Changes have been made to ISO 15996:2017, with clearer specification of design, type testing and marking requirements for valves which incorporate residual pressure devices (residual pressure valves, or RPVs) on cylinders conveying compressed or liquefied gases.
The standard does not apply to RPVs for portable fire extinguishers, cryogenic equipment, low pressure refrigerant gases (cylinder test pressure less than 50 bar), dissolved gases or liquefied petroleum gas (LPG).
The revisions made to ISO 15996:2017 further elaborate on requirements for cylinder valves incorporating residual pressure devices (adding to the requirements outlined in ISO 10297). The purpose of an RPV is to maintain residual pressure inside the cylinder in order to prevent the ingress of air, moisture or other contaminants from external sources. Over time, such contaminants could damage the cylinder’s structural components and thus reduce its reliability and lifespan. In particular, moisture can lead to extraordinarily aggressive corrosion on the inside of the cylinder. By maintaining “residual” pressure (hence the name), also known as “specific pressure”, it is possible to create a positive pressure barrier that prevents the ingress of foreign substances. A classic example of when contaminants might enter the cylinder is if a user leaves the valve open because there is no more gas inside.
There is another important factor to consider, namely that residual pressure devices (RPDs) are divided into two main types: those with a non-return feature to prevent backflow, and those without it. When this technology is present, it prevents cylinders from being refilled with gas unless an appropriate filling connector is used. Having this additional safety measure is especially important when several cylinders containing different gases are connected to the same line, as it prevents a cylinder from being contaminated by gases that are different than the one it is meant to contain. In other words, it prevents mistakes. ISO bodies wanted to revise existing regulations on this issue because the previous standard seemed incomplete, leaving it open to unfavorable interpretations as to how it was to be applied.
The main technical and normative changes
Several changes accompany this latest revision of the standard. The first starts with the scope of the standard itself, which now extends to cover the complete RPV (i.e. the valve including the residual pressure device) and not the residual pressure device only, as was the case before. To that end, the new standard also indicates which tests must be done on the valve, in addition to those to be performed on the residual pressure device. The second major change is that the standard now applies to valves on so-called “cylinder bundles” (meaning several cylinders that are connected to each other), specifically main valves which contain a residual pressure device. A third change worthy of note: it is now clearly specified that this standard does not apply to valves on cylinders containing acetylene. In terms of general technical definitions, the standard no longer defines devices with or without a non-return function as type A or type B: now, residual pressure devices that include the non-return function are defined as “type 1”, whereas those which do not have this function are defined as “type 2”.
Regarding the performance of residual pressure devices, the standard now defines a minimum value of 1.5 bar for the close-off pressure. This parameter used to be established by the valve manufacturer, but now the standard has defined a precise minimum value at which the RPD must be triggered in order to stop the flow. This change will increase the reliability of work carried out at filling stations, as technicians will now have a reference value to guide them. Another undoubtedly significant change concerns the number of endurance cycles that an RPD has to ensure in order to be certified. The previous standard always spoke of 100,000 cycles for both type A and type B devices, but this was a very generic indication and there was no technical explanation to justify these numbers. Now, however, only type 1 RPDs will have to meet this rigorous standard, as it should be. Lastly, there has been an important change in the marking of these devices: indeed, the standard now requires RPDs to be clearly marked as either type 1 (T1) or type 2 (T2). Furthermore, the standard now requires that the letter “A” be added to the T1 or T2 label if the device has a close-off pressure under the minimum standard of 1.5 bar.