Analysis for liquid cryogen spillage in the superconducting cyclotron building at VECC

Roy S., Nandi C., Pal G. and Bhandari R. K.

Variable Energy Cyclotron Center

1/AF Bidhan Nagar, Kolkata 700064, INDIA

The cryogenic system uses liquid helium and liquid nitrogen to cool the

superconducting cyclotron magnet and its cryopanels. In order to assess safety

scenarios subsequent to an unusual leakage of cryogens from the system, a

deterministic analysis has been carried out to estimate the variation of oxygen

concentration with time at several locations of superconducting cyclotron

building. The entire process is simulated assuming evaporated cryogens mixes

instantaneously with air in the confined space, the ventilation system of the

cyclotron building is operational, fresh air continuously enters the confined

volume and mixes instantaneously with air in the confined space.

INTRODUCTION

Liquid cryogens present significant hazards because of their intense cold and substantial change in

density when warmed to gaseous state. Asphyxiation and over-pressurisation are potential hazards caused

due to production of large volume of gas within an enclosed space. A simplified analysis has been carried

out to assess reduction of oxygen concentration. This paper describes an elaborate theoretical model

developed to simulate the variation of oxygen concentration with time, in case of a rupture of cryogen

delivery line and the results generated.

CRYOGEN DELIVERY SYSTEM

The cryogenic system at VECC [1, 2] uses

liquid helium to maintain the superconducting

coil of the cyclotron magnet at 4.5 K and cool

the three cryopanels uses for generating

vacuum in the accelerating chamber of the

cyclotron. The system also supplies liquid

nitrogen to radiation shield of cyclotron

magnet, cryopanel and chevron baffles of

cryopanels. A complex and compact network

comprising vacuum insulated pipelines, valve

box and storage dewars are being used to

deliver cryogens to the cyclotron. A 160 watt

helium refrigerator caters to the 4.5K

refrigeration needs of the cyclotron. Liquid

helium produced in the helium plant is stored

in a 1000 litre liquid helium dewar. Liquid

helium is supplied to the cyclotron from this

dewar. A liquid nitrogen shielded line starts

from the highbay manifold and extends to the Fig. 1. Schematic diagram of cryogen delivery system

LN2 DEWAR

HB

CV1

VL

VM

VT

MV1

VB1

VB2

VB3

VB4VB10 VB6

VB9

VB8

VB7VB5

BL

BT

BMLBMT

MV2

HIGH BAY

VAULT

BASEMENT

CRYOSTAT OF SUPERCONDUCTING CYCLOTRON

CONTROLVALVE

VACUUM PLUG

VACUUM BARRIER

VALVE BOX

MANUALVALVE

LHe DEWAR

FROMPLANT

Proceedings of ICEC 22-ICMC 2008, edited by Ho-Myung CHANG et al. ⓒ 2009 The Korea Institute of Applied Superconductivity and Cryogenics 978-89-957138-2-2

687

basement manifold, passing through vault mezzanine manifold and basement mezzanine manifold.

Vacuum jacketed transfer lines supply liquid helium to the cyclotron magnet and liquid nitrogen to the

upper radiation shield of the cyclotron magnet from the vault mezzanine manifold and liquid nitrogen to

the lower radiation shield of the cyclotron magnet from the basement mezzanine manifold. The liquid

nitrogen system consists of two 1600 litre liquid nitrogen dewars placed at high-bay. Liquid helium and

liquid nitrogen are supplied to the three cryopanels from the basement manifold. The entire delivery

system pipelines are insulated using a coaxial vacuum space. There are eight such vacuum spaces, which

are separated using barriers and sealed using a plug. Figure 1 shows the simplified schematic layout of the

cryogen delivery system.

THEORETICAL MODEL

A theoretical model has been developed considering that the physical process occurs in three stages [3].

Immediately after the rupture, liquid cryogen will flow out and spread over the concrete floor. Phase

change of liquid cryogen will take place slowly and the gaseous phase will diffuse into the air of the

confined space. The mathematical formulation contains several transient terms and a direct solution of the

formulation is not possible. In order to resolve the formulation, the entire process is discretised over

several small time steps. Recurrence relations were established between the parameters to use the current

value of any parameter in the next step.

