urging and inerting (or blanketing) process vessels andequipment are two common, yet distinctive, practices tocontrol the concentration of oxygen, thereby reducingfire and explosion hazards. Purging usually refers to the

short-term addition of an inert gas (e.g., nitrogen or carbon dioxide)to a tank, process vessel, or other piece of process equipment thatcontains flammable vapors or gases to render the space nonignitablefor a specific time period (say, during a maintenance outage). In con-trast, inerting (or blanketing) is the long-term maintenance of aninert atmosphere in the vapor space of a container or vessel duringoperation.

Safe oxygen levelsThe goal of these methods is to reduce a vessel’s oxygen concen-

tration below the limiting oxygen concentration (LOC). The LOC isthe concentration of oxidant below which a deflagration cannotoccur in a specified mixture. The LOCs for common flammablegases and vapors, using a nitrogen or carbon dioxide diluent, are list-ed in Table 1. A safety margin must be maintained between the LOCand the normal working concentration in the system. Conservativecontrol typically uses 2–4 percentage points below the LOC. That is,if the LOC of ethanol using nitrogen as a diluent is 10.5%, the con-trol point would be 6.5–8.5%.

Although nitrogen or carbon dioxide are the most common inert-ing gases, steam is sometimes used. If so, it must be supplied at a ratesufficient to maintain the vessel temperature at 160°F or higher andcare must be taken so that condensation by cooling does not draw inatmospheric air or collapse the vessel by implosion (1).

This temperature is experience-based and is considered sufficient toprevent condensation of the steam, which would hinder the protection

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Inert gases are used to prevent fires and explosions in the vapor spaces of equipment. Correct selection of the oxygen concentration and the inert gas-flow rate are critical to ensuring safety.

George R. Kinsley, Jr.,Environmental ResourcesManagement, Inc.

Properly Purge andInert Storage Vessels

58 www.aiche.org/cep/ February 2001 CEP

Safety

to the vessel from the steam purge.Possible sources of purge or inert-

ing gases include commercially avail-able gases supplied from high-pres-sure cryogenic tanks or standardcylinders, or on-site air separationplants that remove oxygen from theair and recover nitrogen by liquefac-tion followed by absorption, chemicalreaction, or membrane permeation.Cross-connections between thesource of inerting gas and any othersystem should not be allowed. Thegases from an enclosure or vesselbeing purged must be vented at a safelocation. The purging or inerting gasshould be introduced and exhaustedso that effective mixing is ensuredand the desired reduction in oxidantconcentration is maintained through-out the system being protected. Mul-tiple inlets and outlets are desirable topromote diluent distribution.

Several methods may be used toform and maintain a noncombustibleatmosphere in an enclosure. These in-clude batch modes for one-time use,such as purging equipment during ashutdown, and continuous inerting toassure safe conditions during normaloperations. Each method will be de-scribed and illustrative sample designcalculations will be made.

Syphon and vacuum purgingThe common batch purging meth-

ods are syphon, vacuum, pressure,and sweep-through.

Syphon purging involves filling thevessel to be purged with liquid (i.e.,product or water) followed by intro-ducing the purge gas, typically nitro-gen, into the vapor space as the liquidis drained. The required purge-gas vol-ume equals the volume of the vessel.The rate of application corresponds tothe volumetric rate of liquid discharge.Syphon purging may not be appropri-ate if the liquid is above its flashpointdue to evaporation into the space.

Vacuum purging is one of the mostcommon vessel inerting proceduresprovided that the vessel is designed forthe maximum vacuum that can be de-veloped by the source (e.g., a vacuum

pump). The steps in a vacuum purgeare: (1) drawing a vacuum on the ves-sel until the desired level is achieved;(2) relieving the vacuum with an inertgas such as nitrogen to atmosphericpressure; and (3) repeating Steps 1 and2 until the desired oxygen concentra-tion is reached. The amount of purgegas required depends upon the numberof evacuations needed to develop thedesired oxygen concentration.

The oxygen concentration x after kpurge cycles (vacuum and relief) isdescribed by Eq. 1, assuming that thepressure limits PH and PL are identi-cal for each cycle (2):

(1)

The quantity of inert gas requiredmay be calculated by Eq. 2 for k cy-cles, based on the ideal gas law (2):

(2)

Example 1: Determine the numberof purges required and the total con-sumption of nitrogen to reduce theoxygen concentration in a 5,000 galtank to 2% before introducing ace-tone. The temperature is 75°F and thevessel is initially charged with air atatmospheric pressure. A steam ejectoris used that reaches 25 mm Hg abso-lute and the vacuum of each cycle isrelieved with pure nitrogen until thepressure equals 1 atm.

