SAFETY AND LOSS PREVENTION ERT 322 SOURCE MODEL Prepared by; Miss Syazwani Mahmad Puzi School of Bioprocess Engineering

INTRODUCTION Most 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? 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

Limited aperture release Mode of release Wide aperture release Limited 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 - Liquid Flow of liquid through a hole Flow of liquid through a hole in a tank Flow of liquids through pipes

Flow of liquid through a hole 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, dimensionless g 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) Equa. 1

Liquid flowing through a hole Liquid pressurized within process unit External surroundings P = 1 atm ū2 = ū A = leak area P = Pg ū1 = 0 ∆z = 0 Ws = 0  = liquid density Equa. 2 Equa. 3 Co Discharge coefficient

Discharge Coefficient Value Event 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 4.1 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 in a tank Equa. 5 = liquid density A = leak cross sectional area hL ū2 = ū P = 1 atm ū1 = ū Ws = 0 If the vessel is at atmospheric pressure Pg=0 Equa. 4 Equa. 6

Flow of liquid through pipes Mechanical energy balance for the flow of incompressible liquids through pipes, (density is constant) Frictional loss, F Equa 7 Equa. 8 Kf excess head loss due to the pipe/fitting, dimensionless u fluid velocity (length/time)

For fluids flowing through pipes the excess head loss, Kf f fanning friction factor, dimensionless L flow path length d flow path diameter Equa. 9

Determination of Kf 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

2-K Method Excess Head Loss, Kf K1 & K∞ 2-K constants for loss coefficient, dimensionless ID internal diameter Pipe entrances/exit 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 = 0.6. Solve Equa. 12, Co = 0.63 Equa. 12

Problem 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 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. Equa. 13

constant physical properties over the temperature range To to Tb 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; Equa. 14 Assumption: constant physical properties over the temperature range To to Tb Equa. 15

For liquid stored at their saturation vapor pressure, P = Psat, mass flow rate is given by; Equa. 16

Example 4.8 Propylene 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/kg Vfg = 0.042 m3/kg Psat = 1.15 x 106 pa Cp = 2.18 x 103 J/kg.K

Liquid Pool Evaporation or Boiling Mass flow rate Heat flux from the ground Rate of boiling Equa. 17 Equa. 18 Equa. 19

Problem 4.35 Estimate 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/s ks = 0.92 W/m.K Tliq = 109 K Tsoil = 293 K ∆Hv = 498 kJ/kg at 109 K

Example 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.