, Kolkata in collaboration with BARC, Mumbai under a project for reactor safety issues

funded by BRNS, DAE, Govt of India. Unknown crack morphologies concerned were avoided by selecting artificially

generated circumferential slits with rectangular projections on reactor grade Pipelines. Experiments were conducted in

several phases with pressurized (maximum 90 bar) and hot (maximum 250ºc) subcooled liquid water to leak through the

narrow slits on pipe wall oriented perpendicular to pipe flow direction. A bank of Leak flow data has been generated in a

high depressurization flow domain (like 90 bar to atmospheric) and results have been compared with relevant code

predictions, as a function of slit upstream stagnation pressure, stagnation subcooling and slit dimensions.

Thermal hydraulic behavior study of two phase critical flow from narrow slits and concerned code validation demanded a

lot of instrumentation and safety arrangements to achieve proper control over thermal inertia of the huge setup.

KEYWORDS: LOCA, PWR, Sub Cooling, Critical Flow, Leak Test, LBB

INTRODUCTION

Numerous recent pipeline rupture accidents in the oil and gas industries have enforced subsequent stricter safety

design regulations. Interest in the modeling of accidents involving failure of pressurized pipelines started as a result of

research carried out in the nuclear power industry evaluating the critical scenarios of loss of coolant accidents (LOCAs) in

Pressurized Water Reactors (PWRs). The application of Leak before Break (LBB) is for the elimination of protective

structures such as pipe whip restraints, jet impingement shields, etc, which provide protection against high energy line

breaks. LBB methodology has been successfully applied in a number of nuclear power plants. Specially, pressurized water

reactors (PWRs) have been designed to cope up with a catastrophic break on any line for many years. For the Indian

pressurized heavy water reactors (PHWR’s), normal operating conditions are of high pressure (70–100 bars) and high

temperature (260-300ºC). The experimental facilities need to be operated at similar potentially hazardous conditions which

necessitate a robust design backed by detailed analysis. LBB methodology demonstrates that a postulated through wall

crack will ensure a detectable leakage and the flaw will be stable under the severe-most loading with a consequent loss of

pressure. With a capacity of easy detecting a least of mass flow rate 0.05 kg/sec, if 0.5 kg/sec of leak can be shown stable

under the maximum credible loadings, it can eliminate costly shields for postulated rupture and save labor cost,

radiation dosage for normal maintenance and inspection. Real time experiments are necessary to be carried out to estimate

leakage mass flow rate from several stagnation pressure & temperature, slit materials and slit geometries.

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN(P): 2249-6890; ISSN(E): 2249-8001 Vol. 4, Issue 5, Oct 2014, 1-12 © TJPRC Pvt. Ltd.

2 S Ghosh & S. K. Saha

Impact Factor (JCC): 5.3403 Index Copernicus Value (ICV): 3.0

PREVIOUS WORKS

Simoneau (1974) reported two-phase choked flow rate and pressure distribution data for subcooled nitrogen

flowing through a slit. The slit was a narrow rectangular passage of equal length and width (L/DH=43.5). The results

suggested a uniform two-phase flow pattern with vaporization occurring at or near the exit in most cases. Similar results

can be obtained following the theory of Henry (1970, 1979) for non-equilibrium subcooled two-phase choked flow in long

tubes. Some researchers have studied choked flow through narrow parallel-plate channels. As the boundary layer thickness

formed on the plate becomes zero at the outlet of the channel, it appears as a wall of a converging-diverging nozzle,

resulting in the flow to become supersonic at channel outlet. Taguchi et al. (1996) conducted experiments and a 1D

approximate analysis based on boundary layer equations for the low-density gas choked flow through a parallel-plate

channel, with a narrow gap in the regimes from continuum flow to slip flow. Shi et al. and Miyamoto et al. (2001) clarified

the characteristics of the choked flow in the parallel-plate channel with adiabatic and uniformly heated walls through

experiments. Celata et al. (1988) performed experiments at stagnation pressures of 0.5, 1.0, 1.5 MPa and inlet subcoolings

of 0, 20, 40, 60 degrees to study the influence of a non-condensable gas on a steam-water two-phase critical. Chang et al.