Solution:xo = 0.21 lb-mole O2/total molesThe number of purge cycles k is

determined by rewriting Eq. 1 tosolve for k (2):

(3)

Substituting in (2):

(4)

The total nitrogen used is deter-mined from Eq. 2 (1):

(5)

(6)

or 1.66 lb-mol.

Pressure purgingVessels may also be purged by

adding inert gas under pressure and,after the gas has sufficiently diffused,venting to atmosphere. As with vacu-um purging, more than one pressure

V2 = 760 – 25760

5,000 gal ×

1 ft3/7.48 gal = 646.5 ft3

V2 =P2 – P1

P2V 1

k =ln 0.02

0.21

ln 25760

= 0.69 or 1 cycle

k =ln

xkxo

lnPL

PH

V2 = kPH – PL V1

PH

xk = xoPL

PH

k

Table 1. LOCs for selected gases and vapors.

Gas or Vapor LOC for N2/Air LOC for CO2/AirO2, vol. % O2, vol. %

Cyclopropane 11.5 14Natural gas (Pittsburgh) 12 14.5n-butyl chloride 14 —Acetone 11.5 14Carbon disulfide 5 7.5Ethanol 10.5 13Hydrogen 5 5.2Methyl ether 10.5 13Methyl ethyl ketone 11 13.5

Source: Adapted from Ref. 3.

CEP February 2001 www.aiche.org/cep/ 59

cycle may be necessary to reduce theconcentration of oxygen to the desiredlevel. The mathematical relationshipthat describes the pressure purgingprocess is identical to Eq. 1, exceptthat the initial concentration of oxy-gen in the vessel xo is computed afterthe vessel is initially pressurized.

Example 2: Determine the numberof nitrogen pressure purges requiredand the total consumption of nitrogenneeded to reduce the oxygen concen-tration in the 5,000 gal tank described in Example 1 to 1% oxygen. The ni-trogen is supplied to the vessel at 100psig and 75°F.

Solution: Equation 1 is used to de-termine the number of purge cycles.The initial oxygen concentration in thevessel is calculated after the first pres-surization using a simple pressure ratiowhere PL is the starting or atmosphericpressure, whichever is appropriate.

(7)

(8)

Applying Eq. 1, the number of cy-cles is:

(9)

Thus, the number of purge cycles= 1, and the nitrogen consumption iscalculated as follows:

(10)

Note the increased consumptioncompared to vacuum purging (Exam-ple 1), because the vessel must be pres-surized to 100 psig during each cycle.

Sweep-through purgingThis method introduces a purge

gas into a vessel at one opening andwithdraws the mixed gas at anotheropening and vents it to the atmo-sphere (or an air-pollution control-de-vice), thus, sweeping out residualflammable vapor. The quantity ofpurge gas required depends upon thephysical arrangement. Sweep-throughpurging is commonly used when thevessel is not rated for pressure or vac-uum. The purge gas is introduced andwithdrawn at atmospheric pressure.

The relationship between the numberof volumes of oxygen-free purge gas(e.g., nitrogen) that enters the vesseland the reduction in the vessel’s oxy-gen concentration, assuming completemixing, is shown in Figure 1 (3).

An oxygen material balancearound the vessel is (2):

(11)

Equation 11 can be rearranged andintegrated, assuming Co = 0 (2):

(12)

Integrating Eq. 12 yields Eq. 13, thevolumetric flow rate required to reducethe vessel’s oxygen concentration fromC1 (initial condition) to C2 (2):

(13)

Equation 13 assumes perfect mix-ing. Since this is not normally thecase in actual practice, a correctionfactor K is used. Table 2 lists valuesof K for certain conditions. Since lit-tle data exist on defining the degreeof mixing, conservatism recommendsa value of K no greater than 0.25.Equation 13 then becomes (3):

(14)

Example 3: A 20,000 gal storagevessel that contains 100% air must beinerted with nitrogen until the oxygenconcentration is 2% by volume. Howmuch nitrogen must be sweptthrough? Assume K = 0.25.

Solution: The volume of nitrogenrequired Qt is determined by Eq. 14:

(15)

Qt =20,000 gal

0.25ln 0.21

0.02× 1 ft3

7.48 gal

= 25,148 ft3 N2 or 9.41 air changes

Qt = VK

lnC1

C2

Qt = V lnC1

C2

Q = dto

t

= V dcc

C1

C2

V dcdt

= CoQ – CQ

V2 =100 – 14.7 – 14.7

100 + 14.7×

5,000 gal = 582.8 ft3

or 11.65 lb–mole

k =ln 0.01

0.03

ln 14.7114.7

= 0.54

xo = 0.2114.7 psia

100 psia + 14.7 psia

= 0.03 lb–mole

xo = 0.21PL

PH

■ Figure 1. Dilution ratio for purging at atmospheric pressure, assuming complete mixing.

