Fugitive Emissions

Gestión Ambiental

Tema 5

Fugitive Emissions

• An average sized manufacturing plant have 3000-30.000 components (pumps, valves, compressor, seals pipe flanges) that can leak

• Even well maintained equipment there is some unintentional releases (Fugitive emissions)

• These emissions may be:– Continual leakage of small amounts of process fluids due to

faulty process equipment– Sudden, major leaks due to equipment failure

Standards normally define

• Chemical streams that must be monitored• Types of components (pumps, valves,

connections,…) to be monitored• Measured concentrations that indicates a leak• Frequency of monitoring• Actions to be taken is a leak is discovered• Length of time in which an initial attempt and an

effective repair of the leak must be made.• Actions that must be taken if a leak cannot be

repaired within guidelines.

Sources and amounts

• Fugitive emissions can originate: – At any place where equipment leaks may occur.– Pipe connections– Evaporation of hazardous compounds from open-

topped tanks or reservoirs– Dust from activities as construction, demolition, traffic,

waste collection, agriculture

The cumulative impact from the thousands of components with small emissions can be staggering.

Typical distribution of fugitive emissions in process plants

Measuring fugitive emissions

• Use of direct measurement equipment– Time consuming

• Methods for estimating the fugitive emissions– Average emission factor approach– Screening ranges approach– EPA correlation approach– Unit-specific correlation approach

Average emission factor approach

• Based on knowledge on the number and type of each component, the service of each component is in, the total organic concentration of the stream, and the time period each component was in service.

• ETOC = FA WFTOC– ETOC = TOC emission rate from a component (kg/h)– FA = Average emission factor for the component (kg/h)– WFTOC = Average mass fraction of TOC in the stream serviced by the component

• The calculation should be used only to determine whether the aggregate of units is emitting more VOCs than allowable

• It does not account for specific differences at an individual facility

Emission factors

SOCMI = Industria química orgánicaSOCMI, Synthetic Organic Chemical Manufacturing Industries

Screening ranges approach

• It is more exact than the average emission approach because it relies on screening data from the facility, rather than on industrywide average values.

• It is assumed that components with screening values greater or lesser than 10.000 ppmv have a different average emission rate.

• The application of this method is similar to the previous one, except that the number of components leaking less and more than 10.000 ppmv are calculated separately.

Screening ranges approach

ETOC = (FG NG) + (FL NL)

• ETOC = TOC emission rate for an equipment type (kg/h)

• NG = Units with screening values >10.000 ppm

• NL = Units with screening values <10.000 ppm

• FG = emission factor for sources with screening values >10.000 ppm, kg/h source

• Fl = emission factor for sources with screening values <10.000 ppm, kg/h source

Screening ranges approach factors

EPA correlation approach

• Predict mass emission rates as a function of screening values for a particular equipment value. Correlations relating screening values to mass emissions rates for SOCMI process units and for petroleum units are listed.

• The default-zero leak rate is the mass emission rate associated with a screening value of zero. This provides an emission rate for components where the screening rate was below the detection limit of the organic vapour analyser.

EPA correlation approach

Unit-specific correlation approach

• More exact and expensive method• It requires the collection of screening

values and corresponding mass emissions data for a statistically significant number of each piece of process unit equipment.

• It is necessary to obtain data for different screening ranges. 1-100, 101-1,000, 1,001-10,000, 10,001-100,000, >100,000 ppmv

Controlling fugitive emissions

Two primary techniques are used for reducing fugitive emissions from equipment:

• Modifying or replacing existing equipment

• Implementing a leak detection and repair program.

Equipment modification

• Installing additional equipment that eliminates or reduce emissions

• Replacing existing equipment with sealless types

Most fugitive emissions come from leaking valves, due to deterioration of the packing material. To control these emission must be considered:

- Component monitoring

- Stem sealing

- Mechanical conditions

Use of sealless diaphragm valves

Valves

Valves

Other equipment

• Pumps and compressors– Routing leaking vapours to a closed vent system – Dual mechanical seal between which a barrier fluid is circulating

at a pressure higher than the pumped fluid– Sealless pumps (diaphragm pumps, magnetic drive pumps,…)

• Pressure relief valves– These emissions are not considered fugitive emissions – Used of closed vent systems and a flare or by use of rupture

disk-pressure relief valve combination

• Flanges and other types of pipe connectors– The emissions rate per connector is usually low