Evaluation of the spillage rate

As the cryogen carrying line ruptures, cryogen will come out of the inner tube and fill the annular space.

The vacuum in the vacuum space will be lost. The cryogen will come in contact with the pipe at room

temperature, the annular volume will be pressurized and the plugs will come out. Gradually the entire

annular space will be filled with the liquid cryogen. After sometime liquid cryogen will come out through

the plug and spread over the floor. Various parameters like static head, length of pipeline, number of

valves and bends, etc. influences the flow rate. In order to asses the worst situation, it is assumed that the

inventory of liquid cryogen is maximum at the onset of leakage. The initial inventory of the liquid

nitrogen is taken as 1300 litres at a pressure of 850 kPa and for liquid helium it is taken 800 litres at 0.3

kPa.

The principle of energy conservation was applied between the liquid of the supply tank and the plug

at leakage point, neglecting the acceleration of liquid, to estimate the velocity of cryogen stream coming

out of the plug. Various head loss factors due to valves and bends were taken in considering the loss

coefficient KT.

]1[2

)()]([

.

)(2

221Tc

LCryo

Kg

tVtZZ

g

PtP

(1a)

44332211 .... vvvvvvvv

p

C

T KNKNKNKNd

fLK (1b)

Spillage rate of the cryogen was obtained from following equation;

pldLCryo AtVCtQ ).()( 2. (2)

In case of liquid nitrogen, pressure head and static head continuously reduce as the liquid is drained

out from the supply dewar which remains in closed condition.

ttQtVttV LCryoLCryo ).()()( (4)

T

LCryo

A

ttVttZ

)()(

(5)

688

)}({

)()}.({)(

1

1ttVV

tPtVVttP

LCryoT

LCryoT

(6)

In case of liquid helium system, it is assumed the helium plant will continue to maintain a constant

pressure at the liquid helium dewar.

Evaporation of liquid cryogen

Initially, the flow rate of the liquid cryogen being high enough, the entire liquid cryogen may not undergo

phase change instantaneously. The leaked cryogen will be splashed over the floor of the building in the

form of liquid puddles. Thickness of these puddles will depend on surface tension, contact angle and

density of the liquid. There is no detailed data available about the contact angle of liquid helium and

liquid nitrogen with concrete. Puddle thickness is estimated taking the contact angle of water on concrete

and rounded off to 0.5mm. It was also assumed that the floor is perfectly horizontal and a continuous film

of liquid cryogen spreads over it. The splashed liquid cryogen will be gradually evaporated due to heat

addition from various sources. In this analysis, heat transfer by conduction from concrete floor has only

been considered. The concrete is modeled as a semi-infinite plate maintained at an initial temperature of

25OC. The transient heat flux to the splashed cryogen [4] and floor area covered by it, are given by

following equation;

t

TTktqcond

'..

)()( 01

(7)

LCryoLCryo

LCryo

th

tmtA

.

)()(

(8)

A fraction of the total mass of cryogen splashed over the floor will be evaporated into gaseous phase

while remaining liquid cryogen will spread over the floor.

fg

cond

GCryoh

tAtqtm

)().()( (9a)

ttmtQtmttm GCryoLCryoLCryoLCryoLCryo .)().()()( (9b)

Change in oxygen concentration

There are three different demarcated zones in the superconducting cyclotron building, i.e., highbay, vault

and basement. Each of these zones is considered as a control volume according to the leakage location.

The control volume is also ventilated at a constant rate of two air changes per hour through an inlet.

Leaked gas enters within the control volume through one inlet. Since there is no accumulation of mass

within the enclosure, the rate of outflow is obtained from the sum of inflow and gas evaporation rates.

It is assumed that the evaporated cryogen mixes instantaneously with the air of control volume and attains

chemical equilibrium. This assumption is reasonable as there are a number of air circulators inside each

control volume. Condensation of the ambient oxygen is not considered.

GCryo

GCryo

Gcryo

tmtQ

)()(

(10)

QtQtQ GCryoout )()( (11)

Concentration of oxygen is obtained using mass balance equation,

VC

Oout

OOV

ttconctQQtconcttconc

.

2

22

.)().(.21.0)()(

(11)

689

RESULTS

Spillage of liquid nitrogen

Variation of oxygen concentration for spillage of liquid nitrogen at different locations was computed.

