SAFETY AND LOSS SAFETY AND LOSS PREVENTIONPREVENTION

ERT 322ERT 322

SOURCE MODEL

Prepared by;

Mdm. Syazwani Mahmad PuziSchool of Bioprocess Engineering

INTRODUCTIONINTRODUCTIONMost accidents in chemical plants

result in spills of toxic, flammable and explosive materials.

Accidents begin with an incident, which usually results in the loss of containment of material from the process.

The material has hazardous properties, that might include toxic properties and energy content.

Typical incidents might include the rupture or break of a pipeline, a hole in a tank or pipe, runaway reaction or fire external to the vessel.

Once the incident is known, source models are selected to describe how materials are discharged from the process.

What are source models?What are source models? Constructed from fundamental/empirical equation

representing the physicochemical process occuring during release of materials.

Sometimes, we modified the original models to fit the specific situation

Only can be applied once the incident has been identified Provide technical information;

Rate of discharge Total quantity discharged State of discharge

Dispersion Model – to describe how the material is transported downwind and dispersed to some concentration levels.

Fire & Explosion Model – to convert the source model information into energy hazard potentials (eg. Thermal radiation, explosion overpressure etc.)

Effect Model – to evaluate potential loss/damage on people, properties and environment

Mode of releaseMode of releaseWide aperture

releaseLimited aperture

release

-Releasing a substantial amount of material in a short time-Large hole developing in process unit

slow release of material that causing non immediate effect to upstream

Example: overpressure explosion, explosion of a storage tank

Example: leaks in flanges, valves and pumps; ruptured pipes, cracks and relief system

Source Models - LiquidFlow of liquid through a holeFlow of liquid through a hole in a tankFlow of liquids through pipes

Flow of liquid through a Flow of liquid through a holehole A mechanical energy balance describes the various

energy forms associated with flowing fluids:

P pressure (force/area) density (volume/mass)ū average velocity of the fluid (length/time)α velocity correction factor, dimensionlessg acceleration due to gravity (length/time2)gc gravitational constant (length mass/force time2)z height above datum (length)F net frictional loss (length force/mass)Ws shaft work (force length)mass flow rate (mass/time)

.

2

2 m

WFz

g

g

g

udPs

cc

.

m

Equa. 1

Liquid flowing through a Liquid flowing through a holehole

Liquid pressurized within process unit

External surroundings

P = Pgū1 = 0∆z = 0Ws = 0 = liquid density

P = 1 atmū2 = ūA = leak area

gcomPgACAuQ 2

Co Discharge coefficient

gc

o

PgCu

2

Equa. 2 Equa. 3

Discharge Coefficient Discharge Coefficient ValueValueEvent Co

Sharp-edged orifices, Re > 30,000

0.61

Well-rounded nozzle ~1.0

Short section pipe attached to a vessel (L:D 3)

0.81

Unknown/uncertain 1.0

Problem 4.1Problem 4.1

A 0.20-in hole develops in a pipeline containing toluene. The pressure in the pipeline at the point of the leak is 100-psig. Determine the leakage rate. The specific gravity of toluene is 0.866.

Flow of liquid through a hole Flow of liquid through a hole in a tankin a tank

= liquid densityA = leak cross sectional areahL

ū2 = ūP = 1 atm

ū1 = ūWs = 0

L

gc

omgh

PgACQ

2

t

A

AgCgh

Pg

A

A

gCt

t

oo

L

gct

o

e

22

21

o

L

t

o

egh

A

A

gCt 2

1

If the vessel is at atmospheric pressure

Pg=0

Equa. 4

Equa. 5

Equa. 6

Example 4-2Example 4-2

A cylindrical tank 20 ft high and 8 ft in diameter is used to store benzene. The tank is pedded with nitrogen to a constant regulated pressure of 1 atm gauge to prevent explosion. The liquid level within the tank is presently at 17 ft. A 1-in puncture occurs in the tank 5 ft off the ground because of the careless driving of a forklift truck. Estimate (a) the gallons of benzene spilled, (b) the time required for the benzene to leak out, and (c) the maximum mass flow rate of benzene through the leak. The specific gravity of benzene at these condition is 0.8794.

Flow of liquid through Flow of liquid through pipespipes

Mechanical energy balance for the flow of incompressible liquids through pipes, (density is constant)

Frictional loss, F

.