(2002) also performed a series of two-phase critical flow tests at high pressure with a non-condensable gas by using a test

section with an inner diameter of 20.0 mm and a length of 300 mm. As the degree of subcooling (Tsub) strongly affects in

the determination of critical flow rate (Gc), it is necessary to establish the relationship between two parameters

(i.e, Tsub versus Gc). However, the range of subcooling covered in the previous experiments is not sufficiently large

(i.e, less than 30˚C) to quantify the effect of subcooling on the critical flow phenomena. Quantification of the relationship

between Tsub and Gc may help developing a simple critical flow model. In addition, experimental data for small diameters

with real time pipe slits are extremely rare. Present experiments endeavor to seek a longer range of variation in subcooling

as well as stagnation pressure.

SET UP DESIGN

As mentioned earlier, the set up aims to investigate leak flow behavior in predefined domain of stagnation

properties (70 to 90 bar pressure with maximum temperature upto 250 ºC. Transient Thermal- hydraulic analysis has been

carried out in the leak test facility for an open loop system. The facility can be operated by either batch mode or by

continuous mode. The experimental loop was thoroughly investigated including the possibility of pressure surge upstream

or downstream of the slit. Thorough observation provides three sections of the setup identified, namely (i) Primary system,

(ii) Secondary system, (iii) Tertiary system.

Figure 1: Leak Test Facility

An Experimental Leak Test Facility for Sub Cooled Water Leakages 3


Primary System

Firstly water at atmospheric pressure and temperature is taken from hot well or sump and is stored in an overhead

tank after treatment. In order to avoid corrosion in carbon steel parts in PHT loop, PH of water is kept within 9.8 to 10.3

and the conductivity within 20 to 30 micro-mho The same is achieved by incorporating a water treatment plant

(WTP, Figure 2). An Overhead Tank (OT) stores treated water for batch mode operation. One centrifugal pump is used to

suck the water from sump and send it to the OT through WTP. In WTP, Iron removing filter and Water Softener deliver the

water with following specification: iron < 0.2 ppm as Fe and Hardness < 5ppm as CaCO3. A reciprocating pump (P2) is

used here to force the water to BC from OT. This is low pressure and low temperature loop where pressure is maximum 2

bar and temperature is maximum 45ºC i.e. ambient temperature. Pipe work is mainly done by 40 NB poly vinyl chloride

(PVC) / GI (galvanized iron) pipes. From OT to inlet of P2 pipe is made of 40 schedule mild steel.

Figure 2: Water Treatment Plant

The next phase is high temperature high pressure loop which starts when treated water in buffer chamber sent by

P2 from OT is pressurized by nitrogen gas as topping fluid and then is heated externally by hot thermic fluid.

Buffer Chamber is a very important part of the facility having its capacity of 1250 litre or 1.25 m3. It actually acts as a heat

exchanger (Figure 3). There is a cylindrical shell and two dished ends at the top and bottom of the cylindrical shell.

There is a jacket around the cylindrical shell and insulation around the jacket. The jacket has helical baffle inside to pass

through a hot thermic fluid Therminol 59 which ultimately heats up the water in BC. When the pressure and temperature of

water in BC reach the desired values, then water is allowed to flow through the test section via mass flow meter.

Maximum pressure and temperature of water in HPHT is 90 bars and 250ºc respectively. Buffer Chamber (BC) with

nitrogen gas as topping fluid takes care of the possible pressure surge during the flow of high pressure and high

temperature compressed liquid water through slit in pipe.

Test section is another important part (Figure 4) which contains a cracked test pipe inside housing. The housing

has a rectangular shape with hemi cylindrical part on top. The curvature of the hemi cylindrical portion is designed on the

basis of some experimental data that will prevent the possible jet impingement of steam from the pipe slit. The rectangular

portion is rested on four legs. Test pipe of different diameters are kept on two supports specially designed.

Suitable packing is there for making the arrangement airtight. Two temperature elements (TE: 2A and TE: 2B) are wall

mounted on the test pipe to know the temperature of water inside. One pressure transmitter (PT: 3) is attached to know the

pressure in the housing. This pressure is necessarily the outlet pressure of the leak to be considered for future analysis.



Figure 3: Buffer Chamber (Top Half) Figure 4: Test Section

Water from either side will enter into the test pipe to produce a pseudo stagnation point in the pipe just before

flow through slit. One 100 mm NB pipeline from the bottom of test section goes to hot well to drain any accumulated

steam and water from test section. Another line from the top of test section goes to a vent tank containing water and a 30

feet high standpipe. This stops any building up of pressure in test section beyond 1 bar. All high pressure and high

temperature pipelines are made of stainless steel (SS316). High pressure (not high temperature) pipe line as well as high

temperature (not high pressure) pipeline is made of mild steel. Pipelines from out let of P2 to BC, from bottom of BC to

hot well and from bottom of BC to test section is made of 20 NB schedule 40 SS316. Pipeline from test section to hot well

is mad of 100 NB schedule 20 MS pipe.