Initially oxygen concentration reduced to 19% at same rate irrespective of location of spillage (Fig. 2). It

was observed that the duration of spillage was more for the leakage point at lower elevations of the

distribution system. Oxygen concentration in the leakage zone remains below 19% for larger intervals as

spillage occurs through lower points. It was observed that minimum oxygen concentration reaches

15.18% after 16 minutes for the spillage through plug located at lower basement region as maximum

static head is available at the lowest point.

Fig. 2: Variation of oxygen concentration due to LN2 leakage at different locations

Spillage of liquid helium

Reduction of oxygen concentration was also evaluated for liquid helium spillage at different locations. It

is expected that the lower location shall be affected most. Leakage of liquid helium through the lower

basement plug was evaluated. Initially, spillage rate being greater than the ventilation rate, oxygen

concentration reduces rapidly (Fig. 3). The concentration of oxygen falls below 15% in about 9 minutes

and oxygen concentration will be below 15% for about 13 minutes. After the end of the spillage, oxygen

concentration starts to increase at a slow rate. The oxygen concentration in the affected zone improves to

19% after a period of 52 minutes.

Fig. 3: Variation of oxygen concentration due to LHe leakage at lower basement plug.

690

DISCUSSIONS

A comparison of two graphs (Fig. 2 and Fig. 3) reveals that the consequences of liquid helium

spillage is more severe than that of liquid nitrogen. This is because of the larger expansion ratio of liquid

helium and the constant pressure head at the liquid helium supply dewar. In case of nitrogen system,

gradually decreasing pressure head itself causes reduction of spillage rate.

NOMENCLATURE

P1(t) Pressure within the tank at any time ‘t’, (Pa).

P2 Pressure within the vault, (Pa).

Zc Fixed height of tank bottom above point of leak, (m).

Z(t) Height of liquid cryogen within tank, (m).

)(2 tV Velocity of liquid cryogen stream, (m/sec).

t Time, (sec).

LCryo Density of liquid cryogen, (Kg/m3)

KT Total head loss factor due to pipe friction, valves and bends.

Nvi Number of On-Off valves, Control Valves, large and sharp bends.

Kvi Head loss coefficient of On-Off valves, Control Valves, large and sharp bends.

Lc Total length of pipe up to the plug, (m).

f Friction factor.

dp Pipe diameter, (m).

Cd Coefficient of discharge for plug.

Apl Area of opening through plug, (m2).

)(tQLCryo Volumetric flow rate of cryogen (m

3/s)

AT Cross-sectional area of the tank, (m2)

)(tVLCryo Liquid volume within the tank, (m3).

)(tqcond Transient heat flux, (W/m2).

k Thermal conductivity of concrete, (W/m-K).

T1 Current temperature of the concrete surface, (K) .

T0 Initial Temperature of concrete, (K).

' Thermal diffusivity of concrete, (m2/s).

hfg Enthalpy of evaporation of liquid cryogen, (J/Kg).

)(tA Area covered by liquid cryogen, (m2).

LCryoth Thickness of splashed cryogen film, (mm).

)(tmGCryo Mass flow rate of cryogen getting evaporated, (Kg/sec).

)(tmLCryo Mass liquid cryogen spread over the floor, (Kg).

Q Ventilation rate of blowers, (m3/s).

)(tQGCryo Leak rate of cryogen gas, (m

3/s).

)(tQout Outgoing volumetric flow rate, (m

3/s).

)(2 tconcO Oxygen concentration at time ‘t’, (Vol %).

VCV . Volume of the control volume, (m3).

REFERENCES

1. G.Pal, et.al, VECC Superconducting Cyclotron Cryogen Distribution System, 18th International

Cryogenic Engineeering Conference. (ICEC 18), Mumbai, (2000), p. 451-454

691

2. T.K. Bahttacharyya, et.al., control and Instrumentation for the VEC Superconducting Cyclotron

Cryogen Delivery System, Asian Particle Accelerator Conference APAC, (2007), p. 452-454

3. J. R. Delayen et.al, Cryogenic Safety Manual, Argonne National Laboratory, (Sept., 2001), p. 35

4. M. N. Ozisik, Heat Transfer: A Basic Approach, McGraw Hill Book Co., Intl. Edition (1985), p

121

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