Bulletin 5009 [Rev: 6/10/00]
Overfilled Cl2 Gas Cylinder Precaution

Chemical Injection Technologies
Technical Bulletin
Overfilled Chlorine Gas Cylinders
Cautions, and Preventive Measures
WHAT IS THE PROBLEM?
Periodically, we see problems associated with chlorine gas
cylinders which have been filled with liquid chlorine beyond
their rated capacity. Usually the problems manifest themselves
as liquid chlorine observed flowing through the chlorinatorvacuum system, or ice forming either on the cylinder valve oron the chlorine gas flow meter.
There have been a few instances where overfilled cylinders
have actually caused damage to the gas chlorinator vacuum
regulator, due to extremely high pressures generated in thechlorinator inlet valve area. Luckily, these incidents have been
extremely rare, although in recent years we have seen amarked increase in total problems caused by overfilledcylinders.
HOW MUCH CHLORINE SHOULD A FULL CYLINDERCONTAIN?
Although there are still some cylinders in use today that aredesigned to hold only 100 pounds of liquid chlorine, thestandard cylinder is designed to hold a MAXIMUM of 150
pounds of liquid chlorine. At 72N F, 150 pounds of chlorine
will occupy only 89% of the total volume of the cylinder. Underno circumstances should a chlorine packager fill a chlorinecylinder with more liquid chlorine than is specified by The
Chlorine Institute and the U.S. Department of Transportation.
WHY IS IT DANGEROUS TO OVERFILL A CYLINDER?
The pressure of chlorine gas (Vapor Pressure) is a function of
the temperature of the liquid chlorine in a cylinder. Figure 1
shows the vapor pressure of liquid chlorine at varioustemperatures. The curve assumes that there is sufficient space
above the liquid for the vapor (gas) to exist. Notice that at
normal ambient temperatures, the vapor pressure is relativelylow (compared to liquid propane or oxygen, for example) andat 80N F the pressure would be approximately 100 PSIG. Evenat 120N F, the vapor pressure is less than 200 PSIG.
These are the pressures that would normally be expected toexist in a gas chlorination system. All gas chlorinator
manufacturers design their chlorine gas inlet valve assemblies
to handle pressures many times that which would be expected.
However, when a cylinder is overfilled, the values given inFigure 1 may no longer be valid.

Overfilled Cl2 Gas Cylinder Precaution, Rev1-10062000

The reason is that liquid chlorine expands with an increase intemperature. That expansion is allowed for by only partiallyfilling the cylinder, so the expanding liquid will have a place togo. Recognizing that the vapor pressure can build up very
rapidly when the temperature gets higher, cylinder valves areequipped with a fusible metal relief device or fusible plug.
Most valves have a threaded plug containing the fusible metalscrewed into a tapped hole in the valve body, below the valve
seat. The fusible metal is designed to yield or melt between
158N F and 165N F to relieve VAPOR pressure and prevent
container rupture if exposed to fire or other high temperature.
The relief device is only activated in the event of a temperature
increase. It is not a pressure relief valve.
When a cylinder is filled beyond its intended capacity,
increasing temperature will cause the liquid chlorine to expand.
At some point the liquid may completely fill the cylinder (knownas “skin-full”) and Instead of building gas vapor pressure, it
starts to build HYDROSTATIC pressure. While gases arecompressible, liquids are either incompressible or only slightlycompressible. FIGURE 2 shows the relationship of chlorine
liquid volume to temperature, in a container loaded to its
authorized limit Notice that the liquid volume becomes greateras temperature increases. If the cylinder was filled to its
authorized limit, with enough gas area above the liquid to allow
the liquid to expand and compress the gas, the volume of theCopyright © 1994 Chemical Injection Technologies, Inc., Bulletin 5009