The most important instrument of the whole facility was the flow meter whose working was crucial. A coriolis

type mass flow meter (Figure 5) was attached just upstream to the test section in order to ascertain the perfect measurement

of leak through slits. The specialty of this flow meter is that, it was able to measure the velocity of flow along with flow

rate in liters/min. This velocity can be taken as the inlet velocity for a subcooled flow to be subjected to huge

depressurization rate while coming out from the slits to atmospheric co-ordinates. This data is very beneficial for CFD

analysis of the aforementioned phenomenon with respect to cavitation flashing flow. The mass flow of a u-shaped Coriolis

flow meter is given as , where Ku is the temperature dependent stiffness of the tube, K a shape-dependent

factor, d the width, τ the time lag, ω the vibration frequency and Iu the inertia of the tube. As the inertia of the tube depend

on its contents, knowledge of the fluid density is needed for the calculation of an accurate mass flow rate everything is

done by the meter itself and one needs to bother only about the digital outputs.

Figure 5: A Coriolis Type Mass Flow Meter, Working Principle, Standard Output Device

Secondary System

It is the source of pressurization of the experimental facility (Figure 6). The infrastructure is built up in a separate

room called nitrogen room from the point of view of safety. Nitrogen gas is stored in a bank of 48 cylinders at a pressure of



130 bars. 24 cylinders are connected to one manifold and the rest 24 are connected to another manifold. A coil shaped Tail

pipe made of copper actually connects a cylinder to manifold with bull noses on the two ends. Two such manifolds are

connected to a single pipe called the main manifold through globe valves GLVN1 and GLVN2. Pipe work is mainly 12.7

mm internal diameter 16 SWG (standard wire gauge) seamless copper pipes for flow of nitrogen at ambient temperature.

But in the pipe line from PCVN to BC there is a possibility of high temperature due to conduction heat transfer. So this line

is made of 20 NB 80 schedule MS pipes.

Figure 6: Secondary System

Tertiary System

It is the source of heat addition of the experimental facility including the following parts. Thermic fluid heater

(TFH) is the main component of heating system which consists of a double start helical coil, burner, fuel pump,

photoelectric sensor, and FD fan. Burner takes the air and high-speed diesel fuel as mixture. Flame is generated with the

help of burner electrode and flame is sensed by photoelectric sensor. Thermic fluid is circulated through the helical coil.

FD fan is there to force fresh air in the TFH and help the flue gases to exit from TFH through chimney. It is located in the

machine room. Actually thermic fluid expands 7 liters per 100 liters of oil per 100ºC rise in temperature.

Deaerator / Expansion tank actually comes into the picture owing to the above consideration. It is placed just outside the

room of test section. A Diesel tank (HSD) is used to store HSD. It is placed just outside the machine room. A Thermic

fluid circulating pump helps to circulate the TF in the experimental facility. It is placed in the machine room. A chimney is

located outside the machine room. It helps flue gases to exit at a high altitude to reduce pollution. High temperature

pipeline is made of mild steel. 76.2 mm (3inch) NB ERW (Electric Resistance Wieldable) pipes are used for the purpose.

Study of the Control Panel

The Control panel is the beauty of instrumentation (Figure 7). It actually builds up the controls of different

instruments used in the experimental facility. It has been incorporated in the experimental facility to bring the complexities

under control. Any kind of irregularities can be noticed by suitable alarms and actions are taken either by automatically or

mechanically through control panel. Most of the alarms and interlocks are routed through a CPU board. In a separate room

called control room this control panel has been placed. Two CCTVs with eight cameras including one rotating camera, two

quad processors and one tilt controller are there. Three phase indications, phase reversal indications, different starter ON,

OFF, trip indications, Firing indications and wrong phase sequence indications are there in the control panel. Temperature

indicating controller (TIC): There are 4 TIC’s in all on the control panel. Three are for the Thermic Fluid Heater and one is

for the Overhead Tank. There are two alarm annunciators in the control panel with 8 channels each. When some fault

appears in the experimental facility, corresponding alarm gets on. If somebody accepts the alarm by pushing the switch



hooting and blinking will be stopped but the light will be on unless the fault is removed. A microprocessor based

programmable Scanner makes the basis of the Data Acquisitions system. All recorded Data are shown in its display.

