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Pressure relief devices (PRD) have been used in cylinder valves since the 1930s to protect full cylinders from rupture in a fire. Further pressure-activated PRD's can also protect a cylinder that is overfilled from bursting.
In layman's terms, the PRD can be compared to the safety mechanism of a pressure cooker. If the pressure cooker fails to release excess pressure, the inner pressure will rise above controllable levels. The safety valve, generally placed on the cooker's lid, is designed to activate and let out the excess pressure to prevent an explosion.
The need for safety devices was first identified when steam boiler explosions took place on ships and factories in the 1800s and early 1900s. James Watt used the Papin design for a safety valve for steam engines.
In 1952, the Compressed Gas Association (CGA) published CGA S-1 giving minimum requirements considered appropriate for safety devices for cylinders having a water capacity of 1000 pounds or less.
It must be noted that PRD's do not protect the cylinder from rupture under all conditions, for example, a partially charged cylinder in a fire. The use of PRD's is beneficial if they operate under design conditions, but there are also risks associated with their usage. The risks arise due to the possibility of premature activation or leakage through the PRD, resulting in unexpected release of the product.
It is not this blog's purpose to take a position "for" or "against" the use of PRD. Regulations mandate the use of PRD's in many cases (e.g. for high-pressure liquefiable gases) and prohibited for use with highly toxic gases. In other cases, mainly when PRD is optional, it is provided if requested by the buyer
The activation of PRD is a "Notable" event, and the activation of PRD occurring within design rated parameters of pressure, temperature, or both should be accepted if not welcomed. The PRD is designed to release gas content (partial or full depending upon the type of PRD) to protect the cylinder from bursting and prevent severe damage to life and property.
Unfortunately, every occurrence of PRD activation (thankfully few and far between) has been reported as a complaint doubting the quality of the product (even if the valves were put in service many years back). We cannot recall any instance where the customer admitted that an increase in ambient temperature or overfilling was the likely cause for the PRD to actuate.
PRD's from reputed valve manufacturers have invariably performed reliably in the field and have helped save lives and damage to the surroundings due to activation as designed. Improvements in the quality of raw material used to manufacture burst discs / fusible alloy and precise machining affecting the performance of the PRD's have made new generation PRD's as reliable as they ever were since they were put to use.
Next time you come across PRD activation, try to consider the possibility that the PRD performed its duty and helped ensure safety. After all, when equipped with the cylinder valves, PRD's are essential safety devices meant to release the contents before the pressure inside the cylinder reaches above the burst pressure of the cylinder and prevents its rupture.
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