Acetylene Cylinders: Welding gas Equipment

Acetylene Cylinders: Welding Gas Cylinders

Acetylene is an extensively used gas in welding industry. It is used for both welding and cutting operations. In most fabrications industries, acetylene cylinders are a common sight. It is important to understand the right storing pressure, behaviour of acetylene, how it is stored inside the cylinder, how to operate the cylinder safely, etc. for overall safety of personnel and equipment.

Acetylene Cylinders: Welding gas Equipment
The figure below illustrates the section of a typical acetylene cylinder. For oxyfuel gas welding process, these cylinders come in three capacities of 60, 100 and 300 cubic feet.

Properties Of Acetylene
Acetylene is a hydrocarbon, with each molecule containing two atoms of carbon and two of hydrogen. The carbon atoms are bonded to each other by a triple bond, while one hydrogen atom is bonded to each of the carbon atoms.

Acetylene is an explosive gas under high pressures. When stored at a pressure greater than 15 psi (103.4 kPa), it breaks down from the heat, and may explode. But at higher pressures of 29 psi (203 kPa), it becomes self-explosive, and a slight shock can make it explode spontaneously.

Hence it is clear that it is a dangerous gas, and its’ safe handling is an imperative. Due to its’ explosive nature, it is not stored as a single entity. It is mixed with a solvent. A porous mass is stuffed into the cylinder. This mass acts like a heat sink and minimizes dangers of a flashback.

Other than its’ explosive nature, acetylene is a fine gas. It has high amount of chemical energy stored in its’ molecules that gets released during combustion at the welding torch. It combines with air to produce a intense hot yellow, luminous flame.

The acetylene on combustion in presence of oxygen produces concentrated flames of high temperature, that are enough to melt most metals. Though it should be noted that the calorific value of acetylene is less than that of LPG and other synthetic gases, the usability of acetylene for industrial welding operations is high, due to many reasons.

Acetylene, when used with oxygen, produces the highest flame temperature of any of the fuel gases. It also has the most concentrated flame but produces less gross heat of combustion than liquid petroleum gases and synthetic gases.

While acetylene is nontoxic gas, it can asphyxiate, and cause suffocation when present in high concentrations. Also, it is an anesthetic. It is manufactured by reaction of calcium carbide with water in an acetylene generator unit.
Acetylene gas cylinders for welding
Storage Of Acetylene Cylinders
Acetylene is not stored as a single entity inside the cylinder. The cylinder is filled with porous material such as crushed brick, charcoal, balsa wood, etc. Over the porous material, acetone is added in large quantities, till the porous material is saturated by about 40%. The acetone absorbs acetylene, and stabilizes it during storage.

This porous material acts like a sponge that absorbs acetone, which in turn absorbs acetylene.

The storage pressure of acetylene in the cylinders is kept below 15 psi (103 kPa), because above this pressure – the gas becomes unstable. Shocks or sudden exposure to heat can cause the cylinder to explode. Therefore, filled cylinders should be carefully stored sufficiently away from furnace, operations involving open flames, or sparks coming from a welding or cutting torch.

The cylinder should be stored in an upright condition, and care should be taken to ensure that it does not hit unnecessarily with other objects or the ground. While loading/unloading from the transport vehicle, the cylinder should not be dragged, rolled, or made to slide on its’ sides.

Also, during storage and during welding/cutting operation, the cylinder should be securely fastened so that it does not fall over.

When the gas is drawn from the cylinder during welding or cutting, the rate of drawing should not exceed 1/7th of the capacity of the cylinder per hour. For example, if the cylinder contains 300 cubic feet of acetylene, the rate of drawing should not exceed about 40 cubic feet per hour.

This is necessary to avoid drawing acetone out of the cylinder, which can impair the weld quality severely. When and if a higher rate of gas supply is required, multiple cylinders should be used, and the supply should be made through a manifold.

Safety Features Of Acetylene Cylinders

For the safety purposes, acetylene cylinders are provided with safety plugs (See figure 1 above) that give way when exposed to high temperatures or pressures.

The safety plug is basically a hole plugged with a metal alloy piece that melts when exposed to 100°C (212°F) or 500 psi (3450 kPa). When the temperature or pressure inside the cylinder exceeds these values, the metal piece melts and allows acetylene to escape out before higher pressure/temperatures can develop inside the cylinder.

The hole is of quite small diameter such that if the escaping gas is ignited outside the Acetylene cylinders, the flame does not invade the cylinder.

The cylinders come fitted with valves that must be opened when required. The valve body is made of brass, with a stainless steel square stem. The stem must be opened or closed with a wrench when it is required. The outlet of the valve contains a thread, which connects the pressure regulator to the cylinder. A protective metal cap is screwed on this thread to prevent damage during storage or transport.

The thread must be in good condition. If it is damaged, the cylinder should be set aside for return to supplier.

The inlet connection of the regulator fits on the threaded valve stem. The union nut tightens the connection.

Cylinders should be stored in open areas, or well ventilated spaces. So this was a little about acetylene cylinders. Please do let know your thoughts in the comments section below.

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