The scanner is connected to a computer for storage and data processing was done by software called SERCAP, able to

record 3data every second per channel.

Figure 7: Control Panel

GENERATIONS OF SLITS

Slit generation was by far the most critical section of experimental arrangements. The validity as well as

feasibility of the experimental results is strongly dependent on the accuracy of slit dimensions produced. Reminding the

test matrix slits to be produced was very narrow in the range of less than a millimeter. In this context Amos and Schrock's

(1983) report on critical discharge through slits for USNRC, reminds the use of a U shaped block along with an insert

block and spacer block to create a rectangular opening of particular slit width in the range of 0.13mm to 0.38mm.

However, the present experimental setup was intended to test a real time annular through wall opening which will have a

rectangular projection. It was difficult as well as intricate production work subjected with minute slot dimensions.

To create through wall narrow slits at any circumferential location of pipe wall precision advanced machining was

mandatory. In this case an Electric Discharge Machining was selected to create the minute slots in a rectangular fashion.

The specialty of the machine was the use of a wire as the electrode i.e. a Wire EDM. The whole situation can be

conceptualized by the 3D modeling done using CAD (Figure 8)

Figure 8: Wire EDM Slit Cutting Process on a Round Pipe

As per predetermined test matrix, slits were cut on the pipes. A Wire Electro Discharge Machine was used to

prepare the slits on Seamless 4” NB SS-304L pipes. 3 slits were cut with Slit Width (SW) 0.25mm, 0.38mm, 0.67mm

respectively. Slits were circumferential with an inside & outside arc length of 4.373 mm and 7.75 mm respectively.

Slits can be considered as having a rectangular Slit Opening Area (SOA) on the basis of average arc length 6.178mm and

respective slit widths. Due to the minimum wire size available was 0.25mm, the first slit has undergone some over burn



with respect to the tolerance and the slit width achieved was 0.268mm.

Figure 9: Standard Operating Procedure

EXPERIMENTATION

On the next phase experiments were carried out for a set of inlet pressure and temperatures. Due to complexities

at different zones of the setup the whole experimental operation was transformed into a series of simpler steps called as a

Standard Operating Procedure (SOP) shown in Figure 9. Transient flow behavior at a good range of subcooling with a

maximum pressure of 90 Bar was thoroughly studied. To ensure safe operation a bank of safety arrangements were

incorporated into the system design and programmed into several instrumentation modules. The whole data acquisition

system included 16 channels associated with several important Transducers, valves and controllers at variety of locations.

RESULTS AND DISCUSSIONS

To attain a steady state condition was a bare necessity to show a stable leakage flow for considerable period of

time. Therefore pressure and temperature of flow was maintained by proper control of the setup during any test run.

For carbon steel pipe slits critical flow was found on development in both cold and hot runs. A code validation program

showed conformity between experimental data and thermal hydraulic code C-SFA (B Ghosh et al, 2004) prediction.

However the code under-predicted the flow rate on an average 20% (figure 10).

Figure 10: Experimental Code Validation SOA=0.057cm2, SW=.038cm, SD=0.8cm

The aforementioned code estimates the pressure drop by calculating the several different forms of pressure drops

experienced by the sub cooled water in the leakage path. The location of flashing inception was assumed as prescribed by

Henry et al. (1970) for homogenous equilibrium flow approximations. More options of the code predicted leak flow on the



basis of homogeneous frozen flow concept with zero vaporization within the domain. Here, the primary parameter of

material dependence is the friction of depressurizing jet with the slit channel wall. However, the high depressurization rate

and phasic acceleration makes the transit time so small that flow has got the least chance to be majorly affected by pipe

wall friction. And the fact is prominent from the figure 11 where a subsonic unchoked release shows a potential liquid core

region affirming single phase flow within the leakage path. Thus the flow might be governed by single phase pressure drop

terms. A certain degree of superheat due to instant decrease of pressure delays the flashing process resulting in the core

liquid zone.

