All cylinders must be equipped with an over-pressure safety relief device such as a rupture disk or fusible plug.
The spring-loaded relief valve is another type of over-pressure safety relief device that is common to the commercial refrigerant sector.
Relief valves are designed to relieve vapour pressure only.
These valves must never come into contact with liquid refrigerant as the refrigerant expands faster than the relief valve’s ability to bleed off the refrigerant vapour.
The rupture disk is a flangible disk typically welded into the cylinder shoulder.
Should the cylinder pressure exceed rating of the rupture disk a coin will burst and the cylinder’s contents will vent, preventing an explosion.
The fusible plug has a soft material with a low melting point.
It is threaded into the bottom of the cylinder.
When temperatures exceed 130°F it will vent the contents.
The spring-loaded relief valve is a loaded spring and seat valve set to open when the pressure exceeds the safety rating.
The pressure forces the spring to open lifting the valve from its seat and venting the contents through the relief port.
Filling Reusable Cylinders
Little attention has been paid to the dangers that technicians face when recovering refrigerant into reusable cylinders.
A good safety record exists in this area primarily because refrigerant has been packaged at the factory in disposable “discharge only” cylinders.
Disposable cylinders are filled in a controlled environment at the factory, according to prescribed industry standards set by the cylinder manufacturers.
With the return of reusable cylinders for the storing and removal of refrigerants between equipment and storage or reclaiming sites, the potential for problems and accidents will increase.
Like all fluids contained in a pressurized cylinder, the liquid refrigerant will expand and the gaseous refrigerant will increase in pressure as the temperature increases.
All pressurized vessels are potentially explosive.
Anyone witnessing the results of a ruptured cylinder knows the devastating force it unleashes.
Returning to reusable cylinders presents technicians with the very important safety issue of preventing the overfilling of refrigerant cylinders when recovering refrigerant and the resulting personal injury, property damage and financial liability.
For more information, see Storage Guidelines within the Code of Practice.
There are three primary causes of a ruptured cylinder:
Under certain conditions, liquid refrigerants will expand to a point where the cylinder can no longer contain the pressure.
Underwriters’ Laboratories of Canada (ULC) has scientifically determined the maximum volume of refrigerant that can be safely stored in a cold cylinder and later transferred to locations is 80%.
This expansion condition can exist when a cylinder is filled to a level above 90% of the capacity of the cylinder in the cool of the morning.
When temperatures are hot, refrigerant can expand to completely fill the cylinder, causing the cylinder to rupture.
Overfilling can also occur when recovering refrigerants that contain oil.
Oils are less dense than refrigerant and therefore weigh less.
Care should be taken to ensure that the cylinder is filled to 70% of its capacity by weight.
Caution must be exercised to never overfill the cylinder with virgin refrigerant to more than 80% of its weight by volume.
When recovering refrigerant of an unknown quality, such as contaminated refrigerant from a compressor burn-out, a good rule to follow is to fill the cylinder to no more than 70% of its total volume by weight.
Misapplication of Relief Valves on Cylinders
Technicians often mistakenly believe that cylinders equipped with pressure relief valves are safe from reaching dangerous pressure levels.
The reality is that relief valves are designed to relieve vapour pressure only, not liquid pressure.
When a cylinder is overfilled with liquid refrigerant, the relief valve does not provide the protection it was designed for.
Liquid refrigerant under pressure when exposed to a pressure relief valve, will expand faster than the relief valve’s designed capacity to bleed off the refrigerant gas, resulting in an over-pressured cylinder and possible rupturing of the cylinder.
Whether the cylinder ruptures or not, the relief valve will vent refrigerant into the atmosphere, which is illegal and harmful to the environment.
False Sense of Security
Many technicians gain a false sense of security from the knowledge that cylinders are tested, inspected and certified every five years.
There is concern that the corrosive effects of acid in recovered liquid refrigerant could seriously reduce the ability of cylinders to withstand design pressures.