2

2 m

WFz

g

g

g

uPs

cc

Equa 7

c

f g

uKF

2

2Equa. 8

Kf excess head loss due to the pipe/fitting, dimensionlessu fluid velocity (length/time)

The frictional loss, F represent the loss of mechanical energy resulting from friction and includes loss of friction and loss from flow through length of pipe, fitting, such as valve, elbows, orifice, and pipe entrances and exits.

For fluids flowing through pipes the excess head loss, Kf

f fanning friction factor, dimensionlessL flow path lengthd flow path diameter

d

fLK

f

4 Equa. 9

Determination of KDetermination of Kff

2 methods◦Computational method based on

Fanning friction factor◦2-K method

Event Formula

Laminar flow

Turbulent flow in rough pipes

Smooth pipes, Re < 100,000

Re

16f

d

f7.3log4

1

4/1Re079.0 f

2-K Method2-K MethodExcess Head Loss, Kf

K1 & K∞ 2-K constants for loss coefficient, dimensionless

ID internal diameter

Pipe entrances/exit

)1

1(Re

1

inches

f IDK

KK

KK

Kf Re

1

Equa. 10

Equa. 11

Discharge coefficient, Co for liquid flow through a hole;

For a simple hole in a tank with no pipe connections and fittings, the friction is caused only by the entrance and exit effects of the hole.

For Re > 10,000, Kf entrance = 0.5 and, Kf

exit = 1.0, thus ∑Kf = 1.5. Solve Equa. 12, Co = 0.63

f

o KC

1

1Equa. 12

Problem 4.11Problem 4.11

Compute the pressure in the pipe at the location shown on Figure below. The flow rate through the pipe is 10,000 L/h. the pipe is commercial steel pipe with an internal diameter of 50mm. The liquid in the pipe is crude oil with density of 928 kg/m3 and a viscosity of 0.004 kg/m.s. The tank is vented to atmosphere.

Flashing liquidFlashing liquid Flashing normally occurs when a liquid stored

under pressure above their normal boiling point is experiencing sudden ambient environment causing the liquid flash to vapor, sometimes explosively.

If the tank develops a leak, the liquid will partially flash into vapor.

The process is rapid and assumed to be adiabatic

Excess energy, Q contained in the superheated liquid is given by;

m is the mass of the liquid, Cp is the heat capacity of the liquid (energy/mass deg), To temperature of liquid before depressurization and Tb is the depressurized boiling point of the liquid.

)(bopTTmCQ Equa. 13

Q is the energy that vaporizes the liquid.

let ∆Hz is the heat of vaporization of the liquid, now, the mass of liquid vaporized, mv is given by;Fraction of the liquid vaporized, fv is given by;

z

bop

z

v H

TTmC

H

Qm

)(

z

bopv

v H

TTC

m

mf

)(

Assumption: constant physical

properties over the temperature range To

to Tb

Equa. 14

Equa. 15

For liquid stored at their saturation vapor pressure, P = Psat, mass flow rate is given by;

p

c

fg

v

m TC

g

v

AHQ

Equa. 16

Example 4.8Example 4.8Propylene is stored at 25°C in a tank at its saturation pressure. A 1-cm diameter hole develops in the tank. Estimate the mass flow rate through the hole under these conditions for propylene:

∆Hv = 3.34 x 105 J/kgVfg = 0.042 m3/kgPsat = 1.15 x 106 paCp = 2.18 x 103 J/kg.K

Liquid Pool Evaporation or Liquid Pool Evaporation or BoilingBoiling

Mass flow rate

Heat flux from the ground

Rate of boiling

Lg

sat

m TR

MKAPQ

2/1)(

)(

t

TTKq

s

gs

g

v

g

m H

AqQ

Equa. 17

Equa. 18

Equa. 19

Problem 4.35Problem 4.35Estimate the vaporization rate

resulting from heating from the ground at 10 s after the instantaneous spill of 1500 m3 of liquefied natural gas (LNG) into a rectangular concrete dike of dimensions 7 m by 10 m.

αs = 4.16 x 10-7 m2/sks = 0.92 W/m.KTliq = 109 KTsoil = 293 K∆Hv = 498 kJ/kg at 109 K