The unsteady expansion waves in the form of shock dominates the flow when pressure difference is more tending

to a critical choked release involving more unknown parameters in a compressible field. In most cases researchers have

found the flashing to happen at the exit zone or later in case of several L/D ratio ducts and orifices. Therefore a proper

choice of friction characteristics (especially friction factor), governs the code compatibility with practical observations with

smooth or rough slits. Chang K. S. (1999) has discussed about friction factor selection and their dominance while Ghosh B.

et al. (2002) have also enlightened this field.

Figure 11: The Choked and Unchoked Accidental Release Showing a Potential Liquid Core in Both Cases While Unsteady Expansion Waves Governing the Sonic Choked Flow

Present work exploits a study of thermal hydraulic parameters showing their interdependence quite clearly.

Figure 11 and Figure 12 shows a comparative judgment between MS and SS pipe slits of almost similar opening

dimensions. At 80 bar stagnation pressure maintained in the Buffer chamber, the slit flow rate was found to increase with

increasing sub cooling values in both of the cases.

Higher degree of subcooling was achieved at higher pressure keeping the temperature around 240 ºC. Certainly it

increased more delay in flashing and single liquid phase dominance resulting in higher flow rate. Here the analytical code

over predicted when the flow was analyzed on a fixed pressure perspective (Figure 12 a) while the same was under

pedicted in few other observations made on a constant subcooling basis (Figure 12 b).

The mild steel pipes results were affected by high corrosion levels and increased friction. So better conclusions

can be made following the results of stainless steel pipe slits (Figure 13). The basic trend of data continues to be similar in

both of the analysis made at constant subcooling and constant stagnation pressure. The mass flux increases for increased

pressures.



(a) Variation of Leak Flow with Change in Sub Cooling at 80 Bar

(b) Flow Behavior at 60 Degree Sub Cooling

Figure 12: Flow Behaviors at Constant Pressure and Sub Cooling for MS Pipe Slit, SOA 0.057cm2

(a) Variation of Flow with Sub Cooling at 80 Bar

(b) Flow Behavior at 57 Degree Sub Cooling

Figure 13: Flow Behaviors at Constant Pressure and Sub Cooling for Several SS Pipe Slits



More results with 4th, 5th slit with increasing slit sizes and slit opening area (COA) have been depicted in

Figure 14 to show the C_SFA code validation exercise. Analysis is stressed here on a fixed subcooling basis and the code

prediction conform the experimental trends. However, a 10 to 20% under prediction was found in all cases as shown in the

overall plot (Figure 10) too.

(a) Variation of Flow with Pressure at 67 ˚C Sub Cooling with COA 5.91×10-6 m2

(b) Variation of Flow with Pressure at 39 ˚C Sub Cooling with COA 8.05×10-6 m2

Figure 14: C-SFA Code Prediction Compared to Experimental Observations

CONCLUSIONS

The experiments done with narrow slits of varied dimensions involved a lot of thermal intertia to be handeled.

However the error analysis provided an associated uncertainity with leak flow rate are varying between 0.186% and

_1.98% of the estimated value with 95% level of confidence which is quite satisfactory. The C_SFA code validation study

indicated the physical correctness of the experimental results. Moreover the success of a perfect code prediction lied in the

selection of friction factor selection and the use of a suitable discharge coefficient. Although the discharge coeffiecients

were set 1.0 for most cases in this study, a suitable value can be ascertained from the comparative plots. Analysis were also

made on the basis of L/D ratio changes with varying slit dimensions. However most of the data trends conformed the

results found by Amos & Shrock (1983, 1984). Repeatability tests were done on few slits to confirm the present trends and

behavior with satisfactory affirmations. As a whole stable leakage at around 0.5 kg/s through narrow slits resulting into a

flashing two phase exit flow at nearby critical flow velocities was confirmed. Such leakage can be sensed well by modern

instrumentations and sorted down without hampering useful production time at lower costs.



ACKNOWLEDGEMENTS

The author is thankful to the Board of Research in Nuclear Sciences, Department of Atomic Energy, and Govt. of

India for financing such project of experimental code validation which opened up many uncertains domains of critical

flashing flow through high pressure pipeline leakages. Many thanks to Mechanical Engg. & Production Engg. Department

of Jadavpur Univeristy for housing such project giving chance to explore new domains of fluid flow and plant safety.

Sincere gratitudes for other investigators and collaborators too.

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AN EXPERIMENTAL LEAK TEST FACILITY FOR SUBCOOLED …. Mech...structures such as pipe whip...

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