Printed in USA

liquid will increase. At 72N F the liquid occupies 89% of thetotal volume of the cylinder, but at 120N F it occupies 95% ofthe volume. Under any normal operating condition, This is nota dangerous situation, and the pressures are well within thedesign range of all gas chlorinator vacuum regulators, by
several orders of magnitude.
When the cylinder is overfilled, there is less room for thisexpansion to take place. In fact, it is quite possible for theliquid to expand and completely fill the cylinder when thetemperature rises. Sunlight hitting the cylinder or increased
ambient air temperature can then create a dangerous situation
where the HYDROSTATIC pressure can reach VERY HIGHPRESSURES. Depending on the amount of overfilling and thetemperature increases, pressures can go into the THOUSANDSof POUNDS PER SQUARE INCH.

valve sealing area, allowing very high pressure chlorine gas
and liquid chlorine to enter the vacuum regulator. Since allmodern gas chlorinators are constructed of engineering
plastics to avoid corrosion and chemical attack, their ability tohandle extremely high pressures in such a situation is very
limited. The normal operating environment is usually less thanatmospheric pressure, and such high pressures can cause
rupture of the regulator body, releasing chlorine gas into thesurrounding atmosphere.
The good news is that the chlorine release would be controlled
by the cylinder valve orifice, in much the same way as therelease though the fusible plug. As the gas and liquid expand
into the atmosphere, the remaining liquid in the cylinder willrapidly cool, thereby slowing the release and quickly lowering
the pressure. Overfilled cylinders always represent thepotential danger of a cylinder wall rupturing, causing a very
rapid chlorine release and the possibility of turning the cylinder
into a flying projectile.
WHAT PRECAUTIONS CAN USERS TAKE?
We recommend that all cylinders be weighed when received.
The Tare weight (empty weight) of the cylinder is stamped on
every chlorine cylinder. Deduct the tare weight from the total
cylinder weight, and if it is more than a few pounds over 150
pounds, call the chlorine supplier and explain the situation so
he can immediately retrieve the cylinder and safely evacuate
the contents. In any case, the cylinder should be placed in asafe area and kept cool.

This is a very different scenario than a cylinder which has been
filled to its authorized limit, exposed to a large increase intemperature. In this case, only a very small increase intemperature can produce a very significant increase inpressure.

HOW DOES OVERFILLINGCHLORINATORS?

AFFECTGAS

As mentioned earlier, all gas chlorinator manufacturers overdesign their equipment to handle cylinder pressures in excessof that which would be expected to be encountered using acylinder loaded to its authorized limit. Overfilled cylinders,
however, represent a situation which cannot be foreseen, andpressures that can vary from as little as a few hundred PSIG
to over 2000 PSIG and more.
Such pressures could cause failure of threads or materials
within the chlorine gas inlet area. Such failure can range frommild “venting” of gas past the inlet valve, to a rupture of theOverfilled Cl2 Gas Cylinder Precaution, Rev1-10062000

Unfortunately, many scales that are used to weigh the contentsof chlorine cylinders today, are designed to “Dial out” the tare(empty) weight of the cylinders and thus read only theremaining pounds of chlorine. In such cases, we recommend
using a small, inexpensive bathroom scale. Caution should beexercised when moving chlorine cylinders onto such a scale
since the height of the weighing platform is usually higher than
the scales used exclusively for weighing chlorine cylinders. Asmall ramp is often placed next to the scale to facilitate easyrolling of the cylinders on and off the scale. Placing the scale
in a small scale “pit” is also a safer and more convenientmethod.

RECENT EXPERIENCES
As mentioned above, we have seen more instances ofoverfilled cylinders in recent years. We are not in the chlorine
packaging business, so we cannot give an accurate opinion
on the cause for this apparent increase in incidents.
Our experience, however, has shown that most suppliers ofchlorine cylinders are conscientious about following strict
procedures in filling the cylinders.. Human error is always apossibility, even when a company takes extra measures toinsure compliance.
We urge all users of chlorine gas to take the simple
precautions outlined above, and to be aware of the potential forreceiving an overfilled chlorine cylinder.

Copyright © 1994 Chemical Injection Technologies, Inc., Bulletin 5009

Printed in USA