Equipment modifications to reduce fugitive emissions

Fugitive emissions from storage tanks

• Important source of fugitive emissions

• There are 6 basic tanks design– Fixed-roof tanks– External floating roof tanks– Internal floating roof tanks – Domed external floating roof tanks– Variable vapour space tanks– Pressure tanks

Fixed-roof tanks

• Vertical or horizontal• Constructed above or below ground• Steel or fibreglass• Freely vented to the atmosphere or equipped

with a pressure/vacuum vent• Fugitive emissions are caused by changes in

pressure, temperature and liquid level.• They are the least expensive , but are generally

considered the minimum acceptable equipment for storing liquids because of their potential to release fugitive emissions.

External floating roof tanks

• Open-topped cylindrical steel shell equipped with a plate roof that floats on the surface of the liquid.

• The roof rises and falls with the liquid level in the tank. The floating roof is equipped with a rim seal system, which contacts the tank wall and reduces evaporative losses of the stored liquid.

• Fugitive emissions should be limited to:– Imperfect rim seal system– Fittings in the floating deck– Exposed liquid on the wall when liquid is withdrawn and the roof

lowers.

Internal floating roof tanks

• Has a permanent fixed roof and a floating roof inside• Evaporative losses are minimized by installing a floating

roof inside.• The space between the fixed and the floating roof is

generally freely vented, so any vapours that moves into the space will be vented to the atmosphere.

Domed external floating roof tanks

• Similar to the previous one• Is usually the result of retroffiting an existing

floating roof with a fixed roof to block the wind and minimize evaporative losses.

Variable vapour space tanks

• Equipped with expandable vapour reservoirs to attributable to temperature and pressure changes.

• Use a flexible diaphragm membrane to provide expendable volume.

• May be either separate gasholder units or integral units mounted on a fixed roof tank

• Losses are limited to tank filling times when vapour is displaced by liquid and the tank´s vapour storage capacity is excedeed.

Pressure tanks

• Used for storing organic gases and liquids with high vapour pressure

• Equipped with a pressure/vacuum vent that is set to prevent venting loss from boiling and breathing loss from temperature and barometric pressure changes.

• Losses from these tanks should be minimal, provided that the vent is well maintained and the tanks are not overpressurized.

Tanks

Tanks

Emissions estimations

• Looses from fixed-root tanks can occur:– Continually while the liquid is standing in the tank– Working losses when liquid is being added or withdrawn from

the storage tank

• Assuming that the tanks are substantially liquid and vapour tight and operate at atmospheric pressure:

• LT = LS + LW

• LT = total losses

• LS = standing storage losses

• LW = working losses

Ls = 365 VVWVKEKS

VV = vapour space volume, ft3

WV = vapour density, lb/ft3

KE = vapour space expansion factor, dimensionless KS = vented space saturation factor, dimensionless

– KE = TV/TLA + (PV – PB)/(PA – PVA)TV = daily temperature range, ºRTLA = daily average liquid surface temperature, ºRPV = daily pressure range, psiPB = breather vent pressure setting range, psiPA = atmospheric pressure, psiPVA = vapour pressure at daily average liquid surface temperature, psi

– KS = 1/ (1 + 0,053 PVA HVO)HVO = vapour space outage, ft

Standing storage losses

Working losses

LW = 0.0010 MV PVA Q KN KP

• Mw = Vapour molecular weight, lb/ft3

• Q = annual net throughput (tank capacity (bbl) times annual turnover rate), bbl/yr

• KN = Turnover factor, dimensionless– For turnover > 36/year, KN = (180 + N)/6N– For turnover < 36/year, KN = 1

• KP = working loss product factor, dimensionless – For crude oils = 0,75– For all other liquids = 1,0

Emissions control

• Emissions from organic liquids in storage occurs by:– Evaporative losses during liquid storage– Changes in the liquid level during filling and emptying operations

• Emissions from fixed-roof tanks can be controlled by:– Installing an internal floating roof and seals (60-99% efficiency)– Vapour exchange (90-98% efficiency)– Vapour recovery systems to convert them to a liquid product (96-

99%)

Fugitive emissions from waste treatment and disposal

• Many units require vigorous mixing and turbulence

• Other units contain more quiescent liquid but require large expanses of surface area exposed to the air.

• Land application of wastewater is another significant source of fugitive emissions.

• Options:– Cover the equipments– Minimizing turbulence at points where is not needed