Encyclopedia of Environmental Science and Engineering, Volume I and II

PREVENTION OF TOXIC CHEMICAL RELEASE 1025

content of the air inside the equipment to a point that acety-

lene (or other gas) may be subsequently introduced with-

out forming an explosive mixture. From Figure 1 it may

be observed that the safe condition will be reached when

the oxygen content within the vessel or equipment has been

reduced by the introduction of nitrogen to below approxi-

mately 6.6% by volume, point F where FC drawn tangent

to LDU at D intersects the vertical axis AB. Any mixture

of this combustible gas, nitrogen and air having an oxygen

content represented by a point below this line FC may be

diluted with the combustible gas without forming an explo-

sive mixture (see Appendix).

WHEN SHUTTING DOWN

Likewise, when shutting down or withdrawing equipment

containing acetylene from service or when purging from

combustible to inert gas, the object is to reduce combustible

gas content to such a point that the air may subsequently

be introduced without fear of forming an explosive mixture.

Figure 1 shows that a safe condition will be reached when

the combustible gas content within the equipment or piping

has been reduced to below 4.8% by volume, denoted by

point E.

On occasions the mixtures of acetylene, nitrogen and air

are to be diluted with a further quantity of inert gas so as to

render them nonexplosive or incapable of forming explosive

mixtures on later addition of air. Object here is to add suf-

ficient nitrogen to convert the existing composition of the

mixture to some composition corresponding to a point to the

left of LD and DE. If we assume that the mixture is again

denoted by point X, then on addition of nitrogen, successive

mixtures will be formed along XB. Point where XB crosses

DE denotes the safe endpoint composition. Then on the fur-

ther addition of air (intentional or accidental) the line of mix-

tures will fall to the left of LDE.

In practice, it is desirable to decrease the proportions of

oxygen and combustible gas below the determined values to

provide a factor of safety. To prevent explosion in chemical

processing, it is necessary to keep the air–gas or–vapor or

mixture of the gases, air and nitrogen, below the LEL within

the equipment most likely to be affected. An established safe

practice is to keep the mixture well blended with air or inert

gas so that calculated or experimental values never reach

more than 25% of the LEL or other low limit as determined

from Table 1 in temperatures below 250°F. Placing system

into service or out of service should be done when system

has been depressurized to atmospheric pressure.

DEPRESSURIZING TO ATMOSPHERE

More often than not, combustible-gas releases to atmosphere

do not contain air. When handling acetylene in its pure

state a maximum pressure of 15 psig is usually used. Thus

on release to atmosphere large quantities of the gas may

be emitted through pressure relief valves to flares or high

stacks. Then again the gas may be returned to gas holder

storage. If released to atmosphere, inert gas purging should

be provided. An excellent reference on inert-gas purges of

stacks on pressure releasing has been published * ; for process

equipment see review.^12

SAMPLING OF MIXTURE CONTENTS

During startup or shutdown of systems, sampling of mix-

tures within equipment should be conducted downstream of

the point of injection of nitrogen to ensure proper mixing

of gases before sampling. Taking samples from stratified

streams should be avoided. For piping systems sampling

should be taken downstream of elbows and open valves. In

anticipation of this elbows and turns as well as valves should

be designed into the system originally. In actual testing it has

been observed that form 6 to 10 volume changes are neces-

sary to purge equipment or piping systems. Sampling should

begin after this purge.

Purging to a point below the LEL may be done using

an explosimeter^ †^ that has been properly calibrated for the

gas–air–nitrogen mixture. Laboratory testing procedures

may also be used where samples are washed† with acetone

and the volume of acetylene absorbed deducted from the

entire sample to give volume of the remainder gases as air

or nitrogen.

Spark-resisting tools should be used to prevent sparking

and possible ignition of combustible gas. Work shoes with

rubber soles and so steel studs should be used. In addition,

continuous sampling of the workroom atmosphere should be

conducted using explosimeters built for that purpose.

INERT GAS SUPPLY

Where rigid purity of inerting gas is not a requirement, the

use of inert gases obtained from the combustion of hydrocar-

bon fuels may be used. This decision is one to be based on

experience and processing needs.

Gas analysis can now be accurately measured and con-

trolled in tenths of a percent range. These inert gas genera-

tors are provided with rugged instrumentation and controls

for hazardous environments as found in desert and offshore

installations. The analysis can even be accurately measured

down to the ppm range where instrumentation maintenance

is present. The same caliber of technician is needed whether

or not the nitrogen source is cryogenic or hydrocarbon.

Such inert gas generators are currently supplied with

automatic trimmer control which analyzes the discharge gas

and automatically controls the air-to-fuel ratio. For critical

installations they are even equipped with inexpensive auto-

matic vent and alarm systems. For more information and

their technical manual, write to C.M. Kemp Manufacturing

Company, Glen Burnie, Maryland 21061.

† Explosimeters are available from Mine Safety Appliances

Company, Pittsburgh, PA 15235 and Davis Emergency Equipment

Company, Newark, New Jersey.

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