Frac

tion

of O

rigi

nal C

once

ntra

tion

Number of Volumes of Purge Gas Injected

1.0

0.8

0.6

0.4

0.2

00 1 2 3 4 5 6

InertingContinuous inerting methods are

fixed- and variable-rate (or demand).Fixed-rate application involves con-tinuous feeding of inert gas into anenclosure (e.g., vessel) at a constantrate and the corresponding release ofa mixture of inert gas and flammablevapor that has been picked up in thevessel’s headspace. To ensure that thevessel is completely protected, therate must be sufficient to satisfypeak-demand requirements.

The peak demand for continuousinerting is typically controlled by themaximum liquid withdrawal ratecoupled with potential temperaturechanges. For a vessel containing aflammable liquid, the inert gas de-mand based on liquid withdrawal isthe capacity of the largest pump usedto withdraw liquid or the maximumpossible gravity outflow rate,whichever is greater.

The maximum demand from atemperature change will occur in out-door tanks operating at or near atmo-spheric pressure as a result of thesudden cooling from a summer thun-derstorm. The rate of inert gas neces-sary to prevent the vessel’s pressurefrom falling significantly below at-mospheric pressure is determined asfollows: (1) for tanks over 800,000gallons capacity, 2 ft3/h of inert gasper square foot of tank surface (shelland roof); and (2) for smaller tanks, 1ft3/h of gas per 40 gal of tank capaci-ty. Optionally, an inert gas rate corre-

sponding to the mean rate of changeof the vapor-space temperature of100°F/h may be used. The rates cor-responding to a temperature changeand liquid withdrawal must be addedtogether to determine the peak rate.These numbers are experience-basedand are found in Refs. 1 and 3.

Variable rate or demand inertinginvolves feeding inert gas into thevessel at a rate that is a function ofdemand. Demand is based on main-taining a pressure within the vesselthat is slightly above that of the sur-rounding atmosphere (e.g., ~1 in.H2O).

Variable application offers an ob-vious advantage over continuous inthat inert gas is supplied only when itis actually needed, thereby reducing:(1) the total quantity of inert gas re-quired; (2) product loss; and (3) dis-posal problems. A disadvantage isthe dependence upon flow control de-vices actuated by very low pressure-differentials that are sometimes diffi-cult to maintain.

For variable-rate application, aninerting system is required to main-tain such an atmosphere in the vaporspace above the liquid. Ideally, thissystem should include an automaticinert-gas addition valve or backpres-

sure regulator to control the oxygenconcentration below the LOC. Thecontrol system should feature an ana-lyzer to continuously monitor theoxygen concentration and allow inertgas to enter the space to maintain theoxygen concentration at safe levelswith a reasonable margin of safety.An increase in the concentrationabove the set point should initiate analarm and shut down the operation.Where the oxygen concentration can-not be continuously monitored, itshould be designed to operate at nomore than 60% of the LOC andchecked on a regularly scheduledbasis.

Figure 2 shows one simplemethod of flow control that can beused with a continuous introductionof purge gas. Figure 3 depicts amethod than can be used with vari-able rate application.

Practical considerationsOperation of a system with an

oxygen concentration low enough toprevent a deflagration does not nec-essarily mean that incipient fires areabsolutely prevented. Smolderingcan occur in fibrous materials at lowoxidant concentrations, later result-ing in a fire or explosion when ex-

Safety

60 www.aiche.org/cep/ February 2001 CEP

■ Figure 2. One method of flowcontrol for a fixed-rateapplication.

CheckValve

Strainer

From N2Distribution

Header

StyreneMonomer

Storage TankContaining Styrene

Monomer

ToProcess

A

Orifice Plate–Sized Based on Peak Demand

Alarm for HighO2 Concentration

O2 Analyzerin Vessel

Vapor Space

Drain

Table 2. Mixing efficiencies for selected ventilation arrangements.

Efficiency, K

Single Exhaust Multiple ExhaustSource of Supply Opening Openings

Infiltration through cracks, open 0.2 0.3–0.4doors, or windowsPowered air supply through:• Grills and registers 0.3 0.5• Diffusers 0.5 0.7• Perforated ceilings 0.8 0.9• Pipes and ducts 0.25* 0.25*

* Conservative recommendation based on NFPA 69 (3).

posed to a higher concentration. Thephysical and chemical properties ofthe flammable or combustible materi-als involved will govern the type andrequired purity of the purge gasneeded.

The following factors should beconsidered in the design of any sys-tem to reduce the oxidant concentra-tions to safe levels:

• Required reduction in oxidantconcentration;

• Variations in the process, pro-cess temperature and pressure, andmaterials being processed;

• Purge gas supply source andequipment installation;

• Compatibility of the purge gaswith the process;

• Operating controls;• Maintenance, inspection, and

testing;• Leakage of purge gas to sur-

rounding areas; and• Need for breathing apparatus by

personnel.The minimum achievable oxygen

concentration will depend upon thepurity of the inert gas, the vessel’sgas-tight integrity, and the presence ofback diffusion from the exhaust line.To maintain a given oxygen concen-tration, it may be necessary to contin-ue to supply inert gas even after at-tainment of the desired oxygen level.

Flowing inert gas will removeheat from the vessel (e.g., a heatedreactor or oven). Care should betaken that this heat removal will not

upset a reaction or cause exhaust sys-tem damage. The flowing gas willalso carry volatile materials (sol-vents) from the vessel. Considerwhat these materials will do in theexhaust system. Traps, filters, or ab-sorbers may be necessary to treat theexhaust.

Finally, there can be no doubt thatwithout nitrogen (or other inert gas)purging and inerting, many more peo-ple would be injured or killed by fireand explosion. Nevertheless, we havepaid a heavy price for the benefits ofnitrogen. Many employees have beenasphyxiated by it. During the periodfrom 1960 to 1978, one group ofcompanies reported that 13 employ-ees were killed by fire or explosion,13 by toxic or corrosive chemicals,and 7 by nitrogen.

If a person enters an atmosphereof nitrogen, he or she can lose con-sciousness without any warningsymptoms in as little as 20 s. Deathcan follow in 3–4 min. A person fallsas if struck by a blow on the head. Inone incident in which the author wasinvolved, a viewing hatch in a vesselcontaining vegetable oil was re-moved, but the nitrogen was keptflowing to protect the material fromoxidation. A supervisor did not askfor an entry permit as he intendedonly to peek in. Fortunately, someonenoticed that he was no longer movingand he was rescued in time. Whenev-er nitrogen (or other asphyxiatinggas) is used for inerting or purging,employee training, strict adherence toa confined space entry procedure andpermit system, and vessel labeling area must. CEP

CEP February 2001 www.aiche.org/cep/ 61

G. R. KINSLEY, Jr. is principal-in-charge andsafety and health practice area leader for Environmental Resources Management, Inc., Vernon Hills, IL ((847) 680-6868; Fax: (847) 680-6847; E-mail:george_kinsley@erm.com). He has served in the U.S. Army Corps of Engineers,and is a Certified Safety Professional, a U.S. patent holder, and a professionalmember of the American Society of SafetyEngineers. He holds a BSChE from DrexelUniversity and an MBA from WidenerUniversity (Chester, PA).

Literature Cited1. “Inerting and Purging of Tanks, Process

Vessels, and Equipment,” Factory Mutu-al Loss Prevention Data Sheet 7-59, Fac-tory Mutual Engineering Corp., Nor-wood, MA (1977).

2. Crowl, D. A., and J. F. Louvar, “Chem-ical Process Safety — FundamentalsWith Applications,” Prentice Hall, UpperSaddle River, NJ, pp. 195–200 (1990).

3. “Standard on Explosion Prevention Sys-tems,” NFPA 69, National Fire PreventionAssociation, Quincy, MA (1997 edition).

NomenclatureC = concentration of oxygen in the vesselCo = inlet oxygen concentration (0, when

inerting with nitrogen)k = number of purge cyclesK = correction factor in Eq. 14 for degree of

mixingPH, P2 = initial high or atmospheric pressurePL, P1 = initial vacuum pressureV = vessel volumeQ = volumetric flow rate of inerting gast = timeV1 = total volume of tank or enclosureV2 = volume of inert gas required, measured at

PHxk = final oxygen concentration after k purge

cyclesxo = initial oxygen concentration under vacuum

■ Figure 3. Typical setup for flow control in a variable-rate application.

CheckValve

Strainer

Drain

From N2Distribution

Header

StyreneMonomer

SolenoidValves

ToProcess

O2 Analyzerin Vessel

Vapor Space

Flow ControlValve, SizeBased on

Peak Demand

Storage TankContaining Styrene

Monomer

S

S

S